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TECHNI CAL DESI GN AND SERVI CE ASPECTS

OF

DI GI TAL MOBI LE SATELLI TE COMMUNI CATI ON SYSTEMS

Yutaka YASUDA, Masayoshi OHASHI , Fumaki SUGAYA, Yasuo HI RATA

KDD Megur o R

b

D Laboratori es

1-23, Nakameguro 2I chome,

ABSTRACT

Gl obal mobi l e communi cati on servi ces s uch

as mari t i me, aer onaut i cal and l and mobi l e

communi cati ons are ef f ecti vel y pr ovi ded by

usi ng t he sat el l i t e. Si nce t he power and

bandw dt h avai l abl e f or respect i ve car r i er

are sever el y l i m t ed under t he mobi l e

sat el l i t e communi cati ons envi r onment , t he

appl i cat i on of d i gi t al t ransm ss i on

technol ogi es i s very at t r act i ve to ut i l i ze

the l i m ted sate l l i t e resources ef f i c i ent l y.

The vari ous new i ntegr ated di gi t al communi -

cat i on serv i ces wi l l be al so ef f ect i vel y

provi ded i n the di gi t al t ransm ssi on systems.

I n t hi s paper , t he key techni cal el ement s f or

des i gni ng the di gi ta l mobi l e sate l l i t e

communi cati on syst ems ar e cl ari f i ed together

wi t h the di scussi ons on t he f ut ure t rend of

t he communi cati on servi ces to be of f ered f or

mobi l e user s.

1.

I NTRODUCTI ON

Aut hors had desi gned and devel oped

several di gi t al mari t i me communi cati on

sys t ems

so

f ar, and conduct ed the fi el d

experi ment s by usi ng I NMARSAT (I nter nat i onal

Mar i t i me Sate l l i te Organi zat i on) sate l l i te i n

1984 and 1986 i n order t o i nvest i gat e t hei r

perf ormance characteri st i cs under t he act ual

mari t i me s atel l i t e communi cati on envi r onment

[Ref s.

1-41.

Such a di gi t al voi ce- gr ade

mari t i me sat el l i t e communi cati on system i s

cal l ed Standard- B i n I NMARSAT and i s goi ng

t o be i ntr oduced i n i t s second gener ati on era

[Ref . 51. Based on t he experi ence of

desi gni ng t he di gi t al mari t i me communi cati on

syst ems, we have al so devel oped aeronauti cal

di gi t al voi ce-gr ade communi cati on subsyst em

t o be used f or an experi ment al aer onaut i cal

satel l i t e communi cati on system The f i el d

t r i al of th i s syst emhas been successfu l l y

conduct ed by usi ng passenger ai r pl ane vi a

I NMARSAT Paci f i c ocean r egi on sat el l i t e.

I n thi s paper , bas i c conf i gurat i on o f

di gi t al mobi l e satel l i t e communi cat i on

syst ems i s r evi ewed, and t he key di gi t al

Meguro- ku, Tokyo 153, J apan

t r ansm ssi on t echnol ogi es such as voi ce

codi ng, FEC ( For ward Err or Corr ecti on),

di gi t al modul ati on and synchr oni zati on are

di scussed t aki ng account of our experi ence of

t he har dwar e i mpl ement ati on and t he resul t s

of t he f i el d experi ment s. Then, t he

t rade- of f studi es on t he sat el l i t e e. i . r .p.

r equi r ement s and t he ant enna G/ T of t he mobi l e

t erm nal ar e made based on t he l i nk budget

cal cul ati on. Fur t hermore, new servi ces whi ch

w l l be ef f ect i vel y of f ered to mobi l e users

i n t he di gi t al t ransm ssi on syst ems ar e

st udi ed. These new servi ces cover t he

f acsi m l e and data t r ansm ssi on combi ned wi t h

pers onal computer communi cat i on. The

poss i bi l i ty of o f f er i ng vi deo t ransmss i on

servi ces f or mobi l e users are al so di scussed.

2 KEY TECHNI CAL ELEMENT FOR SYSTEM DESI GN

2.1 System Conf i nur ati on

Fi gure 1 shows the overal l conf i gur ati on

of t he mobi l e sat el l i t e communi cati on syst ems.

The l ow gai n antenna i s used f or mobi l e eart h

stat i ons , whi l e the re l at i vel y hi gh gai n

ant enna i s

empl oyed f or t he ground ear t h

Sate i te

CImEKm

C -

b a n d

NCS

Terrestrial)

s

*Or* *

GES

s

L - b a n d

SES

Aeronau t i ca l Ear th Sta t i on

:

Sh ip Ear th Sta t i on

Ground Ear th Sta t i on

:

Network Coordinat ion Stat io n

Fi g. 1 Overal l Syst em Conf i gurat i on f or

Mobi l e Satel l i t e Communi cati ons

1094

34.2.1.

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988 IEEE

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station to enable stable communication with

mobile terminals under the severely

power-limited condition. As for the

access/control of the communication channel,

the demand assignment technique is generally

applied in order to share the limited

satellite channel resources with many mobile

users. In the IlwABSAT system, the network

control station (NCS) is provided in each

ocean region to carry out the centralized

control for the demand assignment operation.

At the mobile earth station, the antenna

and BF unit (up- and down-converters) are

combined with the communication channel

unit.

of the digital voice-grade communication

subsystem. The communication channel bank

composed of the multiple digital communication

subsystems is provided at the ground earth

station. It will be connected at IF stage

with the antenua/RF facilities for ground

earth station which may be commonly used for

different kinds of mobile services.

Figure

2

shows the basic configuration

2.2 Kev Dinital Technolonies

For voice-grade digital communications,

the low bit rate

4.8

kbit/s

-

16 kbit/s)

voice coding scheme which can provide good

voice quality at the given bit rates must be

employed in order to cope with the limited

power/bandwidth constraints of the

communication channel. The APC-MLQ coding

[Ref. 61 is going to be used in the INMARSAT

digital maritime communication system

(Standard-B) and the aeronautical

communication system, because it can provide

high voice quality at reasonable hardware

size and

is

robust to the transmission bit

errors caused in the mobile communications

environment.

advantage that the coding bit rate of the

voice codec can be easily changed according

to the system requirements if it

is

so

designed.

This coding scheme also has an

As for the FEC coding, the effective

coding scheme with high coding gain

1s

required in order to save the power

requirement for the satellite and/or for

mobile earth stations. The soft decision

Viterbi decoding for convolutional code

is

da ta

vo ice

Fig.2

Block Diagram of Digital Communication

Subsystem

widely used for such a purpose. Regarding

the coding rate of the convolutional code, the

punctured codes with the rate higher than 1/2

[Ref. 71 will be suitable in the case that

the bandwidth saving is strongly required for

each channel.

severely degrades the bit error rate

performance of the communication channel, bit

interleaving of the transmission bits (after

FEC coding) will be effective to minimize such

a performance degradation at the FEC decoder.

However, in this case, the increase of the

signal delay due to bit interleaving and

de-interleaving must be taken into account.

When the multipath fading

The frame synchronization is also very

important for the stable operation of the

digital communication equipment. Under the

mobile satellite communication environment,

since the link C/A (carrier-to-noise power

ratio) becomes very low and fluctuates because

of the signal level variation due to various

factors such as multipath fading, the frame

synchronization scheme robust to such a severe

link condition must be employed. In order to

establish a frame synchronization quickly and

to maintain the synchronized status stably,

the framing bits (unique word) of the fixed

length and the fixed bit pattern are

periodically inserted into the transmitted

bit stream. When the coherent demodulation

technique is employed, the influence of the

phase ambiguity caused in such a demodulator

can be also removed by observing the pattern

of the framing bits in the demodulated bit

sequence.

As for the modulation technique, the

power and bandwidth efficient digital

modulation such as

filtered QPSK is generally

desired. However, when a Class-C HPA is used

for the mobile terminal, it will be necessary

to employ a family of constant envelope

modulation schemes such as offset QPSK and

MSK in order to reduce the out-of-band

emission as much as possible at the HPA

output. Furthermore, since the demodulator

must operate at very low C/N condition when

the FEC code with high coding gain is

employed, the stable operation of the carrier

and clock recovery circuit under such a

condition

is

also very important requirement

for the demodulator, When the bit rates of

the transmission channel becomes very low (in

the order of a few kbit/s), the 2-phase

modulation (e.g. BPSK) rather than the 4-phase

modulation such as QPSK

is

applied to ease

the carrier/clock acquisition and tracking.

2.3 ExamDle of Channel Parameters

The basic channel parameters

of

the

INMARSAT Standard-B and aeronautical

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communication systems are shown in Table 1

for reference. Figure 3 shows the frame

formats of the digital voice-grade

communication channels for these two systems.

In the aeronautical communication system, the

bit interleaving technique of the block size

of 384

(=

64 x 6) bits is applied to the

coded data sequence to cope with the burst

errors caused by the fast fading under the

aeronautical transmission environment. The

total signal delay due to this interleaving

and de-interleaving is around 36 msec.

Informat on

bit rate

16 kbit/s 9.6 kbit/s

(option: 9.6 kbit/s)

Modulation Offset QPSK

-

~ ~~

Transmission

bit rate

Carrier spacing

T X ~ilters Square root raised-cosine Nyquist

I

filter with 60 roll-off

24 kbit/a 21 kbit/s

20 kHz

17.5 kHz

before FEC coding-

r a t e

1 / 2 )

G T

of mobile -4 dBK -13 dBK

terminal (low

G T:

-10 dBK)

O A T ~ V C ~ C EATA VOICE

M T A VOICE

FIE FIELD FIE FIELD

- - - - - - - - - - - - - -

FIE FIELD

I

2

2

25

25

RF - c h a n n e l

r a m e = 80

ms4

PR EAM BL E

ONLY)

BURST MODE

U W

C O D ED D ATA FB

- - -

448

80

148

+1872 b i ts

3. TRADE-OFF STUDY ON TRANSMISSION PARAMETERS

3.1 Reauired C/Bo Versus Information Bit Rate

Figure 4 shows the theoretical relation

of the required C/Bo (C: carrier power, No:

one-sided noise spectral density) versus

information bit rates to obtain the bit error

rate (BER) of lo-? In this figure, the

employment of ideal coherent PSK modulation

and the F E based o n the soft decision Viterbi

decoding are assumed. As for the coding rate

of convolutional codes for Viterbi decoding,

the rate 1/2 code with constraint length K=7

and the rate 3/4 and 7/8 punctured codes

derived from it are selected. The required

C/No to obtain objective BER for the given

information bit rate (Br) has been calculated

by the following equation, assuming the

theoretical BER versus Eb/No (Eb: energy per

information bit) performance for respective

F E C code [Ref. 71.

C/Bo(dB) = Eb/No(dB) + 10 loglOBr

(1)

s B

VOICE VOICE VOICE VOICE

b e f o r e FEC c o d ing - F I EL D F I EL D F I EL D F I EL D

4

r a t e

3/41 1 3

124 7

320 I

L 3 5 1

E

:

F r a m i n g B i t s

S B Su b - b a n d s i g n a l l i n g f i e l d

(a) Standard-B System

frame

=

500

ms -

F -channel

BLOCK

2

- - - - - - - - - - BLOCK27

k 2 0 4 d

I X for sub-band r l g n a l l l n g and user data )

(b) Aeronautical System

When the rate 1/2 F EC code is applied to

the 9.6 kbit/s information channel for

example, we can see from Figure

4

that the

required C/No at BER

=

is about

44

dBHz. If additional bits are inserted at the

transmission channel after

F EC

coding for the

frame/burst synchronization and for other

purposes such as a signalling transmission,

the transmission channel bit rate must be

increased according to the ratio of such

additional bits. Therefore, in such a case,

it is necessary to take account of an extra

margin in the required C/No.

60

BER = 10-5

30

12 2.4 4.8 7296

16

32

64

I n fo r ma t i o n

B i t Rate k b i t / s )

No

Cod ing

7 / 0

314

I

/ 2

(X)

Fig.3 Frame Format of Voice Communication Fig.4 Required C/No vs. Information Bit

Channel (specified in INMARSAT) Rate (Objective BER =

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3.2 Li nk Budget Trade-of f

The channel C/No was cal cul ated as a

f unct i on of t he G/ T or t he e.i .r. p. of t he

mobi l e t er m nal assum ng the t ypi cal

t r ansm ssi on par amet er s of t he I NMARSAT

sate l l i t e l i nk. Si nce the overa l l l i nk C/No

i n t he mobi l e sat el l i t e communi cati on syst ems

i s general l y determ ned by t he l i nk parameters

bet ween t he mobi l e eart h st at i on and t he

sate l l i te , the L -band sate l l i te e. i . r .p. i n

t he f orward (C- to-L) l i nk and t he L- band

sat el l i t e G/ T i n the return (L- to-C) l i nk

were t aken as par ameters i n t hi s cal cul ati on.

Satel l i te I r p

d B W)

20

-15

- I O 5

0

Mobi le Term nal G/ T (dBK)

Fi g. 5 Tot a l L i nk C/No vs. G/ T of Mobi l e

Term nal

Satellite G / T d B K )

-

I O

-

I2

-14

-

f

m

z

U

Fi g. 6 Tot al L i nk C/No vs. EI RP of Mobi l e

Term nal

Fi gur es 5 and 6 show t he per f ormance of

t he t o t al s at e l l i t e l i nk

C/No

ver sus G/ T and

e. i . r .p. of t he mobi l e eart h stat i on. I n

t hese f i gures, t he l i nk paramet er s f or t he

I NMARSAT cur r ent Standar d- A, Standard- B and

aer onaut i cal voi ce- gr ade channel s are marked.

The exampl e of l i nk budget cal cul ati ons f or

t hese syst ems ot her t han t he Standar d-A i s

shown i n Tabl e

2,

whi ch al so i ncl udes the

l i nk budget f or a l ow G/ T -10 dBK) shi p

ear t h st at i on syst emusi ng J apanese

Experi ment al Test Satel l i t e (ETS- V) [Ref . 8 1 .

Si nce t he ETS- V satel l i t e has t wo L- band spot

beams, t he satel l i t e e. i . r.p. and G/ T i n t he

l i nk bet ween t he satel l i t e and mobi l e ear t h

st ati on are f ar hi gher t han those of I NMARSAT

gl obal beam sat el l i t e. Theref ore, t he

hi gh- speed dat a tr ansm ssi on such as

of 64

kbi t / s i nf or mati on rat e wi l l become possi bl e

even f or t he l ow G/ T mobi l e t erm nal . ( See

Sect i on 4.)

Tabl e 2 Exampl e of Li nk Budget Cal cul ati ons

[ Satellite System ] INKARSAT ETS-V

(INTELSAT-V/MCS)

Std B Aero.

low-G/T SES

(a) Ground-to-mobile link

(Frequency) (6.42 GHz) (5.96 GHz)

(47 degs.)5 degs.)

UD

link:

GES elevation

55.5 62.0 66.0

ES EIRP (dBW)

200.9 199.4

ath loss (dB)

0.4 0.2

bsorption

loss

(dB)

Up link C/No (dBHz) 68.8 75.3 87.0

Satellite G/T (dBK) -14 -8

Satellite:

Satellite gain (dB)

161.3 166.5

Satellite C/IMo (dBHz) 61.3 67.8

--

U

Frequency) (1.54 GHz) (1.54 GHz)

32.9

atell ite EIRP (dBW) 15.5 22.0

Path loss (dB) 188.5 187.6

0.4 0.2bsorption loss (dB)

63.7

own link C/No (dBHz) 51.2 48.7

-10

obile G/T (dBK) -4 -13

L i n k

Performance (unfadedl:

63.7

otal C/No (dBHz) 50.7 48.6

(b) Mobile-to-ground

link

UD l i n k : (Frequency) (1.64 GHz) (1.64 GHz)

Mobile terminal elevation (5 degs.) (47 degs.)

Mob ile EIRP (dBW) 33 25.5 27.2

Pat h loss (dB) 189.0 188.2

Absorption loss (dB)

0.4 0.2

Satellite G/T (dBK)

-13

-4.9

Up link C/No (dBHz)

59.2 51.7 62.5

Satellite:

Satellite gai n (dB) 150.9 165.1

Satellite C/IMo (dBHz) 62.5 55.0 --

Down link: (Frequency) (4.2 GHz) (5.2 GHz)

Satellite EIRP (dBW) -5.5 -13.0 3.9

Absor ption loss (dB) 0.4 0.2

GES G/T (dBK) 32 33.7

Pat h loss (dB) 197.2 198.2

Down l i nk

C/No (dBHz) 57.5 50.0 67.8

Link Derformance (unfaded):

Tot al C/No (dBHz) 54.5

47 O

61.4

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It should be also noted that the multipath

fading margi n is not included in the link

budget calculations shown

in

Table 2.

Therefore, wh en the elevation angle of the

mobile earth station is low, the appropriate

fading margin must b e taken into account.

The required C/No under the unfaded condition

is determined from the BER objective shown in

Figure

4

by adding hardware implementation

margin, etc. The amount of the fading margin

depends on the antenna size of the mobile

earth station and the elevation angle of it

toward the satellite. Figure

7

shows the

measured results of the fading depth versus

elevation angle of the ship earth station

antenna under the actual INMARSAT maritime

satellite communication link. These data

were obtained by the field experiment on the

digital ship earth stati on systems using a

high-G/T

-4

dBK) parabolic antenna and a

low-G/T (-10 dBK) short-backfire antenna with

fading reduction capability [Refs.

4,

91.

4.

SERVICE CONSIDERATION

In the digital mobile satellite

communication systems, the various integrated

digital communication services will be

provided

in

addition to the conventional

voice services. These new services include

high-speed facsimile transmission, personal

computer (PC) communications and compressed

video/picture transmission, etc. In this

section, the future evolution of such

communication services is considered, taking

account o f the possible technical progress.

6

z

M 4

2 2

a

c

a

.-

A

0

OHigh G/T

o L o w - G / T F R

A

FR

0

0 0

0

0

o p

0 0

O N )

:

OFF)

0

0

0 1 1 1 1 1 1 1 1 1 1 1

4 8

10

12

14

Elevation Angle deg.)

High-G/T: 85c m parabolic antenna (G/T = 4 dBK)

Low-G/T : 40cm short backfire (G/T = -10 dBK)

with fading reduction (FR) capability

Fig.7

Measure Results of Fading Depth vs.

Elevation Angle (maritime)

Voice

In the INMARSAT Standard-B system, the

APC-MLQ voice coding of 16 kbit/s is going to

be used

in

order to provide toll quality voice

communication services. In the aeronautical

communication systems, 9.6 kbit/s APC-MLQ

voice coding is also going to be used. In

the future land mobile communication systems,

further reduction of voice coding rate (e.g.

4.8 kbit/s) may be needed i n order to

minimize the power and bandwidth requirement

per channel. Suc h a low rate voice coding

will be also effectively employed when two or

more voice channels must be integrated into

single digital communication channel. For

instance, two independent voice channels of

4.8

kbit/s can be integrated

in

a single

9.6

kbit/s digital voice-grade channel in the

aeronautical communication systems in which

the multiple voice communication capability

for passengers and cabin crews is strongly

desired.

Facsimile

In the current INMARSAT Standard-A system

using analog companded FM modulation, 63

facsimile transmission of up to 2.4 kbit/s i s

guaranteed. Thi s capability is also

maintained wh en 16 kbit/s APC voice codec is

applied in the INMARSAT Standard-B system.

Furthermore, in the mobile satellite

communication systems wit h a digital

transmission channel at the bit rate of 9.6

kbit/s or more, the

63

facsimile transmission

at a speed of 9.6 kbit/s becomes possible by

providing appropriate analog-digital interface

at the ground earth station. In this case,

the facsimile signal is transmitted in the

digital form via digital satellite

communication channel, whereas in the

terrestrial analog PSTN (public switched

telephone network), it is transmitted through

the conventional V.29 voice-band modem

incorporated i n the 63 facsimile equipment or

through the V.32 modem which enables both-way

9.6 kbit/s data transmission via 2-wire local

line. Whe n the digital communication

channels become widely available in the

public terrestrial networks, the satellite

data channel of the arbitral bit rate is

directly connected to suc h a terrestrial data

channel without using voice-band modem.

such a case, higher speed 64 facsimile

transmission suc h as 16 kbit/s will also

become available for the relatively high G/T

mobile earth station systems.

Personal Computer Communication

In the mobile satellite communication

systems such as maritime and aeronautical

ones, the data transmission by using the

personal computer is an effective mean to

deal with the various communication services

in an integrated digital form. For the

In

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aer onaut i cal appl i cat i ons, t he use of a handy

l apt op PC comput er wi l l be pract i cal and ver y

att r act i ve i n order to communi cat e f r omt he

cabi n wi t h t he user on the gr ound.

per sonal comput er communi cat i on i ncl udes t he

f i l e t r ansf er and database access i n addi t i on

t o t he convent i onal message exchange. I f t he

appropr i ate i mage scanner and/ or a f acsi m l e

equi pment i s combi ned wi t h t he PC t erm nal ,

t he f i gur e/ i mage t r ansm ssi on bet ween mobi l e

user and t he gr ound user wi l l become

an

att r act i ve servi ce. The hi gh speed and

err or- f r ee both- way dat a tr ansm ssi on bet ween

PC t erm nal s i s r eal i zed by usi ng t he HDLC

speci f i ed i n t he OS1 (Open Syst ems

I nt erconnect i on) protocol .

i t i s i mport ant t o sel ect t he appropri at e

packet si ze as wel l as t he wi ndow si ze f or

t he ARQ cont r ol t o get t he maxi mum t hroughput

at t he mobi l e satel l i t e communi cati on channel

wi t h f ai r l y l arge s i gnal t r ansmss i on del ay.

Compr essed Vi deo/ St i l l Pi ct ur e

speed dat a t r ansm ssi on channel such as 64

kbi t / s can be al so pr ovi ded.

can accommodat e t he i ntegrat ed audi ohi de0

t ransm ssi on f or v i sual t el ephone servi ces by

usi ng t he hi ghl y compressed voi ce/ pi ct ure

codi ng equi pment . I n

t hi s year, we are

pl anni ng to conduct such an exper i ment on the

64

kbi t / s audi ohi de0 t r ansm ss i on over

mari t i me communi cat i on channel vi a J apanese

ETS- V sat el l i t e by usi ng the I NVI TE

64

system

devel oped by KDD [ Ref . l o]. I n addi t i on t o

t hi s , t he st i l l pi ct ur e

of

qual i f i ed col or

i mage wi l l be al so t ransm t t ed vi a l ower rat e

channel such as 9.6 kbi t / s .

The

I n such a system

I f t her e i s enough power margi n, t he hi gh

Such a channel

5. CONCLUSI ON

The key techni cal el ement s i n desi gni ng

t he di gi t al mobi l e sat el l i t e communi cat i on

syst ems were i dent i f i ed and t he tr ade-of f

st udi es on the tr ansm ssi on parameters of t he

sat el l i t e l i nk wer e made based on our

exper i ence of t he exper i ment al syst em

devel opment and the r esul t s of t he f i el d

tr i al s. Then, t he f ut ure evol ut i on of the

vari ous new communi cat i on ser vi ces t o be

of f ered i n the di gi t al mobi l e communi cat i on

sys t ems wer e di scussed.

I n the di gi t a l mobi l e sate l l i t e

communi cat i on sys t ems whi ch w l l be oper ated

under t he condi t i on of l ow C/ N and wi t h a

si gnal l evel f l uctuat i on due to mul t i pat h

f adi ng etc., i t i s the most i mpor t ant

r equi r ement t o est abl i sh t he f r ame

synchr oni zati on qui ckl y and mai nt ai n i t

s tabl e i n addi t i on to t he s tabl e

carr i er/ cl ock tr acki ng under such a sever e

l i nk condi t i on.

Thi s was one of t he maj or

r esul t s whi ch we obt ai ned t hrough t he var i ous

f i el d experi ment s on t he di gi t al mari t i me and

aer onaut i cal satel l i t e communi cat i on systems.

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