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UNIVERSITY OF NAIROBI
TITLE: GSM FREQUENCY PLANNING
PROJECT NUMBER- PRJ070
BY
NAME: MUTONGA JACKSON WAMBUA
REG NO: F17/2098/2004
SUPERVISOR: DR. CYRUS WEKESA
EXAMINER: DR. MAURICE MANG’OLI
Project report submitted in partial fulfilment of the requirement for the award of the degree of Bachelor of Science in Electrical and Electronic Engineering of the University of Nairobi.
Date of Submission: MAY 2009
DEPARTMENT OF ELECTRICAL AND INFORMATION ENGINEERING
Dedication
I dedicate this project to my parents, Timothy and Ruth, my brothers and sisters for
believing in me and standing by me all through my life. Without you I would not be what
I am today.
ii
Acknowledgements
This undergraduate project was done at the Department of Electrical and Information
Engineering, University of Nairobi, between November 2008 and May 2009. To begin
with I would like to thank my supervisor for this project Dr. Cyrus Wekesa for all the
help and support during my work. He not only supervised and guided every development
of this project but also accorded me his own material and moral support. I am greatly
indebted to his selfless sacrifice.
Big thanks to Abraham Nyete and Eng. Fwamba, both of Safaricom Kenya Limited, for
their assistance which was invaluable. It was of great help and inspired me to put extra
hours of work into this project. I would also like to thank my friend Paul Musau for
helping me with ideas and proof reading this project report.
Last but not least I would like to thank my family and all my friends for the support
during all my studies. It would not have been possible to manage everything without you.
iii
Abstract
Frequency planning is one of the most expensive aspects of deploying a cellular network.
If a set of base stations can be deployed with minimal service and planning, the cost of
both deploying and maintaining the network will decrease. This project explores the
automatic frequency planning problem and proposes a cost effective frequency reuse
strategy. This development is considered for a frequency and time division multiple
access (FDMA and TDMA) system i.e. GSM with a limited number of frequency bands
that requires different frequency allocations for each base station to mitigate co-channel
interference. A channel allocation algorithm is proposed which includes a channel
acquisition and a channel selection scheme. The proposed distributed dynamic channel
allocation algorithm is based on resource-planning model, where a borrower need not to
receive replies from every interfering neighbor, it can borrow a channel from that
neighbor whose all group members replies with common free channels first. The
proposed algorithm makes efficient reuse of channels and evaluates the performance in
terms of blocking probability.
iv
Contents
Dedication……………………………………………………………………………..ii
Acknowledgements……………………………………………………………………iii
Abstract………………………………………………………………………………...iv
Acronyms used………………………………………………………………………..vii
List of figures…………………………………………………………………………..ix
1 INTRODUCTION 1
1.1 Overview………………………………………………………………………….1
1.2 Objective………………………………………………………………………….2
1.3 Outline……………………………………………………………………………2
2 BACKGROUND 4
2.1 Shared Medium Schemes………………………………………………………..4
2.1.1 FDMA…………………………………………………………………….4
2.1.2 TDMA……………………………………………………………………5
2.1.3 CDMA……………………………………………………………………6
2.1.4 SDMA…………………………………………………………………….7
2.2 Introduction to GSM Networks…………………………………………………7
2.2.1 Entities within a GSM Network………………………………………….9
2.2.2 The Radio Interface………………………………………………………12
2.2.3 Channel Structure………………………………………………………...13
3 FREQUENCY PLANNING 15
3.1 Radio Network Planning…………………………………………………………15
3.1.1 The Automatic Frequency Planning (AFP) Problem……………………..16
3.1.2 The Cellular Concept and Frequency Reuse……………………………...18
3.1.3 Selection of Frequency Reuse Patterns……………………………………19
v
3.2 The Mobile Environment and Interference………………………………………21
3.2.1 Adjacent Channel Interference……………………………………………23
3.2.2 Co-channel Interference…………………………………………………..24
3.2.3 Methods of Reducing Co-channel Interference…………………………...25
3.2.4 Path Loss…………………………………………………………………..27
3.3 Channel Assignment strategies………………………………………………….....28
3.3.1 Fixed Channel Allocation (FCA)...………………………………………..29
3.3.2 Dynamic Channel Allocation (DCA)……………………………………...29
3.3.3 Hybrid Channel Allocation (HCA)………………………………………..31
4 PROPOSED FREQUENCY REUSE STRATEGY 33
4.1 The Frequency Reuse Pattern……………………………………………………..33
4.2 System Model……………………………………………………………………..34
4.3 The Proposed Algorithm ………………………………………………………….36
4.4 Explanation of the Algorithm……………………………………………………..40
5 SIMULATION RESULTS AND ANALYSIS 41
5.1 Description of Results……………………………………………………………..41
5.2 Analysis……………………………………………………………………………43
6 CONCLUSIONS AND RECOMMENDATIONS 44
6.1 Conclusions of the Results…………………………………………………………44
6.2 Future Development………………………………………………………………..44
7 REFERENCES AND APPENDICES 45
References …………………………………………………………………………….45
Appendices…………………………………………………………………………….46
vi
ACRONYMS USED
GSM Global System for Mobile Communications
BTS/BS Base Transceiver Station / Base Station
MA Multiple Access
FDMA Frequency Division multiple Access
TDMA Time Division Multiple Access
CDMA Code Division Multiple Access
SDMA Space Division Multiple Access
SMS Short message Service
MHZ Mega Hertz
kHZ Kilo Hertz
MS Mobile Station
SIM Subscriber Identity Module
BSIC Base Station Identity Code
BSC Base Station Controller
BSS Base Station Subsystem
MSC Mobile Switching Centre
VLR Visitor Location Register
HLR Home Location Register
AuC Authentication Centre
EIR Equipment Identity Register
TCH/F Traffic Channel/ Full rate
TCH/H Traffic Channel/ Half rate
BCCH Broadcast Control Channel
FCC Frequency Correction Channel
RACH Random Access Channel
vii
PCH Paging Channel
AGCH Access Grant Channel
SCH Synchronization Channel
AFP Automatic Frequency Planning
TRX Transceiver
FCA Fixed Channel Allocation
DCA Dynamic Channel Allocation
HCA Hybrid Channel Allocation
BCO Borrowing with Channel Ordering
BDCL Borrowing with Directional Channel Locking
SINR Signal to Interference Noise Ratio
AWGN Additive White Gaussian Noise
DTX Discontinuous Transmission
viii
LIST OF FIGURES
Figure Page
2.1 FDMA……………………………………………………4
2.2 TDMA……………………………………………………5
2.3 CDMA……………………………………………………6
2.4 GSM900 Channels……………………………………….8
2.5 A portion of the GSM framing structure…………………9
2.6 Outline of the GSM Network Architecture……………..10
2.7 FDMA-TDMA………………………………………….12
2.8 Frequency band allocations……………………………..12
3.1 Radio Network Planning Process……………………….16
3.2 An example of GSM Network…………………………..17
3.3 7-cell cluster……………………………………………..19
3.4 Adjacent channel interference……………………………23
4.1 The cellular system model……………………………….36
4.2 Defining cell neighbours ………………………………...37
4.3 Sectorised system model…………………………………37
4.4 Flow chart of the channel allocation algorithm……….....39
5.1 Blocking probability graph……………………………....42
A1 Hexagonal Geometry………………………………….....46
A2 Coordinates for hexagonal geometry……………………47
ix
1
CHAPTER ONE
INTRODUCTION
1.1 Overview
Cellular mobile networks such as GSM are made up of cells, each indicating a coverage zone
with a base transceiver station (BTS) either in the center of the cell or at the corner
boundaries. A hexagon is used to represent a single cell as it would make network diagrams
tidier and that it is closest to the ideal cell shape of a circle. Each BTS (in a cluster) is
allocated a different carrier frequency and each cell has a usable bandwidth associated with
this carrier. Because only a finite part of the radio spectrum is allocated to cellular radio, the
number of carrier frequencies available is limited. This means that it is necessary to re-use the
available frequencies many times in order to provide sufficient channels for the required
demand. This introduces the concept of frequency re-use and with it the possibility of
interference between cells using the same carrier frequencies.
With a fixed number of carrier frequencies available, the capacity of the system can be
increased only by re-using the carrier frequencies more often. This means making the cell
sizes smaller. This has two basic consequences:
a) It increases the likelihood of interference (co-channel interference) between cells
using the same frequency.
b) If a mobile station is moving, it will cross cell boundaries more frequently when the
cells are small. Whenever a mobile crosses a cell boundary it must change from the
carrier frequency of the cell which it is leaving to the carrier frequency of the cell to
which it is entering, a process called handover. It cannot be performed
instantaneously and hence there will be a loss of communication while the handover
is being processed. If the cell sizes become very small handovers may occur at a very
rapid rate.
Thus GSM frequency planning is a major issue in the design of a cellular system which must
achieve an acceptable compromise between the efficient utilization of the available radio
spectrum and the problems associated with frequency re-use.
GSM is one of the fastest growing cellular communication systems in the world. The large
numbers of users, increasing usage of mobile services as well as new services force operators
2
to increase the capacity offered by the networks. In many of the cellular systems, increasing
the capacity means increasing the available bandwidth and using more efficient planning of
the deployment of the base stations. Common ways to increase the capacity are the use of
smaller cells, sectorization of the cells and better assignment of frequencies to mitigate
intercellular interferences. Smaller cells increase the cost of deploying the network. This is
because this scenario requires more base stations and the network requires more planning in
the deployment and frequency assignment. The main concept of cellular communication is
the use of small low-power transmitters and frequencies that can be reused in as small
geographic areas as possible. The frequency reuse will be a key aspect in this project.
1.2 Objective of project
When designing a mobile network there are many things one needs to consider. One of these
is the frequency planning, crucial in a FDMA system. This becomes an important challenge
as the cell sizes decreases. Frequency planning takes lot of time for the operator, especially
when using small cells and it is costly. Even though the planning is expensive, it is very
essential because buying licensed frequencies and bandwidth is even more expensive. So, the
operator wants the best frequency reuse, with as little effort in management as possible, and
at low cost. A frequency planning scheme that has the same frequency reuse factor but
minimal co-channel interference and maintenance cost would be very valuable. The objective
of this project was to develop a strategy to facilitate a cost-effective frequency reuse in GSM
networks.
1.3 Outline
This project report is organized as follows;
Chapter two gives a theoretical background of GSM systems which includes shared medium
schemes, entities within a GSM network, the radio interface and channel structure.
Chapter three discusses the frequency planning procedure, the automatic frequency planning
problem, the cellular concept and frequency reuse, the mobile environment including
interferences and path loss, and the different channel assignment strategies.
In chapter four, a frequency reuse strategy is proposed and theoretical analysis presented.
System model is developed and the proposed strategy represented in form of an algorithm
and a flow chart.
3
Chapter five analyses and gives a detailed discussion of the results obtained, which are
represented in form of a table and a graph. These results show a way that frequency planning
can be solved in a less expensive fashion. It is understood that a real deployment of such a
system would require more extensive tuning of the algorithm. It is hoped that an operator
could expand this to an actual network.
In chapter six conclusions and suggestions for future work are given.
At the end of this project report a list of references used is given and also appendices on the
fundamentals of cellular geometry and simulation pseudocode and Matlab code for the
proposed algorithm.
4
CHAPTER TWO
BACKGROUND Below is a brief introduction to different kinds of multiple access (MA) schemes using a
shared medium. A brief description of GSM is also given to the extent of what is needed for
understanding this report.
2.1 Shared Medium Schemes There are many different ways to divide the shared medium to get multiple user access. In
this chapter, three basic types of medium division protocols are described. These are
frequency division multiple access (FDMA), time division multiple access (TDMA) and code
division multiple access (CDMA). FDMA and TDMA are explained since they are the
protocols used in GSM. CDMA is a protocol used in 3G wireless networks. The knowledge
of these three protocols will help in understanding GSM and the solution proposed in this
report.
2.1.1 FDMA
Frequency Division Multiple Access (FDMA) subdivides the available medium into a set of
narrow bandwidth channels to be shared among different users. In the example in Figure 2.1
channel 1 has been split up in 6 equally large channels. There is no need for the channels to
be equally wide. When the medium is split up, there is no restriction in assigning one user
more frequency slots in order to give him higher capacity.
Fig 2.1 FDMA
5
2.1.2 TDMA
Time division Multiple Access (TDMA) subdivides the capacity of the total channel into a
number of timeslots. Given a particular timeslot, a user has all the available bandwidth at his
disposal. All the users of the medium will then take turns in transmitting. Figure 2.2 shows an
example where channel 1 has been subdivided in 6 channels. As in FDMA, the slots do not
have to be of equal size, but most of the time they are made equal. To create channels with
higher capacity a user can be assigned two or more timeslots.
Fig 2.2 TDMA
The advantages of TDMA are:
a) It offers the capability of overcoming channel fading by appropriate channel
equalization.
b) Flexible bit rates are possible, i.e. both multiples and sub-multiples of the standard bit
rate per channel can be made available to users.
c) It offers the opportunity of frame-by-frame monitoring of signal strength and bit error
rate to enable either base stations or mobiles to initiate handover.
The disadvantages of TDMA are:
a) On the uplink it requires high peak power in the transmit mode. This is a particular
problem for handheld portables with limited battery life.
b) To realize the full potential of digital transmission requires a significant amount of
signal processing. This increase power consumption and also introduces delay into the
speech path.
6
2.1.3 CDMA
The Code Division Multiple Access (CDMA) protocol does not split up the available medium
in terms of frequency or time. Instead all transmissions over-lap, and the correct data is
identified by a unique identification code at the receiver and the transmitter. CDMA is a form
of Direct Sequence Spread Spectrum communications. This means that the digital data x(n) is
coded at a much higher frequency. The code that is applied to the data is pseudo-random,
which means that it is constructed in a deterministic fashion, and therefore reproducible, but
such that the final code will appear random. At the receiver the same code is correlated to the
received signal to extract the data. There are three key points that explains CDMA.
a) The bandwidth is spread using a code that is independent of the data.
b) The receiver uses a code that, synchronized to the received signal, will extract the
received data. First of all, because of the code being independent from all other codes
it will allow multiple users to access the same frequencies at the same time. And
second, since the codes are pseudo-random all the data transmitted by other units,
than the two communicating with each other, will look like noise.
c) With this modulation the signal occupies a bandwidth that is much wider than
necessary to transmit the data. Because of this, one will receive a couple of benefits
such as greater tolerance against interference and disturbance on specific frequencies.
It is possible, by using CDMA, to overlay GSM with an access scheme with high
tolerance to noise and that does not add much interference to the existing system.
Fig 2.3 CDMA
7
2.1.4 SDMA
Space division multiple access (SDMA) is used in all cellular communication systems. The
idea behind SDMA is allowing multiple cells to use the same radio frequency channels. For
this multiple access scheme to work it requires that the users are separated sufficiently far
apart to minimize the co-channel interference. The distance the frequencies can be separated
with is called the reuse distance. Larger distance demands usage of more frequencies. With
frequency planning this distance is made sufficiently large with as few frequencies as
possible. This will be one of the main issues in this project.
2.2 Introduction to GSM Networks Throughout the evolution of cellular communications, many different systems have been
developed apart from each other, resulting in huge problems when it came to compatibility.
The GSM was developed with this in mind and intended to solve these problems. GSM was
the first digital communication system deployed and used in the world. Even now with newer
systems available, GSM stays in use and keeps growing almost all over the world. GSM is a
digital standard that uses Time Division Multiple Access (TDMA) as a multiple access
technology, and its benefits include:
• Support for international roaming i.e. a GSM subscriber may move with his phone
and use it within all countries covered by GSM.
• Excellent speech quality especially in a harsh environment such as in mobile radio.
• Wide range of services (SMS, voice, videotext, etc).
• Extensive security features - a digital communication system provides a ready
platform where encryption techniques may be used to safeguard information
transmitted over the air.
GSM has three dedicated bands that are used. The three bands are usually called GSM900,
GSM1800 and GSM1900. The 900 MHz band was the original band, but as the demand grew
bigger it was extended with the other two frequency base bands. The allocated frequency
band for the GSM systems available is as shown in table 2.1.
8
Table 2.1 Allocated frequency bands for GSM systems
GSM System Uplink Band(MHZ) Downlink Band(MHZ)
GSM900 890 - 915 935 – 960
GSM1800 1710 - 1785 1805 - 1880
GSM1900 1850 - 1910 1930 -1990
The total available bandwidth of GSM900 is divided into 124 200 kHz bands (FDMA) and
each group of 8 users transmits through a 200 kHz band sharing the transmission time
(TDMA). In Fig. 1.1, the eight shaded time slots all belong to the same connection, four of
them in each direction. Note that:
• Transmitting and receiving does not happen in the same time slot and it takes time to
switch from one to the other;
• If the mobile station assigned to 890.4/935.4 MHz and time slot 2 wanted to transmit
to the base station, it would use the lower four shaded slots and the ones following
them in time, putting some data in each time slot until all the data has been sent.
890.2 MHz890.4 MHz
914.4 MHz
935.2 MHz935.4 MHz
959.8 MHz
1
2
124
1
2
124Base
ToMobile
MobileTo
Base
ChannelTDM Frame
Freq
uenc
y
Time
Fig 2.4 GSM900 channels
9
Each TDM slot consists of the following
0 1 2 3 4 5 6 7
000 Information Sync Information 000
Voice/data bit
35757 263
148-bit data frame sent in 547 μsec
8.25-bit(30 μsec)guard time
Fig 2.5 A portion of the GSM framing structure
Each data frame starts and ends with three 0 bits, for frame delineation purposes. It also
contains two 57-bit Information fields; each one having a control bit that indicates whether
the following Information field is for voice or data. Between the Information fields is a 26-bit
Sync (training) field that is used by the receiver to synchronize to the sender’s boundaries.
The 26-bit Sync sequence represents a known pattern of bits that the receiver can lock onto
and adjust itself to, enhancing reception.
Properties of GSM900
• Each frequency band is 200 kHz wide;
• The system has 124 pairs of simplex channels;
• Each channel supports eight separate conversations on it, using time division
multiplexing;
• Each currently active station is assigned one time slot on one channel pair
2.2.1 Entities within A GSM system
Figure 2.6 shows the architecture of a GSM network. In this section some of the most
important entities and their function in GSM systems have been listed, and described in brief.
Mobile Station
The mobile station (MS) is the equipment used to access GSM networks. This is usually the
only part of the system that the user can see, and probably the part where the units differs the
most in quality and available services. Therefore the standard specifies their interaction with
the network strictly. These units are independent of the network-providers. The SIM
10
(Subscriber Identity Module) is as small card, which has to be inserted in the MS for it to
work, except while making emergency calls (112). The SIM is the link between the operator
and the MS. This card uniquely identifies the user of the MS.
Base Transceiver Station (BTS)
The Base Transceiver station (BTS), or more shortly known as the base station (BS), is the
entity in the system that handles the communication with the MSs in the network. Most of the
BTSs have several transceivers, and some of the time the different transceivers communicate
on different radio frequencies. Later in the report the hexagonal cells that make up the
network will be discussed. Each of these cells contains one BTS which is uniquely
distinguished by its Base Station Identity Code (BSIC). The BTS is in charge of all the
communication in the cell. The BTSs are connected to a Base Station Controller (BSC)
through a special interface (A-bis interface). The BTS is the network entity that this project is
mostly concerned with.
Fig 2.6 Outline of the GSM network architecture
Base Station Controller (BSC)
One Base Station Controller (BSC) controls many BTSs. It is the entity that handles part of
the call setup phase and tells each BTS when there is need for handovers between different
11
cells. A BSC together with all the BTSs that it controls are often referred to as the Base
station Sub System (BSS).
Mobile Switching Center (MSC)
The Mobile Switching Center (MSC) is a switch connected to one or several BSCs. Its main
function is to switch speech and data connections between BSCs, other MSCs and mobile and
non-mobile networks. It is also connected to many different registers that are used to verify
each MS and call in the network.
Visitor Location Register (VLR)
It is integrated with the MSC to reduce the signaling link load between the two nodes. It is a
database containing information about all mobile subscribers currently located in the service
area of one MSC. Each MSC therefore has its own unique VLR. The VLR is responsible for
keeping track of a mobile’s position to the nearest location area.
Home Location Register (HLR)
It is a database in GSM which stores permanent subscriber’s data. It can be integrated in a
MSC/VLR node or implemented in a stand-alone node. When a subscriber registers in a new
MSC/VLR, the HLR function is to forward the subscriber information to that particular
MSC/VLR. Each time a subscriber changes MSC/VLR service area, it informs the HLR of its
new ‘address’ so that he/she can be reached when there is a call for him.
Authentication Center (AuC)
This is a database which prevents operators from fraud. It provides HLR with authentication
parameters and ciphering keys. It is implemented on an external computer, connected to the
HLR. The switch is also in charge of authenticating the mobile.
Equipment Identity Register (EIR)
It is a database that contains mobile equipment identity information, and includes list of
stolen, unauthorized or defective MSs. It checks whether the MS is stolen, and if so, prevents
any calls being made to or from it.
12
2.2.2 The Radio Interface
The multiple access scheme used in GSM is a combination of FDMA and TDMA. This
means that the available bandwidth is split up in a larger number of frequency bands, and on
top of that each band is then divided in time to increase the amount of access channels. Going
back to the examples in Figure 2.1 and 2.2 (FDMA & TDMA), and combining these two, a
structure of the available bandwidth as in Figure 2.7 is obtained.
Fig 2.7 FDMA-TDMA
As previously mentioned, GSM has three dedicated base bands, the GSM900, GSM1800 and
the GSM1900. Each of these bands has a collective bandwidth of 50 MHz each. The 50 MHz
are divided in two 25 MHz bands, one used for uplink and the other for downlink. The two 25
MHz bands are then divided into 125 carrier frequencies each separated by 200 kHz. i.e. there
are 125 frequency for uplink and 125 for downlink, each 200 kHz wide. All 125 frequencies
are allocated in pairs so that each uplink/downlink pair is separated with exactly 45 MHz. In
Figure 2.8 we can see the bandwidth locations and separation relative to each other. The
structure is the same for GSM900, GSM1800 and GSM1900.
Fig 2.8 Frequency band allocations
Each of these 200 kHz bands is divided into 8 full-rate channels by using TDMA. These full-
rate channels will either be given a specific usage or split up in even smaller channels. The
total bit rate for one band is 270.833 kbit/s, and each channel is 22.8 kbit/s. When making a
call in a GSM network the MS will be assigned one out of all these channels. A channel in
GSM is one of these 200 kHz bands, discussed above, given a specific area of usage.
13
2.2.3 Channel Structure
There are many different channels in GSM. The two most common channels used for
communication between a MS and a BTS are the TCH/F and TCH/H, which is Traffic
channel/Full-rate and half-rate. A full-rate channel is assigned one timeslot every 4.615 ms
and the half-rate channels gets as the name suggest half of a full-rate. This means that the
half-rate channels gets the entire available spectrum at their use, for a timeslot once every
9.23 ms. There is also a channel called TCH/8 which is an eighth of the full-rate channel.
These kinds of channels are mainly used as control channels. Below are some of the most
commonly used channels in GSM networks.
Traffic channels
A traffic channel (TCH) is used to carry speech and data traffic. Traffic channels are defined
using a 26-frame multiframe, or group of 26 TDMA frames. The length of a 26-frame
multiframe is 120 ms, which is how the length of a burst period is defined (120 ms divided by
26 frames divided by 8 burst periods per frame). Out of the 26 frames, 24 are used for traffic,
1 is used for the Slow Associated Control Channel (SACCH) and 1 is currently unused.
TCHs for the uplink and downlink are separated in time by 3 burst periods, so that the mobile
station does not have to transmit and receive simultaneously, thus simplifying the electronics.
In addition to these full-rate TCHs, there are also half-rate TCHs defined. Half-rate TCHs
effectively doubles the capacity of a system once half-rate speech coders are specified (i.e.,
speech coding at around 7 kbps, instead of 13 kbps). Eighth-rate TCHs are also specified, and
are used for signaling.
Control channels
There are many different control channels within the GSM specification that are assigned
different areas of usage. Common channels can be accessed both by idle mode and dedicated
mode mobiles. The common channels are used by idle mode mobiles to exchange the
signaling information required to change to dedicated mode. Mobiles already in dedicated
mode monitor the surrounding base stations for handover and other information. The
common channels are defined within a 51-frame multiframe, so that dedicated mobiles using
the 26-frame multiframe TCH structure can still monitor control channels. The common
channels include:
14
Broadcast Control Channel (BCCH)
Continually broadcasts, on the downlink, information including base station identity,
frequency allocations, and frequency-hopping sequences.
Frequency Correction Channel (FCCH)
Used to synchronize the mobile to the time slot structure of a cell by defining the boundaries
of burst periods, and the time slot numbering. Every cell in a GSM network broadcasts
exactly one FCCH and one SCH, which are by definition on time slot number 0 (within a
TDMA frame).
Random Access Channel (RACH)
Slotted Aloha channel used by the mobile to request access to the network.
Paging Channel (PCH)
Used to alert the mobile station of an incoming call.
Access Grant Channel (AGCH)
Used to allocate an SDCCH (Stand Alone Dedicated Control Channel) to a mobile for
signaling (in order to obtain a dedicated channel), following a request on the RACH.
Synchronization Channel (SCH)
This channel supplies the time synchronization that all the MS’s needs to be able to
distinguish which time slot is up, and when to transmit. This channel periodically transmits a
distinguishable code that each MS synchronizes with. All the MS’s within the cells get the
same sense of time as the serving BS, but that “local” time will be different for all cells
within the net. Since all the BSs are asynchronous, it will allow the MSs to hear short periods
of control information from different BSs between the transmissions from the operating
BS. With this knowledge the mobile station can prepare for a handover if it would be
necessary.
15
CHAPTER THREE
FREQUENCY PLANNING 3.1 Radio Network Planning
The objective of network planning and design is to provide wireless telephony services in a
serving area in the most cost-effective manner. In the case of existing system, the objective is
to expand and augment its facilities so as to add new features and capabilities or increase its
capacity in case the system has reached its coverage limit. The design usually involves
determining the number of base stations and their locations that would provide the necessary
coverage in the serving area, meet the desired grade of service, and satisfy the required traffic
growth so that the total startup cost is minimized and the rate of return maximized. The
network planning process and design criteria vary from region to region depending upon the
dominating factor, which could be capacity or coverage. The design process itself is not the
only process in the whole network design, and has to work in close coordination with the
planning processes of the core and especially the transmission network. A simplified process
just for radio network planning is shown in Figure 3.1.
The process of radio network planning starts with collection of the input parameters such as
the network requirements of capacity, coverage and quality. These inputs are then used to
make the theoretical coverage and capacity plans. Definition of coverage would include
defining the coverage areas, service probability and related signal strength. Definition of
capacity would include the subscriber and traffic profile in the region and whole area,
availability of the frequency bands, frequency planning methods, and other information such
as guard band and frequency band division. The radio planner also needs information on the
radio access system and the antenna system performance associated with it. The pre-planning
process results in theoretical coverage and capacity plans. There are coverage-driven areas
and capacity-driven areas in a given network region. The average cell capacity requirement
per service area is estimated for each phase of network design, to identify the cut-over phase
where network design will change from a coverage-driven to a capacity-driven process.
While the objective of coverage planning in the coverage-driven areas is to find the minimum
number of sites for producing the required coverage, radio planners often have to experiment
with both coverage and capacity, as the capacity requirements may have to increase the
number of sites, resulting in a more effective frequency usage and minimal interference.
16
Candidate sites are then searched for, and one of these is selected based on the inputs from
the transmission planning and installation engineers.
After site selection, assignment of the frequency channel for each cell is done in a manner
that causes minimal interference and maintains the desired quality. Frequency allocation is
based on the cell-to-cell channel to interference (C/I) ratio. The frequency plans need to be
fine-tuned based on drive test results and network management statistics. Parameter plans are
drawn up for each of the cell sites. There is a parameter set for each cell that is used for
network launch and expansion. This set may include cell service area definitions, channel
configurations, handover and power control, adjacency definitions, and network-specific
parameters.
The final radio plan consists of the coverage plans, capacity estimations, interference plans,
power budget calculations, parameter set plans, frequency plans, etc.
Fig 3.1 Radio network planning process
3.1.1 The Automatic Frequency Planning (AFP) Problem
The frequency planning is the last step in the layout of a GSM network. Prior to tackling this
problem, the network designer has to address some other issues: where to install the BTSs or
how to set configuration parameters of the antennas (tilt, azimuth, etc.), among others. Once
the sites for the BTSs are selected and the sector layout is decided, the number of TRXs to be
installed per sector has to be fixed. This number depends on the traffic demand which the
corresponding sector has to support. The result of this process is a quantity of TRXs per cell.
A channel has to be allocated to every TRX and this is the main goal of the AFP. Essentially,
three kinds of allocation exist: Fixed Channel Allocation (FCA), Dynamic Channel
Allocation (DCA), and Hybrid Channel Allocation, discussed later in this report. In FCA, the
channels are permanently allocated to each TRX, while in DCA the channels are allocated
dynamically upon request. Hybrid Channel Allocation schemes (HCA) combine FCA and
DCA.
17
The most important parameters to be taken into account in GSM frequency planning are to be
explained. Consider the example network shown in Fig. 3.2, in which each site has three
installed sectors (e.g. site A operates A1, A2, and A3). The first issue is the implicit topology
which results from the previous steps in the network design. In this topology, each sector has
an associated list of neighbors containing the possible handover candidates for the mobile
residing in a specific cell. These neighbors are further distinguished into first order (those
which can potentially provoke strong interference to the serving sector) and second order
neighbors. In Fig. 3.2, A2 is the serving sector and the first order neighbors defined are A1,
A3, C2, D1, D2, E2, F3, G1, G2, and B1’’’, whereas, if we consider C2, second order
neighbors of A2 are F1, F2, C1, C3, D2’, D3’, A3’’, B1’’, B3’’, G1’’, G3’’, and E1’’’.
Fig 3.2 An example of GSM network
As stated before, each sector in a site defines a cell; the number of TRXs installed in each cell
depends on the traffic demand. A valid channel from the available spectrum has to be
allocated to each TRX. Due to technical and regulatory restrictions, some channels in the
18
spectrum may not be available in every cell. They are called locally blocked and they can be
specified for each cell. Each cell operates one Broadcast Control CHannel (BCCH), which
broadcasts cell organization information. The TRX allocating the BCCH can also carry user
data. When this channel does not meet the traffic demand, some additional TRXs have to be
installed to which new dedicated channels are assigned for traffic data. These are called
Traffic CHannels (TCHs). In GSM, significant interference may occur if the same or adjacent
channels are used in neighboring cells. Correspondingly, they are named co-channel and
adjacent channel interference. Many different constraints are defined to avoid strong
interference in the GSM network. These constraints are based on how close the channels
assigned to a pair of TRXs may be. These are called separation constraints, and they seek to
ensure the proper transmission and reception at each TRX and/or that the call handover
between cells is supported. Several sources of constraint separation exist: co-site separation,
when two or more TRXs are installed in the same site, or co-cell separation, when two TRXs
serve the same cell (i.e., they are installed in the same sector).
3.1.2 The Cellular Concept and Frequency Reuse
The radio access part of the wireless network is considered of essential importance as it is the
direct physical radio connection between the mobile equipment and the core part of the
network. In order to meet the requirements of the mobile services, the radio network must
offer sufficient coverage and capacity while maintaining the lowest possible deployment
costs. One main issue in cellular system design reduces to one of economics. Essentially there
is limited resource transmission spectrum that must be shared by several users. Unlike wired
communications which benefits from isolation provided by cables, wireless users within close
proximity of one another can cause significant interference to one another. To address this
issue, the concept of cellular communication was introduced, where a given geographic area
is divided into hexagonal cells.
Each cell is allocated a portion of the total frequency spectrum. As users move into a given
cell, they are then permitted to utilize the channel allocated to that cell. The virtue of the
cellular system is that different cells can use the same channel given that the cells are
separated by minimum distance according to the system propagation characteristics,
otherwise intercellular or co-channel interference occurs. The minimum distance necessary to
reduce co-channel interference is called the reuse distance. The reuse distance is defined as
the ratio of the distance, D, between cells that use the same channel without causing
19
interference and the cell radius, R as shown in figure 3.3. R is the distance from the centre of
a cell to the outermost point of the cell in cases where the cells are not circular. Typically
spatial separation, either in distance, angle, or polarization of electromagnetic fields is
exploited for frequency reuse. In such a system capacity per unit area must be sufficient for
the density of users and their usage patterns.
D = Reuse distance
r = radius of cell
Figure 3.3 7-cell cluster
3.1.3 Selection of Frequency Reuse Patterns
Patterns
A pattern is a number of cells grouped together, with each cell allocated a certain number of
channels, which are pairs of two frequencies to enable full-duplex communication. This
entire group of cells is known as a cluster. One cluster serves a complete set of frequencies
ranging from the entire allocated spectrum of the operator. The cluster pattern is then
repeated throughout the required coverage area.
D
ij
r
20
Patterns come in fixed numbers, and are derived from the formula, N = i² + ij + j² (refer to
Appendix A). Typical cluster sizes include 3, 4, 7, 9, 12, 19 and 21; with the most common
configuration being a 7-cell cluster, shown in figure 3.3.
Factors of Consideration
Factors to consider when selecting patterns include co-channel interference, which is the
radio interference caused by placing two cells, which have been allocated the same channel,
too close together. This causes deterioration of signal quality and in severe cases, might cause
a call to be temporarily or permanently disconnected, affecting the Grade of Service of the
operator.
The minimum distance required between the centers of two cells, using the same channel to
maintain the desired signal quality, is known as the reuse distance (refer to Appendix A). The
longer the reuse distance, the smaller the co-channel interference level will be. However, a
reuse distance that is too long increases the number of cells per cluster, which in turn results
in lower reuse efficiency and less system capacity.
Thus, the frequency reuse pattern should be determined taking into consideration both the co-
channel interference level and the reuse efficiency.
Determining Cluster Size
As mentioned above, the formula N = i² + ij + j² is used to derive possible values of a cluster
size. However, to find exact or more accurate values given certain conditions, formula 3.1 is
used instead:
(3.1)
This formula takes into account the desired carrier power (C), the signal power of interferers
(Ii), the radius of cells (R), the frequency reuse distance to the interferers (Di) and path loss
exponent (γ). Using this formula and the respective parameters, operators can then better plan
their coverage areas by fine tuning those factors.
21
Implementation
In the course of preliminary planning, an operator would have thoroughly researched upon
and determined appropriate operational parameters such as the coverage radius of each cell,
placement of cells, and most importantly, the selection of the frequency reuse pattern derived
from the research and formula 3.1.
After being allocated a spectrum of frequencies from the respective regulator, the operator
will have to implement the plan by first determining the number of channels it can provide.
Then, the operator distributes the channels evenly among the cell cluster. The total number of
cells required for the desired coverage area is then calculated and the chosen cluster pattern is
repeated throughout the coverage area.
Testing will then be done to ensure the levels of co-channel interference and external
interference remain negligible, and following which the network will be fully functional for
commercial use.
Application
Due to the prevalence of frequency reuse, there is rarely a need to create an environment to
simulate application. The main problem that frequency reuse eliminates is the limitation of
allocated spectrum width to accommodate a higher number of mobile subscribers. Once
operators have chosen to implement frequency reuse, there will be an essential need to
strategically plan the distribution of cells and channels, and hence, decisively select a
frequency reuse pattern.
3.2 The Mobile Environment and Interference
Radio or wireless path normally described in wireless systems corresponds to the radio link
between a mobile user station and the base station with which it communicates. It is the base
station that is, in turn, connected to the wired network over which communication signals will
travel. Modern wireless systems are usually divided into geographically distinct areas called
cells, each controlled by a base station.
The focus here is on one cell and the propagation conditions encountered by signals
traversing the wireless link between the base station and mobile terminal. The link is made up
of two-way path: a forward path, downlink, from base station to mobile terminal; a backward
path, uplink, from mobile terminal to base station. The electromagnetic signals generated at
22
either end will often encounter obstacles during the transmission, causing reflection,
diffraction, and scattering of the waves. The resultant electromagnetic energy reaching an
intended receiver will thus vary randomly. As a terminal moves, changing the conditions of
reception at either end, signal amplitudes will fluctuate randomly, resulting in fading of the
signal. The rate of fading is related to the relative speed of the mobile with respect to the base
station, as well as the frequency (or wavelength) of the signal being transmitted.
The important variables that define the mobile environment for a particular area include:
a) Physical terrain (Mountainous or hilly or flat or water).
b) Number, height, arrangement, and nature (construction materials) of man-made
structures.
c) Foliage and vegetation characteristics.
d) Normal and abnormal weather conditions.
e) Man-made radio noise.
Link budget defines the quality of the radio link, measured in terms of decibels (dB) or dBm,
where the ‘m’ stands for milliwatts, of signal power. A simplified link budget is given in
equation 3.2.
rtbmtPS GGLLLFLG −−++++= (3.2)
Gs = system gain in decibels
Lp = free space path loss in dB
F = fade margin in dB
Lt = transmission line loss from waveguide or coaxials used to connect radio to antenna, in
dB
Lm = miscellaneous losses such as minor antenna misalignment, waveguide corrosion, and
increase in receiver noise figure due to ageing, in dB
23
Lb = branching loss due to filter and circulator used to combine or split transmitter and
receiver signals in a single antenna.
Gt = gain of transmitting antenna
Gr = gain of receiving antenna.
3.2.1 Adjacent Channel Interference
The carrier to adjacent ratio is defined as the signal strength ratio between a serving carrier
and an adjacent carrier. Interference from adjacent channels may sometimes be serious in a
mobile radio environment. Consider for example figure 3.4 where two mobile stations, MS1
and MS2, are transmitting on two adjacent channels. If the distance r2 from mobile MS2 to
the BTS is, say 10 times or more greater than the distance r1 of MS1, the signal received at
the BTS from MS1 may be over 40 dB higher than the signal from MS2. Furthermore,
because of fading, it is possible that the signal from MS2 is in a deep fade while the other
signal is above its local mean. Consequently, it becomes necessary to assign the channels in
such a way that there are no adjacent channels in the same set. This can be done in the
following way. Suppose that there are only 21 voice channels in the available spectrum block.
The channels may then be divided into seven sets, each with three channels, using channels
1,8, and 15 in set 1; channels 2, 9, and 16 in set 2; channels 3, 10, and 17 in set 3; and so on.
Figure 3.4 Adjacent channel interference
24
However, in the previous assignment, even though no cell contains an adjacent channel, any
two adjacent cells will always have some adjacent channels between them. For example, cell
1 has channels 1, 8, and 15, and cell 2 has channels 2, 9, and 16. If base stations are located at
the center of a cell and use omnidirectional antennas, adjacent channels will cause
interference in any cell. This can be avoided by use of sectorised cells. By simply allocating
the channels to the directional antennas in a specific way, the interference due to adjacent
channels can be reduced to a satisfactory level. It is also possible to control adjacent channel
interference completely through filtering of both the transmitter and receiver to ensure that no
energy is transmitted outside the desired band and that the receiver filters out any unwanted
out-of-band energy.
3.2.2 Co-channel Interference
It is well known that one of the major limitations in cellular wireless telephone networks is
the so-called co-channel interference. In the case of TDMA networks, such as GSM/GPRS,
the co-channel interference is mainly caused by the spectrum allocated for the system being
reused multiple times (“frequency reuse”). The problem may be more or less severe,
depending on the reuse factor, but in all cases, a signal received by a handset will contain not
only the desired forward channel from the current cell, but also signals originating in more
distant cells.
The carrier-to-channel interference ratio (C/I) is the fundamental parameter in calculations of
reuse factors. It determines the degree of reuse and spectrum efficiency that can be achieved
in a cellular architecture. It is not possible to reuse frequencies in adjacent cells as the
boundary conditions would be characterized by nearly balanced signal levels from competing
transmitters and no communication would be possible. It is the isometric contour of co-
channel interference, not of signal level, that defines the cell boundary. The amount of
separation between cells using the same frequency is driven by;
a) The C/I ratio that is required to achieve the desired transmission quality.
b) The fade margin that is necessary to take care of statistical fluctuations in desired
signal level induced by the mobile environment. The fade margin is determined by
the maximum fades likely to be experienced, given the particular set of
countermeasures e.g. antenna diversity, being employed by the receiver.
25
3.2.3 Methods of Reducing Co-channel Interference
Cellular systems are limited by interference. In these systems, multiple co-channel
interference, though controlled, is a normal situation, and is the main factor to determine the
service area/capacity. The goal is to allow the higher interference level in order to reuse the
available frequencies within the smallest area. As quality of service depends on the
carrier/interference ratio (CIR) more than on the signal/noise ratio, there is a certain trade-off
between quality and capacity that can be tolerated by the system.
Mobile markets have the larger growing rates among telecommunication markets. For this
growing rate to continue, higher levels of capacity and quality are needed. This means that all
possible techniques must be used to improve such features in order to reach a progressive
enhancement of the radio and network performance. Current implementation of GSM has
some powerful mechanisms intended to reduce the effect of interference such as slow
frequency hopping (SFH), discontinuous transmission (DTX) and power control, among
others.
1. Slow Frequency Hopping(SFH)
Slow Frequency Hopping (SFH) involves changing the frequency of the channel in every
transmitted burst (217 hops per second) providing frequency diversity and interference
averaging. This allows randomising the risk of interference and improving the behaviour of
the channel (for the selective fadings). The frequency hopping can be classified in two
categories: base band hopping and synthesiser hopping. Synthesiser hopping uses only one
transmitter for all burst belonging to a specific connection and the base band hopping uses as
many transmitter frequencies in the hopping sequence as possible. Synthesiser hopping is
more efficient and flexible. If we consider the way of changing the frequency, the hopping
can be also cyclic or random.
There are many factors affecting the performance of the SFH which include;
• Number of hopping frequencies: The higher the number of hopping frequencies the
better the system performance as it improves frequency diversity. However, using
more than 8 hopping frequencies does not provide a significant improvement due to
the GSM interleaving period of 8 bursts.
26
• Hopping frequencies separation: The larger the frequency separation between the
hopping frequencies, the better the system performance as the effects of propagation
become more uncorrelated.
• System load: It has a direct influence in the SFH performance. Low system load
means lower interference probability in each hopping frequency and therefore, more
benefits from SFH.
• Frequency plan: When SFH is activated, a conventional frequency reuse scheme
based on a worst case interference situation is spectrally inefficient. Results show the
limitations of different frequency plans introducing this additional parameter in the
analysis, proposing a reuse scheme. Tighter reuse schemes can be achieved with the
use of the SFH providing more capacity.
2. Discontinuous Transmission(DTX)
DTX is used to suspend the radio transmission during the silence periods. This exploits the
observation that only 40-50% during a conversation does the speaker actually talk. DTX
helps also to reduce interference between different cells and to increase system capacity. It
prolongs battery charge life. The DTX function is performed by means of:
• Voice Activity Detection (VAD), which has to determine whether the sound
represents speech or noise, even if the background noise is very important. If the voice signal
is considered as noise, the transmitter is turned off producing then, an unpleasant effect called
clipping.
• Comfort noise. A side-effect of the DTX function is that when the signal is
considered as noise, the transmitter is turned off and therefore, a total silence is heard at the
receiver. This can be very annoying to the receiving user since it appears as a dead
connection. In order to overcome this problem, the receiver creates a minimum of
background noise called comfort noise. Comfort noise eliminates the impression that the
connection is dead.
3. Power Control
The power that is transmitted both from the mobile equipment and from the base station has a
far-reaching effect on efficient usage of the spectrum. Power control is an essential feature in
27
mobile networks, in both the uplink and downlink directions. When a mobile transmits high
power, there is enough fade margin in the critical uplink direction, but it can cause
interference to other subscriber connections. The power should be kept to a level that the
signal is received by the base station antenna above the required threshold without causing
interference to other mobiles. Mobile stations thus have a feature such that their power of
transmission can be controlled. This feature is generally controlled by the BSS. This control
is based on an algorithm that computes the power received by the base station and, based on
its assessment, it increases or decreases the power transmitted by the mobile station.
3.2.4 Path loss
Path loss (or path attenuation) is the reduction in power density (attenuation) of an
electromagnetic wave as it propagates through space. Path loss is a major component in the
analysis and design of the link budget of a telecommunication system. This term is
commonly used in wireless communications and signal propagation. Path loss may be due to
many effects, such as free-space loss, refraction, diffraction, reflection, aperture-medium
coupling loss, and absorption. Path loss is also influenced by terrain contours, environment
(urban or rural, vegetation and foliage), propagation medium (dry or moist air), the distance
between the transmitter and the receiver, and the height and location of antennas.
The strength of the transmitted signal decreases in power relative to the distance between the
transmitter and the receiver. The standard rule for the path loss is that the signal strength
decreases as a factor R-γ, where R is the distance and γ is an environment dependent variable,
called path loss exponent. The value of γ varies between around 2-6, and for normal urban
environments the number is relatively close to 4. Lower values can appear in canyon-like
environments, for example at actual canyons or streets with tall buildings around.
Causes of Path Loss
Path loss normally includes propagation losses caused by the natural expansion of the radio
wave front in free space (which usually takes the shape of an ever-increasing sphere),
absorption losses (sometimes called penetration losses), when the signal passes through
media not transparent to electromagnetic waves, diffraction losses when part of the radio
wave front is obstructed by an opaque obstacle, and losses caused by other phenomena.
28
The signal radiated by a transmitter may also travel along many and different paths to a
receiver simultaneously; this effect is called multipath. Multipath can either increase or
decrease received signal strength, depending on whether the individual multipath wavefronts
interfere constructively or destructively. The total power of interfering waves in a Rayleigh
fading scenario vary quickly as a function of space (which is known as small scale fading),
resulting in fast fades which are very sensitive to receiver position.
Path Loss exponent
In the study of wireless communications, path loss can be represented by the path loss
exponent, whose value is normally in the range of 2 to 4 (where 2 is for propagation in free
space, 4 is for relatively lossy environments and for the case of full specular reflection from
the earth surface, the so-called flat-earth model). In some environments, such as buildings,
stadiums and other indoor environments, the path loss exponent can reach values in the range
of 4 to 6. On the other hand, a tunnel may act as a waveguide, resulting in a path loss
exponent less than 2.
Path loss is usually expressed in dB. In its simplest form, the path loss can be calculated
using the formula
CdL += 10log10γ (3.1)
where L is the path loss in decibels, γ is the path loss exponent, d is the distance between the
transmitter and the receiver, usually measured in meters, and C is a constant which accounts
for system losses.
3.3 Channel Assignment Strategies
Channel Allocation
The capacity of a cellular system can be described in terms of the number of available
channels, or the number of users the system can support. The total number of channels made
available to a system depends on the allocated spectrum and bandwidth of each channel. The
available frequency spectrum is limited and the number of mobile users is increasing day by
day, hence the channels must be reused as much as possible to increase the system capacity.
Once the channels are allocated, cells may then allow users within the cell to communicate
29
via the available channels. Channels in a wireless communication system typically consist of
timeslots, frequency bands and/or CDMA pseudo noise sequences, but in an abstract sense,
they can represent any generic transmission resource. There are three major categories for
assigning these channels to cells (or base stations). These are;
• Fixed Channel Allocation
• Dynamic Channel Allocation and
• Hybrid Channel Allocation.
3.3.1 Fixed Channel Allocation (FCA)
FCA systems allocate specific channels to specific cells. This allocation is static and cannot
be changed. For efficient operation, FCA systems typically allocate channels in a manner that
maximizes frequency reuse. Thus, in a FCA system, the distance between cells using the
same channel is the minimum reuse distance for that system. The problem with FCA systems
is quite simple and occurs whenever the offered traffic to a network of base stations is not
uniform. Consider a case in which two adjacent cells are allocated N channels each. There
clearly can be situations in which one cell has a need for N+k channels while the adjacent cell
only requires N-m channels (for positive integers k and m). In such a case, k users in the first
cell would be blocked from making calls while m channels in the second cell would go
unused. In this situation of non-uniform spatial offered traffic, the available channels are not
being used efficiently.
3.3.2 Dynamic Channel Allocation (DCA)
DCA attempts to alleviate the problem of FCA system when offered traffic is non-uniform. In
DCA systems, no relationship exists between channels and cells. Instead, channels are part of
a pool of resources. Whenever a channel is needed by a cell, the channel is allocated under
the constraint hat frequency reuse requirements can not be violated. There are two problems
that typically occur with DCA based systems.
• DCA methods typically have a degree of randomness associated with them and this
leads to the fact that frequency reuse is often not maximized unlike the case for FCA
30
systems in which cells using the same channel are separated by the minimum reuse
distance.
• DCA methods often involve complex algorithms for deciding which available channel
is most efficient. These algorithms can be very computationally intensive and may
require large computing resources in order to be real-time.
The role of DCA is to allocate channels to cells or mobiles in such a way as to minimize;
a) The probability that the incoming calls are dropped
b) The probability that ongoing calls are dropped
c) The probability that the carrier-to-interference ratio (C/I) of any call falls below a
prespecified value.
The goal is to maximize the frequency reuse. In DCA there is no permanent allocation of
channels to cells. Rather, the entire set of available channels is accessible to all the cells, and
the channels are assigned on a call-by-call basis in a dynamic manner.
DCA schemes can be implemented as centralized or distributed.
Centralized DCA schemes
In this scheme, all requests for channel allocation are forwarded to a channel controller that
has access to the system wide channel usage information. The central controller then assigns
the channel by maintaining the required signal quality. The First Available (FA) is the
simplest method where the first available channel within the reuse distance encountered
during a channel search is assigned to the new call. Future blocking probability is a very
important factor for the channel selection in a new call.
Distributed DCA schemes
In this scheme the decision regarding the channel acquisition and release is taken by the
concerned BTS on the basis of the information from the surrounding cells. As the decision is
not based on the global status of the network, it can achieve only suboptimal allocation
compared to the centralized DCA and may cause forced termination of ongoing calls. This
scheme is based on co-channel distance, signal strength measurement and signal to noise
interference ratio. Centralized schemes can theoretically provide near optimal performance,
31
but the amount of computation and communication among BTSs lead to excessive system
latencies and make them impractical. Therefore, distributed DCA schemes have been
proposed that involve scattering of channels across a network. A channel is selected for a new
call from its cell or interfering neighbouring cells.
3.3.3 Hybrid Channel Allocation
This includes all systems that hybrids of fixed and dynamic channel allocation systems.
Several methods fall within this category.
Channel Borrowing is one of the hybrid allocation schemes. Here, channels are assigned to
cells just as in fixed allocation schemes. If a cell needs a channel in excess of the channels
previously assigned to it, that cell may borrow a channel from one of its neighbouring cells
given that a channel is available and use of this channel won’t violate frequency reuse
requirements. Since every channel has a predetermined relationship with a specific cell,
channel borrowing is often categorized as a subclass of fixed allocation schemes. The major
problem with channel borrowing is that when a cell borrows a channel from a neighbouring
cell, other nearby cells are prohibited from using the borrowed channel because of co-channel
interference. This can lead to increased call blocking over time. To reduce this call blocking
penalty, algorithms are necessary to ensure that the channels are borrowed from the most
available neighbouring cells i.e. the neighbouring cells with the most unassigned channels.
Two extensions of the channel borrowing approach are Borrowing with Channel Ordering
(BCO) and Borrowing with Directional Channel Locking (BDCL).
• BCO systems have two distinctive characteristics;
1. The ratio of fixed to dynamic channels varies with traffic load.
2. Primary channels are ordered such that the first primary channel of a cell has the
highest priority of being applied to a call within the cell. The last primary channel is most
likely to be borrowed by neighbouring channels. Once a channel is borrowed, that channel is
locked in the co-channel cells within the reuse distance of the cell in question. From a
frequency reuse standpoint, in a BCO system, a channel may be borrowed only if it is free in
the neighbouring co-channel cells.
32
• In BDCL, borrowed channels are only locked in nearby cells that are affected by the
borrowing. The benefit is that more channels are available in the presence of borrowing and
subsequent call blocking is reduced.
A natural extension to channel borrowing is to set aside a portion of the channels in a system
as dynamic channels with the remaining (primary) channels being fixed to specified cells. If a
cell requires an extra channel, instead of borrowing the channel from a neighbouring cell, the
channel is borrowed from the common ‘bank’ of dynamic channels.
33
CHAPTER FOUR
PROPOSED FREQUENCY REUSE STRATEGY
4.1 The Frequency Reuse Pattern
A reuse pattern using clusters of 3 cells is proposed, where each of these 3 cells uses a
different subset of the available channels. Each cluster of cells uses the same channels in this
manner. Cells that use the same frequency are far enough apart that the interfering signals
received in any cell from co-channel cells are much weaker than the signals that originate
within that cell. For planning purposes, the cells are considered to be hexagonal, although in
practice their shapes are determined by radio coverage and they are irregular. For K-cell
frequency reuse, the distance between co-channel cells is KD 3= R where R is the cell
radius. In most systems, the interference is much stronger than the noise, and the SINR is
approximately equal to the carrier-to-interference ratio or C/I, which is used in the following
equations. The most significant interference comes from the six closest co-channel cells, so γ
⎟⎠⎞
⎜⎝⎛=
RD
IC
61 (4.1)
where γ is an empirically determined path loss exponent. For free space γ = 2. For suburban
and urban areas γ can be as high as 5 or 6, but is typically between 3 and 4. For the proposed
system model K=3 and γ is taken to be 3.3, thus D = 3R, and the C/I is calculated to be
3.3361
⎟⎠⎞
⎜⎝⎛=
RR
IC = 6.257 (power ratio) or (4.2)
10 log106.257 = 7.963 dB. (4.3)
This is far below the recommended GSM C/I of 9 dB and will cause significant co-channel
interference. In the proposed system therefore the cells are further divided into three sectors
each as shown in figure 4.3. For example cell A3 is divided into sectors A31, A32 and A33.
Each of these sectors is assigned a unique frequency to minimize adjacent channel and co-
channel interferences. Directional antennas are used so that each cell sector sees interference
primarily from two of the six closest co-channel cells. In this case C/I is given by
γ
⎟⎠⎞
⎜⎝⎛=
RD
IC
21 . (4.4)
34
This is a 4.8 dB improvement over the C/I of a system with non-sectorised cells. For a three
sector system with K=3 and γ=3.3, C/I is calculated to be
3.3321
⎟⎠⎞
⎜⎝⎛=
RR
IC = 18.77 (power ratio) or 12.73 dB, (4.5)
which is above the threshold of 9 dB that is required for GSM systems. By the central limit
theorem, as the number of interferers becomes large, the total interference will tend towards a
Gaussian distribution and will resemble AWGN. Using this, capacity is approximated as
[ ]SINRBC T += 1log2 (4.6)
where SINR ≈ C/I. A single frequency band can be reused throughout the area, and the
capacity per unit area becomes
[ ]2
2 1logRK
SINRBAC T
Π+
= (4.7)
where A = area. In practice, the same modulation and coding techniques are used throughout
a communication system, and each technique has a minimum SINR threshold that must be
exceeded to achieve satisfactory performance. To maximize capacity, the minimum K that
will provide acceptable SINR is used. From equation 4.7 it is seen that decreasing R will also
increase capacity per unit area, but this approach is very expensive because it requires more
base stations. Systems that use small cells can also experience higher interference levels
because the path loss exponent γ in carrier to interference equations tends to be distance-
dependent. As the cell radius is decreased, the path loss exponent γ approaches 2 because
unobstructed line-of-sight propagation is more likely in smaller cells. From both equations, it
can be seen that if γ decreases, C/I (and SINR) decrease if the cellular reuse factor K, and
hence the D/R ratio, are fixed. Thus, if the cell radius is decreased too much, K must be
increased to maintain acceptable SINR, and this reduces the capacity per unit area.
Approaches have been proposed that use adaptive antennas to reduce interference and allow a
smaller K and increase capacity.
4.2 System Model
Consider a geographical area divided into 6 × 6 hexagonal cells, each cell having a radius of
R as shown in figure 4.1. The distribution of channels among the cells in the system, based on
resource planning model with cluster size 3 is assumed. The cluster size 3 means all channels
35
in the allocated spectrum are assigned in three adjacent cells, if a channel is being used in any
cell, then none of the remaining two adjacent neighbouring cells can use this channel. The
considered minimum reuse distance (Dmin) is 3R, therefore the number of cells in interfering
neighbours is 6. If a channel r is being used in a cell then none of its interference
neighbouring cells can use r. When a MS wants to set up a call it sends request message to its
immediate BTS, if there exist some free primary channels in the requesting cell, then the BTS
will pick one in such a way that there will be no co-channel interference and channel
utilization is maximized.
If a channel is allocated for supporting the communication session between the MS and BTS,
then two cells can use the same channel only when the physical distance is not less than the
threshold distance Dmin, otherwise their communication session interfere with each other i.e.
co-channel interference.
Some of the channel selection schemes use resource planning model to get better channel
reuse for which the prior knowledge of channel status is required. The rules for using
resource planning model are as follows;
a) The set of cells is divided into a number of disjoint subsets such that any two cells in
the same subset are physically separated with at least a minimum reuse distance. The set of
channels are also divided into equivalent disjoint subsets.
b) The channels in the disjoint subsets are primary channels of cells in the subset and
secondary channels to another subset.
c) When all primary channels are exhausted, then a cell requests the secondary channels.
36
Fig 4.1 The cellular system model
4.3 The proposed Algorithm
The cells are divided into 3 linear disjoint subsets i.e. A, B, C. Each of these 3 cells uses a
different subset of the available channels. Each cell has 6 neighbours. Consider a total of 18
channels available, hence each cell has 6 primary channels with the following channel
numbers;
Subset A: 1, 4, 7, 10, 13, 16
Subset B: 2, 5, 8, 11, 14, 17
Subset C: 3, 6, 9, 12, 15, 18
Channels 1, 2 and 3 are BCCHs and are used to broadcast the base station identity and
channel availability information. The subsets are disjointed to avoid adjacent channel
interference. The following assumptions are made;
• The terrain of the considered geographical area is flat such that channel fading is not
accounted for in this system.
• Neighbours of each cell in the system are defined. For example the neighbours of cell
A3 are defined as B1, C1, B3, C3, B5, and C5 as shown in figure 4.2.
37
Figure 4.2 Defining cell neighbours
• All BTSs transmit at the same power and they are centrally located in the cell.
• Directional antennas are used in the sectorised cells where each cell has 3 sectors as
shown in figure 4.3, where each sector is assigned two channels hence two TRXs
installed.
Figure 4.3 sectorised system model
38
A cell, say A3, needs a channel to support a call request. It first checks free primary channels,
if available it picks the least and marks it as used channel and supports that call immediately.
Otherwise A3 changes to search mode and sends request to all its interfering neighbours, Nx
(i.e. B1, C1, B3, C3, B5, and C5). When a cell in Nx receives a request from A3, it sends a
reply Px-Ux=Bx , where Px are the primary channels in that cell, Ux are the channels in use and
Bx are the unused channels which can be borrowed. When A3 receives reply messages from
Nx , it selects a channel randomly from the largest Bx or if two or more Bxs are equal, it
selects a channel from the first to arrive.
A3 takes confirmation of selected channel to the lender which marks this channel as an
interference channel; the channel cannot be used until it is returned by the borrower. The call
is blocked if no free available channel is left. This is depicted in the flow chart of figure 4.4.
40
4.4 Explanation of the Algorithm
A channel allocation algorithm includes a channel acquisition and a channel selection
scheme. Most distributed DCA algorithms are based on non-resource planning model in
which a borrower needs to consult with every interfering neighbor in order to borrow a
channel. The proposed algorithm makes efficient reuse of channels. The purpose of this
scheme is to assign channels in such a way so that channel utilization is maximized at the
same time maintaining the voice quality.
Channel acquisition
The task of channel acquisition phase is to collect information of free available channels
from the interfering cells and ensure that two cells within the minimum reuse distance do not
share the same channel.
Channel selection
The channel selection phase is concerned with choosing a channel from the number of
available free channels in order to get better channel utilization in terms of channel reuse. The
acquisition phase of the distributed DCA algorithm consists of two approaches namely search
and update. In search approach, when a cell requires a channel, it searches from all its
interfering neighbours to find the currently free available channels from which it selects one
channel according to the underlying selection scheme. In the update approach, a cell
maintains information about free available channels. When a cell requires a channel, the
channel selection scheme is used to pick one available channel and confirms with its
interfering neighbouring cells whether it can use the selected channel or not. After that, when
a cell acquires or releases a channel at any time, it informs its interference neighbours so that
every cell in the system model always knows the available channels of its interference
neighbouring cells. Fast carrier returning should be used to increase channel utilization by
returning the borrowed channels as soon as possible. Consider a cluster size of 3 cells for
example. The number of channel sets is divided into 3 channel disjoint subsets. When a cell
needs to borrow a channel, it has to send request message to its 6 interfering neighbours.
41
CHAPTER FIVE
SIMULATION RESULTS AND ANALYSIS
5.1 Description of Results
One of the most important characteristic of a cellular network is that a certain number of calls
will fail to establish an initial connection. Two measures can be stated for dealing with
unsuccessful call attempts: the never serviced new calls, and the delayed successful calls
(waiting for initial connection). The blocking probability is one of the most important
characteristics for the performance of a cellular network. When a new call arrival occurs and
the network cannot allocate a channel then we say that this call is blocked. The blocking
probability Pbl is calculated from the ratio
receivedcallscallsblockedPBL _
_= . Table 5.1 illustrates the blocking probabilities of the proposed DCA
scheme and FCA scheme which is currently being used by cellular network operators in the
country. These results were generated by simulating the program code dca.m in matlab. The
code is illustrated in Appendix C.
Table 5.1 Blocking probability simulation results Number of calls
Arriving in one cell
Number of calls
blocked in DCA
scheme (nb)
Number of calls
blocked in FCA
scheme (nb1)
Blocking
probability
In DCA (pbl)
Blocking
probability
In FCA (pbl1)
152 5 49 0.0329 0.3224
165 9 48 0.0545 0.2909
170 11 52 0.0647 0.3059
173 12 62 0.0694 0.3584
180 7 71 0.0389 0.3944
213 15 87 0.0704 0.4085
216 11 90 0.0509 0.4167
218 17 84 0.0780 0.3853
226 18 94 0.0796 0.4159
241 8 107 0.0331 0.4440
42
The results in table 5.1 were used to plot the blocking probability curve of figure 5.1 for
comparison of the DCA and FCA frequency reuse schemes.
150 160 170 180 190 200 210 220 230 240 2500
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
prob
abili
ty o
f blo
ckin
g
number of calls arriving in one cell
Blocking probability vs call arrival
FCA scheme
DCA scheme
Figure 5.1 Blocking probability graph
43
5.2 Analysis
The blocking probability of the proposed DCA algorithm is compared with that of FCA. The
DCA algorithm significantly reduces the call blocking rate, due to the channel borrowing and
reduction of the cluster size in resource planning model. In DCA scheme proposed the
number of cell congestion is smaller in contrast with FCA scheme, which means that the
network management is easiest in DCA.
The dropping probability is also an additional and very important characteristic of the cellular
network. When a call is in progress and the required quality conditions are not met then his
call is obligatory driven to termination. The dropping probability Pd is calculated from the
ratio
)__()__(__
numbercalldroppednumbercalltotalnumbercalldroppedPd −
= . This was taken care by
sectorization of the cells to improve the C/I ratio.
44
CHAPTER SIX
CONCLUSIONS AND RECOMMENDATIONS
6.1 Conclusions of the results
In conclusion, this report has described the process of selecting frequency reuse patterns and
implementing them in mobile networks. It has discussed the required considerations in proper
planning, and the key factors in determining appropriate parameters to build a feasible and
robust network. The proposed channel allocation algorithm makes efficient reuse of channels
using resource planning model with reduction of the cluster size. The simulation results
shows that the blocking probability of the proposed DCA algorithm is significantly less than
that of FCA algorithm. The divergence between simulation results and real network behavior
is affected by the structure of the implementation algorithms and the corresponding research
of real network conditions like new call arrival schemes, user movement, etc. A deeper
research requires full customizable software with high level of adaptation in real network
specifications and behavior.
Future Development
As the selection of frequency reuse patterns is more of a concept and process rather than a
technology itself, the continued development into this area is dependent on the individual
standardization bodies or manufacturers who develop the standards. The results obtained are
promising but not conclusive. Comparison with other dynamic frequency allocation
algorithms is warranted. A solution similar to the one demonstrated here might one day be
considered by a cellular provider or implemented in an actual cellular network.
In future as these cellular networks become larger and more complex, extra functions could
be added to the proposed algorithm. These would include mobile environment effects and
different base station locations for a better generalization of the proposed algorithm and extra
functions for more reliable testing and frequency assignment. An algorithm to get a more
self-calibrating network of base stations could be developed to deploy the network that has
implemented the frequency planning algorithm proposed in this project. When the network is
up and running we could have all the base stations registering and saving the received SINRs
during a period of time. Using this data, a more sophisticated algorithm could be developed
that tunes the antennas in a similar manner to what is done by hand nowadays to make the
network close to self operating.
45
CHAPTER SEVEN REFERENCES AND APPENDICES
References
[1] Mischa Schwartz, Mobile Wireless Communications, Cambridge University Press, 2005.
[2] J. Dunlop and D.G. Smith, Telecommunications Engineering, 3rd edition,
Chapman & Hall, University and Professional Division, 1994.
[3] Razavi B., RF Microelectronics, Prentice-Hall Inc., USA, 1998.
[4] Calhoun George, Digital Cellular Radio, Artech House, Inc, Norwood, USA, 1988.
[5] I.A. Glover and P.M. Grant, Digital Communications, Pearson Education Limited,
2nd edition, 2004.
[6] G.S.O. Odhiambo, FEE 522 Lecture Notes, Communication Networks and Information
Theory, University of Nairobi, 2009.
[7] P.M. Papazoglou, D.A. Karras, and R.C. Papademtriou, “A Dynamic Channel
Assignment Simulation System for Large Scale Cellular Telecommunications”, IEEE
Personal Communications, vol.5, No.1, 2004, pp. 10-31
[8] T.S. Rappaport, Wireless Communications: Principles and Practice, Prentice Hall, 2002.
[9] M. Zhang, and T.S. Yum, “Comparisons of Channel Assignment Strategies in Cellular
Mobile Telephone Systems”, IEEE Transactions on Vehicular Technology, vol.38, No.4,
pp211-215, 1989.
[10] M. Gupta, and A.K. Sachan, “Distributed Dynamic Channel Allocation Algorithm for
Cellular Mobile Network”, Journal of Theoretical and Applied Information Technology,
vol.1, No. 1, pp58-63, 2007.
46
Appendices
Appendix A: Properties and Fundamentals of the Cellular Geometry
Let Radius of each cell = R
Let Distance between center of adjacent cells = d
Let Distance between centers of Co-channel Cells = D
Step 1: To find the relation between radius of cell (R) and distance d
Fig A1 Hexagonal geometry
From the figure A1,
CI = R/2; OI = d/2 and OC = R (A1)
By Pythagoras theorem
222 CIOIOC += (A2)
Applying equations (A1) into (A2),
47
222
22⎟⎠⎞
⎜⎝⎛+⎟
⎠⎞
⎜⎝⎛=
RdR (A3)
Making d the subject of the formula,
Rd 3= (A4)
The figure A2 shows the most convenient set of coordinates for hexagonal geometry. The
positive halves of the two axes intersect at a 60° angle, and the unit distance along any of the
axis equals 3 R. The radius is defined as the distance from the center of a cell to any of its
vertices. Based on this, the center of each cell falls on a point specified by a pair of integer
coordinates.
Fig A2 Coordinates for hexagonal geometry
The first thing to note is that in this coordinate system the distance d12 between two points (U1,
V1) and (U2, V2) respectively is:
( ) ( ) ( )( ) °−−+−+−= 60cos2 12122
122
122
12 VVUUUUVVd (A5)
= ( ) ( ) ( )( )21*2 1212
212
212 VVUUUUVV −−+−+− (A6)
48
= ( ) ( ) ( )( )12122
122
12 VVUUUUVV −−+−+− (A7)
Therefore:
( ) ( ) ( )( )][ 12122
122
1212 VVUUUUVVd −−+−+−= (A8)
Using this we can easily verify that the distance between the centers of adjacent cells is unity
and the length of a cell radius R is:
31
=R . In general, as we showed before if the distance between the centers of adjacent cells
is d, then:
3dR = (A9)
Step 2: To find the distance between the centers of large Clusters (D)
Let us have a cell at (0, 0)
Let us have Co-channel Cell at (i, j)
By using Distance formula between two cells (as described before and assuming the previous
coordinate system);
( ) ( )( )jijiD *00 222 +−+−= (A10)
( ) ( )( )jijiD *00 22 +−+−= (A11)
Step 3: To find the radius of the cluster (Rc)
3DRC = (A12)
3
22 ijjiRC
++= (A13)
49
Step 4: To find the number of cells in a cluster (K) (i.e. cluster size)
K = Cluster Area/Cell Area (A14)
Single Cell Area = 6 * (Area of 1 triangle with equal edges R)
= ]2
*21[*6 dR (A15)
From (A4) we have that Rd 3= . Therefore:
Single Cell Area = 2*2
33]321*
21[*6 RRR = (A16)
Following similar reasoning the cluster area is:
Cluster Area = 2*2
33CR (A17)
Substituting equation (A16) and (A17) in (A14) we get,
2
2
RR
K C= (A18)
By substituting (A13) and (A4) in (A18) (assuming unit distance d=1) we get:
ijjiK ++= 22 (A19)
Step 5: To find Co-channel Reuse Ratio / Factor (Q)
Q = D/R (A20)
Substituting equation (A19), (A11) and (A4) in (A20) and also taking unit distance (i.e. d =1),
we get,
KQ 3= (A21)
50
Appendix B: Dynamic Channel Allocation pseudocode.
Partition coverage area and assign channels to cells.
on {call arrival,} {
if ( primary channel available ) {
use smallest channel
}
else {
if (secondary channel available ) {
borrow largest from neighbour with maximum free channels
}
else {
block call
}
}
}
on { call termination } {
if ( secondary channel borrowed ) {
reassign and return
}
}
51
Appendix C: Matlab simulation code for DCA and FCA schemes
%Channel allocation simulation code
sumnA3=0; %sum of calls arriving in cell A3 after a number of iterations
sumnb=0; % sum of calls blocked in DCA scheme in cell A3
sumnb1=0; % sum of blocked calls in FCA scheme for an equal number of calls
%arriving in cell A3
n1=1;
while n1 <= 25 % Defines number of iterations from 1 to 25
nA3=int16(rand(1)*15) %generates random number of calls in reference
%cell A3 i.e. nA3
%Define neighbours of cell of consideration (A3)and generate random number
%of calls in the neighbours of cell A3
nB1=int16(rand(1)*15);%number of calls in cell B1
nC1=int16(rand(1)*15);%number of calls in cell C1
nB3=int16(rand(1)*15);%number of calls in cell B3
nC3=int16(rand(1)*15);%number of calls in cell C3
nB5=int16(rand(1)*15);%number of calls in cell B5
nC5=int16(rand(1)*15);%number of calls in cell C5
sumnA3=sumnA3+nA3;
52
%Channel Allocation through DCA scheme
%Declare condition to be met for supporting a call using primary channels
if nA3<=6
disp('Channel assignment in DCA scheme successful through primary channels');
else
if (nB1<6 || nC1<6 || nB3<6 || nC3<6 ||nB5<6 ||nC5<6 ) % condition of
%neighbour cells to lend free channels
n=nA3-6; % number of borrowed channels
disp ('Channel assignment in DCA scheme successful through borrowed channels');
else
nb = nA3-6 % If no free secondary channel available, call(s) is/are blocked
disp('calls blocked');
sumnb=sumnb+nb;
end
n1=n1+1;
end
%Fixed channel allocation (FCA) scheme
if nA3>6 % Condition for blocking calls in FCA scheme
disp('Corresponding number of calls blocked in FCA scheme is')
nb1=nA3-6 %number of blocked calls in one iteration
53
sumnb1=sumnb1+nb1;
else
disp('Channel assignment in FCA scheme successful')
n1=n1+1;
end
end
%Display results
disp ('Total number of calls recieved')
disp(sumnA3) %Number of calls arriving in one cell (A3)
disp ('number of calls blocked in DCA scheme')
disp (sumnb) %Number of calls blocked in DCA scheme
disp ('number of calls blocked in FCA scheme')
disp (sumnb1) %Number of calls blocked in FCA scheme