174
Design and AnaIysPs of a Multi-Variable Rate Channel Access Scheme for Geostationary Mobile Satellite Networ k Junaid A. Israr, B. Eng. A thesis submitted to the Facuity of Graduate Studies and Research in partial fuISllment of the requirements for the degree of Master of Engineering Ottawa-Carleton Institute for EIectrÏcal Engineering Faculty of Engineering Department of Systems and Computer Engineering Carleton University Januay 1999 O Copyright 1999, Junâid A. Israr

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Page 1: Design and AnaIysPs of a Multi-Variable Rate Channel Geostationary Mobile Satellite ... · 2005. 2. 12. · Table 4-2: Return Downlink budget for a user operating at a data rate of

Design and AnaIysPs of a Multi-Variable Rate Channel Access Scheme for

Geostationary Mobile Satellite Networ k

Junaid A. Israr, B. Eng.

A thesis submitted to

the Facuity of Graduate Studies and Research

in partial fuISllment of

the requirements for the degree of

Master of Engineering

Ottawa-Carleton Institute for EIectrÏcal Engineering

Faculty of Engineering

Department of Systems and Computer Engineering

Carleton University

Januay 1999

O Copyright

1999, Junâid A. Israr

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National Library of Canada

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In this thesis, a TDMA based system for a geostationary (GEO) mobile satellite service

(MSS) system for North America is designed and anaiyzed with the help of an or i ,~a l and

comprehensive software module. With a Large sateIlite antenna diameter, it is relatively easy

to m e m e n t a GE0 MSS system which is capable of supporting a direct link to a hand-held --

user terminal The systern caters to users of various data rates ranging fiom 8 kb/s to 64 kb/s

with two distinct types of user termin&. It c m provide an acceptable qualig of speech

s e ~ c e at 8 kb/s with hand-held terminais and higher data rate services to users with a more

capable terminal Furthemore, a new TDMA fiame structure is developed for the system

whîch provides capacity enhancements up to 1226 (depending on the fiarne contents) relative

to the conventionai TDMA fiame structures. The proposed fiame structure is highly efficient

when the majority of the users in the system operate at data rates higher than 8 kb/s. An

exhaustive iink budget analysis is perfonned to detemine the capacity of the systern The

software module produces a precise multiple spot beam layout for the continent of North

Anmica and performs interference and fiequency assignment andysis. The results f?om the

software module and the linl: budget analysis are used to conduct a detailed bandwidth limited

and power Iimited capacity analysis.

iii

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Acknowledgements

AU praise is to AUah for blessing me with numerous bounties and providing me with the

courage, strength and patience required to complete rny diesis.

1 am sincerely indebted to Dr. Samy El-Hemawey for his guidance and endless patience for

my questions. I wiU never be able to thank Dr. Samy Mahnîoud enough for having £bm belief

in me and for his passionate moral and technical s u p p o ~

1 gratefdy acknowledge the fûnding support provided by the Communications and

Information Technology Ontario (previously TRIO) and the National Science and

Ena~eering Councii of Canada. 1 would also like to express my gratitude towards Mr. Mike

Razi of TMI Communications, Dr. Jim Wight of the Department of Electronics, Carleton

University aod Dr. Lawrence E. May of the Department of Mathematics and Statistics,

Carleton University.

1 would &O like to thank rny grandrnother, my parents, my brothers, my fiancée and fiends

for their constant support and guidance.

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Table of Contents

LIST OF

LET OF

LIST OF

TABLES ....~......~.~...............~.............~........................................... CrS:

F I G E S ..................................................................................... XI

2 . MOBILE SATELL1 TE SYSTEMS .......................................................... 10

............................................................................................... 2 -1 -1 LQW Earth Orbits 11

2 -12 Medium Earth Orbits ......................................... .-... .... 12

2 -1 3 Geostationary Earth Orbits ........................................................................... 1 3

............................................................................ 2.2 SERVICE CHARACTERISTICS 14

2.3.1 Fixed Assignment Schemes ................................................................................. 18 2.3.1.1 Frequency Division Multiple Access (FDMA) ........................................... 19 2.3.1.2 Tirne-Division Multiple Access ('IDMA) ..................................................... 21 2.3.1.3 Code Division Multiple Access (CDMA) ................... ........ .......................... 23

2.32 Random Access Schemes .................................................................................... 23

2.33 Contt-olled Access or Reservation Based Schemes ........................................... 24

2.4 MOBILE SATELLITE PROPAGATION CHARACI-ER~STICS ..................................... 25

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3.1 TRADITIONAL TDMA FRAME STRUCTURES ......... ,,... .......................... 2 8

................... . 4 AN INTRODUCTION T 0 THE DESIGN PROCEDURE 3 9

4.3.1 Siant Range ............................................................................... ,. ..................... 49

4 E a t i ...................................................................................................... 0 9

4.33 Coverage Area (Footprint) ... ... ........................................................................... 50

4.4 SPOT BEAMS GENERATION/LAYOUT ................................................................ 51

...................................................................... 4.4.1 Spot beam Contour Calculaiions 5 2

4.42 Coverage of North America ..........................~................................................ 5 8

............................................................................... 4.5 INTERFERENCE ANALYSIS 59

.............. 4.6.1 UpIink remrn (user teminal to satellite) budget for data rate of 8 kbls 67

........ 4.62 Return downïink (satellite to ground station) budgetfor data rate of 8 kbls 71

4.63 Uplink rettim (user teminal to satellite) budget for data rate of 16.32. 48 and 64 .......................................... kbls ..................................................................... ,...., 76

4.6.4 Remrn downlink (satellite to ground station) budgetfor data rare of 16.32.48 and 64 kbls ................................................................................................................. 79

5 . PURPOSE OF THE SOFTWARE MODULE ........................................ 83

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6.4 FREQUENCY ASSIGNMENT AND TM B ANDWIDTH-LMTED CAPACITY ........... 124

6.4.1 All interfenng users operate at a data rate of 8 kbls: ................ .. ..................... 125

.................... 6-42 The inte$ering mers are a mU: of 8, 16,32, 48 and 64 kals users 130

6.43 Bandwidth-Limited Capacity wirh the extended algorithm ................................ 135

6-5 PO WER-LIMITED CAPACITY CALCULATIONS .................................................. 138

7.4.1 Multiple Access Techniques ............................................................................. 150

7.42 Proposed Frame Structure ......................................................... .. ............. 150

7.4.3 Link Budger Anaiysis .............. ..,. ........... ..... .... ..,. .............................................. 151

7.4.4 Frequncy Assignmenr Anulysis ana! Spot Beam Layout ................................... 152

REFERENCES ~ o o e ~ m * ~ ~ - o - - ~ - ~ - ~ ~ - ~ ~ - ~ ~ - - ~ ~ - - - - ~ ~ - ~ - ~ ~ - - ~ - ~ - - ~ - - - - - ~ ~ - - ~ - o ~ e o ~ e - ~ o ~ ~ ~ * ~ - * ~ ~ ~ ~ 153

viii

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List of Tables

Table 3-1: The time dot lengths (includes overhead bits and guard h e ) for different data rates 34

Table 3-2: Improvement in Capaciq with the proposedframe structure over the tradirional fiame sn-ucture 38

Table 4-1: Return Uplink (user teminal tu the satellite) Budgetfor a user operaîing at a data rate of 8 kbfs 70

Table 4-2: Return Downlink budget for a user operating at a data rate of 8 kbls. Some of the paramerer values are chosen fiom [9/. -- 75

Table 4-3: Return Uplink (user teminal to satellire) b2:dget for high data rate users- 79

Table 4-4: Return Downlink (Satellite to Ground Station) budgetfor high data rare mers81

Table 5-1: Some of the records stored in the 40lar database file. Each row contains the coordinates of a grid-square and the artenuation experïenced by a signal tranrmittedfrom these coordinates with respect tu the boresight of each of the spot beams. Please note that only a fav selected columnî are shown here for illustration purposes. 89

Table 5-2: A sample of records stored in the 'Centres' database file. Again, only a fav selected columm are presented in the table here 90

Table 5-3: A sample of the contents of the file called TJav York High Intemity'. 92

Table 5-4: A sample of maximum inte~erence values for the spot beam of San Diego. 92

Table 5-5: A sample of the neighbour spot beams of the Ottawa spot beam along with the distancesfrorn the center of Ottawa spot beam to the centers of the remaining spot beam94

Table 5-6: The two tables (a) and (b) Iist the CO-channel centers assigned to Frequency 1 and 12, The frequency assigrunent algonthm is initiated fi-orn Columbia South Carolina95

Table 6-1: The names, expected trmc intemiries (TI) and the coordinates of some of the centers of the spot beum 113

Table 6-2: The names, eqected trqffic intemities (TI) and the coordinares of remaining centers of the spot beam 114

Table 6-3: The minimum guard time values ussociated with some of the spot beams - 119

Table 6-4: The minimum guard time values ussociated with remaining spot beam - 120

Table 6-5: The duration of the time slots for diferent data rate users and with différent guard h e values 121

Table 6-6: Improvement in Capacity with the proposedframe structure over the traditional fiame structure 123

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Table 6-7: A typical example of the results obtainedfrom the frequency assignment algorithm. 126

Table 6-8: The bandwidth-limited system capaciv with al1 users operating at 8 kbls. 127

Table 6-9: The bandwidth-Zimited systern capacity with al2 users operating at 8 kbls. 131

Table 6-1 0: A comparison of the systm bandwidth-limited capacily for users operating ar a data rate of 16 kbls with pruposed and traditional frame structures 131

Table 6-11: A comparison of the system bandwidth-Zimited capacily for users operating at a data rate of 32,48 and 64kbls with proposed and iraditional frame structures. 132

Table 6-12: Frequency assignment with the extended algorithm assuming the inteq5ering mers are a mïx of 8kb/s, 16 kbls, 32kb/s, 48 kbls and 64 kbls mers, FB No. = Frequency Band No 136

Table 6-13: Frequency assignment with the extended algorithm assuming the inteq5ennng users are a mix of 8kbls. 16 kbls, 32kb/s, 48 kbls and 64 kbls mers, FBNo. = Frequency Band No 137

Table 6-14: The power required by a satellite to bent-pipe a user operatïng at i kbls and the corresponding time slot duration in the 50 ms m e . 141

Table 6-15: The range of possible values for ng, nI6, n32, n.48 and n@- 142

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List of Figures

Figure 1-1: A flenXIble, multifinctioonal network envisioned for a future wireless system - 2

Figure 2 -1: Elemenrs of a typical mobile satellite communication system. 14

Figure 2-2: Frequency Division Multiple Access, Time Division Multiple Access and Code Division Multip le A ccess 19

Figure 2 -3: Frequency Division Multiple Access [14] 20

Figure 2 -4: Time-division muitipie access 31

Figure 3-1: Physical Sb-ucture for GSM Hypefiame, Supegkme, Muitifi-ame, Frame, and Time Slot 28

Figure 3-2: The MSAT Tm-ST TDMA signaling Channel Frame ~&ture /17] 31

Figure 3-3: An illus~ation of the defrgmentation process in the frame 35

Figure 4-1: An illusnation of the spot beam aitenuarion 44

Figure 4-2 : Diference in the shapes of the spot beam in different regions due tu the spherical nature of earth 44

Figure 4-3: Radiation characteristics of a rotational-symmetric directional anrenna [IO145

Figure 4-4: Bmic Satellite Geornew [14] 48

Figure 4-5: The parameters associated with the plane triangle EOP 49

Figure 4-6: Coverage area footprint) of a satellite [I 01 50

Figure 4-7: The eanh satellite geometry and the vectors required for the anenuation

Figure 4-8: An uplink (mer teminal to the satellite) interference scenario 60

Figure 4-9: Performance of rate 2/3 K = 3 code with Viterbi decoding. Numerical bound and simulation results [22] 65

Figure 4-10: A complete return Iink budget for a ruer operating at a data rate of 8 kb/s66

Figure 5-1: An overall diagram of the sofhvae system components 85

Figure 5-2: An illustration of the grid-squares covering entire North America. 86

Figure 5-3: A breakdowit of the database files. The dorted squares represent a repetition of the endosedfiles. Centre X is a generic name and can denote any one of the IO2 centres chosen tu cuver North America. 88

Figure 5 4 : The initiai so%are sta.mp display 96

Figure 5-5: The extent of attenuarion calcuZarionr perfomed by the so*are. 98

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Figure 5-6: The amunt of atrenuarion eqerienced by a user in the grid-squure ut a latitude of 40.22 O and 10ngin.de of 75.89 . A 18-45 dB value for the Detroit region represents the anenution with respect ro the boresight artenution of the thee~oit region spot beam. 98

Figure 5-7: An arbinary spot beam is shown to illustrate the concept of guard tirne calculations. The spot beam is big enough tu cause signwcant djg'erence in the propagation *es ro the satellite IO4

Figure 5-81 A superimposition of the antenna pattern on the spor beam contours illustrating the non-iinear relationshp between the distance from the boresighr and the attenuationI06

Figure 6-1 : A particular spot beam Iayout fo provide MSS service tu North America I I 7

Figure 6-2: Frequency Assigrnent assming al1 interfering users operate ar 8kbls - 129

Figure 6-3: Frequency Assigrnent asswning the interfering users are a mUr of 8k6/s, 16 kbls, 32kb/s, 48 kbls and 64 kbls mers - - 134

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List of Symbols

rrmimum antenna gain

antenna efficiency

signal wavelength

3-dB bearnwidth of satellite antenna

sateUite antenna diarneter

angle of signal reception at sateIlite with respect to boresight --

latitude of ground station

difference in longitude between ground station and subsatellite point

nadir or tilt angle of the sateIlite

equatorial radius of the earth

satellite altitude measured from the surface of the ex th

satellite altitude measured fiom the center of the eaah

elevation angle

central angle

slant range distance to the sateIlite

extent of coverage area

p2th loss

latitude of center of spot beam

latitude of a gid-square

difference between satellite longitude value and boresight longitude

value (taken positive if boresight is to the West of the satellite)

difference between satefite longitude and -gid-square longitude value

(taken positive if grid-square is to the West of the satellite)

antenna gain characteristics as a function of reception angle

angle of received desired signal with respect to boresight

xiii

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angle of received interference signal with respect to boresight

path loss experienced by the des- signal

path loss experienced by the interfering signal

interference received in spot beam i £rom spot beam j

ratio of data bit energy to noise power densisr

Noise Power, where K = Boltzmann's constant = 1.38 * JK, T =

Noise Temperature in 'K and B = bandwidth in Hz.

ratio of uplink carrier power to thermal noise power

ratio of downlink carrier power to thennal noise power

(3 ratio of carrier power to interference noise power

ratio of carrier power to intermodulation noise power

na, nis, 1132 number of users operating at 8,16, and 32 kb/s respectively

na, number of users operating at 48 and 64 kb/s respecrively

&, S16, S32 uplink signal share for users operating at 8, 16 and 32 kb/s respectively

sa, s a uplink signai share for users operahg at 48 and 64 kb/s respectively

xiv

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Chapter 1

Prologue

1. Introduction

The advancements in niicroelectronics have allo wed new applications for telecommunication

satellites such as mobile and persona1 communications. The future 'wireless systems must

provide services that support ~aditional mobile voice communications as well as a variety of

voice and data services for a wide range of applications. Services supporthg multimedia

capabilities, Intemet access, ima@ng and video conferencing will Iikely be needed in any tme

future wireless system At the same the, global standards organizations are workùig

towards the convergence of many diverse systems existing today (including paging, cordless,

cellular, mobile satellite etc.) into a seamless radio infrasaucture capable of o f f e ~ g a wide

range of services.

F i g e 1-1 represents the breadth of seMces and market se,ments being included in the next

generation wire1ess system architectures. It is evident that a number of different operating

environments, ranging fiom very-high-capacity indoor picocelk to Iarge outdo or terrestrial

cells and satellite coverage, are envisioned for future wireless systems. It is Iargely believed

that close integration between the satellite and t e r r e s a components wiii facilitate the initial

deployment of services via satellite where there is Little or no existing £ixed infrasn-ucnire.

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Figure 1-I, A flexible, muitifunctional network envisioned for afiture wireless sistem

In view of such global firamework, the satelLite network operators are faced with a number of

technical and economic constraints. The satellite systern designer nust choose, among others,

the constellation to use, the spot beam sizes, the rnost efficient deplo yment of ground stations

and multiple access schernes. Since a satellite spot bearn is large, it rnay straddle a nurnber of

counbries and regions with dissimiIar terrestrial networks which presents another complex

challenge to the system designer.

The era of satellite communications opened in the f o m of many experiments in the late m e s

and early sixties with satellites in low altitude orbits. The low altitude orbits presented several

problerns including but not limited to complex antenna tracking and interrupted s e ~ c e due to

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the satellite disappearing over the horizon. By the mid-1960'~~ the geostationary equatonal

orbit (GEO) with its unique char acteristics became increasingly attractive and presented a

ktter solution for communication systerns. During the 1970's and 1980's GE0 satellites

becarne by far the most popular medium for providing commercial satellite based

communication services. However, by the 1 9 9 0 ' ~ ~ competing terresmal technologies such as

cable, miclo wave radio and particularly fiber optic systerns provided attractive and low cost

alternatives for telecornmunications and threatened the position of satellites. Subsequently,

the identification of applications suitable for satellite cornmunications.led to the design and

deployment of satellite systems for long distance mobile communications. SateIlites offer

unique s e ~ c e to users in rural and remote areas which are outside the range of existing

telecornmunication systerns.

The year 1990 was a tuming point for MSS, when Motorola introduced the concept of a low

earth orbit (LEO) satellite system capable of directly serving hand-held terrninak. At that

time, the concept of providing direct links to hand-held terminais was considered infeasible

even with the most advanced satellite systems. However, by 1994, several schernes had k e n

devised ro provide mobile service to hand held phones at all altitudes ran-ghg kom Indium's

780 km to the GEO's 36,000 km A GE0 satellite based system is a strong candidate due to

the long experieiice acquked through years of operational practïce. The field of view

provided by the G E 0 satellite is large and hence very few are required to provide global

coverage. Also, since the satellite appears stationary to users on earth, the complications

associated with antenna tracking, Doppler effects and loss of coverage are mitigated.

Beginning in Iate 1998, systerns supporting comrnunïcation directly between hand-held

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phones and satellites are expected to be in operation. These systems will also see s e ~ c e for

fked telephone and temporary and emergency communications, which will. have signincant

impact in areas and times of geat need.

2.1 Thesis Motivation

For satellite comunication systems to be economically viable, efficient use must be made of

the satellite's limited resources such as bandwidth and po wer. This is particularly important

in MSS systems where a large nurnber of uncoordinated and statisecally bursty users are

expected to share these resources in a mutually cooperative and efficient rnanner. Depending

on the system requireements and the nature of the applications supporteci by the system it is

important to choose the appropnate multiple access technique in the system design step.

Recently, rnany authors Cl], [2], 1331, [4], [5], [6'J have investigated issues related to future

mobile satellite communication systems with the capability of providing personal services.

For exarnple, [l] addresses some of the most si@cant topics related to Tirne Division

Multiple Access (T'DM.) and Code Division Multiple Access (CDMA) that are considered as

candidates for the thkd generation MSS systems. However, not much research has k e n

devoted to investigate the capacity performance of a complete future third generation MSS

systern with its service requirements and s ystem char ac teristics.

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At present, TMI' Communications, operate a geostationary satellite MSAT, providing voice

and low data rate (9.6 kbps) services to users in North Arnenca The system currently

emplo ys Single Channel per Carrier (S CPC) FDMA as system multiple access scheme. This

research effort is undertaken in coIlaboration with TM Communications to seek aitemate

multiple access schemes for a next generation MSS system, providing voice and higher data

rate s e ~ c e s to users in North Arnerica with a geostahonary satellite.

The third generation MSS systerns are expected to provide multiple data rate services to users

with different types of user temiinals. The data rates are expected to range fiom 8 kb/s for

speech to 64 kb/s for applications such as intemet access, file transfers, etc. The user

temiinals are expected to be similar to the hand-held cellular handsets for speech users.

Larger transportable termina are expected to be used for higher data rate senices. The

existing and the planned geostationary MSS systerns provide speech services and low data

rate seMces to users. For example, the Thuraya system planned for the Middle-East prcvides

GSM compatible seMces for users with hand-held, vehicular and transportable user termïnals

[7]. [33 presented an analysis of a public mobile satellite system compatible with the GSM

cellular network. However, most of the a s s q t i o n s made in the analysis rnay not be valid

for a mily third generation satellite system but are more representative of the existing MSAT

system operating in North America For example, the user terminal B requked to transmit at

a peak power of 18.6 W which is prohibitive for a hand-held temlinal Aiso, systerns with

very few spot beams are considered which restnct the ability of the system to utiIize the

T M Communications. Inc.. 1601 TelesatCrt, Gloucester, Ontario, Canada Tel.: (6131 742-0000.

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reciprocal relationship between the spot beam size and the satellite antenna gain. The few

spot beams are also resaictive in utiIinng the concept of fkquency reuse to enhance the

capacity of the system.

Currently, there is a lack in the existing literature of a TDMA fÎame structure that can support

users with mul-variable data rates efficiently. An investigation is perfomed and a structure

is developed and compared with traditional fiame structure for efficiency.

- -

For the third generation systems, a GE0 sateIlite-based system is a strong candidate due to

the long expenence acquired through years of operational practice. The future geostationary

mobile satellite systems being envisageci for North America consist of several spot bearns

(around 100 spot hem) with coverage extending to the Caribbean and Hawaii Islands and

the coastal waters [8]. Also, since a regional system is sought, a LE0 or a ME0 based

satellite system is not very practical.

1.2 Thesis Work Description

In this thesis, a geostationary mobile satellite system with multiple spot beams c o v e ~ g North

America and supporting users with multiple data rates is considered and analysis for its

capacity is presented. The system design involves a thorough investigation and analysis of

several key issues which are surnmarwd in the foilowing:

an understanding of earth satellite geomeay to generate individual spot beams.

Using vec t or analysis, an original mathematical relationship is developed in t his

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thesis to calculate an essential psrameter required for spot beam contour

generation-

an understanding of the a a E c characteristics to determine appropriate spot beam

sizes (contour power levels) and the overall spot beam layout for the desired

coverage area.

an understanding of all link budget parameters to estabkh the power

requirements/con~b:aints of the system Since a new system design is undertaken,

reasonable assump tions regarding performance requirements and communication

terminal characteristics are made. The satellite channel propagation characteristics,

modulation techniques, error correction schemes, satellite amplifier characteristics,

ground station characteristics and the data rates constimte an integral part of the

Link budget and are analyzed accordingly.

a realization of the problem of interference kom CO-channel spot bearns when

attempting fiequency reuse and designing a rnethodology to determine and tackle

the interference. As a result, several possible kequency assignrnents for the

different spot bearns are devise& The ficequency a s s i g m n t is followed by a

bandwidth-Limited capatiq calculation.

an understanding of the parameters involved in the power-Eted capacity analysis.

A power-limited capacity analysis for the proposed design with multiple data rate

users (with different communication terminal capabilities) transrnitting within the

sarne fiame is not available in existing literature. Therefore, an original

methodolo gy is developed to calcuiate the po wer-limited capacity for the proposed

svstem

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A new TDMA fkne smcnire is proposed which provides s igdcant capacity improvements

and reasonable overhead relative to traditional TDMA fiarne structure. A complete r e m

link budget is presented which assists in determining the feasibility of the proposed systern

Considerable effort has been devoted to the design and implementation of a complete

software module during the course of the research. The software module assists in the spot

beam layouts, calculation of guard times, interference analysis based on the Link budget and

fiequency assignrnent. It is shown that the proposed TDMA fiarne structure can provide

siLgd5cant capacity irnprovement by cornparison to conventional appro.aches.

1.3 Thesis Organizatiopz

The chapters of the thesis are organized as fol10 ws:

Chapter 2 presents background infomatÏon related to mobile satellites systems. The chapter

brïefly discusses the different orbits and availability of satellite systems. This is followed by a

short reference to the s e ~ c e characteristics expected for future systems and the

choice/features of the available multiple access schemes to cater to these requirements. Next,

a brief discussion about the link propagation characteristics is provided.

Chapter 3 presents the new TDMA fime structure developed in this thesis, which is suitable

for future geostationary MSS systems. The h e contents and the def?a,gnentation dynamics

are discussed. Tie performance of the proposed kame structure is evaluated and cornpared

to that of the conventional structures.

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Chapter 4 is a description of the complete channe1 access system design. The chapter

provides a brief background about the earth-satellite geometry which Ieads to the spot beam

contour calculations. The coverage of North A M c a with spot beams of Meren t contour

Ievels is presented next. The calculations involved in interference analysis and kequency

assignrnent are presented next with details regarding the link budget

Chapter 5 provides detailed description of the software module designed to aid in design of a

complete multiple spot ba rn MSS operating in North Amerka AU features incorporateci in

the software module are discussed. effort is made to irnplement the theoretical concepts

presented in Chapter 4 in an actual software system in order to derive preliminary results.

In Chapter 6, the results of the preliminary design are presented. The capacity of the system

is calculated assurning bo th p O wer-limitecl and bandwidth-limited scenario S. A CO mparison is

made between the traditional TDMA fiame structure and the proposed fi-arne smicture with

reazistic calculation of guard time values obtaÏned fiom the design. The coverage pattern for

North America is presented folIowed by the interference analysis results for dinerent

frequency assignment schemes.

Chapter 7 summarizes the results O btained in the coursework of this thesis. ConcIusions are

drawn and suggestions are presented for future research work.

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Chapter 2

Background

In this chapter, background information pertaining to issues associated with Mobile Satellite

Service (MSS) systems is presented. The choice of different orbits for an MSS system is

discussed initially follo wed by the desired s e ~ c e characteristics, multiple access schemes and

the channel propagation conditions.

2. Mobile Satellite Systems

The choice of a mobile sateIlite system involves finding the best solution in terms of the

following factors: [9]

a) orbits and availability,

b) satefite,

c) systern capacity,

d) multiple access,

e) dïversity,

f ) signal quality,

g) systerncost.

The constellation deterniines the number of satellites; the altitude and capacity determine the

satellite' s size; and the modulation/mdtiple access method and altitude determine its

10

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complexïty- AU parameters are interdependent, for exarnpIe, a good multiple access system in

connection with a constellation at a reasonable altitude simplifies the satellite, provides good

voice quality, and rninimîzes c o s [IO]. In this chapter, a review of some of the above

parameters is presented.

2.1 Orbits and Availability

Several MSS systems offering voice and data s e ~ c e fkom three different orbfs have k e n

proposed [1][7][9][11]. Three types of satellite systems exist i n c l u e g the low earth orbit

(LEO) satellites, medium earth orbit (MEO) satellites and geostationary earth orbit (GEO)

satellites. The LE0 satellite systems include the Iridium hc.'s Iridium (at an altitude of 780

km), Constellation Communications Inc.'s Aries (1,O 18 km) and Lord-Qualcomm's Globalstar

(1,389 km) [Il]. Medium earth orbit W O ) or Intermediate circular orbit (ICO) systems

include 1-CO'S Project 21 (10,355 km). Ellrpsat's Ellipse (7,800 km) ïs considered as a LE0

but is doser to the ME0 orbits. Geostationary earth orbit (GEO) regional mobile satellite

systems at about 36,000 km include AMSC and MSAT (US and Canada respectively), Aces

(South East Asia) and Thuraya (UAE).

2 -1 -1 Low Earth Orbits

Since the satellites are close to earth (typical altitudes Vary fiom 500 -2,000 km), the Effective

Isotropie Radiated Power (EIRP) requIred for a given link margin is lower. For instance, the

path loss for Iridium at 20' elevation angle i s 18 dB lower than that of ME0 satellites .

Although this allows Indiun to potentially have higher link rnargin yet this is countenveighed

by the fact that Iridium must cope with more shadowing and more severe Ricean fading as it

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is &O seen at Iower elevation angles [l]. Also, the lower elevation angle reduces availability

especially in built up areas. Further, it is no t always possible to see multiple satellites to allo w

diversity. Globalstar, with a higher orbit than Iridium, offers dual satellite diversity with 85%

of time availability Cl], but has a lower link margin than Iridium

In a LE0 system, the field of view (FOV) of each satellite is Iower and therefore a large

number of satellites are required. Since the FOV is small, every satellite need not be in view

of a user temiinal. This requires either networkïng in space with inter:satellite links s i m k to

Iridium or a large number of earth stations as in Glo balstar. Further, frequent handovers are

required because the satellites move rapidly and the beams move with the satellites. The tïme

delay is short for local caIis and is comparable to that for an ME0 for international calls since

the signal is usually relayed over multiple satellites or multiple hops. The satellites move fast

and therefore have higher Doppler frequencies. This may require merentially coherent

communications, which implies E D O ratio higher than that required for coherent schemes

used by GEOs and MEOs.

2.1.2 Medium Earth Orbits

The ME0 satellites operate n o d y at an altitude of 10,355 km Due to the higher altitude,

they have a larger FOV than LEOs and require 10-12 satellites to cover the earth. A

constellation of 12 satellites in 3 planes provides higher average availability than LEOs for

latitudes up to 65'. Furthemore, the availability is more uniform at these latitudes. A larger

constellation of 18 satellites in 4 planes provides diversity for approximately 85% of the time

and inmeases the average avaiiability in urban areas ~ o m about 65-70% to around 90% for

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latitudes up to 50'. The higher availability is achieved because satellites are seen at hïgher

elevation angles and hence divers* is provided. The ME0 satellites orbit the earth at a

slower rate and can be seen for a longer time during a call which also -es the handover

rate relative to a LEO- ME0 satellites require a larger EIRP than LEOs to make up for the

additional path loss. With the larger FOV, a ME0 system requires fewer gateways and there

is no need for intersatellite links. Cl]

2 -1.3 Geostationary Earth Orbits

The GE0 satellites are used for regional systems. With the help of large antennas (> 10

meters) and high power output, the GEOs provide cornpetitive link rnargins. Further, since

the sateIlite is stationary, fuced termhals c m be oriented towards the satellite with high gain

antennas to provide a range of voice and data services. This cm be accoqlished by LE0

and ICO systerns but with greater cornplexity. The satellites are in an equatorial link belt and

hence always south or north of the users. As well, the satellites exist at higher elevation

angles (typically above 30'). This enables the users to position themselves for better

communication. The GE0 systems have a relatively straightfonvard ground station

architecture that does not require multiple tracking antennas since the sateIlite is rehtively

stationary. The aitical issue with GE0 satellites is end-to-end time delay which can reach up

to 500 rns. In a GE0 satefite system, a user at the e d g of the spot beam has a lower Iink

rnargin, which is not the case in ME0 or LE0 satellite systems where the user experiences

more averaging as the sateUite position changes. [l]

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2.2 Service Characteristics

A typical MSS network is designed based on the requkement that a large number of user

termuth are able to sirnultaneously intercomect their respective voice, data, teletype and

facsUnile seMces through a satellite. When designing a systern, it is essential fist to establish

the network senices required and the nature of the circuits needed to cany these s e ~ c e s .

Figure 2-1 depicts an overall cypical mobile satellite communication systern

Figure 2 4 : Elements of a typical mobile satellite communication system.

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An MSS system may carry B e r e n t types of information ranghg from digital voice, of

reasonable quality, to data at various speed (depending on the requirements of the supported

applications) or slow scan video for facsi.de transfers. With advances in technology,

teleconferencing applications can also be supported even though full motion video rnay be

contingent on the availability of large amount of inexpensive bandwidth. There are three

types of services that a typicai MSAT system is expected to provide to its users, narnely:

1. Continuous Voice Service

2. Continuous Data Service

3. Packet Data Service

The continuous services for voice or data are similar in the sense that both of them are based

on circuit switching. A circuit switched comection is one for which a specific transmission

path or circuit is established for dedicated use during the duration of each cal, that is, fkom

the time a call is set up to the time it is terminated. [12]

The djfference between voice and data calls is that a user terminal participakg in a voice CU

uses a voice activity switch (to provide a time assigned speech interpolation advantage in

terms of the power limïted operation of the satellite), whereas no such activity switch is used

for the data cali (a circuit assigned to a data call is assumed to be always active). The voice

activity switch nims the aansmitted signal off fiorn the mobile teminal during idle penods of

voice. This reduces the po wer required by the satellite repeater to the level required by the

active voice sources. [12]

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Until recently, the packet data senice was expected to cars. low-rate, Iow-volume data for

application like messaging, paging or telex-type services. The future systems are expected to

provide hybrïd sateuite/ground-based personal communication service which would include

hgh quality voice, variable bit-rate data (4.8kbps to 64kbps), along with image and position

determination to rnobiIe and fixed users. This would entail the need of [22]:

extending concepts spical to ceEukir network to satellite systems. This is due to the need

to i n t e p t e satellite systems widi terresaial networks.

charme1 sharing to cope with higher variance of aafnc per spot beam This is due to the

increased demand for capacity.

decentralize the network control in order to reduce the signaling information exchanges.

This is to ensure high required link availabiljty.

2.3 Multiple Access Schemes

A key elernent that determines the satellite network complexity and efficiency is the selected

multiple access scheme. Considering the environmental conditions, the optimal scheme

should provide: [4]

highly efficient use of the available nansponder power and bandwidth for single or

multiple spot beam satefites,

resismce to hostile propagation conditions like multïpath fading and shadowing,

adaptability to highly variabIe t r a c patterns, ie., equally efncient for high and Io w trafic

loads and graceful performance degadation in case of non-nominal conditions,

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4. fzexibility to different application, i.e. digital voice, data and mked tr&c,

5. adaptability to different network configurations: star, rnulti-star, partially meshed, ex.,

6. cornpliance with power flux density (PFD) requîrements for user temiinal providhg

minimum RF interference to other services-

A review of the existing access techniques shows that none of them satisfy aI1 these

requirements. Requirement 1 irrrplies coherent detection at the modem side in order to

rnaximize the power efficiency together with some sort of spectral shaping to lirnit the

bandwidth occupancy. Moreover, carrier activation has to be used to rnaximize the satellite

power utilization. The peculiar propagation characteristics of the mobile/ked channel c d

for a pilot-based synchronization system able to keep the mobile in-lock even in the absence

of the communication carier (Le., during speech pauses or between consecutive data bursts).

Requirements 3, 4 and 5 call for avoiding a rigïd fiamhg structure in handhg the traffic

channels. [12]

The multiple access schemes c m be classifed into the folIowing three categones:

Fixed Assignrnent,

Random Access and

Controlled Access techniques.

Fixed Assignment schemes, such as those using fiequency division multiple access (FDMA),

tirne division multiple access (TDMA), or code division multiple access (CDMA),

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pemianently assign a fiaction of the system's resources to ezch user. Such schemes are best

suïted to routes carrying large quantities of steady M c .

For an MSS system with a large populanon of bursty short trafEic sources, a permanent

assignment scheme rnight be insufficient. Random Access techniques like Pure or Slotted

Aloha, may be better suited for such systerns. This is despite the fact that they have to be

operated at low efficiency in order to avoid problerns of instability. [12]

For systems where the information generated by an active user tends to be long (for example,

in voice cab or long data transactions), Controkd Access techniques, rnay be a better

alternative. In these schemes, a fraction of the system's resources (bandwidth or tirne) is set

aside to cany requests for (resource) assignment. Successful requests get an appropriate

assignment which can then be used for the actual message transmission. Circuits may be

assigned in a random access mode. The division of the system's resources rnay be done either

over tirne, as in Reservation Aloha, or over eequency. For telephone speech n&c on

satellite mobile systems and for other fixed thin route networks, a dernand assignment

multiple access (DAMA) scheme is often preferred in which a circuit-switched FDMA,

TDMA or CDMA channel is assigned to the user only of the period of the call for which it is

needed. cl01

2.3.1 Fixed Assignment Schemes

These schemes permanently assign a part of transmission resource to each user in a manner

proportional to the user's requirements. These schernes tend to be suitabIe for users who

generate a&c fairly re,&ly. The actual entity king shared could be either bandwidth,

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time, or a set mutually orthogonal codes, resdting in

19

Frequency Division MultipIe Access,

Time Division Multiple Access or Code Division Multiple Access respectively. Fi-we 2-2

provides a graphical representation of these schemes. Each of these schemes wiu now be

presented brie£ly.

TDMA

Channel N A

Figure 2-2.- Frequency Division Multiple Access, Time Division Mulriple Access and Code Division Multip Le Access

23.1.1 Frequency Division Multiple Access (FDMA)

To illustrate the concept of FDMA, let us consider a satellite sysrem with a total satellite

system bandwidth of B Hz serving N nodes. If the users generate equal arnounts of t r a c ,

access is provided by dividing the bandwidth into bands of (B / N) Hz and permanently

assigning one such band to each user for transmission. If the users generate unequal arnounts

of t r a c , bandwidth can be assigned in proportion to the a&c generated by each user. The

band assigned to a user is available over a l l tirne, or at Ieast, for extended penods. [14]

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Figure 2 -3 : Frequency Division Multiple Access 1141

In some ~Ecumtances, FDMA schemes may be coqaratively inefficient since they require

pard bands between neighboring bands to prevent the transmission in one band fiom

i n t e r f e ~ g with those in the neighboring bands. Given that the system requires pard bands

of A Hz around each user fiequency band, the bandwidth available for to each station for

information transfer is:

Another problem with FDMA systems is that they require the satellite amplifier to be linear in

nature. The high power amplifier V A ) on board the satellite is usually constructed from a

travelling wave tube (TWT) device. An HPA works most eficiently when it is operated close

to saturation but then non-linearities occur. Unfortunately, the linearity requirement of the

FDMA system -lies that the TWT has to be backed off substantially in order tc? operate it

as a hear amplifier. This in turn, leads to inefficient usage of the available satefite power.

For this reason, fked assignment strategies have tended to increasingly shift fiom FDMA to

TDMA as certain technical problems associateci with the latter are overcome. 1101

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2.3.1.2 Time-Division Multiple A ccess (TDMA)

Again, to iuusaate the concept of TDMA, let us consider a system where the available system

bandwidth is B Hz. Ako, consider the total transmission tirne to be divide into fiames of T

seconds each, Let each fizm be M e r subdrvided into N dots of duration T / N seconds

where each dot is permanently assigned to one of the N users for its transmissions- Unlike the

FDMA scheme where a user continuously transmits in irs assigned &equency chamel, in a

TDMA scheme each user transmits over the full system bandwidth of B Hz but only during its

own tirne dot in each frame. This is iUustrated in Figure 2-4.. [14] . -

-- - - - - - - - -

Figure 2 -4: The-division multiple access

Corresponding to pa rd bands in F'DMA scheme, TDMA requires p a r d times to separate

one user's transmission Tom that of the next one in the fiame. Since the transmission from

each user is essentially done in the burst mode, additional prearnbles rnay also required to

enable carrier synchronization (for coherent dernodufation) and clock synchronization (for bit

timing in digital aansmissions). These synchronization procedures are essenaal to allow the

receiver to recover the information containeci in the transmitted burst Cl41

A TDMA system also requires overall system timing so that each user c m unambiguously

detexmine the fiame boundaries and the position of its own dot within the fkame. Any

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possibility of error in this determination wiU nanslate into a longer guard time requüement

and will have a detrimental effect on the overail efficiency of the system The receivers in a

TDMA system must be capable of rapid burst synchronization, that is quickly acquiring

carrier (and possible, clock) at the begmnning of the bursts they are intended to receive

~ 4 1 .

It is evident fkom the above that a TDMA system requires considerably more complex timing

and synchronization than an equivalent FDMA system h o t h e r disadvantage of TDMA is

that the transmissions fiom each mobile station are in the form of bursts of T / N seconds with

the station k ing ide for the remaining duration of the kame. This implies that the

transmïtters in a TDMA systern would have a high peak po wer requjrement even though their

average transmitted power is considerably lower. For MSS systerns with small user terminais,

this rnay be undesirable. Because of their hardware Iimitations, user termina in such systems

would prefer to transmit continuously at the average power level rather than in bursts of high

p O wer.

In a TDMA system, at any given instant only one user would be in the transrnitting mode.

This rnakes it easier to exercise power control in the system by requiring each user to adhere

to pre-specZed transmission power limit. Because of this the system rnay be operated such

that the satellite tramponder is close to saturation limit Cl41

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2.3.1.3 Code Division Multiple Access (CDMA)

For a digital system, it is possible to have a number of users share the sarne bandwidth and

tirne, while keeping them separated by ensurhg that they use different codes which are

orthogonal to each other. Each station is assigned a pseudo-random sequence for

transrnitting its message bits. The mode of transmission is to EX-OR (exclusive-or) each data

bit with specified sequence and transmit the resulting sequence. Knowing the sequence used

for ansm mission the receivers c m then extract the information k ing sent by essentïalLy

process of correlation It is important to note that CDMA systems are inherently more

inefficient than FDMA and TDMA schemes. This is primarily because the high effective

transmission rate (compared to the actual bit rate) requires considerably more bandwidth for

its operations Acquiring and mâintaining dock and carier synchronization in such a system is

also reasonably cornplex.

The main advantage of CDMA is that it inherently provides a processing gain of M and cari be

used in systerns which must transmit at low power. The performance of CDMA can be

M e r iqroved if error detection and correction coding is used. Norrnally, in FDMA and

TDMA systerns, coding leads to undesirable increase in bandwidth. In a CDMA system, the

signal is already spread over a very large s p e c m and the increase in s p e c m due to coding

is not too significant At the same time, coding improves the system performance

enormously.

In environments with noise, this processîng gain may also be used to advantage. Moreover the

inherent anti-jarn (AJ) and anti-interference (AI) properties of the PN sequences provide

certain arnount of security to data transmission in a CDMA system.

2 -3.2 Rarzdom Access Schemes

F i assignment strategies tend to be inefficient when the user population consiçts of a large

number of bursty users. Such low duty-factors users generate aafnc in bursts with long

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inactive periods between bursts; hence it will be inefficient to assign fixed transmission

resources to each of them in a permanent fashion- Random schemes using contention based

strategies for multiple access are more suitable for use in such an environrnent. In these

schernes, a transmitter uses the entire transmission resource of the system only when it is

needed. That is, there is no k e d (permanent) assignment of tlansmission resource in this

fashion a o w s a large number of bursty users to be supported by the system-

2 -3 -3 Controlled Access or Reservation Based Schemes

The random access schemes presented above require channel sensing mechanism which allow

transmissions to k aborted if the channel is sensed to k busy. Since such sensing wodd not

be practical in a satellite system, it would be inappropriate to use these schemes for multiple

access in a VSAT or MSS environment. For these satellite systems, random access schernes

which rnay be applied can provide ody limited values of capacity. Higher capacity requires

the use of more complex collision resolutïon schemes. Even these protocols are only able to

increase capacity to about 48 % [12]. If bandwidth efficiency is a major criterion dong with

the ability to support a large and bursty user population, then a better option is to employ

schemes with ernbedded reservations.

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2.4 Mobile Satellite Propagation Characteristics

Satefite Communications with land mobile temrinals sui3er fiom strong variations of the

received signal due to signal shadowing and multipath fading. Shadowing of the satellite

signal by obstacles in the propagation path (buildings, bridges, trees, etc.) results in

attenuation over the total signal bandwidth. This attenuation increases with carrier fkequency,

ie., it is more marked at L-band than at UHF. For low satellite elevation, the shadowed areas

are larger than for high elevation. Multipath fading occurs because the satellite signal Ïs

received not only via the direct path but also afier k i n g reflected &om objets in the

sunomdings. Due to their different propagation distances, multipath signal can add

destructiveiy resulting in a deep fade.

Therefore, for all types of land mobile systerns, the cornmunication link between the satellite

and the mobile terminal is the most critical part of the transmission path and limits the

performance of the total system The introduction of judicious fade marggin in to the link

margin has substantial consequences for the system cost. The link availability determines the

achievable throughput, efficiency and the resulting message delay. Furthemore, the tirne

varying behavior of the land mobile satellite link must be considered when choosing a

modulation scheme as well as when designing channel access and error protection methods.

Ako, carrier recovery, bit timing and fiame synchronizatbn have to be adapted carefully to

the channel behavior. For these reasons, it is essential to thoroughly investigate

characteristics of land mobile satellite lÏnk.

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Various tables and empirical expressions are available in Iiterature gïving shadowing rnar,gïn

versus environment, the satellite elevation angle and terrestrial coverage probability.

Accordhg to [9], for a suburban or urban environment, a fade margin of 10 dB or more is

required-

The thesis research work has focussed on a geostationary satellite system capable of

providing rnuln-variable data rate services to users in North America with TDMA as multiple

access scheme. As discussed in this chapter, the field of view wïth a geostationary satellite is

large and hence one satellite is s a c i e n t to provide senice to the continent of North America

Also, since the sateIlite is relatively stationary, issues such as the Doppler effect, hand-over

and inter-satellite links need not be examined. The data rates k i n g considered range hom 8

kb/s for speech users with hand-held terminais to 64 kb/s for data users with =ansportable

terminals. The performance of the TDMA frarne structure is evaluated in terms of bandwidth

and power-limited capacity. k cornparison between the proposed TDMA sysrem and a

CDMA system is suggested as future research work.

In the next chapter, the new TDMA karne structure is introduced and choice of its parameters

is discussed. Also, its capacity performance is compareci with the traditional TDMA f Î m e

structure.

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Chapter 3

A Novel TDMA Frame Structure for MSS

3. Introduction

In this chapter, a novel TDMA h e structure is proposed for a regional GE0 MSS system

It accounts for multiple users' data rates (8kb/s to 64kb/s) with hand-held as well as fixed

terminais and permits dynarnic change in user rate d u ~ g the call- The traditional frarne

structures provide variable rate s e ~ c e by assigning several tirne slots to higher data rate

users. This is very inefficient in GEO-MSS systerns since guard time and sorne other

overhead bits for these specinc systerns must exist in each tirne slot The basic idea of the

proposed scheme is to defiagrnent tirne dots kequently in a fiame to rninimize non-

inforniabon bits which leads to higher capacity. A preliminaq study is provided to evaluate

the expected guard time dong with the procedure for the fiame dehgmentation dynarnics.

LnitiaKy, aaditional TDMA £rame structures are discussed with typical parameter values and

time-slot assignment dynarnics. This is followed by a presentation of the new TDMA fiame

structure with its choice of parameters. The tirne slot assignment mechanism and the required

signaling procedure ensues. The chapter concludes with a brief cornparison between the

capacity performance of the traditional and proposed fiame structure.

27

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3.1 Traditional TD2MA Frame Structures

In traditional cellular TDMA systems, each tirne sIot in the fiame is fked in length. In GSM,

for exarnple, each turie dot consists of 156.25 bits in a 4.615ms fiame. Also, the tirne slot is

at a predetemiined location in the fiame and has a fixed arnount of overhead bits associated

with it. The overhead bits rnay include a naining sequence for the equalizer, synchronizahon

bits, and guard tirne- In GSM, they are 26 bits, 8 bits and 30.47pec respectively [LSJ. Fi,we

3- 1 depicts the GSM-TDMA frame structure.

Tai1 Data H Trainino H Data Tai1 Guard I I I 1 I I I

GSM Hyperfkme (3 -48 h)

Figure 3-1: Physicai Simetme for GSM Hyperfame, Supe@ume, Multifrume, Frame, and Time Slot

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The delay equalization process in the receiving path is required to compensate for the spread

in tirne delays resulthg fkom the multïpath propagation. Also, the process is essential when

delay spreads are signincant comparai to the information bit period. In GSM, the

aansmitted bit penod is about 37psec, and delay spreads of about Spsec are cornrnon [la. The delay-spread problem becornes critical and diEcult to solve with higher transmission bit

rate.

The GSM system is spec5ed to allow operation of mobile stations in-ce& when they are up

to 35 km fkorn their base station. As signals fkom ail the mobiles in the cell must reach the

base station without overlapping each other, a guard period of 68.25 bits (252 psec) is

provided in the access burst. This long guard penod in the access burst is needed when the

mobile station attempts its k t access to the base station, or after handover has occurred. To

provide the sarne long guard period in the other bursts wodd be specaally inefncient. The

GSM systern overcornes this problem by using adaptive fiame alignrnent. When the base

station detects a 41-bit random access synchronization sequence with a long guard period, it

measures the received signal delay relative to the expected signal ~ o m a mobile station of

zero range. This delay, called the timing advance, is signaled using a 6-bit number to the

mobile station, which advances its time base over the range of O to 63 bits, Le., in units of

3.69 psec. By this process the TDMA bursts arrive at the BS in their correct cimeslots and do

not overlap with adjacent ones. This process allows the guard period in all other timeslo ts to

be reduced to 8.25*3.69 psec = 30.47 psec (8.25 bits). [25j

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In a terrestrial ceIIular system, a user dernanding high data rate seMces (higher than what c m

k accommodated in a reg& tirne slot) can be assigneci multiples of the r e C W tïme slot-

The overhead associated per time dot is not signincant and multiple assignments of the tïme

16.25 overhead bits slots are feasible. For example, in GSM, the overhead is =10.4%. On

156.25 total bits

the other hand, a GE0 MSS system kame structure is not similar and the assignment of

multiple time dots rnay not be practical. Despite extensive fiterature survey, very few details

are available regarding TDMA fiame structure for any of the recent GE0 MSS systems king

launched or planned. Nevertheless, a fiame smcture for the TMI's MSAT MT-ST signaling

charme1 is presented in Figure 3-2 for cornparison purposes 1171. The MT-ST is a TDMA

charme1 used by the mobile terminal to respond to the requests made by the Earth Station. It

is also very important to no te that for the MSAT system, there are only five large spot beams

c o v e ~ g the whole of North America (a sixth spot bearn covers the Hawaii IsIands).

Therefore, the guard tirne value is excessively large. The future systems king p h e d consist

of several spot beams (around 100) and the guard time value is expected to be relatively

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Figure 3-2: The MSAT TMT-ST TDMA s ignahg Channel F r m e Szructwe 1171

/ \ \ / '4

Although the above MT-ST signaling channel is not meant for data aansmission, the fi_we is

presented to depict the Merence in the number of overhead bits. In this specific fiame

structure, the overhead is

rate services (higher than

Flosh (12 bits)

Prearnble 24 bits

63*14 = 52.61% . Again, as before, a user demanding high data

120 ms

what can be accornmodated in a re,& time slot) can be assigned

Vnique Word (32 bits)

multiples of the re-guhr the slot. In this case, each cime slot has an overhead

assignment of multiple tirne slots is spectrally inefficient

Signal Unit (192 bits)

of 52.61% and

In the next section, the new TDMA kame structure suitable for future MSS systems is

presented and analyzed. A new tirne-slot assignment scheme for high data rate users is

discussed and compared widi the traditional multiple time slot assi_ment mechanism

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3.2 Proposed Frame Structure [18]

For purposes of comparison with the proposed TDMA fiame s a u r n e , a 50ms traditional

fiame smcture 0 p e r a ~ g at 160 kb/s is dehed. Each time slot is designed to accomodate

user data and overhead bits (synchronization bits, equalizer trainirg sequence and head and

tail bits). Also, there must exist a mard tirne between two consecutive tirne slots. The

duration of 50 ms is expected to incorporate an acceptable delay in addiao n to the norrnal250

ms delay associated with the one-way GE0 sateIlite link

The choice of the data rate has a significant impact on the peak power requkements of the

mobile tmnhal If the data rate is very high, the mobile temiinal must transmit with high

peak power during its tirne slot to satir* the carrier-to-noise (GIN) ratio. The high data rate

leads to a high bandwidth value which, due to KT.B', resulU in large noise power. In [g], a

regional geostationary MSS system with speech users is described and a carrier data rate of

45 kb/s is assumed. The users are assumed ro operate with a data rate of 4.8 kbps and each

carrier has 8 users. However, this carrier rate cannot be adopted because of the requirement

to support higher data rate (8kbps - 64kbps) users. If a very low carrier data rate (100 kb/s

for example) is chosen, the system must have several c e r s and the access schemes tends

towards FDMA. A carrier data rate of 160 kbps is chosen to strike a balance between the

two conflicting requirements of low peak power dissipation and few carriers. The overhead

bits including synchronization, equalizer training and head and ta2 bits are assumed to be 40

altogether. As will be shown later, the actual number of bits does not affect the comparison

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of the pe~ormance of the nadiaonal time-slot assignment rnechanism and the proposed

rnechanism-

The minimum data rate to be supported is 8 kb/s which translates into 400 bits of coded user

data in a 50 ms frame. The overhead for the time dot consists of the 40 bits mentioned

above and a guard tirne. The value of the guard time depends upon the size and location of

the spot beam There is sigficant ciifference in the sizes of the spot beams in the proposed

system According to the results fiom Chapter 6, the guard time value c m range fiom 0.7111s

to 7.2ms. Therefore, Ît is proposed that multiple dïscrete guard tirne values be used in the

system For the purposes of the cornparison,-an average guard time value of 2.25 ms is used.

Each time slot is, therefore, (440 bits / 160 kb/s) + 2.25ms = 5.0111s long. Hence, the 50ms

h e has a capacity of ten 8kb/s time slots. If the user wishes to cornmunicate at data rates

higher than 8kb/s (16kb/s for example), the user shall be allocated multiple time slots (two

8kb/s for a 16kb/s user) in the kame. In the traditional Bame sbxcture, there always exists

guard time between any two time slo ts even if a user has k e n assigned two CO nsecutive turie

slots.

In the proposed fiarne structure, the time slots would be variable in leri,@. Also, the

overhead bits per time dot wodd be 40 bits (excluding the guard t h e ) as in EaditionaI

TDMA fiarne structure. In addition, each time slot would have a guard time of 2.25111s

Noise Power = KTB, where K = Boltzmann's constant = 1.38 * lus J/K. T = Temperature in OK and B = bandwidth in Hz.

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inespective of the correspondhg bit rate. The fiame length is still considered to be 50ms and

the transmission rate is 160kb/s The lena& of the time slot assigned to each user will be

detexTnined from the data rate at which that the user wishes to operate. The time slot lengths

for different data rates are listed in Table 3-1.

1 8 kb/s 1 5.0 rns 1

Table 3-1: The t»ne dot length (includes overhead bits and guard tirne) for dzrerent ahta rates

For example, if the user wishes to operate at 64 kb/s, his/her particulas t h e slot wodd spart

3200bits / 160 kb/s = 20ms for user data, 0.25ms for overhead and 2.25ms as ,auad time.

Such variabIe lena@ time slot allocation is expected to improve fiame efficiency considerably.

The @me efficiency is df lned ar the ratio of the time used for revenue-generating trq@c to

the time used to transmit aZZ the bits in the frame. For example, considering the case of

64kb/s user data rate, kame efficiency would be 20ms/(20.25 i- 2.25ms) = 88.89%, much

better than the 20ms/[20 t 8-(0-25 + 2.25)lr-n~ = 50.0% allowed by the traditional TDMA

tirne dot alIocation.

Furthemore, the rime dot allocation for each user shalt be dynamic and would be ailowed to

change periodically, see Figure 3-3. If a user is transrnimng in the last 5.0ms of the curent

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fiame, the user may be made to transmit in a daerent 5.0m segment

35

of the next (or future

nb) fiame. The segment assigrnent for the next (or future nh) hrne wodd be done by the

satefite on-board processor or altematively at an earth gateway with additional time delay.

The objective is to accumulate all the 'in use' t h e slots in one portion of the h e to result

in a contipous segment of ho t in use' space towards another portion of the fiame. When

there is no call, a segment of the kame is available

(subject to the length of the segment).

to be used by a user at any data rate

Current Frame 3.75 msec w +l

Legend: 2-25 m e c M-m F = Free Segment

50 msec Next Frame

Data at 8 kb/s + Overhead Guard Time D8 = 8 kb/s user G = Guard Time D32 = 32 kb/s user

50 msec

F D ~ \ G D ~ , G D32

Figure 3-3: An illustration of the defragmentation process in theframe

The main advantage of this scheme is the increased efficiency due to reduced overhead in the

transmission of users at higher data rates. Furthemore, it is very versatile and flexible. The

following are a few factors that make the scheme attractive.

. ,G F

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A change in number of overhead bits (other than guard tirne) does not affect the capacity

performance (efficiency) of this scheme as drasticaily as it does for the traditional TDMA

case. The 'defkaeomentation' of the h r n e may be done every n fiames (n 2 1). If the

complexity involved in perfonning 'defka,pentation' every fiame is too high, the fkarne

may be 'defiapented' less frequently (only when a call is c1eiu:ed).

It rnay also be better to reassign time slo ts for only one user every time a cal1 is cleared.

The remaining users in the fiarne shall maintain their current time slot assignments. There

should be no signitcant loss in performance because fiame 'defÎagmentation' occurs only

when the call is cleared. If more than one time slot needs to be relocated, it can be done

sequentially as the delay in the process wiU not significantly affect the overail capacity

performance for the expected slow aaEc variations. It is also to be noted that additional

signaling wilI be required to perform the defkigmentation process. However, it is

considered to be very minimal as compareci to the overall capacity provided.

3.3 Signaling required for defragmentation of f rams

It is observed that the 'defkagmentation' of the fiame would be required onty when new c a k

are k i n g admitted or cleared fÏom the system It is proposed that the signahg for

'defragmentation' shall follow ARQ protocoL Although this process might take a relatively

long tirne, it can easily be absorbed in the long call setup and termination time. The Ïmpact of

such an ARQ scheme on the overail perforrnance of the system is suggested for future

research work (see Ckiapter 7).

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3.4 A Cornparison of the Frame Structures

A brief cornparison Ïs made here to evaluate the perfomiance of the proposed TDMA kame

structure in ternis of throughput. More accurate results are presented later in Chapter 6,

when augrnented with the completeness of other system design parameters. At 160kb/s, the

2.25ms ward tirne translates into 360 bits and dong with 40 bits for other overheads, the

total number of overhead bits is 400. With a 50ms fiame operating at 16Okb/s, there are 8000

total bits available. In the proposed system, if the 64 kb/s titre dot is considered, the nurnber

of information

addition of 400

bits including forward error correction W C )

overhead bits, total bits are 3600. Therefore, the

bits is 3200 and with the

fiame has the total capacity

of 8000/3600 = two 64 kb/s users (with 800 bits to spare which can accommodate an 8kb/s

user). The traditional kame structure has a total of ten 8kb/s slots and it can accommodate

one 64kb/s user and two 8kb/s users. Therefore, using the proposed kame structure, there is

a 100% irriprovement in the capacity for 64kb/s users. However, it may be rare to h d all

users in the harne structure operating at a data rate of 64 kb/s. A similar analysis is carried

out for other data rates and the results presented in Table 3-2.

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Table 3-2: Irnprovernent in Capaci~, with the proposedfi-ame structure over the tradinOnal frarne structure

The performance of the proposed fkme structure is enhanced by the Merent long mard time

requirements of GEO-MSS systerns. The performance of the system depends heavily upon

the choice of guard time values and is evaluated with exact guard time values in Chapter 6.

In the next chapter, a complete system design is presented with the specifications and the

cdcuIaeons- The calculations and the design parameters are the basis of the software

module discussed in Chapter 5.

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Chapter 4

S ystem Design

4. An Introduction to the Design Procedure

In the previous chapter, the concept of the new TDMA fiame Structure is discussed with -

prelimlliary results. To veriQ the proposed capacity enhancements, a future MSS system

supporthg 8 kb/s - 64 kb/s data rate users is designed with multiple spot bearns. In this

chapter, calculations of the design parameters are presented. The steps involved in the design

c m best be sumrnarized as foilows:

generation of individual spot bearns,

amiving at a spot beam layout acceptable for the desired coverage axa,

design a methodology for interference analysis in the spot beams,

0 using the specifications and the lïnk budget, cornpute the acceptable interference

levels for possible frequency reuse,

device fkequency assigrment strategy and assign ~equencies to the spot beams,

perforrn link budget analysis for capaciq investigation purposes.

In the next section, the system specifications are presented which are then followed by

calculations pertaining to the spot beam layout generation. The antenna pattern and the earth-

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satellite geornetry are discussed in Setion 4.2 and 4.3 respectively. Section 4.4 explains the

calculations performed to achieve individual spot bearn contour layouts. This is followed by

Interference Analysïs in Section 4.5 and the chapter is concluded with a detailed description

of the link budget parameters in Section 4.6.

4.1 System Specifications

The system is designed to operate in North America, includuig the Caribbean Islands and the

Hawaii Islands. S o m of the following specincations are adopted fiom the MSAT satellite

system operated by TM1 Communications [19]. The rernaining specZcations are detennined

during this research as part of the future system design. The following are the system design

specitlcations:

1. The system shall operate with a total bandwidth of 29 MHz. The link between the user

terminal and the satellite s h d operate in the L-band (1-2 GHz) while the link between the

satellite and the c e n a earth station shall operate in the Ku-band (12.5 - 18 GHz). The L

and Ku-band confguration is most p o p h arnong the current GE0 MSS systems (MSAT,

European Mobile Satellite Dl). The operating conditions under these £kequency bands

are weU-known and the future MSS systerns being designed are intended to replace the

existing ones in the sarne bands.

2. The system shali operate with QPSK as the modulation scheme. QPSK is considered the

most appropriate choice when ease of implementation, bandwidth and power efficiency

and resistance to non-linearities are considered 1171.

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3. The satellite shall operate with a 14 m antenna In order to permit reliable

communications between the satellite and a hand-held mobile terminal, the satellite

antenna must be large. The practical limit of the GE0 mo biIe satellite antenna diameter is

detemillied by the faHing height of the launch vehicle, which for the Atlas IIAS is about

12 or 13 m, corresponding to a gain at 2 GHz of 42 dB [9], Once one has amved at a

large antenna diameter, less RF po wer is required to provide a Iarge circuit capacity. The

currently planned Thuraya system by the Thuraya Satellite Telecommunications Co. shall

operate with a 12 m antenna [7].

4. The system shall support multiple data rates ran,ging nom 8 kbfs to 64 kb/s. These data

rates include error correction detectiodcoding and overhead bits which ciiffer for different

data rates. The exîsting MSAT Circuit-Switched Data cmently provides 2.4 kb/s and 4.8

kb/s throughput and supports k e d and mobile applications such as e-rnail, fax, LAN

access and file transfer [20]. For future M S S systerns, higher data rates in the

neighborhood of 32 kb/s and possibly up to 64 kbfs are being envisaged.

5. The systern shall provide a bit error rate of less than IO-' for the 8 kb/s speech user and

Iess than 10-~ for higher data rates. The existing AMSC system is designed with a nominal

threshold of 1% (IO-') error rate for the 6.4 kb/s speech channel [17]. Also, ~ o m

experience, bit error rate of 10" has proved to deliver acceptable speech quality while a

bit enor rate of IO-' is not uncommon for data users.

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6. The satellite is located at 106.5"W and has an antenna capable of generating multiple spot

beams covering ail of North Arnerica. The current MSAT satellite is located at 106S0W

and it provides senice to North and Central Amenca (including coastal waters to 400

km), Mexico, Hawaii, and the Caribbean [20] with seven large beams. The coverage area

for a future MSS system serving North Amenca is expected to be identicaL However,

the number of spot beams shall be much larger to utilize the high gain of satellite antenna

effectively. The Asian Cellular Satellite System (ACeS), currently- behg deployed in the

Asia-Pacific region is designed to operate with 140 spot bearns. The Thuraya sptem is

expected to have 256 re-configurable spot beams providing services to the Arab States,

Central Asia, India and the East Europe [7].

7. The satellite has a Ku-band amplifier capable of g e n e r a ~ g 80W of power. The Ku-band

satellite amplitier power is an important factor which detemines the total capacity of the

network. It is useful to derive a rehtionship between the network capacity and the

satellite amplifier power and then substitute the value of the amplifier power.

8. The system shall yieId optimum performance do wn to an elevation angle of 1 sO, identical

to the requirement of the curent MSAT system [8].

9. The speech users opera- at data rate of 8 kb/s shall have a voice activity factor of 40%

P l -

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10. Due to the variability of the trafic intensity expected fiom different regions of North

America, the spot beam areas (due to the edge-of-barn power level) are to be

rnanipulated. Therefore, two adjacent spot beams may represent different edge power

levels with respect to the? centers. Therefore, in the forward link* the satellite s h d

dedicate merent power levels to the centers of the spot beams. The impact of such

d y n d c power assignment on the system capacity and satellite power utiIizatïon is

suggested as future research work (Chapter 7). --

The ability to generate spot beams and to create an acceptable layout for the desired coverage

area is an integral part of the MSS system design. The generation of the spot bearns is based

on the antenna pattern, path loss and the earth satellite geomeiq. In principle, a signal

transmitted from an edge of the spot beam is received at the satellite with a power level which

is less (attenuated) by a certain arnount with respect to a signal aanmiitted fkom the center of

the spot beam The amount of attenuation is deterrnined by the antenna pattern and the path

loss. A signal transmitted fkom the satellite to a user terminal at an edge of the spot b a r n

also undergoes similar attenuation with respect to the center of the spot barn due to the

transmit antenna pzttern and the path loss. Figure 4-1 is a very brief illustxation of the

concept of attenuation in the spot beam Also, due to the sphencal shape of the earth, the

shape of the spot b a r n is drastically different in different regions of the desired coverage area

For example, as shown in Figure 4-2, a 3 dB contour may appear as a srnall circle near the

equator but may appear kregular in shape (resembling an ellipse) towards the high latitudes.

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In the next section, the principles of antenna pattern are presented dong with the actual

values used in the system design.

The signal received at the edge of spot beam is x dB aîtenuated with respect to the center of the spot beam. Also tme for vice-versa Center of the Spot transmission

The value of x is detennined i =maximum gain by the antema pattern and / i /

Figure 4-1: An illustration of the spot beum attenuation

The spot beam is hegnidy shaped (resembfing an eILipse) when the satellior is not M y overhead

\ I 'The spot beam is very inepiarin shape

Figure 4-2: Difference in the shapes of the spot beums in different regions due to the spherical nature of earth

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4.2 Antenna Pattern

For communications with handheld termirials, the link budget requires a larger gain of the

satellite antenna As disnissed earlier in this chapter, a 14 rn antenna is chosen. A typical

antenna pattem response is depiccted in Figure 4-3. An antenna has a maximum gain in the

boresight (center of the antenna pattern) direction and the areas around the boresight

expenence less gain- The antenna pattern is identical in both transmission and receptïon

modes,

Figure 4-3 : Radiarion characteristics of a rotational-symmenic direcrional antenna [IO]

The gain of an antenna in boresight direction is given by [IO] :

where:

G,,

q = antenna effitiency; srpical range: 0.55 . . ... 0.65

D = antenna diameter = 14 m

h = wavelength = speed of light / fiequency

Eq. 4-1

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In the proposed system, is chosen as 65% and D = 14 rn In a typical GEO-MSS system,

the link between the user terminal and the satellite dictates the performance of the system due

to the limited capabiliaes of the user temiinal Therefore, the analysis presented here is for

the user terminal and the satellite link. The kequency of interest is the L-band fiequency of

1.6 GHz which yields a wavelengui of 0.1875 m. Hence, G,, = 45.5 dBi-

The gain of the antenna in regions distant fiom the bresight is a fûnction of the angle, a,

with respect to the boresight. The value of a, at which the gain is h s o f that at the boresight

is ofien referred

beamwidth of an

In the proposed

to as 3-dB (half-power) beamwidth, a3&. The 2-sided 3 dB (half-power)

antenna with a diameter D and operated at a certain hquency is [IO]:

fl

system, the 3-dB bearnwidth, a3dB = 1.09375O. A generalized antenna

pattern relating the gain of the antenna, G with a and a 3 d ~ is given in Eq.

O then the signal is received at boresight while if a = ajd~ then the signal is

the maximum gain at the edge of the 3-dB spot beam.

4-3 below. If a =

received wfth haif

Eq. 4-3

The background presented above regarding antenna patterns and gain/attenuation

relationships is very instrumental in the generation of spot beams. As rnentioned earlier, a

spot beam is a contour of a certain gain, for example -3 dB, with respect to the boresight.

The steps involved in generating spot beams can be iternized as foliows.

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1. identify the center of the spot beam (record its coordinates); it wiU serve as the boresight,

2. compte the path loss fiom the satellite to the center of the spot beam,

3. locate the locations in the vicinity of the boresight where the composite effect of the path

loss and the antenna gain is of desired attenuation with respect co the boresight

However, due to the spherical structure of the earth, step 3 is not s~aightfonvard and

requires insight into the earth-satellite geomeq. In the next section, a brief background

regardïng earth-satellite geomeq is presented. This is followed by acnial spot beam contour

calculations in Section 4.4- 1.

The basic problems regarding earth-satellite geomeay involve calculating the distance to the

satellite and the azimuth and the elevation angles of the user terminal antenna The angles are

required by the antenna to point at the satellite. Given the Iatitude, $+, of the user terminal

and dserence in longitude, dl, taken relative to the subsatellite point (a terni used to refer to

a position on earth which lies on the luie jobhg the center of the earth and the satellite), the

angles and the distance can be calculated. Assurring that the earth is spherical with a radius

equal to its mean equatorial radius of 6378 km, these quantitïes can be cdculated using the

geometry in Figure 4-4. The basic ûigonorneaic formulae, needed involving the Iaws of

cosines and sines for plane and spherical triangles are listai in Appendix A-

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0, = latitude of ground station at E Ai = difference in longitude between E and subsateIlite point S

(taken positive if earth station is to the West of the satellite) y = a o s T = nadir or tilt angle ar satellite RE = equatoriai radius of the earth h = sateIlite altitude 8 = angie of elevation at earth

Figure 4-4: Basic Satellite Geometry [14/

From the çphencal ixiangle EMS in Figure 4-4, redrawn in Figure 4-5, the central angle y of

the great circle ES comecting the user terminal E at latitude $g to the subsatellite point S is

given by the cosine law for sides in Eq. 4-4 [14],

cos y = cos#, cosAl +sin #, sin AZcos90' = cos$, cosAl Eq. 4-4

where dl is the ciifference in longitude between E and S. When E and S lie on the same

meridian, Al = O and y= &.

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user position

I RE r

' center of the earth satellite:

Figure 4-5: The parameters associated with the p l m e niangle EOP

4.3.1 Slant Range

The siant range d is detemiined by the law of cosines applied to plane triangle EOP [14],

redrawn in Figure 4-5.

Eq. 4-5

where r = RE + h.

4.3 .2 Elevation

The elevation angle 0 is obtained from the Iaw of sines, which implies the following: [14]

r rsùiy cos0 =-sin y =

d JR; + r' - 2R,r cos y

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1-cos2 #, cos' Al cos0 = (R, +

h2+2R,(R, +h)(l-cos@, cosAl) Eq. 4-7

Nomially, a minimum value for 8, OM, is specified as part of the system specifcations. As

long as the user is at a location where 0 2 O&, a certain quality of service of guaranteed.

4.3 -3 Coverage Area (Footprint)

The coverage area of the satellite is definecl as the area on the earth's surface where a satellitz

is seen with an elevation mgIe 0 2 O-. 0- is called 'crninimum elevation".

Figure 4-6: Coverage area (fooprint) of a sarellite [IO]

The coverage area of a satellite is a spherical cap on the earth's surface. The extent of the

coverage area is characterized by O-, Tm, &, #g ,. The extension of coverage Uicreases

with higher h and lower O-.

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the extent of the coverage area (length of arc) is aven b y

with @, ,, in degrees.

In the next section, the relationships presented above are used to generate a multiple spot

bearns layout for the desired coverage area.

4.4 Spot bearns GenerationlLuyout

As mentioned earlier in this chapter, the system is required to perform well down to elevation

angles of 15". Using Eq. 4-8, 0 ,, = lSO, r = 42,165 km, RE = 6,378 km, the maximum

latitude #g , (assuming Al = 0) = 66-59'. Therefore, the system should provide opamal

service till the latitude of 66-59' and hîgher, if possible. The system is also required to

provide s e ~ c e in the coastal regions, approximately 400 km into the either oceans. With

the above restrictions, the system is designed to provide service to users Iocated

between the latitudes of 10" and 70° and between the longitudes of 50" and 170°. Thz

area d e h e d by the latitudes and longitudes is to be covered with several spot bearns. As

outlined in Section 4.1, the initial step in the generation of spot barn is the identification of

the center of the spot beam For example, New York city c m be at the center of the spot

bearn (boresight). The next step Ïs to seek the contour boundary, which is the Iocus of

locations around the center of the spot beam where the composite effect of the antenna gain

and the path loss result in an x dB loss relative to the center of the spot bearn This step is

explained in detail below.

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4.4.1 Spot beara Contour Calculations

A mathematical closed fonn for the Iocus of a contour of a certain attenuation is seen to be of

a very cornplicated process. The alternative strategy adopted here in this thesis involves

dividing the desired coverage area into srnall grid squares and computing the attenuation Ievel

for each grid-square with respect to the boresight. Although this is considered

cc'mputationally ineficient, it is done only once and with a smart design of a database system,

the overall performance becomes acceptable as will be seen later.

. -

The parameters required for the calculation of the attenuation are the coordinates (latitude

and longitude) of the center of the spot beam (boresight), coordinates of the grid-square and

satellite (longitude only). As rnentioned before, the attenuation for a spec5c grid-square is

derived fiom the composite effect of the path loss and the antenna pattern gain with respect to

the boresight. The path loss, L,, is detennined for both the boresight and the grid-square

location using Eq. 4-10. The differerice in the value of L, for the two locations yields one

source of attenuation or gain depending upon the coordinates. In Eq. 4-10, d is the slant

range distance to the sateIlite deterrnined by Eq. 4-5 and il is the wavelength of the signal.

The second source of attenuation is the antenna pattern. Given the values of the coordinates

of the boresight, Md-square and the satellite, the objective is to caiculate the value of a, the

reception angle with respect to the boresight and then use Eq. 4-3 to calculate the relative

attenuation. Using vector analysis, earth satellite geometry and the antenna pattern, an

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53

onginal relationship is developed to calculate a from the given pararneters. Figure 4-7 shows

the pararneters and the vectors required for the analysis.

~=sareiiite B = user terminai C = boresight . -

1 (center of the spot beam)

$,,- laanide of tire centre of spot beam $,,= latitude of the user terminal

&= difference in longitudes between the

satellite aad the user terminal

.....*..-..*..*...... X

Y

Figure 4-7: The earth satellite geometry and the vecmrs required for the artenuation computations

In Figure 4-7, the point A represents the satellite while B and C are the grid-square and the

center of the spot b a r n (boresight), respectively. Therefore, a signal aansmitted kom C shdl

be received at the satellite with maximum gain ( G d while a signal kom B is received wirh

relatively less gain. The follo wing is a bnef description of the remaining parameters in Fi*me

4-7.

eg,= latitude of the center of spot beam (boresight)

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ggB = latitude of the grid-square

Al, = dinerence benveen the values of the satellite longitude and the boresight longitude

(taken positive if the boresight is to the West of the satellite)

Al, = diBerence between the values of the satellite longitude and the gid-square Ionginide

(taken positive if the g id square is to the West of the satellite)

r = distance £iom the center of the earth to the satellite = 6,378 km t 35,787 km = 42,165

- 3 + + - b + + OA, OB, OC, AB, ACand BC are the vectors used in the analysis.

Since cc is a solid angle, it can be calculated by considering ABC as a triangle and utilizing the

cosine law. According to the cosine law:

The objective nov

:. CC = arcco

Eq. 4-11

Eq. 4-12

I is to reIate each of AC , AB and BC with #gc, #8B . AIc dlB and r. The I l I l I l following analysis relates AC with gg, , dlc. and r. The analy sis is daived fiom Fi-me 3-7. ri

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OC = Jr' cos' $,, cos2 Alc+ r' COS' g,, sui Aic + r' sui2 @,, =RE = 6378h I l

The foUo wing analysis relates

Fi,oure 4-7.

AB with @gB , Ab , and r. Again, the analysis is derived from 11

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Eq. 4-14

Finay, the reIationship between BC and OgB , q$=, AlB . Alc. and r is derived. 17

3

.: BC =r(cos#,, cosAl, -cosga cos~2,)i +r(cos@,, sin Al, -cos#@ s i n ~ l , ) j

+ rkin #,, -sin $à)k

(cos#,, cos N, -cos#,, cosdB )Z+ (cos@,c sin uc -cos& s i n ~ k p

Eq. 4-15

a Once the value of a has been detennined, Eq. 4-3 can be used to calculate the antenna

pain relative to the boresight gain. The antenna gain value c m then be combined with the

path loss value to yield the composite attenuation value. To illustrate the use of the above

equauons and concepts, an example is presented next.

Let us assume that the spot beam is centered in the region of New York at the latitude of

40.45ON and longitude of 75OW. Also assume that the satellite is at a longitude of 106S0W

and the grid-square of interest is at a latitude of 40°N and longitude of 70°W- Therefore #gB

=40°, @gc=4û.450, Al, =70 - 106.5 =-36.5°.Alc= 75 - 106.5 = -31.5O.and r=42,165 km.

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The first step is to calculate the path loss from both the boresight and the grid-square

locations to the satellite. To enable the calculation of the path Ioss, the s h t range distances

fÏomthese locations to the satellite must be calculateci fist. Using Eq. 4-5 and Eq. 4-10, the

distance fiom the boresight to the satellite Ïs 38,334.96 km and the corresponding path loss is

188.19 dB. SimilarIy, the distance k3m the grid-square location to the satellite is 38566.05

km and the corresponding path loss is 188.24 dB. Therefore, in t e m of path loss, the grid

square location receives a signal which is 0.05 dB less in power than the one received at the

boresight location. Note that in a diEferent situation, the path loss at the grid-square rnight be

lower in magnitude than that of the boresight when the distance to the satellite is shorter.

The next step is to compute a and the corresponding antenna attenuation- The values of

1 1 , IÀC~ and FI murt be calculatecl kst . Usinp Eq. 4-13, Eq. 4-14 and Eq. 4-15, the

values of /ÀC/, IGI and BC are 38,566.05 km, 38,334.9 km and 427.7 km respectively. I l Using Eq. 4-12, the value of a is 0.536' which when substituted into Eq. 4-3 yields a gain

value of -4.03dB relative to the boresight gain. Therefore, the composite gain for the

particular grid-square region is the sum of -4.03 dB fiom the antenna pattern and -0.05 dB

fiom the path loss. The total attenuation value i s therefore 4.08 dB with respect to the center

of the spot kam.

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The process can be repeated for several grid-square locations around the center of the spot

bearn and the attenuation values can be stored into a database. The database c m later be

querïed to obtain the coordinates of the locations with any particular attenuation value. For

example, if a 3 dB contour is desired, the grid-square bcations with attenuation value of

approximately -3 dB c m be retrieved and plotted onto a map.

4.4.2 Coverage of North America

The next step in the systern design is to ensure that the desired service area is fully covered

with spot barns. Since a signal transrnitted fiom the center of the spot barn (boresight)

enjo ys maximum gain onboard the satellite receive antenna, an attempt was made to rnake the

centers of spot bearns coincide with d i e s of high population and/or high expected a&c.

In order to enhance the capacity of the system and cater to the varying amounts of naEc

generated £kom dinerent regions of North America, four distinct edge of the spot beam

attenuation levels were chosen. This is similar to the concept of macro, micro and picocefi in

the terreshial ceiluiar systerns. The edge of the spot bearn attenuation level along with its

location determine the size of the spot bearn The variation in the size of the spot beam help

in s e ~ c i n g specxc areas more effectively than others. The four different attenuation levels

are:

1. -1.0 dB (Highest Traffic Intensity, e-g. regions of New York, San Francisco, ..)

2. -1.5 dB (Moderate Traffc Intensity, e.g. regions of Ottawa, Chicago, ..)

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3. -2.0 dB (Low Traffic Intensity, e.g. re$ons of Phoenix, Orlando, ..)

4. -3.0 dB (Lowest TmEc Intensity, e.g. reaons of Oklahoma, Banff Islands, ..)

In absolute practicd ternis, the edge of the spot -bearn attenuation level is identical for a l i spot

beams. However, the center of the spot beam receives more power fiorn the satellite by 1.0,

1.5, 2.0 or 3.0 dB depending on the type of the spot beam This affects the fonvard link

(satellite to the user terminal) performance of the system Since the fonvard link is generally

not considered cntical for a GE0 MSS system, the performance analysis for this link is

suggested as future work (see Chapter 7). The task of covering the desired coverage area

with several spot bearns is quite rneticulous as it involves ensuring minimum overlap between

the spot beams and providing senice to all of the desired coverage area The final

con£ipation of spot beams covering North Amenca with four distinct edge of spot bearn

attenuation levels is given in Chapter 6.

4.5 Interference Analysis

The £inal spot beam con@uration is followed by CO-channel interference and fiequency

assignrnent analysis. The CO-channel interference analysis is vital to ensure an acceptaHe

quality of senrice for a user in a particular spot beam The CO-channe1 interference is

calculated by assurning a user (norrnally t e d as the "wanted signal") is using a fÏequency in

a particular spot beam and that one or more other user(s) are sirnultaneously using the same

parricular fiequency in other spot bearn(s). A scenario Ïliustrating such fiequency reuse and

potential interference is depicted in Fiume 4-8.

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Index c: user (carrier) Index ir interference

Sr receivlng sateUite Tc: transrnitting mobiie termuid q: interferhg terminal

6,: receive angie of the wanted signal. relative to the centre of the spot beam (boresig h t angle)

O,: receive ande of the interferhg signal. relative to the centre of the spot beam of the wanted signal

*--- GJ8): antenna characterisitics of the spot bearn of the wanted signa1

P, Pi: terminai iransmit power Le Li: path Ioss factor -

- ' fi,&,&: fresuencies used in the system

The antenna of the mobiie terminal is assumai to be ------....__......--.- omnidirectiond

Figure 4-8: An uplink (user reminal ro the satellire) interference scenario

The analysis includes calcula~g the level of interference that would be acceptable in a

paaicular spot beam under worst case scenario and the calculation of the actual interference.

The worst case scenario Q l i e s that the "wanted signal" is received at the lowest possible

power IeveL The "acceptable" Ievel of interference is calculated using the link budget for the

user in a spot bearn For example, if the user is in a 3-dB spot bearn, link budget analysis c m

be used to calculate the maximum tolerable amount of interference for the user at the edge of

the contour. Section 4.6-1 deals with the link budget and conchdes with the calculation of

maximum tolerable interference. The interference received corn each of the inte~ering users

(Pi) is given by:

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where Pi = transmit power of the interfering user, Li = path loss for the interferhg signal,

Gc(Bd = antema pattern gain experienced by the interferhg user. The total received

interference is @en by:

where i 2 1.

The value of i cannot be deterrnined beforehand as in the case of terrestrial cellular system

anaLysis. In the analysis for terres- ceUuIar systems, i = 6 CO-channel i n t e r f e ~ g users are

n o d y assumed. Traditional CO-channel interference analysis for FDM-4 and TDMA

satellite systems have often considered the hexagonal spot b a r n S m i C N e S (aU 3 dB) [IO].

The analysis is useful to provide some preliminary insight into the interference value.

However, it is based on several following assumptions which are not valid for the purposes

of the design under consideration.

the coverage area is flat such that all the spot bearns are of equal sizes,

there exists no difference in the slant range of the users to the satellite,

0 the spot beams are all3 dB contours, Le. the power received at the edge of the spot bearn

is 3 dB Iess than that received at the center of the spot beam, and

there is no overlap between the spot beams.

Most of the above assumptions are no? valid in the proposeci systern The overaii coverage

area is the entire North Arnerica which cannot be considered £iat due to the sphencal shape

and hence the curvature of the earth- The distance between two furthest points in a 3 dB spot

beam at the equator may only be about 200 km in diameter wMe it may be about 1000 km

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towards the northem areas, as expIaïned earlier. The contours of same power level are of

=es are much different sizes in different locations of the desired coverage area. The slant ran,

sigrificantly different £kom within the spot beams and they conmbute to the path loss. The

path loss in addition to the reception angle define the spot b a r n contour and therefore the

slant range cannot be ignored. AIso the merences in the s h t range d e h e the ga rd t ime

calculations in Chapter 3.

Because of trafnc variations expected &om different regions of the coverage area, the spot

bearns are not d l 3 dB contours. As explained in Section 4.3.2, the contours are preset at -

1.0 dB, -1.5 dB, -2.0 dB and -3-0 dB. There are significant overlaps in the spot beams and it

is quite dificult and cumbersome to rearrange the spot bearns such that the overlap is

minimized and simultaneously have no gaps between the spot beams. However, the task is

accomplished and the final layout is depicted Iater in Figure 6-1.

The actual interference computations involve calculaihg the interference received fiom a l l the

spot beams into the rernaining spot beams. Assurning a system with a to t d of N spot beams,

for every spot beam i (i E spot km 1, 2, 3, . . ..N), the interference received fiom j ÿ # i, j =

spot bearnl, 2, 3, . . . N) into i is calculated and tabuIated. This c m best be represented usin_e

a matrix notation as follows.

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In the maair above, X, represents the interference received in spot beam i from spot bearnj.

The diagonal elements Xq (i = i ) = O because there is no interfering user in the spot beam with

the wanted signal The calculations of the ma& Ïs a lengthy procedure but it is done only

once and the results are stored into a database for fiequent future use. Once the ma& has

k e n setup, the fiequency assignrnent procedure involves a fast database lookup for

interference vaIues. The next section provides insight into the link budget analysis and the

calculations of acceptable interference values.

4.6 Link Budget Analysis

For a GEO-MSS system, the return u p l M (user temiinal to the satellite) is rnost critical due

to the limited power output capabrlity of the user (hand-held) tenninaI [21] and essentially

dictates the overaU perfomiance and the capacity of the system During the course of the

research, considerable t h e and effort have k e n devoted to understand the impact of different

parameters on a typical GE0 MSS luik budget. Different authors present the sarne

information in several different ways and it is challenging to grasp their explmation widiout

M y understanding and practicing with the fundamentals of the LUik budget [3], (91, [21].

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In the proposed design, only the retum link budget was considered to evaluate the system

performance. Some of the performance specifications of the system are no t initkIly provided

and are determineci during the course of the research, The specifications such as the

acceptable bit error rate (BER) and the code rate have a direct consequence on the system

throughput and capacity performance and rnust be chosen carefdly. After several design

iterations, it is decidcd to provide an upper BER threshold of 10" for the 8 kb/s voice users

and an upper BER threshold of 105 for the higher data rates at 16 kW, 32 kb/s and 64 kb/s.

A Viterbi codec with a 2/3 code rate is employed in with K = 3 constrajnt length . The acmal

information data rates are therefore 5.33 kb/s, 10.6 kb/s, 21.2 kb/s and 42.4 kb/s. An

example provided in [2 11 assumes the voice charnel to operate at a data rate of 4.8 kbls and

therefore a data rate of 5.33 kbls for the proposed system is deemed acceptable. The

performance of the codec is given in Figure 4-9 [22].

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\ - - \ - - - - - - - - - - - - WPER WUHD - - \ -

\ I I I ! 1 l I 1 1 ,

Figure 4-9: Peflonnance of rate 2/3 K = 3 code with Viterbi decuding. Nurne~cal bound and simulation results [22]

Rom Figure 4-9, an EdN, of 3.0 dB is required to achieve an upper BER threshold of IO-'

at the gound station after the cornplete return (user terminal to the ground station via

satellite) link Figure 4-10 dlusaates the composition of a r e m link budget for the proposed

systern with a user operating at 8kb/s. The ccplink, downiink and the overall combined Efl,

are relatai by the following equation.

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As mentioned before, the retum uplink budget is the most crucial and the upiink EdN, is ody

marginally afFected by the h m l i n k (satellite to the gound station) E a 0 . Therefore, an

uplink E6N, of 3.5 dB is chosen such that the overd combined uplink and ûi~wnlink E& is

3-0 dB.

Arnpiifïer RF Power = 80 W Antema Gain = 455 - x dB (x depends on the location of the user)

L V ~ . of C e s = ? Noise Temperanire = 27 dBK

Free Space Loss = 2053 dB Channel Eandwidth = 100 KH2

Fade Margïn = 7 dB Channel Bit Rate = 16û kb/s

Free Space Loss = 288 dB Fade Margin = 10 dB PoIarïsation and Aunosphere Loss = 0-1 dl3

T k Peak Power = 4.6 W for 8kb/s user Antenna Gain = 60 d3i Tx Peak Power = 5-7 W for hîgher data rates Noise Taperanire = 23 dBK Antenna Gain = 3 dBi for 8kb/s user Desired EJN, = 3.0 dB for 8kb/s user Antenna Gain = 5 dBi for higher &ta rates Desired E@l, = 6.0 dB for higha ciaiarates Loss = 0-7 dB

Figure 4-10: A complete return ZNzk budgetfor a mer operating ar a data rate of 8 kbis

Some of the important parameters in the Iink budget are explained next. They are classfied

according to the user data rate. The link budget for the user operathg at a data rate of 8 kb/s

is presented fkst followed by the link budget for a user operating at 64 kb/s. The link budget

for users operating at 16 kbls, 32 kb/s and 64 kb/s is idenhcd because identical user terrnïnals

are assurned to operate at these data rates. Therefore, in the proposed system, there are two

types of terminals; one hand-held terminal suitable for 8kb/s speech communication and

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ano ther transportable terminal suirable for higher data rate communication. The following is

the r e m link budget for a user 0 p e r a ~ g at a data rate of 8 kb/s.

4.6.1 Uplink return (user terminal to satellite) budget for data rate of8 m s

The template used for the Iink budget is similar to rhat in [93. The characteristics of the 8 kb/s

hand-held terminal become evident in the foUowing link analysis-

The user terniinal has a peak power output of 4.6 watts = 6.6 dBW. A peak power

dissipation of 4.6W is of concern to a user with a hand-heid terminal. However, the peak

dissipation of 4.6 W is required when the user is at the edge of its 3 dB contour. The

requirement can be rehxed immensely if the user is located in a more favorable region of

the spot beam If the user is located at the center of the spot beam, the hand-held terminal

is required to transmit at a peak power of 2.3 W = 3.6 dE3W. This power control feature

is not considered in this thesis and is suggested as funire research work in Chapter 7. As

discussed in Chapter 3, Section 3.2, an 8kb/s user ~ansmits 440 bits in a fiame of 8,000

bits. Therefore, on average, an 8kb/s user terminal dissipates -- 440 4.6 W = 23OrnW . 8000

This value is close to that of existing terresaial systerns.

2. The antenna gain for the user teminal is 3 dBi There is a loss of approxirnately 0.7 dB

within the temùnal[9]- Therefore, the user terminal EIRP is 6 t 3 - 0.7 = 8.3dB.

3. Free Space (Path) Loss cm be detemiined using the following formda:

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where h is the wavelength of the signaL For the L-band link (user terminal to the satellite),

the fiequency is assurned to be 1.6 GHz and therefore the wavelength is 18.75 cm. With a

distance of 36,500 lan (which is an approximate distance kom the satellite to its nearest

point on earth), the fkee space loss is 188.0 dB. For the Ku-band link (satellite to the

ground sration), the fiequency is 12 GHz and the fiee space loss is 205.3 dB. Polarization

and other losses account for 0.1 dB [9].

4. A fade margin of 10 dB is chosen so that the system can perform in a satisfactory marner

for no he-of-sight conditions and it can also tolerate shadowing by trees and low

buildings. [3], [9], 1211

5. The channel bit rate is assurned at 160 kb/s and QPSK scheme is applied with a filter roll-

off factor of 25%. Therefore, the channel bandwidth is

The satefite antenna noise temperature is 27 dBK [9]. Therefore, the noise power is:

6. The satellite antenna diameter is 14 m and is therefore capable of providing a maximum

(boresight) gain of 45.5 dB (see Section 4.1). The actual gain of the antenna depends

upon the reception angle as outlined before. For the purposes of calculating the

"'acceptableyy amount of interference, the worst case scenario is assurned where the user is

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at the edge of a 3 dB spot beam Therefore, the gain provided by the satellite antenna is

45.5 - 3 = 42.5 dB.

Therefore, the signal power received at the sateIlite is @en by:

R Y d Signal Power = EIRP - Free Space Loss - Orher Losses + Satellite Antenna Gain -

Fade Margin

= 8-93 - 188-0 - 0-10 t42.5 - 10-0 = -146.6 dBW Eq. 4- 1 8

The uplink noise power is -15 1.6 dBW.

Therefore, the uplink canier signal power to noise power ratio,

(c/N)u =-146.6-(-151.6)= 5dB

The required upii& is 3.5 dB with a (z) code rate. Therefore, the required

upiink energy of coded bit to noise density ratio, (Ej/No ) = 3.5 + 1010~,, ($)= 1.734 dB -

The available (C /N) = 5 dB , while the required (C/N). = 3.78 dB and therefore an

additional 1.22 dB of noise power can be allowed into the system The current thermal noise

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power is -151.6 dBW and the total allowed noise power is (-151.6 + 1.22) = -150.38 dBW.

In absolute terrns, total allowed (thermal + interference) noise is

9-1622e-16 W wMe the cunent thermal noise is 6.9183e-16 W- Therefore, a total of

2.2438e-16 W = -156.49 dBW is allowed as uplink interference noise. With a received

c-er power of -146.6 dBW, the required c-r to interference ratio in the systern (Ch) =

- 146.6 - (-156-49) = 9.89 dB. The cdculations are summarized in Table 4- 1,

1 IDistance to Satellite. km 365001 Frequency, MHz Channel Bandwidth, kHz Bit rate per carrier, kb/s 160.0'

"RF power, W 4.6 Transmitter power, dBW 6.6

Antenna Gain, dBi 3.0 Uplink EIRP, dBW 8.9 Free Space Loss, dB -1 88.0, Fading Marqin, dB

INoise power, dBW 1 -151 -611

Receive antenna efficiency, % 1 65.0

I(c/N)u, total, - dB 1 3.81

peceive antenna diameter, m

Table 4-1: Return Uplink (user teminal to the satellire) Budget for a mer operating at a data rate of 8 kbls

14.0

The r e m downlink (satellite to ground station) budget for a user operating at 8 kb/s is

presented in the next section,

I~eceive antenna gain, dB 42.5

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4.6.2 Retum downlink (satellite to ground station) budget for data

rate of 8 kbls

The satellite relays or 'bent-pipes' the uplink signal (fkorn user temiinal) to the ground

station. The objective of this rerurn downlink budpt is to calculate the amount of satellite

power required to bent-pipe a user operating at a data rate of 8 kb/s. The required satellite

power is a h c t i o n downlink carrier to noise ratio, (C/N), The value of (CLM), ïs

ta noise ratio,

- fl

the required total (overall) carrier to noise ratio, (Ch),, , the uplink canier

( C / N ) and the carrier to inter-modulation noise ratio, (%), . The required

overall ('y N, )mm' is 3.0 dB with a %code rate. Therefore, the required total energy of

coded bit to noise density ratio, ( EAj/N.)wd =3.0+LOLog,,(t)=1.24d~.

ratio, (C N ) wf°' are related by the following relationship [2 211. /

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Using Eq. 4-19, and substïtuting, (%), = 3.78 dB, (CA), = 3.28 dB and

(C/,) ,, = 16.0 dB [9], the required (%), = l5.86dB . The carrier bandwidth is LOO 1cHz

= 50 dBHz and the antenna noise temperature is assumed to be 23 dBK [9]. Wth Boltzman

constant = -228.6 dB/K, the noise power is -155.6 dBW. The required (*/N)~ value of

15.86 dB and noise power of -155.6 dBW imply that the required camer power at the ground

station must be -139.74 dBW.

To detemine the amount of power required fiom the satellite to satisfy the above carrier

power reqeement at the gound station, several gaidattenuation factors necd to be

considered. These factors are discussed next

1. The ground station is assumed to have a large antenna setup capable of providing a gain of

60 dB. The MSAT system operated by TM1 Communications has an 1 1 m antenna at the

ground station which provides comparable gain value [20]. Therefore, the carrier power

must be at a power level of at least -139.74 - 60 = -199.74 dBW before it arrives at the

ground station amplifier.

2. A fade margin of 7 dB is assumed. The r e m downlink is always a line of sight link

between the satellite and the gromd station ad therefore a 7 dB fade margin will sufnce

[93,[2 11. The unfaded carrier power level is therefore - 199.74 + 7 dB = - l92.74 dB.

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3. The kee space (path) loss is cdculated to be 205.3 dB using Eq. 4-10. The carrier power

level at the output of the satellite amplifier rnust therefore be at a power level of -192.74 t

205.3 dB = 12.56 dB. Hence. a carrier with an 8kb/s user demands an EIRP power level

of 12-56 dBW = 18.03 W-

4. The satellite Ku-band antenna gain is assumed to be 25.4 dBi [9]

5. An amplfier back-off factor of 4 dB is assumed. The authors in [ 9 ] assume a back-off

factor of 3 dB which is not considered sufficient [23]-

6- The satellite Ku-band amplifier can provide 100 W = 20 dBW of power [9] to bent-pipe

the incorning upiink signal- An equipment mansrnit loss of 1 dB reduces the available

amplifier power to 80W.

7. Since a non-regenerative repeater is assumed in the system, the available power of 80W is

consumed to relay the uplink composite useful carrier signal and the noise. Therefore,

only a certain portion of the total satellite power of 80W can be considered as available for

g e n e r a ~ g the downlink carrier signal. This portion of the total satellite power k referred

to as the Uplink Signal Share and is calculated based on the ratio of the uplink useful

carrier signal to the composite u p h k signal. It is definecl as follows:

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In the case of 8 kb/s users, the value of (Ch) , = 3 -78 dB and Uplink Signal Share = 0.705

= -1.5 dB. Therefore, if aU the users in the system are assumed to operate at a data rate of

8 kb/s, only 0.705 * 80W = 56.4 W are available to bat-pipe the useful uplink camer

sipal.

8. The amplifier power of 8OW (19 dBW) with an antenna gain of 25.4 dBi, a back-off factor

of 4 dB and an Uplink Signal Share of -1.5 dB yields total EIRP of 38.9 dBW.

9. The calculation of the nurnber of the carriers is considered in the power-limited capacity

calculations, Section 6.5.

The calculations are surnrnarized in Table 4-2.

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Bandwidth, dBHz 50 .O Noise temperature, dBK 23 .O Noise ~ower. dBW -1 55.6

I

iC/N)d, thermal, dB 15.9 I (Cllm), dB 16.0 I(C/N)d, final, dB 12.9 1

Total Return Link I(c/N),. dB 3.8

(EJN,) of carrier, dB (EdN,) of carrier, dB (2/3 code rate)

Table 4-2: Remrn Downlink budget for a user operaring af a data r ~ t e of 8 kbls. Some of the parameter values are chosen Rom [9].

Since the user data terminals for higher data rate users are different than the 8kb/s users, the

link budgets for the former are different fÎom those of the latter. The following is the return

link budget for a user operating at one of the high data rates.

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4.6.3 Uplink return (user terminal tu satellite) budget for data rate of 16,32,48 and 64 kbls

The link budgets for users operating at 16 kb/s, 32 kb/s and 64 kb/s are identical because the

user data terminals are assurned to be operating at the sarne power leveL The calculations are

al l exactly the same as in Section 4-51. However, some values of the parameters change due

to the dif3erent type of terminal used for high speed access. The data terminal used for rates

above 8 kb/s i s more capable than the hand-held terminal Frorn Figure 4-9, an E D O of 6 dB

is required to achieve an upper BER threshold of IO-* at the ground station afkr the

complete return (user t~mùnal to the gound station via satellite) Iink An upiink EdN, of

8.0 dB is chosen such that the overall combined uplink and downlink E&&, iS 6.0 dB. The

following are brief descriptions of the parameters used in the link budget for high data rate

users.

1. The user terminal has a peak power output of 5.7 W = 7.5 dBW. Again, the requirement

can be relaxed if the user is located in a more favorable region of the spot bearn.

2. The antenna gain for the data temiinal is 5 dBi There is a loss of approximately 0.7 dB

within the terniinal [9]. Therefore, the user terminal EIRP is 7.5 + 5 - 0.7 = L 1.8 dB.

3. As before, the path loss is 188.0 dB.

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4. A fade margin of 7 dB is chosen so that the system c m perfom in a satisfactory manner

for be-of-sight conditions as the data terminal is expected to be stationary when

comrnunica~g.

5. As before, the charnel bandwidth is 50 dBHz and the satellite antenna noise temperature

is 27 dBK [9]. Therefore, the noise power is KTB = -228.6 + 27 + 50 = -151.6 dBW -

6. The satellite antenna diameter is kept at 14 m and is therefore capable of providing a

maximum (boresight) gain of 45.5 dB (see Section 4.1). The actual gain of the antenna

depends upon the reception angle as outlined before. For the purposes of calculating the

ccacceptable" amount of interference, the worst case scenario is assumed where the user is

at the edge of a 3 dB spot beam Therefore, the gain provided by the satellite antenna is

45.5 - 3 = 42.5 dB.

Using Eq. 4- 18, the signal power received at the satellite is:

= 11.8 - 188.0 - 0.10 + 42-5 - 7-0 = -140-8 dBW

The uplink noise power is - 15 1.6 dB W.

Therefore, the uplink carrier to noise ratio, (Ch ) , = -140.8 -(-151.6) = 10.8 dB

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The required u p h k ( E//No ). ïsasssumed ta be 8 dB with a 2/3 code rate. Therefore, the

required uplink energy of coded bit to noise denslty ratio,

Hence, the required uplink carrier-to-noise ratio,

(yN). = 6.24dB + 52-04 -50.00 = 8.28 dB.

The availab1e ( C / N ) . = 10.8 dg, while the required (x), = 8.28 dB and therefore an

additional 2.52 dB of interference noise power cm be allowed into the system The current

thermal noise power is -151.6 dBW and the total aIlowed noise power is (-151.6 + 2.52) = -

149.08 dBW- In absolute ternis, total allowed (thermal + interference) noise power is

1.2359e-15 W while the current thermal noise power is 6.9183e-16 W. Therefore, a total of

5-4406e-16 W = -152.64 dBW is allowed as uplink inteflerence mise. Hence, with a

received carrier power of -140.8 oBW, the required carrier to interference ratio in the system

(Ch) = -140.8 - (-152.64) = 11-84 dB. The calculations are summarized in Table 4-3.

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- - -

LOSS, dB B -0-7' Antenna Gain, dBi 5-0 Uplink EIRP, dBW 1 11.9

"Free Space Loss, dB -1 88.0' Faciincl Marain, dB -1 0.0

M~eceive Gtenna efficiencv. % 1 11 65.01 UReceive antenna diameter. rn I if 14.08

- - - - - - - - -- - -

Noise power, dBW (C/N~I . dB 10.9

Table 4-3: Return Uplink (user terminal tu satellite) budget for high data rate users

The return downlink (satellite to ground station) budget for a user operating at high data rates

is presented in the next section.

4.6.4 Return downlink (satellite to gvound station) budget for data

rate of 16,32,48 and 64 kbls

As mentioned before, the number of signals which can be relayed by the satellite is a function

of the available onboard satellite amplifier power and the required duvmlink carrier to noise

ratio, (C/N) - The value of (%), is deterniined by the required total (overall) carnier to

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80

noise ratio, ( C / N ) , , ,the uplink carrier to noise ratio, (C ), and the carrier to inter- A modulation noise ratio, (%), . The calculation of the required (%), is presented next

n ie required overall (E6 /No " 6.0 ~5th a code rate. Therefore, the required rotai

energy of coded bit to noise density ratio, ($1 = 4.24 dB . Hence,

the required (C /N) , , = 4-24 dB + 52-04 - 50.00 = 6.28dB .

Substituting 6.28 dB, 8.3dB and 16.0dB for ( ) ( ) , and c&-to-

intemodulation noise ratio (%), Cg] respectively, the required (%), = 12. l dB . With

the addition of the gain/attenuation factors discussed in Section 4.6.2, a c d e r with a high

data user demands an EIRP power level of 8.8 àBW = 7.58 W.

As per specincations (Section 4.6.2), the satelJite Ku-band amplifier c m provide net power of

80W. The calculations of the remaining parameters of the downlink budget are similar to

those used in the budget of 8 kb/s users. In the case of all users in the system operating at

data rates higher than 8 kb/s, the value of (x) = 8.3 dB and Upiink Signai Share = -0.6

dB. Therefore, only 0.87 * 80W = 69.6 W are available to bent-pipe the useful uplink

canier signaL Table 4-4 is a typical downlink budget assuming identical kame structures in

each carrier with 64 kb/s users.

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H I~ianai share due to u~i ink noise, d~ 1 -0.6fl

1

I

themai, dB 1 1.5: ( ~ / N ) I M , dB f 6-0 (C/N)d, final, dB .I 0.21 A

1

potal Return Link 1

~umberof Carriers, dB E.I.R.PI carrier. dBW

Table 4-4: Return Downlink (Satellite to Ground Station) budget for high data rate zisers

-3 1 .O( 8.8

' ~ad i ng '~a r~ i n , dB -7.0'

As in the case of 8 kb/s users, the calcuiations of the number of the carriers is explainecl in

Chapter 6, Section 6.5.

Receive antenna aain. dB

4.6.5 Sumrnary

In this chapter, the calculations pertaining to the system design are presented. The earth-

satellite geometry and the antenna pattern is used to calculate the spot bearn contours Iayuut

60.0

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for North Amenca The interference analysis and link budget analysis is conducted in detaiL

In the next chapter, the software module designed to use the calculations and perform

capacity anaiysis is presented. The objective and hctionality of each of the software

cornponents is disnissed in detail.

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Chapter 5

Software Module for the Design and Analysis of a Multi-Variable rate

Geostationary Mobile Satellite Network

5. Purpose of the software module

A signifïcant portion of the research conducted in this thesis is to provide a software module

for the design of a multi-variable rate geostationary mobile satellite network. The software

module is developed in ~icrosoft@Visual Basic 5.0 and ~ ic roso f t@~ccess 97. The module is

designed with an easy-to-use Graphical User Interface (GUI) and is intended to perform

cornplex exhaustive caiculations in reasonable time. The developrnent of the software is

initially undertaken to provide some insight into the spot km shapes in the desired coverage

area (North Amenca, including the Caribbean and the Hawaii Islands) and hence facilitate the

calculations of guard times. It is enhanced to perforrn interference analysis and frequency

assignment. The software design involves illuminating North Amenca with multiple spot

beams ficorn a satellite in the geostationary orbit. The concept of spot beams and anrenna and

distance attenuation in the vicinity of the center of spot bearn is presented in the preceding

chap ter-

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5.1 Objectives of the Softu>are

The objectives of the software are as follows:

CalcuIate and tabulate the gain values (relative to the center of the spot beam) due to the

path loss and the satellite antenna pattem satellite in the vicinity of the centers of the spot

bean

Ushg the tabulated relative gain values, generate spot beam contours of any desirable

threshold value for the differential power gain.

M o w the choice of the coordinates of the centers of spot beams and repeat steps 1 and

2 to obtain an acceptable spot beam configuation.

Reaieve and provide attenuation values for any particular location (grid square) in North

Arnenca

Calculate guard time values for each of the spot beams.

Calculate CO-channel interference received fiom each of the spot beams to the remaining

spot beams.

Caiculate (ficorn hk budget) the acceptable interference power Ievel in each of the spot

beam.

Perform Frequency Assignrnent in the spot beams.

Find the overall system capacity (explained in detail in Chapter 6).

The software module is carefully designed to be flexible enough to account for several

different parameters (for example, antenna pattern, satellite location, traffc intensity in a

particular location, link budget parameters, etc.). The graphical user interface aIlo ws the user

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to experirnent with merent parameters and spot beam layouts. In the next section, an

overview of the software module design is presented.

5.2 An Overview of the Software Module

The software module c m easily be decomposed into the following three major cornponents

which are depicted in Figure 5- 1.

The p p h i c a l user interface,

The software managing the interaction between the user interface and the database anà

the necessary calculations requked to produce results,

The database which stores all the caIculations for easy and quick retrïeval.

Figure 5-1 : An overail diagram of the sofrware system components

The main strategy ~dopted to accomplish ali of the above stated objectives involves dividing

North America into a square gnd of 0.022458 O (equiv~lenf to 2 5 km memred at the

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equator) berneen the latitudes of 10" N and 70" ami berneen the longitudes of 40e W and

170" W. The concept is ïilustrated in Figure 5-2.

0.022458' x 0-022458' tOrid-square

(not to scale)

Figure 5-2 : An illustrarion of the grid-squares covering enrire North America.

The information about each of the grid-squares (region enclosed within the 0.022458' square

grid) is stored in the database. The grid-square is designed to lx a record capable of storing

the attenuation with respect to the boresight (of any spot bearn), expenenced by a signal

transmitted hom the coordinates. Upon initialization, the database contains only the

coordinates of each of the grid-squares and the centers of the spot beam Since the area of

coverage extends fiom latitude of 10°N to 70°N and a longitude of 40°W to 170°W, there

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huge number of records, the database is oqanized into several files, each pertaining to a

certain region in North Amenca

5.3 Organization of the Database

The database consists of severd files, each of which is related to the merent features

irnplernented in the software package. Figure 5-3 is an overall diagram of the database files.

nie dotted rectangles represent a repetition of the enclosed rectangle(s). As presented in

Chapter 6, there are altogether 102 centers with their spot beams providlng service to a l l of

North Amerka. Therefore, each of the dotted rectangle represents 102 instances of a file-

The narne 'Centre X' is generic md denotes one of the 102 centres.

The purpose of each of the files in Figure 5-3 is explained next

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Moderate

Center X

htensity

[-] Frequency ; MixedInt

Figure 5-3: A breakzbwn of the database files. The a b ~ e d squares represent a repetition of the enclosedfiles. Cenne X is a generic name and can denote a- one of the IO2 centres chosen to cover North America.

5.3.1 Filenames IOlat, 201at, 301at, 40Zat, 501at and 60lat

These files store the grid-square coordinates dong with the attenuatim values with remect

the center-

latitudes of

File lOlat stores information pertauung to @rd-squares located

10°N and 20°N and between the longitudes of 50°W and 170°W.

between the

S imilarly, the

B e 2020 stores information pertaining to $rd-squares located between the latitudes of 20°N

and 30°N and between the longitudes of 50°W and 170°W and so on. The total coverage

area between the latitudes of 10°N and 70°N and longitudes of 50°W and 170°W is divided

into such files to expedite information retrieval £iom the database and to ease database

management A sarnple of a table h m the file 401at is shown in Table 5- 1.

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Longitude Latitude Detroit Memphis - -

Knoxville StLouis 1 Missouri Pittsburgh

Table 5-1: Some of the record stored in the 401af database Ne. Each row contains the coordinates of a grid-squme and the attenuation arpenenced by a signal rransmirted from these coordinates with respect to the boresight of each of the spot beam. PIease note that only a fm selecred columns are show here for illustrariûn purposes.

Each record in the file contains the coordinates of the grid-square and the attenuation, in dB,

experienced by a signal transmitted fkom these coordinates with respect to the boresight of

each of the spot beams. For example, a signal transmitted from a grid-square at 4û0N and

80°W would expenence an attenuation of 4.06 dB with respect to the boresight of the spot

beam in the Detroit region. The same signal would expenence an attenuation of 22.10 dB

with respect to the boresight of the spot beam in the Memphis region.

5 -3.2 Filename 'Centres'

This file contains information pertainuig to the acîual centers of the spot beams. Each record

in the £ile represents one center, see Table 5-2.

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1 X0 1 YO 1 Intensity 1 Guard Time I~hreshold 1 User Rate

I

Dallas 1 96.50 1 32.50 1 1 1 1.1 1 E-03 1 -149.42 1 8 ColumbiaSC

Houston 1 95.20 1 29.50 1 1 1 1.00E-O3 1 -148.57 1 8 Detroit 1 83.22 1 42.1 3 1 1 1 1 -92E-03 1 -147.68 1 8

(Lat.) 81 .O0

I I 1 I I

Phoenix 1112.101 33.30 1 2 1 1.36E-03 1 -150.94 1 8

(Long.) 34.00

SanDiego Memphis

Table 5-2: A sumple of records stored in the 'Centres' database file. Again, only a f av sekcted c o l m m are presented in the table here

(Trafk) 2

The record contains the center's name, its coordinates (latitude and longitude), the spot bearn

ûafiic intensity expected, the guard time for the spot beam, the user data rate in the spot b a r n

and the interference threshold value for the user in the spot beam The spot beam traffic

intensity dictates the edge-of-spot bearn attenuation leveL The values are as follow:

117.10 90.00

1

'O' means that high aaEc intensity is expected and the spot bearn contour power level =

0.5 dl3 below the boresight.

'1' rneans that moderate trafEc intensity is expected and the spot beam contour power

level = 1.3 dB below the boresight

'2' means that low t r a c intensity is expected and the spot bearn contour power level =

2.0 dB below the boresight

'3' means that lowest trafic intensity is expected and the spot beam contour power level =

3.0 dB below the boresight

(s) 1 -89E-03

2.60E-03 1 -89 E-03 1.1 8E-O3 2.71 E-03

Boiseldaho Knoxville SanAntonio YellowStone

32.43 35.07

WB) -1 48.80

-1 56.58 -1 53.43 -1 48.97 -1 56.58

(kbfs) 8

2 2

8 8 8 8

3 2 2 3

1 16.1 5 ( 43.71 83.95 98.50 108.93

- - .

1.37E-03 1.65E-03

35.96 29.50 45.50

-1 51 -1 O -1 48.33

8 8

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The 'Guard Time' field represents the value of the time calculated for the spot beam and it

depends upon the size and more importantly on the location of the spot b a r n As mentioned

in Section 4.1, due to the spherical shape of the earth, the shape of the spot k m is drastically

different in different regions of the desired coverage area For exampIe, a 3 dB contour may

appear as a s d c ~ c l e near the equator but rnay appear img& in shape (resernbhg an

ellipse) towards the high latitudes- Therefore, the guard time values are Vary significantly in

difFerent spot beams. The calculations of the guxd t h e s are presented in Section 5.5. The

'threshold' field represents the arnount of interference which can be tolerated by a user in the

spot beam The arnount of interference depends upon the user data rate and the calculations

are presented in Section 4-61.

5.3.3 Filename 'Centre X with High, Moderate, Low & Lowest Intensity '

This database file is created to aid in the plotting of the spot bearn contours. The 'X' is

generic and c m be replaced by the actual center narne. For example, the actual database file

may be 'Nau York High Intensi~', 'New York Moderate Intemity' , 'New York Low Intensip'

or 'New York Lowest Inrenriry'. The file ïs a coUetion of the coordinates of grid-squares

that exhibit a desirable attenuation value. For example, the file narned 'New York High

Intemiiy' contains the coordinates of those grid-square which have an attenuation value of -

0.5 + 0.04 dB with respect to the boresight of the New York spot bearn To iUustrate the file

contents, a sample is provided corn 'Nao York High Intemiîy' in Table 5-3. These hles are

generated by scanning the IOlat, 201at, 301ar. 401at. 50lat and 60lat files for the grid-squares

with the appropnate attenuation values for a particular center.

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Table 5-3: A sample of the contents of the file called 'New York High Intemity'.

Lonqi -t . ( O ) 77.09

5.3 -4 Filename 'Centre X Interference'

This file is very instrumental for frequency assignment as it contains the maximum amount of

attenuation experienced by a signal in any CO-channel spot beam with respect to boresight

gain of center X- As before, a separate file is created for each of the different centers.

;itit ( 0 )

40.04

Table 5-4: A sample of maximum inreg%rence values for the spot beam of San Diego.

t. IdB) 0-47

An example is presented in Table 5-4 for the spot beam centered around San Diego. The

Interferhg Center Name

ColumbiaSC Dallas Houston Detroit Phoenix Memphis Miami Boiseldaho Knoxvi Ile SanAntonio YeIlowStone RapidCity LasVegas

table contains the narne of the interfering spot bearns, the attenuation experienced by a user in

Min Signal (dB) -2.00 -2.00 -2.00 -1 99 -2.00 -1.99 -1 -99 -1.98 -2.00 -1.99 -2.00 -2.00 -2-00

Max Inff. (dB)

-22.80 -1 7.80 -20.39 -22.70 -1.61 6 -20.66 -24.67 -1 3.04 -22.81 -1 7.83 -1 3.54 -1 7.59 -0.34

Latitude ("1

30.38 31.54 31 -93 29.85 32.87 29-94 30.53 31 -41 29.79 29.76 34.64 29.65 35.14

Longitude ("1

1 14.96 1 14.40 1 14.35 1 17.79 1 14.46 1 15.47 1 18.80 1 14.44 1 17.63 1 15.86 1 15.68 1 16.24 11 6.58

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the San Diego spot beam, the strength of the interfering signal and the coordinates of the

calculation of the interfering signal strength. For example, let us consider two users who

operate sirnultaneously at the same fiequency and time dot in the San Diego and Miami spot

bearns. In the worst caseo the signal fkom the user in the San Diego spot bearn shalI be

received with an attenuation of -1.99 dB with respect to the maximum antenna gain (at the

boresight). At the same tirne, the user in the Miarni spot beam shall be received with an

attenuation of -24.67 dB with respect to the boresight gain of the San Diego spot bearn

FoIlowing similar logic, if a user in San Diego is operating sirnultanec.usly with a user in Las

Vegas, the signal nom the latter wodd appear -0.3416 dB with respect to the boresight gain

of the San Diego antenna element The interfering signal would be stronger than the actud

desired signal which would appear as -2.00dB with respect to the boresight gain of the San

Diego antenna element. It is to be noted that the information provided in this file is made

prior to fiequency assignrnent. During the process of fiequency assignment, the data in the

me can be retrieved to calculate the actual amount of interference received. The data in the

file can be accessed repeatedly for severd iterations for Werent assignment trials and

confipations- This feature provides a geat deal of system versatiliy and eases up the

cornputationd burden.

5 -3.5 Filename 'Centre X Neighbours'

This file contains a list of the names of the neighboring spot bearns for the Cenae X. For

example, Table 5-5 is a s q 1 e list of the neighbors of Ottawa dong with the distance to each

spot beam's center. The records cm be reaieved in an increasing order of distance. This file

i s specially usefd for the purposes of fiequency assignment. A separate list is rnaintained for

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ail centers. The name 'Centre X Neighbours' rnay be a misnomer as it may imply the

immediate neighbors of Centre X. In the database file, all the centers (except X) in the system

are neighbors of Center X Some of the 'neîghbors' rnay be very far away and that is why a

list of neighbors is maintaineci in increasing order of distance from the center X-

Table 5-5: A sample of the neighbour spot beam of the Ottawa spot beam dong w . h the distancesfrom the center of Oflava spot beam to the centers of the remaining spot beam

Pittsburgh Detroit

5.3.6 Filename 'Centre X AlZLowlnt'

This me is an output of the fiequency assignrnent algorithm The procedure of fiequency

assignrnent can start fiom any cenrer X The file has rndtiple tables; each table contains a list

of centers assigned to a particular fiequency. Depending on the algorithm and the

assumptions regarding the user data rates, the system network rnay require as few as 12 and

as rnany as 14 fkequencies (see Chapter 6 for more details). Therefore, the number of tables

535.81 623.31

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in the database file rnay Vary fkom 12 to 14. The 'AULowInt' label in the filename irnplies that

the fÎequency assignment is done based on the a s s q t i o n that ail CO-channel Ïnterferïng users

are operating at a data rate of 8 kb/s. For example, if the fiequency assignment algorithm is

initiated fkom Columbia South Carolina (ColumbiaS C), the table for ColumbiaS C Frequency

1 may appear as in Table 5-6. According to Table 5-6(a), the spot beams centered around

ColumbiaS C, Memphis, Ottawa, San Antonio etc. spot beams are assigned Frequency 1,

FortWorthWestTexas lColumbiaSC 1 MonterrevSea Memphis San Salvador Ottawa Sable Island SanAntonio NormanWeIls H aiti Hebron

Phoenix

Table 5-6: The two tables (a) and (b) list the CO-channel centers assigned to Frequency I and 12. The frequency assigrnent algonthm is initiated from Columbia South Carolina

5.3 -7 Filename 'Centre X MixedIat'

The purposes of these fies are exactly as described in Section 5.3.6. However, the narne

'Mixedht' irriplies that the fiequency assignment is based on the assurnption that the co-

charme1 interferhg users are a mix of 8, 16, 32 or 64 kb/s users. A particular type of user

type (8 kW, 16 kb/s, 32 kb/s, 48 kb/s and 64 kb/s) is assigned to each spot bearn prior to

fiequency assignrnent. The assignment is done on a random basis. Since no expected af ic

rnodels are available for the proposeci system, it is assurned that on average, half of the

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interferïng users are 8 kb/s speech users with hand-held terminais and the rest are higher

96

data

rate users.

transmission

analysis.

Since all the

c haract eristics,

higher data rate users have the sarne

no distinction need be made between

data

them

terminal power

for interference

5.4 Features Impfemented in the Softu>are Module

Upon startup, the user is presented with an interface s i m k to the one shown in Figure 5-4.

Each of the command buttons can be related to one of the tasks outlined in the objectives

section A brief description of the individual features is given below.

Specific Location the Database

I Interference ( 1 Tier interference I

- - -

Assign Frequency

Figure 54: The initial samare s t a w display

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5.4.1 Add Center to the Database

This feature allows the user to introduce a new center into the database. The user is allowed

to enter the name and the coordinates of the new center into the system The user rnay

directly proceed to allow the software to compute attenuation values for _ed-squares located

t2l 10" north, IO0 south, 10' east and 10' West of the new center coordinates. The

nanslation of 10' into decibels for a power level depends entirely upon the geographicai

location of the center of the spot beam If the center of the spot beam is n e z the equator, 10'

may translate to approxùnately 20dB of attenuation with respect to ihe power level of the

center. Alternatively, if the center is far north towards the polar regions, 10" may result in

only 4dB (approxirnately) of attenuation.

Altematively, the user may ako specfi a specific range (depicted as 'x' and 'y' in Fi,oure 5-5)

in ail directions. Based on the coordinates of the new center and the range, the software

accesses the appropriate database files (IOlar, 201at, 301at, etc.) and calculates the attenuation

values wirh respect to the new cemer coordinates. The cdculations are performed for all the

grid-squares that ex& in the specified/default range-

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attenuated with respect to the boresight of the spot bearn centered in the Detroit region.

Simihrly, the grid-square receives a signal which is 0.063 dB attenuated with respect to the

boresight of the spot beam around the New York region.

5.4.3 Add Table to the Database

This feature allows the user to add a certain region of North Arnerica into the database. For

example, if the region enclosed berween latitudes of 50°N and 60°N and longitudes of

130°W and 140°W is to be included into the database, the user c m spe- these coordinates.

The software then generates grid-squares pertaining to the speci£ied region and updates the

appropriate file (IOlat, 201at, etc.). The user may then wish to add a center (located in the

vicinity of newly generated grid-squares) to the database by specifying its coordinates (see

Section 5.3.1 for details of th-is feature).

5.4.4 Load Map

This feature loads the map of North Amenca in a separate window and allows the user to

scroll to different regions of the map and view the spot bearns. The user is presented with a

pull d o m menu to select the centers. The user can then select the contour level and actually

plot the spot beam conesponding to the center. As mentioned in Chap ter 4 and earlier in this

chapter, four distinct power levels are chosen for the system design. The user can choose

korn (i) high intensity (ü) moderate intensity (E) iow intensity and (iv) lowest intensity. The

user can also delete any spot beam Depending upon the intensity input fiom the user, the

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software selects the correspondbg database @e Centre X with High, Moderate, Low or

Lowest Intensity to plot the spot bearn.

The imposition of the contours on the Noah Amenca rnap is not an easy undertaking. The

mapping of the coordinates ont0 the rnap is not linear unless the scanned rnap projection has

the latitudes and longitudes equally spaced. Unfortunately, because such a projection

introduces severe distortïons, a map projection with equally spaced latitudes and longitudes

does not exist and the software had to be adapted to the 'Mercator' projection [24]. In the

'Mercator' projection, the longitudes are equally spaced but the space between latitudes

inmeases in a non-linear fashion. Using interpolation techniques, a polynomial was generated

to aid in the plot.

5.4.5 Get Contour for a Power Level

The software allows the user to generate contours of one of four power levels for a center.

The user c m choose a cenrer that exists in the database and spec* the intensity (high,

moderate, low and lowest) level for the contour. The software smeys the database for grid-

squares with attenuation values associateci with the cenrer. If, at a particular square-grid, the

difference between the attenuation value and speczed power leveI is less than a certain

threshold, the coordinates of the square-grid are stored into a database nle. This is also

exphineci in Section 5.3.3. The user can then plot the coordinates as they would d e h e the

spot beam pertaining to the specified cenrer and of the specified power level.

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5.4.6 Change Name

This feature pemiits the user to change the narne of any center that exists in the database.

The software alters the center narne in the 'Centers' file and the files pertaining to grid-

squares (IOlat, 202ut, etc). This feature is sought when the user rearranges the spot bearns.

This f e a ~ e allows the user to determine the interference received from one specinc spot

beam into another assuming that there exist a CO-channel user in that spot beam It is

assumed that both users use the sarne Nne slot and the sarne fiequency. A more detaiIed

analysis regarding the Interference and Tier Analysis is presenred in Section 5.7.

5.4 .8 Delete Center

The user c m delete the center f?om the database. AU the information regardhg the center is

purged fÏom the database.

The following section deals with in-depth description of the procedure involved in the

calculations of the results. More specincaly, the calculation of attenuation, interference

analysis and frequency assigrnent are dixussed.

5.5 Attenuation Calculation

As explained in Chapter 4, the cdculation of attenuation involves determinuig both (i) the

path Ioss and (ii) the angle of reception at the satellite with respect to the boresight The

analysis commences with the specification of the center of the spot bearn; the user must input

the coordinates of the center. A completely separate database iïie is maintained for the

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centers. Each centes is a separate record with fields such as its coordinates, spot beam

contour power level, and guard h e value.

The coordinates of the center âre used to determuie the square-grids that s h d be used to

calculate the antenna gain and the path loss. IfXO and YO denote the longitude and latitude of

the center respectively, the software calculates the received attenuation values in a vicinity of

X0 + QO, X0 - bO, Y0 t cO, Y0 - do around the center. The value of a, b, c, d is 10' by default

but can also specified by the user. The software calculates the gain values for ail grid-squares

located between XO + a", XO - bO, Y" + c0 and Y"- do.

As explained in Chapter 4, cenrer coordinates serve as a reference for the path loss and

boresight. Al1 the remaining attenuation values are calculated relative to the cenrer

cddations. An example is presented next to aid in understanding the above details.

Let's assume that a center is chosen in the Miami region with coordinates of longitude = X =

80.15" W and latitude = Y = 25.52ON. As per specifcations, the satellite is assumed to be at

106S0 W. Using Eq. 4-5, the distance between the satellite and the center of the spot beam is

37,196.98 km Using Eq. 4-10, the path loss at the Miami center is -187.93 dB. For this

example, the grid square with latitude = 30°N and longitude = 90°W is selected to

demonsaate the attenuation calculations. The path loss fiorn this grid-square to the satellite is

-187.89 dB. The angle, a, is calculated as 1.663' and the corresponding antenna attenuation

is -14.07 dB- Therefore, the total attenuation is:

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-187.89 t (-14.07) = -201.96 dB.

Hence, a signal t rasmit ted fÎom this grid-square is received at least

-20 1-96 - (-187.93) = -14.03 dB

below in power level as compared to the signal transmitted from the center of the spot beam.

5.6 Calculution of Guard Time

In a TDMA kame structure for GEO-MSS systems, the guard time is a very sigficant

component of the overhead associated with each time do t and therefore must be deterrnined

accurately to help evaluate the performance of TDMA schemes (also discussed in Chapter 3).

The calculation of the pard time involves calculating the minimum and maximum disrances

hom within one spot barn to the satellite. To accornplish t h task, the distance to the

satellite has to be cdculated fkom all the grid-squares contained within the spot b e m The

rnaxlmum and the minimum distances can then be stored and their difference dictates the

value of guard the . This is depicted in Figure 5-7. The equation relating the difference in

distances to the Guard Tirne is given in Eq. 5-1.

D ifference between Max. and Min. D istances tu the Satellite Guard Time = Eq. 5-1

Speed of L i g h

As an example, a center is chosen in the Miami region with coordinates of longitude = 80.15'

W and latitude = 25.52ON. Again, the satellite is assumed to be located at a longitude of

106S0 W- Also, the center is assumed to have a 3 dB contour spot b a r n According to the

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software calculations, the minimum distance to the satellite is 36,942.31 km and it occurs at a

latimde of 23.27ON and a longitude of 83.2lCW. The maximum distance to the satellite is

37,488.28 km and it occurs at a latitude of 27.92ON and a longitude of 76.8g0W- Using Eq.

5-1, the guard tirne is:

The guard tirne value is stored in the 'Centers' He.

Distance to the Satellite shall be calculateci fiom ALL &d-squares within the spot bearn Choose maximum distance Choose minimum distance Calculate the difference Speed = Distance / Time Guard Time = Difference in Distance

Speed of Iight

Figure 5-7: An arbitrary spot beam is show to illusnate the concept of gzcard tirne calculations. The spot beam is big enough to cause significant difference in the propagation times to the satellite

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5.7 Interference Analysis

The so fiware has provision for two h d s of interference analysis, termed as "Interference

Analysis" and ' T e r Interference Analysis". Interference Analysis is useful for determining the

amount of interference ernanating fcom one spot beam to another. To illustrate t k , let us

assume an e x q l e involving two users o p e r a ~ g at the same tirne slot on the same

fiequency- Also assume that one user is operating in the Miami spot bearn while the other

user is operating in the Chicago spot beam To perform worst-case interference andysis, the

user in the Mïami spot beam is assumed to be at the edge of his/her spot bearn For example,

if the Chicago spot beam is a 3.0 dB contour, the user is assumed to be 3.0 dB below the

boresight gain. This ensures that the desirable signal is received with weakest strength. To

calculate the maximum possible interference that can be received by the user in Miami, all the

square grids within the Chicago spot bearn should be scanned to determine the grid square

which provides maximum interference to the user in the Miami spot beam At ks t , the

obvious solution may be to seek the grid square in Chicago spot b a r n that i s nearest in

distance to the Miami spot beam However, due to the sinc function antenna pattern

(existence of side lobes), the gid-square physically nearest to the Miami spot barn rnay not

produce the maximum interfaence. This îs shown in Figure 5-8.

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Figure 5-81 A superimposition of the antenna pattern on the spot beam conruun illumaring the non-linear relatiumhip between the distance from the boresight and the aitenuahon

Therefore, it is imperative to scan and calculate the interference received fiom ail grid-squares

in the Chicago spot beam. The software calculates the maximum interference receÏved fiom

the spot bearn and also assists in locating the exact square grid which is e r n a n a ~ g the

maximum interference.

Tier interference Analysis dows the user to calculate the amount of interference k ing

received fiom severai spot beams simultaneously. The user cm specw the number and names

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of the interferkg spot beams- The user cm &O spece the type of user terminal k ing

employed in each of the i n t e r f e~g spot beams. The software then cornputes the total

received interference power £iom various spot beams. This is very useful as it helps in

determininp the fiequency reuse distance,

5.8 Frequency Assignment Analysis

Frequency Assignment is the next major task in the system design. The system bandwidth is

29 MHz and effort has to be made to assign fÎequencies efficiently. Due to the nature of the

sizes and irregular shapes of the spot bearns, conventional fkequency assignment techniques

based upon the hexagonal analysis (as in terrestrial case) are not suitable for GEO-MSS

systerns [l O].

Two or more spot bearns c m be assigned the same fiequency if the maximum in te rence

received fiom one spot bearn to the remaining spot beams is bel0 w a specifïed threshold. The

process cm be expedited if the interference received fiom all the spot beams to all the

rerriauiing spot beams cm be calculated ofBine and stored into a separate database Bee. The

software can execute once and generate the ma& presented in Section 4.5. The fkequency

assignment can then be c&ed out relatively quickly by querying the database for interference

values. The frequency assignrnent is performed assuming one of the following two distinct

interference scenarios at a tirne:

1- AU interfering CO-channel users are 8 kbls users with terminals identical to the terminal of

the desirable user

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2. The interfering CO-channel users are a randody assigned mùr of 8 kb/s and higher data rate

u s e s As explained before, on average, half of the interfering users are assumed to be 8

kb/s users and the rest are high data users. The temiinals for all the higher data rate users

are identical and are more capabIe than the temiinal of the desirable user.

The analysis for each of the three scenarios is expected to yidd the maximum nurnber of

fiequency bands, N, required to provide s e ~ c e to ail spot bearns in North Amenca with

tolerable interference. The system bandwidth of 29 METZ can then be divided by N to

determine the capacity of each spot beam The steps involved in fiequency assigrnent

follow:

Assign different data rate users randornly to the spot beams

Calculate the threshold interference that can be tolerated by the user in each spot beam

and store it into the 'Centres' £ile database me.

Assign a particular fiequency to a maximum nurnber of spot beams. A frequency, f, can

be assigned to another spot beam, S, if and only if the mutual interference between S and

the spot bearns which have already ken assigned f, is toierable.

To facilltate the calculations, a list of spot beams that can simultaneously use f, is

rnaintained (Section 5.3.6). Also, the interference received fkom each spot b a r n into the

remaining spot beams is akeady tabulateci (Section 5.3.4).

The basic algorithm is @en beIow; most of the details regarding the updating of the

thresholds after the fiequzncy assiament are omitted.

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start from a center X and mark it assigned Add it to 'f, Iist' while unassigned centres left

traverse through the neighbors of X with no frequency assigned yet for each neighbor Y

retrieve interference received from ALL centres in 'f, list' to Y TOTALINT = add the total interference if TOTALINT > threshold of Y then

success = false exit

else for every cenferZ in 'f, list'

retrieve interference received from Y to Z if interference from Y > threshold of Z then

success = false else

success = true end if

end for end if if success = true then assign Y to ' f , list' update the thresholds for the centres in the ' f , list'

end for increment the frequency counter, now try the procedure with f,= f, t 1 (f, list is ernpty)

end while

This algonthm is successhilly iqlemented for the two scenarïos and the results are presented

in the next chapter. The direct search algonthm for frequency assignment explained above

yields a preliminary set of results for cluster size (number of frequency bands). There exists

potential for using more than one fkequency band in one spot beam to ensure maximum reuse

and bandwidth utirizatio n This algorithm is extended and band width-limiteci capacity

analysis is made more accurate. In the next section, the extension to the algorithm is

discussed. The resulting fÏequency assignment corQguration and bandwidth-limited capacity

analysis is presented in the next chapter.

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5.8.1 An Extension ro the Frequency Assignment Algorithm

The existing algorithm alIows one fiequency band to be assignecl per spot beam. Every time a

kequency band is assigned to a spot beam, the spot bearn is m k e d as ccdready assigned" and

is not reconsidered for another kequency band. Due to the fewer number of spot beams

available later in the process of frequency assigrment, an unequal number of spot beams per

kequency band result The algorithm is extended to repeat £iquency assignment and allow

more than one l%equency band to be assigned to each spot beam This results in a siboni.ficant

enhancement of bandwidth-limited capacity of the spot beam with more than one fiequency

band. The centres and their corresponding frequency band(s) assignrnents are presented in

the next chapter.

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Ghapter 6

Results and Capacity Analysis

6. Results

In this chapter, results obtained fiom the softtvare module are presented. They are used later

to evaluate the systern capacity. As expIained in Chapter 5, the software module has been

instrumental in providinp a complete system design The development of the module

produces results which are incrementally incorporated into the system design and capacity

cdculations, The capacity calculations are camed out for the proposed systern assuming two

distinct approaches. The two approaches are as follows:

bandwid t h-limi'ted case (eom fiequency assignment)

power-limited case (fiom the link-budget)

In the case of bandwidth-limited capacity calculation, the fiequency assignment performed by

the software module is utilized. As expked in Chapter 5, identification of the spot beam

centers, the spot b a r n (coverage of North America) layout and the interference analysis

culminate in a particular frequency assignment The fiequency assignment provides an

insight into the fiequency reuse. Based on bandwidth of each carrier, number of carriers is

calculated. The number of users in the systern depends on the nurnber of users

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per canier which in tuni depends on the tratFc rnix (number of 8kb/s, 16kb/sy 32kb/sy 48kb/s

and @k/s users) and the choice of guard time in the fiame. The calculation of the capacity

in the power-limîted case stems fiom the Iink budget. The satellite ampfier is capable of

providing a lunited amount of power which is shared between the carriers in the system The

share of power dedicated to each carria depends on the fiame contents; specifcally on the

received signal power levels in each time slot in the frame.

In the bandwidth lirnited case, the capacity calculations are conducted for both the traditional

and the proposed fiame structures and the results are compared. The po wer -k ted capacity

calCuLations are almost indifferent for the two fiame structures. This is because there is no

sate~tepowerconsumedduringtheguardEime(majonty)portionoftheoverhead. Inthe

next section, results pertaining to the spot beam centers, the spot beam (coverage of North

Arnerica) layout, interference analysis and guard tirne calculations are presented.

6.1 List of Centres

The choice of the coordinates of the centres is a meticdous undertaking as it involved

ensuring that the entire Noah Amenca is covered completely with spot beams. The objective

is to try and cover all grid-square locations in North A d c a with several spot beams (of

different edge-of-spot ba rn power levels) with Mnimum overlap taking into consideration

variability of traffic intensity fiom a spot bearn to another. Since a mathematical optimization

solution (if one exists) is beyond the scope of the thesis, an ad-hoc approach is adopted to

yield an acceptable spot bearn configuration. The list of the cenees and their CO ordinates are

presented in Table 6-1 and Table 6-2.

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ayj j o autos JO s a ~ m ~ p ~ o o ~ a y ~ pu^ (ld sagpuaq 3-4 pana& ' s ~ ~ u m l ~ . : 1-9 q p ~

O'Z 1 1 Z tAal MN) a4!u)lMollaA ST8E L'SO 1 E s 6 u u d ~ o p ~ ~ o l o ~ (=?-F'Yv)

00'68 & easoqxayyjlng ~ c - 6 ~ 86-0 1 1 E (mn) u o ! ~ ~ u n y p u ~ i ~ (PuqpunoPfaN)

00'9s E ~ I E P ~ ~ O H OP'9E W'91 C E ( w o w d s ~ 6 a ~ s e l OÇ'E8 & ( ' u ~ ~ M N ) J O W H I O J e 3 W 6 - b EZ.86 Z ( = q w w ) 6ad!uu!m t'LI1 Z ( ~ 9 ~ ) ~ ~ ~ 1 ~ 3 ZE'PP ZE'EOl E (~0)lea *SI &11p!de8 0Ç.96 Z CJJal MN) a y e l u l l h ~ Os'ÇP €6'80 1 E ( ~ u ! u J o ~ ~ M)UOlSMOllaA

( = ~ ~ a e y s g ) E'.bOL Z uooWsES OF62 O S 8 6 Z (=ml) O!UOJU~UES OL'L8 2 (o-muo) ~ W P ~ S 96'SC S6'€8 Z (a=sauual> alpwouy 00'89 t y?MsurWMaN CL'€$' SL.911 E (OY~PI) O~BPjaS!Oa El'P9 L ( ~ p ~ l plemp3 aJuki) 13d ZS'SZ 1 9C.08 E (=PYW) !we!yy F601 € ( m - ~ a ~ ) x d e l LO'SE 100'06 2 (aassawal, s! y d w a yy 00'86 E (w-m) 3adaWenqal &VZ& 1 L'L 1 1 2 1 ( i . 1 0 ~ ~ ) o6a!a ues W C E ~!uJo4!l~3lo4ln!3 OE'EE 1 L'Z L 1 2 (euoz-w) xyaoy d FZO C E (w~w) ~Je!Wpen!3 El'ZP ZZ'E8 1 ( ~ i i l v w ) ~!oriaa O'szL E (!!!EH) !!E?MEH 05.62 OZ'S6 1 (=ml> uoisnoH 00'68 E ( 0 3 0 : ~ ) a43adm3 OS'ZE OS'96 1 (=ad sqea

t (=J!lo=3 'SI

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Guadalupelsland (~exko) 1 3 1 21 -5 31 .O0 Labrador (Nowfoundland) 12 71.00 52.00

!PortlandOregon (oreson) 1 122.3 45.50 0 ma ha (Nebraska) 3 97.00 42.50 Monterrey (~exi-~o) 3 100.5 24.66 MexicoCity (~exl-~o) 2 98.60 19.80 Durango (Mexico) 3 105.0 23.50 GjaoHaven orth th ~ e ç t ~ e r r - ) 2 94.00 70.00 Frobisher (NO& est Ter.) 3 73 -70 63-00 Newfoundland 3 53.00 52.00 Sab[elsland INovaScotia) 3 58.00 44.50

- -

Haiti 1 3 ( 73.00 1 19.00

Belcherlslands ( ~ ~ r t h west Terr.) 3 78-75 53.00 SanPedroSula hiexicol 3 89.00 14.50 - - - --

Fairbanks (Alaska) 1 2 ( 146.0 1 64.50 Bathurstlnlet (NaRh w ~ t ~ern.1 2 1 07.5 70.00 H abana (Cuba) 3 84.00 23.00 Jamaica (Carn'beanl . 3 78.00 17.00

1 .

H ebron (Newfoundland) 13 )60.00) 63.00 NewYorkSea (New YO&) 3 70.25 39.00 ManzanillaSea (Mexico) 3 107.5 19.00 Bemudalsland 3 65.00 32.50 BelizeCitySea ( ~ e x i a ) 3 82.50 17.00 DawsonCreek [sntish Columbia) 1 1 22.5 56.00 S t Kitts (Caribbean} 3 67.00 17.20 Dorninica (Caibbean) 3 60.00 15.00 SanSalvador IM-) 3 73.00 24.00

Table 6-2 : The m e s , expected nqffic intemities (TI) and the coordinates of remaining centers of the spot b e m

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6.2 Spot beam coverage of North AmenCa

As shown Ui Figure 6-1, there are altogether 102 spot k a m s chosen in the design of the

system The edge-of-spot beam power b e l is one offou. values with respect to the center of

the spot beam Fi,we 6-1 depicts the spot beam coverage layout produced with the aid of

the sofcware module developed. This layout is also the basis for the interference analysis,

fiequency assigrnent and the calculation of the guard time values. h Figure 6-1, different

colours are used to dinerentiae between the spot beams with dinerent edge-of-spot beam

power levels according to trafEc intensity expected. The edge-of-spot beam power levels

and their respective colour codes are as follows:

-0.5dB (High T r S c Intensity) Red

- 1 -3dB (Moderate Traffic Intensity) Blue

a -2.0dB (Low Trafic Intensity) Magenta

-3.0dB (Lowest T r a c Intensity) White

As can be seen in Figure 6-1, the coverage extends to aU parts of North Amerka including the

Caribbean and the Hawaii Islands, coastal regions and areas situated at a latitude of more

than 70°N. The choice of the edge-of spot beam power Ievel is dependent on the ~ a a i c

intensity expected £rom the users in a particular region. Since no formal t r a c

characteristics/rnodels are availab1e to the author, the expected traffc intensity has largeIy

been based upon the population in a particuk area However, it is also known [8] that the

current Tm's operated MSAT systern is experiencing its highest t r a c concentration in the

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West beam (specifically the British Columbia region in Canada). Therefore, spot beams with

moderate rna£tïc intensity power levels have k e n assigneci to the region in spite of the

relatively Iow regional population. It should also be noted that the spot beam coverage

includes areas in both the Pacific and the Atlantic oceans to serve ships and crukes where the

terressial cellular services are unavailable,

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6.3 Guard Time Values

The minirnum guard time calculations are perfonned for ali spot beams in Fi,oure 6-1. As

explained in Chapter 5, the p a r d rime values are calculatecl by computing the maximum

clifference between the propagation dehys fiom locations within a spot beam to the satellite.

The guard time values associated with each spot beam are presented in TabIe 6-3 and Table

6-4. They range from 0.76080 ms to 7.1869 ms. It is obvious that the range of the guard

time values is quite large and the choice of only one value will result into a great deal of waste

in the overall systern capacity. For ease of implementation, the guard time is assigned one of

the following four values; 1.40, 2.25, 3.5 and 7.2ms, with the number of correspondhg spot

beams; 24,29,26 and 23, respectively.

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0 -8939 Knoxville 1.8963 0.9880 OklahomaCity 1.9042 1 .O013 Detroit 1.9295

Durango 1.0381 LasVeqas 1.9401 LaPaz Dallas ,Monterrey Campeche 1 1 -1 494 DesMoines

I ~ o n t e r r e y ~ e a GuaymasMexico

Phoenix

1 .O71 5 1.1108 1.1 341

San Antonio 1 -1 856 ColoradoSprings 1.9926 1-9840 1

,SanDiego NewOrteans Habana ~StLouisMissouri JacksonVille GulfMexicoSea MobileAlabama

1 -2480 1 -2938 1.3396 1.3458 1.3524 1.3595 1 -3644

Jamaica EIPaso

Table 6-3: The minimum guard time values associared with sorne of the spot beams

Pittsburgh Haiti PortlandOregon

1.3775 1.3992

FortWorth Wes tTexas Memphis

ArkansasCity GrandJunction SanSaIvador SanJoseSea S t Kitts Reddin~California JacksonVilleSea

1.5896 1.61 03

1.9614 1.9737 1.9740

2.051 7 2.1 108 2.1 833 2.21 48 2.3867 2.3908 2.4078

Ottawa Omaha

1.6369 1.6529

,

1 -51 50 1 -5277 1.5428 1 5502 1 5620

2.423 1 2.4729

1 ~ominican~epublic 1 ~ e l l o w ~ t o n e

2.6934 2.71 47

Lonqtac Banff

Duluth PortlandMaine

2.7450 2-79 19

2.51 84 2-5 186

RapidCity Boiseldaho Thunderbay

2.5856 2.601 7 2.6382

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Table 64: The minimum guord rime values associated with remining spot beams

As discussed in Chapter 3, the time d o t duration in a kame depends on the user data rate and

the overhead bits. In the MSS system proposed in this thesis, the guard time has a drastic

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impact on the fiame capacity. Table 6-5 shows the duration of the time slot for dinerent data

rate users with the four distinct p a r d time values rnentïoned earlier. Note that the tune slot

du.ratïon are only applicable for the proposed frame structure; explained in Chapter 3.

8 kb/s 1 2.75 ms 1 4.15 rns 1 5.0 ms 1 6.25 rns 1 9.95 rns 1

32 kb/s 1 10.25 ms 1 11.65 ms 1 12.5 ms 1 13-75 ms- 1 17.45 ms 1 I l l 1 1

Table 6-5: The duration of rhe time dots for different dara rate users and wiîh different guard time values

According to Table 6-5, duraiion of an 8 kb/s user tirne slot c m be 4.15, 5.0, 6.25 or

rns, depending upon the spot bearn This large difference in the duration of the tirne slo ts

9-95

has a

sibonificant impact on the capacity of the system In traditional fkarne smcture, a user

dernanding high date rate seMces is assigneci multiple time slots with the above guard time

values exîsting between each time dot.

the traditional frame structure is given,

In the next

with regard

section, a cornparison of the proposed and

to the capacity of the fiame smctuxes.

6.3.1 Cornparison of the Proposed and the Traditional Frame Structures

At a fiame rate of 160kb/s, the 1.40rn.s guard tirne translates into 224 bits. Along with 40 bits

for other overheads, the total number of overhead bits is 264. With a 50rns fiame operating

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at 160kb/s, there are 8,000 total bits availab1e. In the proposed system, if the 64 kb/s time

slot is considered, the number of information bits including forward error correction (FEC)

bits is 3,200 and with the addition of 264 overhead bits, total bits are 3,464- Therefore, the

fiame has the total capacity of 8000/3464 = two 64 kb/s users (with 1,072 bits to spare

which c m accommodate a l6kb/s or an 8kb/s user). With a pard time of 1.4 ms, the

traditional fiame structure has a total of twelve 8kb/s tirne slots and it can accommodate one

64kb/s user and four 8kb/s users. Therefore, using the proposed fÏame structure, there is a

100% ïmprovement in the capacity for 64kb/s users.

In Table 6-6, a 538 capacity ïqrovement is shown with a data rate of 64 kb/s and a s a r d

tirne value of 1.4ms. The capacity irnprovement of 53% is calculated using the following

formula:

T o t . Capacity (prop. stnict.) - Total Capacity (trad. structure) Capacity hprovement =

Total Capacïty (trad-structure)

A similar andysis is canied out for other data rates and the resdts presented in Table 6-6.

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Table 6-6: Improvement in Capacity with the proposed frome smcture over the traditional ffame structure

The proposed TDMA kame structure promises a capacity inrprovement of 175% in some

particular scenarios (for data rates of 48 kb/s with a guard tirne of 7.2 ms, see Table 6-6). In

some cases with large guard tirne values, the maditional fiame cannot support any 64 kb/s

user at aLi and therefore the proposed fiame structure is the only option. The capacity

Ïmprovement varies fiorn 0% to 175% depending upon the user data rate and the spot beam

guard time value. If ail users operate at 8kb/s, there is no difference between the proposed

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and the traditional fiame smctures. The ciifference in the time dot assignment mechanism for

higher data rate users results in signii?cant capacity improvement, as shown in Table 6-6. It

should be noted that several other combinations of mixed data rates (ran_eing fiom 8 to 64

kb/s) are possible and again the efficiency enhancement will be in the range of O to 1758.

6.4 Frequency Assinment and the Bandwidth-Zimited Capaeity

As mentioned in Chapter 4, the total system bandwidth is chosen typicdy to be 29 MHz

which is divided among a number of distinct fiequency bands. The kequency bands can then

be re-used Ieading to increased network capacity. The fiequency assignment yields the

number of distinct kequency bands requixed in the network. The process of assiping

kequency bands to different spot beams in one cluster depends heavily upon the mutual

interference between the spot beams in various neighboring clusters sharhg the same

kequency bands. For the proposed system, the fiequency assignment algorithm is

hplemented for two assumptions (mentioned shortly) regarding the interfering user terrninals

(explained in Chapter 5). The algorithm is iqlemented separately for bo th the assumptions

and two distinct sets of results are obtained. Briefiy, the two assumptions are as follows:

all interfering users are 8kb/s users (temiinal peak power = 4.6 W, antenna gain = 3

Bi), or

the interferïng users are a random mix of 8kb/s, 16 kb/s, 32 kb/s and 64kb/s users.

The data temllnals for the 16, 32, 48 and 64 kb/s users have a peak power

dissipation of 5.7 W and an antenna gain of 5 Bi).

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The analysis Ïs conducted for each of the assumptions separately and the results are provided

in the next section.

6.4.1 Al2 interfering usen uperate at a data rate of 8 kbls:

The fiequency assignment algorithm (explained in Chapter 5) is executed 102 times; a

dzrereent spot beam is assumed as the initial center every the . Each execution of the

software module yields a particular fkequency assignment codiguration. A typical result fiom

one execution is surnmanZed in Table 6-7. In this partic* example, 14 fiequency bands are

required and therefore the bandwidth available per spot bearn is 29MHz / 14 = 2.07 MHz.

The list of spot bearns which can use a particda-r fiequency (Frequency 1, for example)

without causing unacceptable interference to the CO-channel spot beams is also provided by

the software module-

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1 Memphis Mobile Alabama 1 1 . . 1 1 Newfoundland 1 1 Ottawa 1 Cincinnati 1 Des Moines 1 1 11 San Antonio 1 Grand Junction 1 La Paz I 1 r - r

- --

Pittsburgh 1 Las Vegas

II Port Nelson 1 Jarnaka Saskatoon I I I l - . - 7

PEI

Table 6-7: A typical m p l e of the resuits obtained fi-om the frequency assignmenr algorith-

The results in Table 6-7 are obtained when the kequency assignment algorithm is executed

assuming Columbia, South Curulina as the initial starting point. However, to achieve

maximum capacity per spot beam, the number of fiequency bands (kequency reuse factor or

the cfuster size) is desired to be low.

When aIl i n t e r m g users are assumed to be opcrating at a data rate of 8 kb/s, the lowest

value for the number of frequency bands is 13 and is obtained when the fiequency assignment

aigorithm is initiateci fiom Hebron (Newfoundland), Boise (Idaho) or New York Sea (the

coastal region near New York). Therefore, the bandwidth allowed per spot bearn is 29MHz /

13 = 2.23 MHz Shce each carrier occupies a bandwidth of IOOKHz, there enst 223MHz 1

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IOOKHz = 22 cam-ers per spot beam. There &s a total n u d e r of 102 spot beam and

therefore the system bandwidth-iimited capaciq is 22 * 102 = 2,244 carriers. The fiequency

assignrnent is depicted in Figure 6-2. AU the spot bearns sharing the same IÎequency band are

represented with the same color in the figure.

If the bandwidth-limited capacity of the system is desired in terms of the number of 8, 16, 32,

48 and 64 kb/s users, the fiame structure for each carrier must be established first- For

example, if a l l users in the system are assurned to operate at a data rate of 8 kb/s and guard

tirne values of 1.4,2.25,3.5 and 7.2ms are distributeci arnong 24,29,26 and 23

carriers respectively out of a total of 102 carriers.

Guard Time Value 1-40

--

Tuble 6-8: The bandwidth-limited system capcity with ail mers operating at 8 kbls.

There is no difference between the traditional and the proposeci fiame structures in the

bandwidth-limited capacity for the 8 kb/s users. This is because the number of 8 kb/s users in

the 5Orns fiame is identical in both cases.

No. of carriers with Guard T h e value

(24/102) * 2244 = 528

3 -50 7-20

In this case, since all in terfe~g users are assurned to operate at a data rate of 8 kb/s, the

overall system capacity cm be calculated only with 8 kb/s users. In the next section, the

8 5

(26/102) * 2244 = 572 (23/102) * 2244 = 506

No. of 8 kb/s users in the 50 rns h e

12

4576 2530

Bandwidth-Limited System C a p a c i ~ for 8 kbls mers with traditional & provosed frame structure

-

Total No. of 8 kb/s users 6336

19,822

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analysis is repeated assumhg that the in te r fe~g users are a mk of 8, 16' 32, 48 and 64 kb/s

users.

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6.4.2 The intelfering users are a mix of 8, 16, 32, 48 and 64 kbls users

FolIowing the other assurnptim where the i n t e r f e ~ g users are assumed to be a mix of users

operathg at different data rates (a more realistic assumption), the lowest value for the number

of fkequency bands is 12 and is obtained when the firequency assignment algorithm is initiateci

fhom Lynn Lake (Nurth West Manitoba), Carol Harbor (North West Temtorïes) or Dawson

Creek (British Columbia) . The procedure used to obatin the value of 12 is identical to that

presented in Section 6.4.1. Therefore, the bandwidth allowed per spot beam is 29MHz / 12

= 2.41MHz. Since each carrier occupies n bandwidth of IOOKHz, there enst 2.41MHr /

lOOKHz = 24 cam-ers per spot beam. There meX2sts a total nwnber of 102 spot b e a m and

ther~ore the system bandwidth-limited capaciîy is 24 * IO2 = 2,448 cariers. The fiequency

assignrnent is depicted in Figue 6-3. The resdt is unexpectedly better than the case with aU

i n t e r f e ~ g users operating at 8 kb/s. This is because in diis case, although some of the co-

charme1 interferhg users use a more capable terminal, they can simultaneously tolerate more

interference,

If the bandwidth-limited capacity of the system is desired in t e m of the number of 8, 16, 32,

48 and 64 kb/s users, the frarne stnicture for each carrier must be established £irst- Table 6-9

illustrates the overall system capacity achieved when dl users in the systern operate at a data

rate of 8 kb/s.

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Guard Time 1 No. of carriers with 1 No. of 8 kb/s users 1 Total No. of 8 kb/s 1 Value Guard Time value in the 50 ms fime users 1.40 (24/102) * 2448 = 576 12 6912

Table 6-9: The bandwidth-rimited system capaciry with all zaers operating ar 8 kb/s.

7.20 1 (23/102) * 2448 = 552 1 5 Bandwidth-Limited System Capaciq for 8 kbfs mers with traditional & proposedframe structure

The increase in the number of available carriers leads to a direct increase in the number of the

users that can be supported in the system From Table 6-9, the capacity o f the system

increases from 19,822 users (Table 6-8) to 21,624 users. Table 6-10 iUusaates the overd

system capacity achieved when ail users in the system operate at a data rate of 16 kb/s.

2760

21,624

I System Capac i~ for 16 kbls users with:

Guard Time (GT) value

1.40 2.25 3.50

l

hprovement = 35.74% . ..

Table 6-10: A cornparfson of the system bandwidth-limited capacity for zrsers operdng ar a data rate of 16 kb/s with proposed and traditionulframe structures

7.20 552 4.0 2208 2.5 1380 B andwidth-Limi ted prup. f i m e 14,677 trad.frame 10,812

No. carriers with GT

576 696 624

The proposed fiame structure is very efficient as compared to the traditional h e structure

when higher data rates are used in the system. As shown in Table 6-10, the system capacity is

sigficantly better with the proposed kame structure. With the proposed karne sbxcture,

the capacity increases fÎom 10,812 l6kb/s users to 14,677 users, a marked improvement of

No.16kb/s users in

prop. frame 7.5 6.6 5.7

TotalNo. of

16 kb/s users 4320 4593 3556

NoA6kb/s users in txad.

k e 6 5 4

Total No. of

16 kb/s users 3456 3480 2496

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132

35.74%. The capacity enhancements for data rates higher than l6kb/s are illusaated in Table

Guard Time (GT) value cauiers

with GT N Sysfem Capac i~ for 32 kbls users with:

P?o,32kb/s users in

prop. frame 4-2

Improvernent = 67.23%

Sim ilarly,

TotalNo- of

32 kb/s users 2419

Bandwidth-Limited 1 prop. frame 6,446 trad. fi-ame 3,517 1 System Capacity for 48 kbls mers with:

No.32kbls users in trad-

h e 3 .O

Improvement = 83.28%

TotalNo. of

32 kb/s users 1728

Tuble 6-11: A cornpanson of the system bandwidth-limited capaciry for mers operating ar a data rate of 32,48 and 64kbls with proposed and tr~ditionalfrnme stnrctures.

Badwidth-Limited System Capaciv for 64 k%/s mers with:

Table

fkarne

prop- @ m e 5,158 nad-frcrne 2,323 1 Improvement = 122.04% 1

6-11 presents an abndged cornparison of the

stnicture when the users in the system operate

improvements O ffered

at data rates of 32, 48

by the proposed

and 64 kb/s. As

expected, the capacity improvement offered by the proposed fiame stnicture as compared to

the wditional fiame structure is more pronounceci at high data rates. The performance of the

traditional fiame structure is not admirable at high data rates due to allocation

Nne and overhead bits in each time dot. With the proposed f i m e structure,

of the pard

the capacity

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improves to up to 122.04% when ail users in the system are assumed to operate at a data rate

of 64 kb/s-

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6.4.3 Bandwidth-Limited Capacity with the extended algorithm

As discussed in Section 5.8 and 5.8.1, the kquency assignment algori th is modified to

allow more than one fiequency band to be assigned to a spot beam The algorithm is

executed assuming the interferhg users operate at mixed data rates. The resulting frequency

band assignment is given in Table 6- 12 and Table 6- 13. In Table 6- 12, spot b a r n center 'San

Diego' is assigned fiequency bands 3, 10, 11 and 12 which results in a bandwidth capacity

improvernent of 300% as compared to single fkequency assignment per spot ba rn for 'San

Diego' alone. SimilarIy, 'Hawaii' can reuse the entire frequency band of 29 MEIz as the

interference fiom CO-charme1 in te r fe~g spot beams is minimal.

As discussed in Section 6.4.2, when the interferhg users are a mix of users operating at

daerent data rates, the 10 west value for the number of fkequency bands is 12 . Since the to ta1

system bandwidth is 29 MHz, the bandwidth per fiequency band is 29MHz / 12 = 2.41MHz.

Since each carier occupies a bandwidth of IOOKHz, there e;nSt 2-41MHz / 100 KU7 = 24

carriers per spot beam- With only one frequency band per spot bearn, the system

bandwi&-limited capacity is 24 * 102 = 2,448 cnmers. The extended algorithm aIlows for

more than one fiequency band to be assigned to each spot beam, interference pennittïng.

From Table 6-12 and Table 6-13 , there exrexrst a total of 145frequency ban& which n ~ m h t e

into a bandwidth-Zimited capaciw of 24 * 145 = 3,480 carriers. The extension to the

algorithm results in a capacity irnprovement of 42.1 % as calculated below.

3480 -2448 x100 = 42.1% 2448 In rnost cases, MSS systerns are power Zimited. However, due to large coverage area and

different time zones; while the system may not reach its power-limited capacity, the demand

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for bandwidth in one particular region may exceed the available bandwidth. Hence, it is

desirable to rnaxïmize bandwidth-limited capacity.

Table 6-12: Frequency assignrnent with the wtended algofi th assuming the inteq5enng users are a mU: of 8kb/s, 16 kbls, 32kb/s, 48 kb/s and 63 kbls users, FB No. = Frequency Band No

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Table 6-13: Frequemy nssignment with lhe er~fended aigorithm asszuning the inmering mers are a mk of 8kbls, 16 kbls, 32kb/s, 48 kbls and 64 kbls users, FB No. = Frequency Band No

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The power M t e d capacity of the proposed systern Ïs dictated by the avâilable satellite

amplifier Ku-band r e m downlink (return = satellite to earth station) RF power and the

frarne duration. Since the user terminal to earth station is most critical, it is used to

denve the system capacity. The underiying concept is to allocate the RF power available

onboard the satellite among several carriers judiciously until no power remains to support

further carriers. In the remm uplink (mobile user terminal to the satellite), several users (in

their respective hrnes) access the satellite and are then 'bent-pipe' for the return downlink.

The arnount of power consumed at the satellite for the 'bent-pipe' operation vanes for each

canier according to the power Ievels of each user in the fiame.

The task is to calculate the total number of m e r e m rate users who c m be sustained in several

carriers within one h m e duration The concept of kame duration is introduced here to

provide a mechanism for assigning a certain amount of weight to each type of user. For

example, if a 8 kb/s user requires p W of power during the fiame to 'bent-pipe' and his/her

time slot duration is z ms, then the energy consumed by that user in a 50 ms fiame is p * z.

At the same t he , if a 32 kb/s user requires (2.5' * p) W of power during the frarne and

his/her tirne slot duration is (3.2' * z) ms, then the energy consumed by that user in a 50 ms

frarne is (8 * p * 2). Therefore, in this specinc example, a 32 kb/s user uses eight rimes as

much energy as an 8 kb/s user- This comparative mechanism c m be instrumental in

determining the sensitivity of the system power consumption to each of the user data rates.

l Arbitrarily chosen to serve as an exarnple

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The total useful satellite energy available within one h n e duration of 50 ms is detennined by

the total return satellite RF amplifier power available and the signal share value for each user

Ïn the fiame. In this thesis (chaprer 4), a value of 80 W was used as the total available power.

As in chapter 4, if an antenna gain of 25.4dB and a back-off of 4dB is assurned, the total

EIRP available to 'bent-pipe' ail caniers is 40.43 dBW. However, the EIRP available to

'bent-pipe' the useful carrier part of the composite (carrier + noise) uplink signal depends

upon the uplink s ipa l share value for each user in the frame.

As discussed in Section 4.6.2, a non-regenerative repeater is assumed in the system, the

available power of 80W is consumed to relay the upiink composite useful carrier signal and

the noise. Therefore, only a certain portion of the total satellite power of 80W can be

considered as avaiiable for generating the downlink carrier signaL This portion of the total

satellite power is referred to as the Uplink Signal Share and is calculated based on the ratio of

the uplink useful carrier signal to the composite uplink signal.

If the power required by each data rate user (Section 4.6.2) and his/her time sIot duration in

the kame (Section 3.2) is known, the energy required for each data rate user can be

calculated,

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Let Pi denote the power required to bent-pipe a user operating at a data rate of i kbls (i = 8,

16,32,48 or 64) and let Ti denote his/her time dot duration within a TF, (50 m) kame- AIso,

let ni represent the number of users operating at a data rate of i kb/s and let Si represent the

uplink signal share for each user operating at a data rate of i kb/s.

As calculated in Section 4.6.2, Sg = 0-70 (uplink signal share for an 8 kb/s user) and Sig = S31

= Sa = Sa = 0.87. Therefore, the total power available to bent-pipe multiple cankrs, P,r

with multiple data rate users is deterrnined using the following relationsm:

where Sh = S16 = S31 = Sa = Sa = 0.87.

Therefore, the total available energy, Es, is,

Emr = Pmt * TF,-

Let Ei denote the energy requued for a user operating at a data rate of i kb/s and let EE

represent the total energy required for a users operating a t i kb/s.

Then, EE=n;* Pi*Ti

Let the total energy required for & users operating at data rates be denoted by E,

Accordingly,

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The power-limited capacitv of the system is calculated bv seIectin9 a combination of different

values of ni - such that the foUowino condition is satisfied-

The power required by each data rate user, Pi (Sections 4-6.2 and 4.6.4) and the time slat

duration, Ti (Section 3.2) is given as follows:

Table 6-14: The power required by a satellite to bent-pipe a user operating ar i kbls and the corresponding time do t duration in the 50 msficune.

Using the values in Table 6- 14 and substituting the values of EIRP(aiier backk061 (40.43dB W or

1 1,041 W), TFr (50 ms), Sg = 0.70 and Sh = 0.87, Eq. 6-2 can be re-written as follows:

L i=8J6.32,48.61 A

2 (n8 * 18.03 * 2.75)+ (n, * 7.58 * 5.25)+ (& *7.58 * 10-25)+

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Any combination of values of na, n16, n32, n~ and that satisfies the above inequality and

also rninimizes the daerence between the two sides of the equation d e t e e e s the capacity of

the system. There also exist an upper bound to the values of na. nid, n32, n a and n~ which

c m be individually deterrnined using Eq- 6-1 and setting the remaining ni values to O. For

example, the maximum vahe of n8 can be deterrnined by setçing nl6, na, n48 and n~ equal to

O and evaluating Eq. 6- 1. The range of values for each of ns, n16, nn, ndg and n64 are given in

Table 6- 15-

Table 6-15: The range of possible values for na, q,, na and n*.

From Table 6-15, the power-constrained system capacity is limitai to 7,794 & kb/s users

simultaneously accessing the system. Similarly, 12,068 M kb/s, 6,181 22 kb/s, 4,154 @ kb/s

and 3,128 @ kb/s mers can simultaneously access the system. To aid in deteminhg

various candidate combinations, a ~ a t l a b @ program was executed. The ~ a t l a b @ code is

presented in Appendix B. It is assumed that the satellite power is not consurned during the

guard times and therefore the above analysis is applicable with different guard time and voice

activity values. Table 6-16 lists some of several values of m, tqo, nz, ma and n64 that satisfy

Eq. 6-3.

Max. No. of caniers in the system

Range of values Max. No. of users per 50 ms fiame

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Table 6-16: The values of Q, n16, n32, and rw that satifi Eq- 6-1.

6.6 Summary of Results

As in the case of any GEO-MSS system, it is evident from power-Iimited and bandwidth-

limited analysis that the proposed geostationary MSS system is power limited. The maximum

capacity achievable in terms of the number of users operating at a data rate of 8 kb/s is 21,624

when the systern is bandwidth-Iunited (only one frequency band per spot kam) and 7,794

when the system is power-limited. However, upon cornparison with the traditional fiame

structure, the propo sed fiame structure promises a capacity enhancement as high as 122%. In

the proposed system there are 102 spot beams required to provide s e ~ c e to the desired area

of coverage. The total bandwidth needs to be divided into 12 bands each of which c m be

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1 44

reused many cimes (Section 6.4.2). If a spot bearn is dowed to use more than one fiequency

band (Section 6.4.3), the bandwith-limited capacity i s improved by 42.1 %.

In the next chapter, the research is concluded and the main contribution s and finding 's are

highlighted. Also, avenues for possible extensions of this research effort and related topics

are idenfiecl and discussed.

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Chapter 7

Conclusions and Recommendations for Future Research

7. Conclusions

In this thesis, a complete TDMA based system for a geostationary (GEO) mobile satellire

service ( M S S ) system for North America is designed and analyzed with the help of an ori,ginal

and comprehensive software module. With a large satellite antenna diameter, it becomes

possible to irrrplement a GE0 MSS system which is capable of supportkg a direct link to a

hand-held user temiinal The system caters to users of various data rates ranging fiorn 8 kb/s

to 64 kb/s with two distinct types of user termulals. It can provide an acceptable quality of

speech service at 8 kb/s with hand-heId tenriinals and higher data rate services to users with a

more capable terminaL Furthemore, a new TDMA fiame stnicme is proposed ior the

system which provides capacity enhancements ranging Srom 0 8 to 1224 (depending on the

fiame contents) relative to the conventional TDMA hrne structures. An exhaustive link

budget analysis is performed to determine the capacity of the system The major conclusions

are presented in the following secaons.

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7.1 New TDMA Frame Structure

A comparative performance analysis of the proposed TDMA f r m e saucture with the

traditional TDMA kame smcture yields results which heavily depend upon the choice of the

user data rate and guard time values. The higher data rate users ernploy a larger user terminal

and therefore in a fiame with different rate users, the signal is received at the satellite with

varying power levels and difEerent voice activiaes within the time slo ts. The performance of

the proposed fiame structure is enhanced by the inherent long guard time requirements of

GEO-MSS systems. The p a r d time values in the system range from 0.76 rns to 7.18 m.

For the purposes of analysis, four distinct guard time values of 1.4 ms, 2.25 ms, 3.5 ms and

7.2 ms are considered. The capacity enhancements promised by the proposed TDMA h r n e

structure Vary ftom 0% to 122 % depending upon the data rates of the users in the fiame and

the guard tirne values. When all the users in the £rame operate at a data rate of 8 kb/s, there is

no merence between the traditional fiarne structure and the proposed frame structure.

However, in the case of higher data rate users, the dynamic rearranpment of variable-size

time slots in the proposed fiame as opposed to static ked-size multiple tirne slot assi,.nmerit

results in much better utili;ration of the h r n e and hence the increased capacity.

7.2 Spot Beam Design and Layout

The multiple spot beam MSS system designed during the course of the research provides a

graphical visual illustration of the spot beam Iayout for the entire continent of North Amenca

Using vector analysis, an original rehtionship is developed to associate the coordinates of

particular location on earth (assurning the earth is a sphere) and the angle of signal reception

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at the satefite antema with respect to the boresight of the spot beam The difference in the

shapes and sizes of the spot bearns in different regions of Noah Arnerica is clearly evident

with the final spot bearn layout (see Figure 6- 1). This yields in different guard h e values for

the spot beams and the values are calculated separately for each spot beam The software

module also performed interference analysis for the spot beam layout based on user de5ned

required EdN,, user terminal's peak power capabilities, channe1 fading characteristics,

channel bandwidth, satellite antenna noise temperature, etc. Using the results h o m the

interference analysis, fiequency assignment algorithm is implemented assuming different types

of t r a c m e s in the system The results fiom the frequency assignment algorithm yield

the bandwidth-lirnited capacity of the system in temis of the number of carriers. niis

capacity can be iranslated into the nurnber of users by assurning certain iraffic charactenstics.

There are two trafnc confgurations assurned for the kequency assignment and bandwidth-

limited capacity cdcuIations. In the e s t confignation, a l l i n t e r f e ~ g users are assurneci to be

8 kb/s speech users with hand-held termùials. Based on the interference analysis and

hquency assignment, 13 distinct fkequency bands are required in the system when aU the

interfering users are 8 kb/s speech users. Since the total system bandwidth is 29 MHz, the

bandwidth allowed per spot beam is 29 MHz / 13 = 2.23 MHz. Since each tank occupies a

bandwidth of 100 H z , there exist 2.23 MHz / 100 kHz = 22 carriers per spot beam There

exist a total of 102 spot beams and therefore the systern bandwidth-iïmited capacity is 22 -

102 = 2,244 carriers, assuming one fiequency band is assigned per spot beam When all users

in the system are assurned to operate at a spec5c data rate, the capacity cm be calculated in

terms of the number of users and a cornparison between the proposed and traditionai kame

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suucture can be done. The proposed fkame structure promises a capacity enhancement of

O%, 35.795, 66.8%, 85.23% and 122.0% when all users in the system operate at data rates of

8, 16,32,48 or 64 kb/s respectively.

In the second configuration, the interferhg users are assumed to be operating at data rates of

8 kb/s, 16 kb/s, 32 kb/s, 48 kb/s and 64 kb/s with user tennInals of different capabilïties.

Again, based on the interference analysis and fiequency assignrnent, 12 distinct fkequency

bands are required in the system The bandwidth allowed per spot beam is 29 MHz / 12 =

2.41 MHz. Since each carrier occupies a bandwidth of 100 kHz, there exist 2.41 MHz / 100

lcHz = 24 carriers per spot beam There exist a total of 102 spot beams and therefore the

system bandwidth-limited capacity is 24 102 = 2,448 carriers, assurning one fiequency band

is assigned per spot beam If multiple frequency bands are assigned to each spot beam

(interference perrnitting), the system bandwidth capacity is improved b y 42.1 5%- %th

multiple fiequency bands assigned per spot barn, the systern can support 3480 carriers. The

results are better than the case with dl interfixing users operating at 8 kb/s. The reason is

that while some of the CO-charme1 ïnterfering users employ a more capable terminai, they can

simultaneously tolerate more interference.

7.3 Power Limited System Capacity

The total amount of satellite amplifier po wer dictates the total capacity of the system in ternis

of the number of carriers. The arnount of power allocated to each carrier depends upon the

data rate of users operating in the ficame. The situation in this system Ïs unique because in a

kame with different rate users, the signal is received at the satefite wïth varying power levels

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for different tirne duration within the tirne slots. To help investigate the power-limited

capacity of the system in terms of the number of users, the concept of energy is used instead

of power. Given the total power availabe at the satellite and the fixed came duration, the

total available energy is calcuLated. Also, given the nxed time slot duration of each user in the

kame and the power required by each user for signal relay, the energy requked by each user

c m be calculated, The task is to determine the total number of different rate users which can

be accommodated in a kame across several canïers with the given total satellite energy.

Since there are five data rate users, there exist no unique solution and therefore a range of

solutions is presented in this thesis (Section 6.5). The rnaximurn number of 8, 16, 32, 48 and

64 kb/s users that cm be supporteci in che system are 7,794, 12.068, 6,181, 4,154 and 3,128

resp ec tivel y.

7.4 Recomnzendations for Future Research

Sùice several aspects of the system channel access scheme were analyzed in this thesis, many

issues can k identifiecl for future r e m c h work. The assumptions made regarding several

parameters can be thoroughly investigated and the capacity performance cdculations can be

refined. The issues requiring M e r investigation can be broadly class5ed as one of the

following:

Multiple Access Techniques

SignaLing required for the proposed frame structure

Link Budget Analysis

Frequency Assignrnent Analysis and Layout

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In the next sections, each of the above is discussed briefly.

7.4.1 Multiple Access Techniques

The design of the software module and the spot bearn layout obtained £kom it are not limited

to evahate the performance of a TDMA based system only. The layout can easily be

extended to assess the performance and capacity of an FDMA based system or a CDMA

based system Several authors Cl], [2], 141, [a, [25], [26] have compared the capacity

performance of different multiple access schemes for a GE0 MSS system However, a

complete system design with multiple spot beams for North A m c a has not k e n exploreci.

The proposed TDMA system can be compared with a counterpart CDMA system in terrns of

capacity and throughput perfomance and complexity-

7.4.2 Proposed Frame Structure

The ïmproved performance with the proposed h n e structure is achieved at an expense of

additional signaling. The impact of the additionai signaling in terms of delays involved and

the additional power requirements fÏom the satellite for the signaling channel is to be

investigated. TypicaIly, the long delay associated with a GEO-MS S system is the foremo st

reason of concem. Since it is proposed that the signaling be c-d out during caIi setup or

termination, the delay can be absorbed in the long c d setup and termination times. Ho wever,

an exact analysis is preferred to justiQ the performance iqrovements promised by the new

proposed frarne structure.

Also, the proposed hrne structure can be compared with the currently planneci and soon to

be launched ACeS and Thuraya systems. Both these systerns have adopted TDMA as their

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multiple access scheme. However, very little derails are available at present, regarding the

fiame structures used in these systems. With more details readily available shortly? a

comparaiive analysis of the assurnptions and the capacity performance c m be undertaken.

7.4.3 Link Budget Analysis

In the analysis of the proposed system, a non-generative satellite systern is considered with no

on board processing capabiliàes. The performance of the system is expected to improve

dramatically wïth the inclusion of an onboard processor iti the system The link budget

andysis changes as the uplink (user terminal to the satellite) and the downlink (satellite to the

gourid station) becorne independent. Also, in the curent analysis, the modulation schemes

are assurned identical for both the uplink and downïink. The modulation schernes can be

different depending upon the operating environments in both the links. In the thesis, a dual-

hop luik is assurned for communication between two user terminais of the system If two user

tenriinals wish to cornmunicate with each oeher, the information kom one user is relayed to

the ground station via the satellite £kt (one c o q l e t e r e m link) and then forwarded to the

other user temùnal again via the satellite (one complete fonvard link). This causes additional

dehy in the system The analysis in the thesis can be repeated assurning a single-hop

approach where the ground station is not reqUn-ed and a direct satellite luùc is established

berneen two user teIlTLiXZals wishing to cornmunicate with each other.

Also, in this thesis, only the return link is analyzed as it is mo st CnticaL Ho wever, the forward

Iink needs to be investigated as well. Since the spot bearn contours can represent different

power levels at their respective edges, this affects the power transmitted by the satellite to the

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center of each spot beam The power utilkation onboard the satellite for the forward link is

an important issue to be investigated. Ako, the effect of adjacent channel interference shodd

be incorporated in the Iink budget analysis.

7.4.4 Frequency Assignment Analysis and Spot Beam Layour

In this thesis, the process of fkquency assignment is based on a direct search method.

However, if more precise information regarding aafnc characteristics, fkequency plan and

interference characteristics is available, the spot bearn layout and kequency assignment may

be ïmproved- Any aïteria required to enhance capacity with alternate eequency assignrnent

can be easily included into the exiscing ~icrosofi@ Visual Basic algorithm If precise traffic

characteristics are available, the spot beam layout c m be made accurate and the spot bearn

overlap c m be reduced.

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Appendix A

The following are the reference forrndae for plane and sphencal geometry. [14]

For any spherical h g l e ABC whose side lenghs a, b, and c are measured by the great circle

arcs subtended at the center of the sphere:

sinA - sinB - sinC - - - - (sine law) sina sinb sine

cosa = cosbcos c +sin b sin ccos A (cosine law for sides)

cosA=-cosBcosC+sin BsinCcosa (cosinelawforangles)

For any plane riangle ABC

c' = a' + b' -2ab cos C (iaw of cosines)

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Appendix B

The Matlab code used to determine the dBerent combinations of n8, n16,n32, n48 and n64

that sa@ Eq. 6-1 is given below. Sorne of the results obtained £iom this algorithm are

presented in Table 6- 16.

m m 8 = 7794; maxnl6 = 12068; m m 3 2 = 6181 maxn48 = 4154 maxn64 = 3128 energyfactor = 11041 * 50;

r e s u l t = [O O O O O O O];

f o r a = 0: maxn8 for b = O: maxnl6

f o r c = 0 : maxri32 f o r d = 0 : maxn48

f o r f = 0: maxn64 s u m v a l = a + b t c + d + f ; i f (sumval -= O )

leftsum = ((0.7*a) + 0.87 * (b + c + d t f)) * energyfactor / sumval;

rightsum = ($9 -58 * a) + '7 -58 * ((5-25 b) + (10 -25 * C ) (15-25 * d) t (20.25 * f ) ) ;

difference = leftsum - rightsum; end

i f (difference > 0) & (d i f fe rence c 5) r e s u l t = [ r e s u l t ; a b c d f leftsum rightsum]

end; en6;

end; end;

end; end;

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