Frequency Planning 4 GSM

8/7/2019 Frequency Planning 4 GSM

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KUNGL TEKNISKA HÖGSKOLANInstitutionen förSignaler, Sensorer & SystemSignalbehandling100 44 STOCKHOLM

ROYAL INSTITUTEOF TECHNOLOGY

Department ofSignals, Sensors & Systems

Signal ProcessingS-100 44 STOCKHOLM

Autonomous Frequency Planning for GSM

Networks

Niklas Jalden

February 2004

IR–SB–EX–0404



1



Abstract

Frequency planning is one of the more expensive aspects of deploying a cellularnetwork. If a set of base stations can be deployed with minimal service andplanning, the cost of both deploying and maintaining the network will decrease.

This thesis explores the problem of designing an algorithm that lets base stationsdetermine their own parameters with respect to other base stations. This devel-opment is considered for a frequency and time division multiple access (FDMAand TDMA) system with a limited number of bands that requires different fre-quency allocations for each base station to mitigate intercell interference. Thegoal of this thesis was to design an algorithm that determines the appropriatefrequency band for a base station to use based on data it received from listen-ing to other base stations. Because most network systems are not designed forbase-station-to-base-station communication, periodic training times when basestations can communicate with each other had to be inserted. The task includesboth algorithm development and suggestions on how to transmit the messagesbetween the stations. The output from this project will include: A description

of the algorithm, a discussion about roughly how much information needed tobe shared between base stations and how often the training should take placeis given.

i



Acknowledgements

This master thesis was performed at the Department of Signals, Sensors andSystems (S3) at the Royal Institute of Technology (KTH), between June 2003and December 2003.

To begin with I would like to thank my supervisor for this thesis Dr. SarahKate Wilson for all the help and support during my work. I want to send bigthanks to Wireless@KTH for awarding me with a stipend for this thesis. It wasof great help and inspired me to put extra hours of work into this project. Iwould also like to thank my friend Niklas Lithammer for help with ideas andproof reading this thesis report. Big thanks to Cidney Lau, my father andmy brother Joakim for proofreading and giving comments. Extra gratitude toJoakim for the crash course in LATEX. Last but not least I would like to thankmy family and all my friends for the support during all my studies. It wouldnot have been possible to manage everything without you.

iii



Contents

1 Introduction 1

1.1 Purpose of pro ject . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Background 3

2.1 Shared medium schemes . . . . . . . . . . . . . . . . . . . . . . . 32.1.1 FDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.1.2 TDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.3 CDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.4 SDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 GSM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2.1 Entities within A GSM system . . . . . . . . . . . . . . . 62.2.2 The radio Interface . . . . . . . . . . . . . . . . . . . . . . 72.2.3 Channel Structure . . . . . . . . . . . . . . . . . . . . . . 82.2.4 Control Channels and their function . . . . . . . . . . . . 82.2.5 Features in GSM . . . . . . . . . . . . . . . . . . . . . . . 92.2.6 Frequency assignments and frequency reuse . . . . . . . . 9

2.3 Test environments . . . . . . . . . . . . . . . . . . . . . . . . . . 102.3.1 Path loss . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.3.2 Fading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3 Problem Statement 13

3.1 Transmission Method . . . . . . . . . . . . . . . . . . . . . . . . 133.2 Type of information needed . . . . . . . . . . . . . . . . . . . . . 143.3 The best frequency choice . . . . . . . . . . . . . . . . . . . . . . 153.4 How to test the algorithm . . . . . . . . . . . . . . . . . . . . . . 15

4 The simulator 17

4.1 Simulation environment . . . . . . . . . . . . . . . . . . . . . . . 174.2 Simulator structure . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.2.1 Simulated transmission method . . . . . . . . . . . . . . . 204.2.2 Simplifying assumptions . . . . . . . . . . . . . . . . . . . 214.2.3 Simplifications . . . . . . . . . . . . . . . . . . . . . . . . 224.2.4 Differences comparing to real situations . . . . . . . . . . 22

v



CONTENTS vi

4.3 Channel model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234.3.1 Antenna Pattern . . . . . . . . . . . . . . . . . . . . . . . 234.3.2 Link Budget . . . . . . . . . . . . . . . . . . . . . . . . . 254.3.3 Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 26

5 Presentation of the algorithm 27

5.1 Key points for understanding the algorithms . . . . . . . . . . . . 285.1.1 CGI number . . . . . . . . . . . . . . . . . . . . . . . . . 285.1.2 Sub-CGI number . . . . . . . . . . . . . . . . . . . . . . . 285.1.3 Designated Base station . . . . . . . . . . . . . . . . . . . 295.1.4 Designated Distance . . . . . . . . . . . . . . . . . . . . . 295.1.5 Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295.1.6 Hop Distance . . . . . . . . . . . . . . . . . . . . . . . . . 305.1.7 The Least Interfering Frequency Set . . . . . . . . . . . . 30

5.1.8 Reliability of information . . . . . . . . . . . . . . . . . . 305.2 Algorithm in Brief . . . . . . . . . . . . . . . . . . . . . . . . . . 315.3 CDMA as transmission protocol . . . . . . . . . . . . . . . . . . 325.4 Description of the algorithm . . . . . . . . . . . . . . . . . . . . . 33

5.4.1 When to transmit the data . . . . . . . . . . . . . . . . . 345.4.2 Information that triggers change in frequency . . . . . . . 365.4.3 How the best frequency choice is calculated . . . . . . . . 365.4.4 Using perfect grid sets . . . . . . . . . . . . . . . . . . . . 395.4.5 Calculating the frequency choice . . . . . . . . . . . . . . 405.4.6 Antenna tuning . . . . . . . . . . . . . . . . . . . . . . . . 42

6 Evaluation of the simulations 45

6.1 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456.2 Assumptions and simplifications . . . . . . . . . . . . . . . . . . 46

7 Simulations and evaluation 49

7.1 Perfect Grid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497.2 Semi-random grid . . . . . . . . . . . . . . . . . . . . . . . . . . . 527.3 Messages per station . . . . . . . . . . . . . . . . . . . . . . . . . 567.4 Reliability of the frequency plan . . . . . . . . . . . . . . . . . . 57

8 Conclusions and future Work 60

8.1 Conclusions of the results . . . . . . . . . . . . . . . . . . . . . . 608.2 Ideas on improvements . . . . . . . . . . . . . . . . . . . . . . . . 618.3 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61



List of Figures

2.1 FDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.2 TDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.3 Example GSM network . . . . . . . . . . . . . . . . . . . . . . . 62.4 FDMA-TDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.5 Frequency band allocations . . . . . . . . . . . . . . . . . . . . . 82.6 Frequency reuse of 7 . . . . . . . . . . . . . . . . . . . . . . . . . 102.7 Channel fading . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.1 Hexagonal structure . . . . . . . . . . . . . . . . . . . . . . . . . 184.2 Simulator structure . . . . . . . . . . . . . . . . . . . . . . . . . . 194.3 Transmission concern . . . . . . . . . . . . . . . . . . . . . . . . . 214.4 Hexagonal grid structure. BS centered and randomly positioned 224.5 Plot of attenuation . . . . . . . . . . . . . . . . . . . . . . . . . . 244.6 Antenna gain pattern . . . . . . . . . . . . . . . . . . . . . . . . 24

5.1 CGI and Sub-CGI explained . . . . . . . . . . . . . . . . . . . . . 295.2 Flowchart: Location information phase . . . . . . . . . . . . . . . 335.3 Flowchart: Frequency decision and briefing . . . . . . . . . . . . 355.4 Frequency calculation example. Network from BS:1’s point of view 375.5 Choice of perfect grid sets . . . . . . . . . . . . . . . . . . . . . . 415.6 No antenna tuning . . . . . . . . . . . . . . . . . . . . . . . . . . 425.7 Using antenna tuning . . . . . . . . . . . . . . . . . . . . . . . . 43

7.1 Frequency plan in perfect grid simulation . . . . . . . . . . . . . 507.2 Antenna direction in perfect grid simulation . . . . . . . . . . . . 517.3 Signal power from serving BS in perfect grid simulation . . . . . 517.4 Interference power in perfect grid simulation . . . . . . . . . . . . 52

7.5 Signal to interference power in perfect grid simulation . . . . . . 537.6 Frequency plan in perfect grid simulation . . . . . . . . . . . . . 547.7 Semi-random antenna direction . . . . . . . . . . . . . . . . . . . 557.8 Signal to interference power in semi-random grid simulation. . . . 557.9 Plot of the average number of messages per base station. . . . . . 567.10 PDF of simulation success . . . . . . . . . . . . . . . . . . . . . . 577.11 Outage as a function of center offset . . . . . . . . . . . . . . . . 58

vii



List of Tables

2.1 Channels in GSM . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4.1 Link budget assumptions . . . . . . . . . . . . . . . . . . . . . . 26

5.1 LIFS calculation example database for BS1 on antenna 1 . . . . 37

7.1 Perfect grid simulation parameters . . . . . . . . . . . . . . . . . 497.2 Perfect grid simulation results . . . . . . . . . . . . . . . . . . . . 507.3 Semi-random grid simulation parameters . . . . . . . . . . . . . . 537.4 Semi-random simulation results . . . . . . . . . . . . . . . . . . . 54

viii



Chapter 1

Introduction

Cellular communication is one of the fastest growing telecommunication systemsin the world. The large numbers of users, increasing usage of telephony servicesas well as new services force operators to increase the capacity offered by the net-works. In many of the cellular systems, increasing the capacity means increasingthe available bandwidth and using more efficient planning of the deployment of the base stations. Common ways to increase the capacity are the use of smallercells, sectorization of the cells and better assignment of frequencies to mitigateintercellular interferences. Smaller cells increase the cost of deploying the net-work. This is because this scenario requires more base stations and the networkrequires more planning in the deployment and frequency assignment. The mainconcept of cellular communication is the use of small low-power transmittersand frequencies that can be reused in as small geographic areas as possible.The frequency reuse will be a key aspect in this thesis.

1.1 Purpose 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 becomesan important challenge as the cell sizes decreases. Frequency planning takeslot of time for the operator, especially when using small cells and it is costly.To get reliable planning, good predictions or real measurements of the signalpropagation are needed. Even though the planning is expensive, it is veryessential because licence frequencies and bandwidth is even more expensive. So,

the operator wants the best frequency reuse, with as little effort in managementas possible, and at low cost. An autonomous frequency planning scheme thathas the same frequency reuse factor but minimal maintenance cost would bevery valuable. This thesis presents a scheme where base stations determinetheir own frequency assignment and sectorization based on low-power signalsfrom other base stations

1



CHAPTER 1. INTRODUCTION 2

1.1.1 Outline

The thesis’ contributions to this problem consist of the following:

• The idea of having Base stations communicating with each other at lowpower.

• Algorithm for choosing frequency sets.

• Algorithm for choosing sectorization and tuning of antenna direction

• Simulation and analysis of the proposed system.

These results show a way that frequency planning can be solved in a less expen-sive fashion. It is understood that a real deployment of such a system wouldrequire more extensive tuning of the algorithm. It is hoped that an operator

could expand this to an actual network. The simulation and evaluation of theproposed algorithm requires a network to test it on. GSM was chosen as anexample network for the following reasons:

• GSM is a FDMA/TDMA network with frequency division duplexing.

• Part of the challenge was developing and determining how to fit the algo-rithm into an existing network standard.

Simulations shown in the report are made in networks using micro cells, butthe algorithm has been tested and works the same way for macro and pico cells.Some adjustments for different setups might be needed, and these are explainedlater in the thesis.



Chapter 2

Background

Below is a brief introduction to different kinds of multiple access (MA) schemesusing a shared medium. A brief description of GSM will also be given to theextent of what is needed for understanding the report. Different kinds of testenvironments along with channel models for wireless communication are alsoshown with respect to understanding this thesis. For further information aboutMA-schemes and GSM see [6, p 644].

2.1 Shared medium schemes

There are many different ways to divide the shared medium to get multipleuser access. In this chapter, three basic types of medium division protocols are

described. These are frequency division multiple access (FDMA), time divisionmultiple access (TDMA) and code division multiple access (CDMA). FDMAand TDMA are explained since they are the protocols used in GSM. CDMAis a protocol used in some newer wireless networks and is presented as a keypoint for this frequency planning solution to work. The knowledge of these threeprotocols will help with understanding GSM and the solution proposed in thisreport. Further information of multiple access schemes can be found in [6] and[5]

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 largechannels. There is no need for the channels to be equally wide. When themedium is split up, there is no restriction in assigning one user more frequencyslots in order to give him higher capacity.

3



CHAPTER 2. BACKGROUND 4

1

Time

>

>Frequency

Time

>

>Frequency

1 2 3 4 5 6

Figure 2.1: FDMA

1

23456

1

1

Time

>

>Frequency

Time

>

>Frequency

Figure 2.2: TDMA

2.1.2 TDMA

Time division Multiple Access (TDMA) subdivides the capacity of the totalchannel into a number of timeslots. Given a particular timeslot, a user has allthe available bandwidth at his disposal. All the users of the medium will thentake turns in transmitting. Figure 2.2 shows an example where channel 1 hasbeen subdivided in 6 channels.

As in FDMA, the slots do not have to be of equal size, but most of the timethey are made equal. To create channels with higher capacity a user can beassigned two or more timeslots.

2.1.3 CDMA

The Code Division Multiple Access (CDMA) protocol does not split up theavailable medium in terms of frequency or time. Instead all transmissions over-

lap, and the correct data is identified by a unique identification code at thereceiver and the transmitter. CDMA is a form of Direct Sequence Spread Spec-trum communications, see [6] and [5]. This means that the digital data x(n)is coded at a much higher frequency. The code that is applied to the data ispseudo-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.

1. The bandwidth is spread using a code that is independent of the data.

2. The receiver uses a code that, synchronized to the received signal, will ex-tract the received data. First of all, because of the code being independentfrom all other codes it will allow multiple users to access the same frequen-cies at the same time. And second, since the codes are pseudo-random allthe data transmitted by other units, than the two communicating witheach other, will look like noise.

3. With this modulation the signal occupies a bandwidth that is much widerthan necessary to transmit the data. Because of this, one will receivea couple of benefits such as greater tolerance against interference anddisturbance on specific frequencies.

The key reason for using CDMA in the solution to our problem lies within point3 above. It will be possible, by using CDMA, to overlay GSM with an accessscheme with high tolerance to noise and that does not add much interference tothe existing system. This is further explained in section 5.3.

2.1.4 SDMA

Space division multiple access (SDMA) is used in all cellular communicationsystems. The idea behind SDMA is allowing multiple cells to use the sameradio frequency channels. For this multiple access scheme to work it requiresthat the users are separated sufficiently far apart to minimize the co-channelinterference. The distance the frequencies can be separated with is called the

reuse distance, and this is further explained in section 2.2.6. Larger distancedemands usage of more frequencies. With frequency planning this distance ismade sufficiently large with as few frequencies as possible. This will be one of themain issues in this thesis. While talking about using SDMA in the report, I referto spreading the frequencies with with large physical distance, and not otherimplementations of SDMA such as separating users through multiple antennabeamforms.

2.2 GSM

Throughout the evolution of cellular communications, many different systemshave been developed apart from each other, resulting in huge problems when it

came to compatibility. The GSM was developed with this in mind and intendedto solve these problems. GSM was the first digital communication system de-ployed and used in the world. Even now with newer systems available, GSMstays in use and keeps growing almost all over the world. The GSM stan-dard mostly provides recommendations. The requirements arise only when itcomes to the interfaces between entities to ensure compatibility. GSM has threededicated bands that are used. The three bands are usually called GSM900,




BSC

MSCBTS Other networks

mobile and

non-mobile

>

>

MS

Figure 2.3: Example GSM network

GSM1800 and GSM1900. The 900 MHz band was the original band, but as thedemand grew bigger it was extended with two extra frequency base bands.

2.2.1 Entities within A GSM system

In Figure 2.3 we see a small example of a GSM network. In this section someof the most important entities and their function in GSM systems have beenlisted, and described in brief. For more information about the different entitiesin GSM see [6].

Mobile Station

The mobile station (MS) is the equipment used to access GSM networks. This isusually the only part of the system that the user can see, and probably the partin where the units differs the most in quality and available services. Thereforethe standard specifies their interaction with the net strictly. These units areindependent of the network-providers. The SIM (Subscriber Identity Module)is as small card, which has to be inserted in the MS for it to work, except whilemaking emergency calls (112). The SIM is the link between the operator andthe MS. This card uniquely identifies the user of the MS.

Base Transceiver Station

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 MSsin the network. Most of the BTSs have several transceivers, and some of the timethe different transceivers communicate on different radio frequencies. Later inthe report the hexagonal cells that make up the network will be discussed. Eachof these cells contains one BTS which is uniquely distinguished by its cell globalidentification number (CGI). The BTS is in charge of all the communication inthe cell. The BTSs are connected to a Base Station Controller(BSC) through




1

Time

>

>Frequency

Time

>

>Frequency

1 2 3 4 5 67 8 9 10 11 12

13 14 15 16 17 18

19 20 21 22 23 24

25 26 27 28 29 30

31 32 33 34 35 36

Figure 2.4: FDMA-TDMA

a special interface. The BTS is the network entity that this thesis is mostlyconcerned with.

Base Station Controller

One Base Station Controller (BSC) controls many BTSs. It is the entity thathandles part of the call setup phase and tells each BTS when there is need forhandovers between different cells. A BSC together with all the BTSs that itcontrols are often referred to as the Base station Sub System (BSS)

Mobile Switching Center

The Mobile Switching Center (MSC) is a switch connected to one ore severalBSCs. Its main function is to switch speech and data connections betweenBSCs, other MSCs and mobile and non-mobile networks. It is also connected tomany different registers that are used to verify each MS and call in the network.To read more about the different registers in GSM and the usages, as well asmore about all the entities that make up the network, see [4].

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 fre-quency bands, and on top of that each band is then divided in time to increasethe amount of access channels. Going back to the examples in Figure 2.1 and2.2 (FDMA & TDMA), and combining these two, a structure of the availablebandwidth as in picture 2.4 is obtained. As previously mentioned, GSM has

three dedicated base bands, The GSM900, GSM1800 and the GSM1900. Eachof these bands has a collective bandwidth of 50 MHz each. The 50 MHz aredivided 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 sepa-rated 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 up-link/downlink pair is separated with exactly 45 MHz. In picture 2.5 we can see




25 MHz 25 MHz

45 MHz ><

< <> >

Uplink Bands Downlink Bands

>frequency

1 1

<>200 kHz

Figure 2.5: Frequency band allocations

Channel Description Usage

TCH/F Traffic channel / Full Sp eech and data transmissionsTCH/H Traffic channel / Half Sp eech and data transmissionsTCH/8 Traffic channel / one eighth Data & control information transmissions

SCH Synchronization channel Time synchronization in cellFCCH Frequency correction channel Frequency correction within cell

PAGCH Paging and access grant channel Send request for communication

BCCH Broadcast control channel Transmits broad cast messages in a cell

Table 2.1: Channels in GSM

the bandwidth locations and separation relative to each other. The structure isthe same for GSM900, GSM1800 and GSM1900. Each of these 200 kHz bandsare divided into 8 full-rate channels by using TDMA. These full-rate channelswill either be given a specific usage or split up in even smaller channels. Thisis further described in section 2.2.3 The total bit rate for one band is 270,833kbit/s, and each channel is 22,8 kbit/s. When making a call in a GSM networkthe MS will be assigned one out of all these channels. A channel in GSM is oneof these 200 kHz bands, discussed above, given a specific area of usage.

2.2.3 Channel Structure

There are many different channels in GSM. The two most common channelsused for communication between a MS and a BTS are the TCH/F and TCH/H,which is Traffic channel/Full-rate and half-rate. The channels described in theprevious section (2.2.2) is the same as a full rate channel. A full-rate channelis assigned one timeslot every 4.615 ms and the half-rate channels gets as thename suggest half of a full-rate. This means that the half-rate channels gets theentire available spectrum at their use, for a timeslot once every 9.23 ms. Thereis also a channel called TCH/8 which is an eight of the full-rate channel. Thesekinds of channels are mainly used as control channels. Further explanation of the usage of the control channels is given in the next section. Tabulated in

Table 2.1 are some of the most commonly used channels in GSM networks.

2.2.4 Control Channels and their function

As seen in Table 2.1, there are many different control channels within the GSMspecification that are assigned different areas of usage. One of these is the SCH,a dedicated channel that has to be implemented in all cells of the network.




This channel supplies the time synchronization that all the MS’s needs to beable to distinguish which time slot is up, and when to transmit. This channelperiodically 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 theBSs are asynchronous, it will allow the MSs to hear short periods of controlinformation from different BSs between the transmissions from the operatingBS. With this knowledge the mobile station can prepare for a handover if itwould be necessary.

2.2.5 Features in GSM

GSM uses a feature called discontinuous transmission (DTX) to mitigate theinterference. DTX is a kind of variable bit rate transmission. This means

that the bit rate transmitted will decrease if there is nothing to send. In datacommunication this is easily understandable: one transmits only when one hassomething to transmit. When it comes to speech, this is done in a similar way.In general each person only speaks actively during 40% of a conversation and tobenefit from this a function called Voice Activity Detection (VAD) that detectswhen the user speaks and when the user is silent is added. By only transmittinguseful information the total amount of transmissions is decreased, hence thenetwork interference is decreased as well. Both these functions are explainedmore thoroughly in [6].

2.2.6 Frequency assignments and frequency reuse

In a GSM network each of the base stations will be assigned a transmit carrierfrequency, or in the sectorized cells, one frequency band per sector. To miti-gate the interference the carrier frequencies are separated over a wide area, asdescribed in section 2.1.4 (SDMA). The larger the distance between reuse of frequencies, the lower the interference. Bandwidth is expensive, and thereforethe operators want to reuse the frequencies in the smallest area possible, withas little interference as possible. Usually a frequency reuse of 7 is used in GSMnetworks, see [6], [5], [4], which means that 7 frequencies are used in total in thenetwork. In Figure 2.6 a small GSM network with hexagonal cells and frequencyassignment is shown. Note that the carrier frequencies are only numbered from1-7, and has nothing to do with actual frequency.In the net, shown in Figure 2.6, 7 frequencies in total are used. All the frequen-cies are separated as much as possible, according to earlier discussions. This is

the same as separating the frequencies with 3 sectors. For example, frequency 3is used in sector 2 in cell 27, and the closest sector where it is used again is sector1 on cell 28. The distance between these two sectors is 3 since you need to passat least 2 sectors to get there. By using this pattern in assigning frequencies anumber of combinations of frequency assignments or, as they’re called in thisthesis, frequency sets at the base stations will appear. The optimal frequencyplan for 7 frequency reuse make use of the following 7 frequency sets:




72

6 4

86

1 7

91

3 2

103

5 6

115

4 1

143

5 6

155

4 1

164

7 3

177

2 5

182

6 4

194

7 3

207

2 5

212

6 4

226

1 7

231

3 2

266

1 7

271

3 2

283

5 6

295

4 1

304

7 3

313

5 6

325

4 1

334

7 3

347

2 5

352

6 4

387

2 5

392

6 4

406

1 7

411

3 2

423

5 6

436

1 7

441

3 2

453

5 6

465

4 1

474

7 3

Figure 2.6: Frequency reuse of 7

{[1 3 2] [3 5 6] [5 4 1] [4 7 3] [7 2 5] [2 6 4] [6 1 7] }

The shaded cells in Figure 2.6 have the same frequency sets, and the distancebetween two of these sets is called the reuse distance. This network uses a perfecthexagonal grid setup. More about why hexagonal cells are used in the networklayouts is explained in section 4.1. However, the hexagonal grid of base stationsdoes not exist in real environments. In this thesis the hexagonal grid structureis modified so that the algorithm can be tested in a more realistic environment.

This modification includes randomizing the position and the heights of the BSin the cells. This is further described in section 4.1 of test environments. Whenmoving the station location so that they are no longer in the center of thecell, the frequency sets shown above might not be the best choice to mitigateinterference. Different sets of frequencies will be proposed, and to keep thedefinitions from getting mixed up, these sets described here in this section andshown in Figure 2.6 will be defined as Perfect Grid Sets (PGS).

2.3 Test environments

A central factor when simulating a wireless communication system is decidingthe propagation model. In all environments, the transmitted signal power isattenuated and the signal is distorted while propagating from the transmitterto the receiver. Different environments affect the signal in different ways, soit is of great importance to test the system in a simulated environment thatresembles the reality where the system is to be deployed. The models used anddescribed within this report are the UMTS recommendations ITU-R M1034, see[1]. These models are part of the world wide standard for test environments.




r(t)

d(t)

Figure 2.7: Channel fading

2.3.1 Path loss

The strength of the transmitted signal decreases in power relative to the distancebetween the transmitter and the receiver. The standard rule for the path loss isthat the signal strength decreases as a factor R−α, where R is the distance andα is an environment dependent variable. The value of α varies between around2-6, and for normal urban environments the number is relatively close to 4.Lower values can appear in canyon-like environments, for example at actualcanyons or streets with tall buildings around. Further information about path

loss is given in [1]

2.3.2 Fading

In many cases while using a mobile service, there is no line-of-sight 1 for thesignal transmitted between the two communicating units. Instead the signalis reflected on various objects which results in detecting multiple copies of thetransmitted signal at the receiver. The positive and negative interference willcause fading in the channel. The fading characteristics of the channel are depen-dent on the propagation environment. There are two types of fading: long termand short term fading. The long term fading is the attenuation of the signalpower due to the propagation distance. The short term fading is rapid changesin signal power due to multi-path propagation and scattering. In Figure 2.7 we

can see the two types of fading. d(t)is the long term fading, and r(t) is the shortterm fading. In this report the long term fading is taken into account, but theshort term fading has not been considered. The reason for neglecting the short

1By line-of-sight it means that the receiver can ”see” the transmitter without obstacles inthe way. This will result in the reception of one strong signal and some delayed copies due toreflection against different objects.




term fading is that we are looking at the works case scenario in terms of inter-ference levels. Short-term fading which can affect both signal and interferencepositively and negatively is therefore ignored in this study as the we concentrateonly on the propagation affects. This study will show the weakest spots in thenetwork, and it is these spots that are most likely to experience MSs with tolow SINR. The interferences assumed in the network are much higher than usualsituations and this will in most cases be more than enough to cover even thefast fading term. Further explanation of the simulations and the attenuation of the transmitted signals are given in chapter 4.



Chapter 3

Problem Statement

In the previous section, the background to the autonomous frequency planningproblem was given. In this section, the problem is outlined and divided intofour individual modules necessary for solving the problem. These modules are:

• Transmission method.

• Type of information needed.

• The best frequency choice.

• How to test the algorithm.

Each of these separate modules will be given a proposed solution, and can be

changed individually, if one wishes. The four separate problems are explainedfurther in the following subsections, in the same order as stated above.

3.1 Transmission Method

As described in the background, different base stations have different opinions of the ”real” time. This is an advantage to a moving MS that needs to change theirserving base station because it allows it to hear many different base stations’control signals. With knowledge of the other base stations the MS can preparefor a possible handover, see [4]. However, this is a problem for the proposedidea of BSs talking to each other. A base station is designed for BS to MScommunication and not BS to BS as proposed here. The differences in time

results in some loss in synchronization between all stations, and therefore analready existing GSM channel cannot be used to transmit the data neededfor the frequency assignment. The main reason for this is that usage of anexisting channel requires synchronization and for the transmitter to be givenan operating channel. A second reason is that if a base station is to transmitto another base station the received power will probably be to low to receive inthe usual manner.

13



CHAPTER 3. PROBLEM STATEMENT 14

In addition to BS not being synchronized to each other and low power, theyare not designed to talk to each other. There are two ways to go about this:

1. Listen to existing BS communication and use that information for fre-quency selection. Due to the lack of synchronization and the fact that thereceived power from one BS to another will be too small, this option isnot worth pursuing. In addition, it would be good if more informationthan just the used frequency could be transmitted. This extra informa-tion could assist the base stations while making the choice of frequencysets.would like more information.

2. To overlay a signal on top of existing communication (an applique) so thatmore information can be transmitted between the stations.

The most desirable solution would be to overlay GSM with a transmission

method thats low power, hence adds low interference to the existing GSM com-munication, and that is relatively immune to interference from other systems.This is why alterative 2 is suggested

3.2 Type of information needed

In a real GSM system, the BS, in most cases, terminates the air transmission androutes the call from the MS to a control center that determines who the receiveris, and switches the call to the appropriate interface. This means that thereis no communication in between two separate base stations, and therefore theyhave no knowledge about each other. In fact in normal GSM operation, they donot need any knowledge about one another. The task of autonomous frequency

planning demands certain knowledge about a base stations surroundings forit to be able to choose a frequency that does not interfere with others. Theknowledge needed for a base station to choose a frequency is:

• What frequencies are the BSs allowed to use?

• What other base stations can be heard?

• What frequencies do these other stations use?

• Is there a frequency that the BS is allowed to use that is not already usedby another station that can be heard?

These requirements lead to the following discussion. First the BS need to know

what frequencies are allowed so is does not interfere with other operator’s radiospectrum. Then the BS need to know the frequencies of its neighbors, since theywill be the most-likely interferers to the BS as it will be to them. Further more,all stations in the network are of equal importance in making the total networkfunction in terms of coverage. When designing an algorithm for frequency as-signment, collisions in frequency choices will occur. We will then need a way torank the base stations. This rank will determine who gets to keep its frequency



CHAPTER 3. PROBLEM STATEMENT 15

choice and who will back off. Besides this we will need a fixed point in the netthat will start the frequency assignment, since the algorithm is such that thebase stations will adopt their frequency choice to what they have heard. Bydeciding one station that will be a master station, and that starts the frequencyassignment, and then having the other stations adjusting frequencies to adjustwhat they’ve heard, the assignment will spread like a drop of water. Section5.1.3 describes what privileges this stations gets, and what criterion is used forthe selection of this station.

3.3 The best frequency choice

With the knowledge of the surrounding base stations and their choice of fre-quency, the BS of interest needs to pick the best frequency, or in a sectored cell,

the best frequency set. There are many ways to decide which set to choose, andone of the contributions of this thesis is an algorithm for how to find the bestfrequency choice. This is the most important and probably also the most diffi-cult problem to solve, since the change in frequency at one station will have animpact not only on itself but on all the other stations that can hear it. Anotherissue that adds to the complexity in this task is that This thesis is proposing todo the frequency assignment based on information transmissions between BS toBS. The BSs will choose the least interfering frequency (LIFS) for them selvesbased on information received in BS-BS communication, but in the end the goalwill be to minimize the interference in the communication between a MS and aBS from other units.

3.4 How to test the algorithmAfter the frequency assignment has settled, the problem of how to evaluate theresult remains. Since the quest has been to automatically assign frequencies toeach base station, the straight-forward way to test the network capacity wouldbe to measure the interferences due to the frequency planning. By measuringthe signal-to-noise-interference ratio (SINR) at various mobile positions, we canevaluate the performance. From these measurements we can estimate the per-cent of users with no outage. We will need a way to simulate the communicationbetween the base stations and the mobile units for any specific position withinthe net to determine if the received signal strength is adequate.



Chapter 4

The simulator

Because I am proposing an overlay to existing GSM systems, existing simulatorscannot evaluate the proposed algorithm. This necessitated the developmentof a simulator that can evaluate the performance of the frequency selectionalgorithm. The simulator required three parts:

1. Code simulating GSM networks

2. Code simulating the GSM frequency planning algorithm.

3. A way to evaluate the frequency assignment after planning is done

The simulator in this work is kept as simple as possible. It supports what isneeded to test the reliability of the proposed algorithm within some basic test

environments and for some specific setups for different types of networks. Thealgorithm was developed under the assumption that all BSs transmit at equalpower. In the simulations the base stations transmit at maximum power andthe spreading gain from CDMA is set to 5 dB. These are only values used inthe simulations and can be altered by the operator deploying this method. Norestrictions in data packet sizes or packet structures have been made either.This report recommends what types of information and transmissions whichwill be needed for the frequency assignment to work. In the section of futurework, some extra information that might help in making the algorithm morestable and reliable are presented and discussed. In section 4.3.3 the relationshipbetween transmit power and spreading gain is explained further. The structureof the chapter is such that first the simulation environment will be described.

Then an explanation of the structure of the simulator that is used is given, andfinally an explanation of the channel models used in the simulations.

4.1 Simulation environment

The environment used is an extension of the typical hexagonal grid that isusually used to illustrate a mobile network. Each of the hexagonal cells contain

17



CHAPTER 4. THE SIMULATOR 18

Figure 4.1: Hexagonal structure

one base station. The reason that the cells are usually modelled in hexagonalshape comes from the following. Each base station has three antennas whosebeams are separated by 120 degrees. Placing these 3-sectored base stations ina hexagonal grid is a way to cover most of the geographical area as possible ascan be seen in Figure 4.1.

The optimal position of the base stations, in a totally flat environment, isin the center of each cell, but due to physical, political and economic reasons itis not always possible to place an antenna at those coordinates. To model this,the simulator can randomize the base station offset from the center by roughly0-100% of the cell radius.

The randomization of the center offset has a normal distribution in x and y

separately. The reason for using a normal distribution is because it is assumedthat the BSs will be positioned as close to the center as p ossible. The offsetspecified in the simulator is the expected offset for the al the BSs.

In addition, the relative height of the base station can be randomized. Therandomization of the BS center and height leads to irregular propagation pat-terns. Such irregular patterns are found in the field and are important in testingthe robustness of the algorithm.

4.2 Simulator structure

In this section a short description of the structure of the simulator used is given.A figurative structure is supplied. The boxes that represent the structures willbe described along with their usage.

As can be seen in Figure 4.2, there are two different views to the simulatorsystem. These two are the network and the BS. The reason for the two viewsis the following. An operator has the information about the whole net andthe ability to make changes anywhere it suits them, but a base station onlyhas information about itself. The net is the global structure that contains allinformation for the simulation, and that the simulator is constructed around.It contains all the data concerning the network and it is the access point that




Network

Direction

Antenna Gain pattern

Sub-CGI number

Antenna

x3

Base Station

x?

Information Database

Nr of base stations

Network type

Background noise

Available frequencies

Control and overview variables

etc...

Information Database

Network view

Base station view

>

>

(What the operators sees)

(What the BS sees)

Figure 4.2: Simulator structure

sets all the global variables such as the total number of base stations and theirformation. It is here where GSM base band, thermal noise, background noiseand to what extent to vary height and position of the base stations is specified.

To simulate that the base stations are unaware of each other, the net containsX number of base stations that will work independent of each other. All thedata dependencies are one way, so each block/structure knows of everythingbelow, but nothing above. This means that the network is aware of all thenetwork variables and the base stations, who in their turn knows nothing of thenet and other stations. Each of the base stations will be given a location and aCGI number from the network. To simplify the simulations, the simulator onlyconsiders BS contained in a circular vicinity about the BS-of-interest as possiblereceivers. Each BS has an array that contains the neighboring BS it is assumedthe BS-of-interest might hear. The information about these base stations is notused for information gain but only as help in whom to transmit to. In a realnetwork this will not be used, because the base stations will broadcast their data

independent of if the other base stations can hear it or not. More about whatstations to transmit to and why these stations are chosen as possible receiversis explained in the chapter about the assumptions. Each base station will haveone database that is used for saving the information it receives from the otherstations around it in the information exchange part. In the sectored cell, eachbase station will have three antennas each with its own frequency band. Forsimulation purposes, that make the code more understandable, each antenna




keeps track of its direction, gain pattern for all angles and a database of whatit has heard of. These 3 separate databases are used to make the simulationcode easier, and contain exactly the same information as the total database forthe station. This means that they are of programming concern only and has noeffect on the algorithm structure. All the databases are empty at the beginningand will grow as the simulations go on, and as the base stations receive messagesfrom their neighbors.

4.2.1 Simulated transmission method

The simulator uses a simple model for transmission. When transmitting mes-sages, the received power from the transmitting antenna is calculated. If thereceived power is above a specified threshold needed for reception the data isassumed received. The data handovers are done in basic programming struc-

tures, and if this system should be deployed this needs change. As describedearlier, this thesis will present an autonomous frequency assignment algorithm,and show what type of data is needed for it to converge. Therefore the sizeand structure of the data packet is beyond the scope of this thesis. No recom-mendations for structures or sizes will be given. The size of the packets willbe of importance because as the information accrued by the BS increases; theamount of transmitted information will grow. This means either the packet sizegets bigger, or you need a way to break the information up over several packets,i.e. packet fragmentation. In real communication, there are possibilities of er-ror within the packets, which can arise from a bad channel corrupting the data.The algorithm has no built in function to handle those kinds of errors, and forthe moment it is only assumed that the transmitted packets contain as muchoverhead in terms of redundancy code, like cyclic redundancy check (CRC), see[3, p.430-432], needed to detect such errors. Should an erroneous packet bereceived it is immediately dropped, since none of the data can be trusted. Tosimulate this, a certain percentage (x%), adjustable in the simulator, of all thereceived packets sent during frequency assignments are discarded at the recep-tion. x is dependable on the quality of the channel. In this theses a fairly goodchannel is assumed, and only 1% of the packets are dropped. For informationon redundancy codes and how they work see [6, error detection/correction].

As seen in Figure 4.2, one variable that can be set is the available frequencies.This can be done in two ways. You can both specify the available PGS and theavailable frequencies individually. As explained in section 2.2.6, GSM networksusually uses a reuse of 7. 7 frequencies in perfect hexagonal structure make useof 7 sets. In the networks where we have large scattering of the stations the

7 frequencies might not always be enough to reduce the interference. This iswhy the function where you can specify both sets and separate frequencies isadded. So in cases where the interference cannot be minimized by the use of 7frequencies, the operator can allow the base stations to use one, or more, extrafrequency bands.




Figure 4.3: Transmission concern

4.2.2 Simplifying assumptions

To speed up the simulations and make the code simpler, the following assump-tions are made. At the creation of each base station, a list of all possible receiversfor that station is calculated. This list contains only the antennas in a vicinityof the BS under consideration. In the simulator the size of this near-vicinityis variable, but for most of the simulations presented here the vicinity encom-

passes all the BSs in a hop distance of two from the transmitter. A hop isdefined as the number of transmissions to the station that originated the infor-mation. The hop distance is further explained in section 5.1.6. The reason forthis choice is that the probability for a signal to propagate as far as 3 cells awayand still be detectable is very small. Therefore, the assumption that receivedpower is below the threshold needed for a message to be received correctly isdone, and simulation-wise, no transmission is even considered to those stations.Figure 4.3 shows an example of one station and the surrounding stations thatwill be considered in a transmission In reality, there is a small but still non-zero-probability that an environment exists where propagation allows stationsin far ends of the net to hear/interfere each other. This vector, containing theCGIs, of possible receivers is as described earlier saved at the base station even

though it contains information the stations normally would not know of. Theinformation is only used to find the receivers and does not affect the algorithm.The area of possible receivers can be varied in the simulator. A smaller areawill result in larger probability that possible receivers are not considered in thetransmission, but will be less computational heavy for the simulator. A largerarea means smaller probability that possible receivers are not considered in thetransmission, but is computational heavy. The most realistic simulations are is




Figure 4.4: Hexagonal grid structure. BS centered and randomly positioned

those when this area encompasses the whole net.

4.2.3 SimplificationsTo test this algorithm, a set of test network environments was developed. Noreal network data was available, so therefore the algorithm was tested on a setof simulated networks. To simulate different types of environment and obstaclestwo cases was used: an ideal hexagonal grid and a semi-random grid.

• The ideal hexagonal grid has a base-station perfectly centered in eachhexagonal cell.

• The semi-random grid placed base-stations in random locations withinhexagonal cells.

The position of the base-station within the hexagonal grid was limited to an

offset of x% of the cell radius from the cells center. The value of x is differentfor every network setup and it is given a value for all the presented simulationin chapter 7. By randomizing the position of the antennas some problems incoverage and collisions of antennas can occur. More about this and what prob-lems this will have are described in section 4.2.4. The base station antennas arepreferably mounted on tall objects to be able to cover larger areas and serveall the units within its cell. When mounted on a building for example, not allthe antennas will get uniform height. This non uniform height is simulated byrandomizing the antenna heights. Different heights of the antennas will affectthe propagation distance, see Equation 4.1.

4.2.4 Differences comparing to real situations

When simulating BS locations where the BS has a random offset from the center,some difficulties can arise. The most common problem is when antennas getpositioned to close to each other and therefore create larger interference thanthe amount that can be handled at the BSs and MSs. In the same time therewill be large areas without base stations and this will lead to low received powerfor the MSs used there. In picture 4.4, in the lower rightmost three cells, one of




these cases is shown. This problem will probably not be seen in real deployednetworks because positioning the base stations in this manner is not defensiblein any aspect. In the same way that the randomization of the positions canlead to situations that you normally would not encounter in a real network,the randomization of height will give similar problems. These problems are notquite as severe, but it can create difficulties.

4.3 Channel model

The channel model used in the simulations comes from the ETSI model TS-100-557 developed from COST 231, see [1]. This is a model, generally excepted, forthe path loss in urban and suburban test scenarios where the buildings are of nearly uniform height. The attenuation L, of the signal as it propagates can be

described with the following formula:

L = 40(1 − 4 × 10−3∆hb)log10(R)− 18log10(∆hb)+21log10(f ) + 80dB (4.1)

R is the separation distance between the two communicating parts in kilome-ters. f if the carrier frequency, which here is assumed to be 1800 MHz sincewe are studying the GSM1800 band. ∆hb is the base station height in meters,measured from the average rooftop level. The average number for ∆hb is 15m, and this is also the mean used in our simulations. In Figure 4.5 a mesh of how the attenuation varies depending on the distance from the base station andthe height of the BS is shown. Remember that as the attenuation grows, thereceived power will decrease. This is further explained in 4.3.2. In the simula-tions two different kinds of transmissions will be used. These two transmission

types are BS-BS and BS-MS. Thi COST 231 model is originally designed towork for BS-MS transmissions, so in the simulation of the frequency assignmentwhen information exchange between BS-BS is made the model has been slightlychanged. As said the ∆hb is originally the height of the BS antennas. Whenwe now consider transmissions between two base stations the ∆hb representsthe relative height between the two stations. The formula is valid for antennaheights between 0-50 m over ground, and using the relative height between twoantennas will not result in heights outside this range. The reason for this is thatI assume the antenna pattern to be vertically uniform.

4.3.1 Antenna Pattern

In Figure 4.6 you can see the antenna polarization pattern that is used in the

simulations. It is the typical pattern used for simulations of a GSM networkwith sectored deployment recommended in the UMTS standard, see [1]. Thispattern gives the transmission gain for a BTS and the reception gain will beequal, if the receiver is another BTS. The gain is found by calculating the anglebetween the communicating parts and reading the value from the plot in Figure4.6. Keep in mind that this is only the horizontal polarization pattern shown.The reason for plotting this pattern in Cartesian coordinated instead of polar




010

2030

4050 0

1000

2000

3000

40000

20

40

60

80

100

120

140

160

180

Distance from antenna (m)Antenna height (m)

A t t e n u a t i o n ( d B )

Figure 4.5: Plot of attenuation

150 100 50 0 50 100 150

22

20

18

16

14

12

10

8

6

4

2

0

Degrees

d B

Figure 4.6: Antenna gain pattern




coordinates as seen in some data sheets, for example see pictures in [7, p.11], isfor simulation reasons. The antenna gain for a specific angle is given by indexingthis gain vector with appropriate angle. As describer earlier each antenna hasone of these gain patterns saved for simulation reasons, and the maximum gain,0 degrees in the plot, is centered for the angle that the antenna is directedtowards.

4.3.2 Link Budget

The link budget estimates the received power. It is calculated as follows.

P rec = P tx + Gtx + Grx − L − N thermal − 10log10(bandwidth) + S Gain (4.2)

• P rec is the received power.

• P tx is the transmission power

• Gtx is the transmission gain according to the angle of the transmission. Itcan be read out of the plot above. Note that the gain is negative.

• Grx is the reception gain, and is collected from the plot above accordingto the received angle.

• L is the attenuation calculated depending on the distance and height of the station.

• N thermal is the thermal noise in the system. This is set to -174 dB/Hz

• S gain is the spreading gain received depending on the length of the spread-

ing code used in the CDMA coding. This spreading gain is only availablein the transmissions between BS-BS, during the CDMA transmissions.The link budget will be the same except for the C Gain in the BS-MS sim-ulations.

• The bandwidth used is depending on the transmission method. In theevaluation of the system we are transmitting on a regular GSM channelwith the bandwidth of 200 kHz.In the information exchange simulationswhile using CDMA, all the available bandwidth is used i.e. 200 × 7 kHz

As described shortly in the background section, the CDMA protocol is usedin the transmissions during the frequency assignment section. The link budgetis used in the simulation to estimate the expected interference during the BS-

to-BS communication stage. A link budget is calculated to see if the receivedpower is sufficient for reception of data. It is also used to estimate the SINRexperienced by a mobile in the network after the frequencies have been assigned.In this case one link budget between the MS and each of the BSs in the networkare calculated. These values will represent the received signal power and theinterference powers. This is further explained in chapter 6.




Thermal noise in network -174 dB/HzCoding gain from CDMA 5 dBMaximum transmit power 20 W (43 dBm)

Table 4.1: Link budget assumptions

4.3.3 Assumptions

The network and environment variables used in the simulations are tabulatedin Table 4.1. In the simulations done in this report, the thermal noise is set to-174 dB/Hz. This value could vary in different systems; but is fairly good as anaverage. Throughout the simulations a spreading gain of 5 dB is used. As canbe seen in the Table, the spreading gain is fairly low, while in the same time,the station transmission power is far too high. In reality one would probably

use a much lower transmission power to avoid adding to much interference tothe existing GSM network. For the algorithm to work the operator might needto decide the transmission power they want to use, and thereafter adjust theamount of spreading gain needed for reception of the message. As described insection 4.2.1 a message is assumed to be received if the received power calculatedfrom the link budget is above a specified threshold. The gain is dependent on thelength of the code used to spread the spectrum in CDMA, and the length of thetime during which the BS transmits. Longer transmission time will give higherspreading gain, but the frequency assignment will take longer time. Higher gainwill allow the user to have lower transmission power, which will result in lowerinterference on GSM systems.



Chapter 5

Presentation of the

algorithm

In this chapter a presentation of the algorithm is given. First some key pointsto help with understanding the algorithm are given and explained, followedby a brief description of how the algorithm works. The point with the brief description is not to give full understanding of the algorithm, but to serve asa first view of the proposed solution, or a recap for the ones who have readthe report. After that the algorithm will be described in full using examplesand flowcharts to give enough information needed to reconstruct and verify thesimulations. The chapter will be finished with a discussion of why we need totune the direction of the antennas and the enhancements this can result in as

well as a short presentation of the algorithm used for tuning.

27



CHAPTER 5. PRESENTATION OF THE ALGORITHM 28

5.1 Key points for understanding the algorithms

This section describes some key information to help with the understandingof the discussions made to reach the goal of the algorithm. Some of the keypoints are information that is sent between the stations during the frequencyassignment phase, and others are definitions made by the BS to help with thedecision in choosing frequency sets. For the algorithm to work, the BSs mustpass key information values to each other. Below are the most important onesdescribed, and they will later on be put in context in the following subsections.These five key points are information sent between the base stations and thefirst four of them are needed for the frequency assignment algorithm.

1. CGI-number

2. Sub-CGI number

3. Designated Base station

4. Designated distance

5. Location

The location is data that is sent, but it is not used for the frequency planningalgorithm. It is used in a feature of a basic idea on how to tune the antennadirections on the base stations to be able to cover the physical area better. Thefollowing three key points are definitions made at the base stations and aretherefore not transmitted between the BSs.

1. Hop distance

2. Least interfering frequency set

3. Reliability of information

5.1.1 CGI number

In every GSM net each cell has a unique identification number called the CGI(Cell global identification) number. This number is used to identify transmissionfrom a specified BS to the MSC, for example in localization of emergency calls. Itis also used to determine the nets hierarchy. In the algorithm the CGI numberwill be transmitted within each data packet to specify the sender, just as aregular postal address or IP address in data networking.

5.1.2 Sub-CGI number

This number is not an existing number within a regular GSM net and is thereforea new feature needed. The sub-CGI number is used for specifying the numberof the transmitting antenna on the base station. This numbering is simple,and will range from 1 to 3 within a 3 sectored cell. So for example when BS




Base station CGI = 5

sub-CGI: 1

sub-CGI: 3sub-CGI: 2

Figure 5.1: CGI and Sub-CGI explained

5 transmits on antenna 2 the transmission address will be 5.2. In Figure 5.1

we can see how the sub-CGI is numerated on a base station is done. When abase station defines a set of frequencies, the frequencies are listed in the sameorder as the sub-CGI numeration. For example, if a station had the set [4 56], it would mean that antenna 1 had frequency 4, antenna 2 frequency 5 andantenna 3 had frequency 6.

5.1.3 Designated Base station

The designated base station (DBS) is the master station within the area of interest. The DBS will be the station that chooses frequency first and thereafterwill act as the fixed point that the rest of the antennas will adopt themselves to.The title of DBS will not come with extra work or responsibilities, but is added just for the need of a fix point to help the frequency convergence. This term

has its influences from routing protocols, such as RIP and OSPF, see [3], whichhas an entity called designated router that is the master router in an area.

5.1.4 Designated Distance

The designated distance is a value that specifies the number of hops (trans-missions between base stations, see section 5.1.6) to reach the designated basestation. This value will also help the convergence of the frequency assignment,since the smaller the number of hops to the DBS the larger the probability of being closer to the DBS is. The closer a BS is to the DBS, the more likely itwill have a correct frequency assignment.

5.1.5 LocationThe location is information that has to be known within GSM networks to beable to do localization of emergency calls made in the network. So the onlyextra feature is that the base stations spread this data among each other soeven the neighbors around them will get aware of their location. This locationinformation is used within the basic algorithm for the tuning of the antennas,and is not needed for the frequency planning algorithm.




5.1.6 Hop Distance

The hop distance is a term borrowed from some routing protocols, such as RIPor OSPF, and has the same function as in those protocols. A hop is definedas a transmission between two base stations. This means that a transmissionof information from A to B with hop distance of 5 will pass 4 stations beforegetting to the receiver. Hop distance is used in two ways in this algorithm. Thefirst is to specify the distance to the designated station, and in this case it servesas a level of trust of the data saved. The second use is for the network operatorto be able to specify the amount of information they want each station to have.Every base stations grade its data according to the number of hops is neededto reach it.

5.1.7 The Least Interfering Frequency Set

When the base stations receive a signal/packet from another base station itsaves data as well as the received power. When deciding which frequency to useon an antenna, or alternatively, which frequency set that should be used on thebase station, it looks through the 3 separate databases (one for each sectoredantenna). It compares each choice of frequency set on all three antennas andin the case of frequency collision in a sector it checks the received power of that frequency. All these interfering powers are summed together over all threeantennas on each set, and the one with lowest power in total is defined as theLeast Interfering Frequency Set (LIFS). Further explanation on how to calculatethe LIFS is given in section 5.4.3.

5.1.8 Reliability of information

During the frequency assignment, a lot of data is sent in the network. Thisnecessitates the base station to grade the reliability of the information theyreceive.

Depending on the amount of information the operator wants each base sta-tion to know, each base station will sends what they know from other stationsthat are x hops away or less. The number x is variable in the simulator and isdescribed further in section 5.4.

The BSs will grade this information reliability depending on the number of hops to the originator of the information. Information heard directly from theoriginator will be graded 1:st order, and information from 2 hops away is gradedas 2:nd order and so on. If a BS should hear of information about the samestation from two or more different directions it only saves the information withthe lowest received distance.

The reliability of the information is also graded on the distance to the DBS.BSs closer to the DBS will have higher probability of having the correct fre-quency assignment, and is therefore valued higher. Remember the analogy of the drop of water.




Another feature in the algorithm is that the base stations treat the PGSas being more reliable than the LIFS. When a base station picks a set and thechosen set is a PGS, this data is transmitted as the frequency choice. If a stationchooses a LIFS it holds the transmission for a while. If a station should choose aLIFS and it is the same set as it chose the last time, this information is assumedreliable and therefore transmitted. This is done to keep the data flow as low aspossible, and only transmit reliable information.

5.2 Algorithm in Brief

Different kind of messages can be sent in the net. There are two types imple-mented in the simulator that is used in the simulations. The first is the locationupdate message that contains information about the sending BS and its physical

position.The second type is the frequency update message that tells which frequencyor frequency-set a base station has chosen. Other possible message types thatcan be implemented are error messages, that can be used for telling nearbystations when the made a choice of frequency that collides with the plan. De-pending on how much the operator wants each base station to know, they candecide on how many steps, in terms of hop distance, they want base stationdata to travel. Briefly the location and frequency planning is done as follows.Part 1:

1. The BS transmits CGI-, sub-CGI-number and location.

2. Received data is saved. If the information has a hop distance within that

the BS is obligated to tell about, this data is broadcasted to its neighbors.This part is only used to transmit basic information that helps in deciding whowill be the DBS, and the distance for the other stations to the DBS. Betterinformation about the surroundings will help the BSs to decide the frequencychoice in phase 2.

Part 2:

1. Each base stations decides who they think the designated base station inthe net is.

2. If the BS is the DBS, it chooses a frequency set that it will use. This setis always a PGS. Then go to point 4.

3. If the BS receives information that collides with its own choice of frequencyit calculates a new best choice of frequency. This choice can either be aPGS or a LIFS, depending on the information database at the BS. If thechoice of frequency is a choice that it should tell its’ surrounding stationabout go to point 4. The new calculated choice can be the same as theset it had before.




4. Transmit the information. This information can either be informationthat has been heard about from other stations and that should be passedon, or its own choice of frequency.

5. If the BS hears of frequency changes from a base station, it will update isdatabase to the new knowledge. If the change is within the hop distancethat the BS obligated to tell about, go to point 4.

6. If the BS receives information of a lower CGI than its assumed DBS, itmeans that it has had the wrong information of the DBS. It will immedi-ately update the information of DBS to the new CGI it heard of. Then itmakes a new calculation of its frequency choice. If the new frequency set isthe different from the set it had the BS should transmit this information.If its is not a new set, the BS will only transmit the information aboutthe DBS. Go to point 4. *

∗ It is not important to know of the correct designated base station when thealgorithm begins. In every message that is transmitted the knowledge of theDBS is included. Should a BS start with incorrect knowledge of the DBS it willget that information as the ”wave” of correct frequency assignment reaches it.When the knowledge of the new DBS is received it immediately drops earlierknowledge it has and adapts itself to the new data.

5.3 CDMA as transmission protocol

The first of the sub problems stated in the problem definition was how to solvethe transmission. Since the base stations that will exchange information are

not synchronized, we either need a way to synchronize them or find a trans-mission protocol that does not require total synchronization. The answer tothis problem lies within the CDMA protocol. Using this medium access schemewill also give us benefits beyond communication ability. As we remember themodulated signal occupied a much wider bandwidth than was necessary. Thiswill therefore give us the possibility to transmit at lower power/Hz, resulting inless interference to all the units using GSM services in the net.

Overlaying CDMA on top of GSM has been proposed in [2]. While the au-thors in [2] propose full CDMA communication overlay on top of GSM, thisthesis proposes sending only small amounts of data. Short informational datamessages occasionally will render much less interference than full communica-tion. To make it even easier on GSM, it is proposed to insert periodical trainingtimes when the BSs can update theirs frequency assignment.

By having the base stations evaluating frequency choice when traffic is nor-mally low the frequency assignment feature can be added to an existing networkwithout interfering the regular users. One idea is to have the base stations com-municate and decide new frequency sets over a longer period, and after theassignment is done all the BSs switches to the new frequency sets at same time.This would make the change of frequency work without the MS ever knowingabout it.




Transmit

Update Message

?Did the BS receive new Data?

Update data

Heard of new DBS?

?

Update BS

<

Begin data update

no

yes

yes

no

<

<

< <

<

< <<

<

Figure 5.2: Flowchart: Location information phase

5.4 Description of the algorithmAs described in section5.2, (algorithm in brief) the algorithm is divided into twoparts. These two are the ”location information briefing” and the ”frequencydecision and briefing”. Below the two parts are explained separately and flowcharts are given. The boxes in the picture and the information used in thedecision making are explained later.

Location information briefing

The first part is for the BS to learn about its surroundings and letting itssurroundings know the BS exist. In Figure 5.2 a flow chart of the locationinformation briefing is depicted. The messages that are transmitted in this

section contains:

1. CGI number.

2. Sub-CGI number.

3. Location.

4. CGI of the station the BS thinks is DBS.

5. Its designated distance. (distance to the station mentioned above)

6. The CGI number of and the distance to the ones the BS has heard of ata hop distance of x.

The hop distance of x can be an integer from 0 to infinity depending on howmuch the operator wants the base stations to know. By choosing x = 0 the BSonly transmit data about itself, with x = 1 it will include the CGI of the onesthe BS has heard directly and so on.

If the BS should receive a message about another station with a hop distancewithin the hop-distance of what it is obligated to forward information about,this will trigger a new message. To keep the data packages as small as possible




the BS will only need to transmit its CGI and the new data that it has heard of,but to keep the simulator as simple as possible all the known data is transmitted.

Frequency decision and briefing

The second part is: depending on what it knows choose a frequency and broad-cast its choice of frequency. In Figure 5.3 we can see a flowchart of the frequencydecision algorithm. The boxes in the picture are explained separately. The mes-sages that are transmitted in this part are called frequency update messages andcontain the following.

1. CGI number

2. Sub-CGI number

3. Frequency choice4. CGI of the station the BS thinks is DBS.

5. Its designated distance. (distance to the station mentioned above)

6. Its neighbor’s CGI numbers and the distance to them *

7. Its neighbor’s frequency *

The last two parameters, highlighted with *, above are transmitted depend-ing on if they are within the hop distance of what the BS is obligated to forwardinformation about.

When a frequency update message is received, the BS first needs to consider if it needs to change its own frequency assignment set. How to determine whetherthe BS needs to change its own frequency and how this is done is explained inSection 5.4.3. The information that decides if the BS should report its choiceof frequency is explained in section 5.4.1. Finally, the information that resultsin a possible change is explained in Section 5.4.2.

5.4.1 When to transmit the data

When BS receives a frequency update message from anyone of its neighbors is,there are three possibilities that this message will trigger a response message.These three are:

1. The BS receives a message that makes it consider a change of frequencyand the result is that it does change, the BS will send a message, if andonly if it has chosen a PGS.

2. The BS receives a message that triggers a change in frequency and itchooses a set that is not a PGS, but it is the best choice and the samechoice you made the last time. 1

1The reason for not transmitting information about any choice of frequency directly is away to keep the information flow low. Remember section 5.1.8 of reliable information.




?

Calculate

Frequency choice

Transmit

Frequency message

Update data

Is the BS the DBS?

Should the BS tell about this data?

?

Does this data interfere?

Did the BS change frequency?

?

?

Did the BS receive new data?

?yes

yes

yes

no

If the BS heard of a more

Correct DBS, then Update

after update information

section has finsihed

begin frequency assignment

<

<

>

<

<

<

<

<

<<

<

End

no

<

<

yes

no

<

<

<End

<

yes

no

no

Figure 5.3: Flowchart: Frequency decision and briefing

3. You receive a message about a change in frequency at another base station,and that base station is within a hop distance that makes you obligatedto forward the change.

In point two above, it is seen that the BS will only transmit its frequencyinformation if it has chooses the same LIFS set twice. This is to keep unreliableinformation from spreading in the network. At some occasions, a BS will onlyreceive one message that triggers change, and even if the station has chosen aLIFS it needs to tell his neighbors of this. So, the station will be allowed totransmit information about a LIFS without choosing it two times, but to keepthe information flow low, the station has to wait a longer period of time, orverify that the frequency data is reliable before transmitting. This will give usthese extra cases when a station will transmit a frequency message.

1. A base station has chosen a LIFS, and for a longer period of time it hasnot received any information that has made it change its choice. This set

is then considered reliable.

2. A base station has chosen a LIFS, and after receiving 5 additional fre-quency messages from other stations that does not interfere with the choiceof LIFS, this information is considered reliable.

The best amount of time to wait before transmitting information about a LIFShas not been evaluated in this thesis due to restriction in the simulator. The way




that it is implemented is that when the transmission of frequency informationmessages stop. The base stations, that has chosen a LIFS and not broadcastedthis information yet, will send their information. This might start up the fre-quency assignment once again if a collision of two LIFSs has occurred. In thesame manner any base station can restart the frequency assignment if they findthat there is a frequency collision between one of its sectors and a neighbor’s.But for this to happen the BS has to be able to find a better set to choose beforerestarting.

5.4.2 Information that triggers change in frequency

There are many different kinds of messages that will trigger a change in fre-quency. To ensure that frequency change is made when it is necessary the basestation only transmit reliable information, see section 5.1.8. The base station

also categorizes the other stations in its vicinity so that some stations are morelikely to trigger a change in frequency. More on how these stations are prior-itized is explained in section 5.4.3. The information that triggers change at aBS is:

• The BS hears information about another cell using the same set of fre-quencies as it does. Change is only considered if the frequency informationis fully reliable, i.e. 1:st order.

• A frequency collision occurs. This means that the transmitting sector atthe sending base station uses the same frequency as the receiving sector.

• The BS receive 1:st order information about frequency use at another basestation, and the receiving BS has not made a choice yet. This happenswhen the assignment has started and the BS waits for information formthe DBS.

5.4.3 How the best frequency choice is calculated

The third sub problem of this thesis was how to determine the best frequencyset to use. This section shows how the calculation of the LIFS is done at theBS.




2

3

5

7

7

6

15 6

BS:2

BS:3

BS:4

DD = designated Distance

DD=1

DD=2

DD=3

BS:1 DD=2

Figure 5.4: Frequency calculation example. Network from BS:1’s point of view

Stations heard BS 2 BS 3 BS 4Frequency used 3 5 6 7 2 5 6 1 7

Sub-CGI 3 2, 3 2Received power 9 5, 6 7

Table 5.1: LIFS calculation example database for BS1 on antenna 1

To make the choice of frequencies to use, some information is needed. Eachbase station has a database containing the information from all the base stationsthat it has heard of. The database contains:

1. Base station CGI.

2. Base stations Sub-CGI.

3. Frequency choice of each base station.

4. Received power.

5. Receiving antenna.

6. Designated distance.

In this description a network with 3 sectored cells and 7 available frequenciesis assumed to be used. To help describe the procedure consider the followingexample illustrated in Figure 5.4. The BS of interest is BS:1, and we are specif-

ically looking at antenna 1. Tabulated in Table 5.1 is the database for antenna1 on BS:1. Remember the numbering of the antennas, and sub-CGI numbersfrom section 5.1.2, and Figure 5.1.

The received powers in Table 5.1 are values given to each reception. It isbased on the received power in dB, and is used to help determining the frequencychoice that interferes the least. A frequency that has not been heard of is giventhe value 0, and should not be seen as a received signal power of 0 dB.




When a message triggering a change of frequency is received the base stationgenerates three lists, one for each of its antennas. The lists contains all theavailable frequencies within the net. For each of the frequencies it calculatesthe interfering power, which is the sum of the received powers from all theantennas using that specific frequency. Using the example above BS1 will haveheard of antenna 3 on BS2, antenna 2 and 3 on BS3 and antenna 2 on BS4.The generated interference vector will therefore be.

S 1BS1 = [ 7 5 0 0 6 9 0 ]

Then a matrix containing all the possible combinations of allowed frequencies iscreated. The matrix is structured in the way that row 1 will have the frequencychoice for antenna 1, row 2 for antenna 2, and so on. By using the matrix rowsas index to the interference power vectors we will get a matrix containing the

interference powers that each choice of frequency will lead to.

N =

1 1 2 ... ... n

2 3 1 ... ... n − 13 2 3 ... ... n − 2

Summing the rows together and taking the minimum of the result will give theindex to the least interfering frequency set.

S itot =3

k=1

N (k, i)

j = min(S itot)

LIFS = N (:, j)

Using this technique will give us one set with low interference, but some un-wanted problems may occur. Many of the possible frequency sets will have thesame interference power, and by choosing the minimum a special pattern of fre-quency sets will be more commonly chosen within the net. This is because thealgorithm that finds the minimum among a set of equal numbers will usuallyselect the first minimum value it finds. This problem will lead to higher concen-tration of some frequencies in a smaller area while some other frequencies arenot used at all. To overcome this, one solution would be to randomly choose oneof the sets with equal interference power. Another solution, the one used here, isto modify the interference vectors slightly, and use all the available information.The new vectors will not only contain the received powers of frequencies heard,but also the frequencies used in the neighboring cells that can not be hearddirectly, but that the BS know is being used by the neighboring BSs. Thesefrequencies will be weighted at 1% of the received power level from the antennathat uses them. Using the example above the interference vector at antenna 1will be:

S 1BS1 = [ 7 5 9 × 1% 0 6 + 9 × 1% 9 + 7 × 1% 7 × 1% + 6 × 1% ]




With this modification the number of possible sets with equal interference powerwill decrease greatly and the frequencies will be spread more uniformly. Thiswill make the frequencies spread more evenly and results in lower interferencewithin the net, as described in section 2.1.4 (SDMA). If there still are two ormore sets that have the same interference ratio, the set that minimizes thenumber of matching frequencies with the surrounding cells is the one chosen.This is to separate the frequencies as much as possible, as described in section2.1.4 (SDMA). With this choice of best frequency set, the station will pick theset with least interfere while spreading the frequencies as much as possible inthe net, which is what is wanted in the spacial division. Now that we havechosen the optimal frequency set with respect to the surrounding stations, wecan modify the choices to further improve the convergence of the algorithm.In case of a collision, which will occur when all the available frequencies aretaken by the stations surrounding the BS-of-interest, it will pick the set that

interferes the least, no matter of direction. After the collision, the two collidingbase stations will send messages back and forth until they have chosen sets thatbenefit both of them. This leads us back to our original thought when designingthe algorithm. We wanted, with the use of a fixed point in the designated basestation, to have the frequency assignments spread as the waves propagate froma drop of water in a tank. This is not the case now, since in a collision anybase station may pick a frequency that makes the station closer to the DBSreconsider their choice of frequency set. To overcome this, all the base stationsweight their information depending on where they heard it from. The receivedpowers in the interference vectors get multiplied by 2 if it is received from astation with lower designated distance. Using the example again we get:

S 1BS1 = [ 7 5 9× 2× 1% 0 6 + 9× 2× 1% 9× 2 + 7 × 1% 7× 1% + 6× 1% ]

The result of this is: when a collision occur the base station would rathermake the change outwards to the stations with higher designated distance, thandisturb the lower who probably already settled within their surroundings. Byusing the choice of weights it is more difficult to bounce the assignment back-wards but not impossible. In the simulations made during the development of the algorithm, the weights based on the designated distance resulted in a re-duction of transmitted messages. i.e. the reliability of information in this wayminimizes the number of iterations needed for convergence in case of collisions.The choice of using the weights of 1% and 200% are no based on any studies,are chose only because the give the result that is sought. This set with theminimum interference power is, as described in section 5.1.7, called the LIFS.

5.4.4 Using perfect grid sets

As described earlier in the background section of frequency assignment andfrequency reuse, when having a number of frequencies to use and placing themin optimal distance from each other only a finite number of sets will be used.As with 7 frequencies, we will get 7 sets. The best solution to the problem of automatic frequency assignment would be if the frequencies got assigned in this




pattern. A problem is that the best plan when deciding frequency depending onwhat you’ve heard and assigning frequencies to their position relative to eachother will not always coincide. As described in the section above, on how tocalculate the best frequency choice, the base station will pick the frequency setthat interferes the least to them. By picking the LIFS, they will make the bestof the situation for themselves, but this choice could make the planning moredifficult for others. In some cases we can, even when using the refined way of picking the LIFS, see higher concentrations of some frequencies in some areas.When knowing that neither the LIFS nor the PGS will make the best solutionwhen looking at the whole network, it led to the discussion of a mixture of boththe sets from the perfect grid plan as well as the LIFS calculated depending onwhat you heard.

5.4.5 Calculating the frequency choice

Now when we know of the LIFS, calculated from what has been heard and theperfect grid sets, the problem is in choosing which of the sets to use. Listedbelow we see the strengths and weaknesses in the two ways of choosing frequencyset.

LIFS

+ Best in the BS point of view

- Computational heavy

- Selfish way of choosing

PGS

+ Best in the networks point of view

+ Easy to choose

- Not optimal for the base stations

The PGS might not be the best choice at the moment, but when the fre-quency assignment is finished, the PGS and the LIFS could be the same. Bymaking it easier for the other base stations to choose frequency, the total amountof interference in the net can be minimized. We know that the best choice of frequencies should be such that the BS gets the set that interfere the least, andif possible a set that makes it easier for the other stations in their choice of frequencies. By combining the two ways of choosing frequency sets, we will see

if this can be achieved.The perfect grid sets are structured in the network in a pattern such thata base station receiving information about any one of these sets would, if heknew the senders location, only have one set to choose. The problem is toknow where the sender is located, and this kind of information is normally notknown by the BSs. As said before the benefit of using PGS is the simplicity inchoice of sets, and it will be tried to be kept this way. Normally a base station,




2

6 4

6

1 7

1

3 2

5

4 1

4

7 3

7

2 5

Figure 5.5: Choice of perfect grid sets

assuming that he is surrounded by stations at all sides, will have three stationsthat it can hear two sub-CGI numbers from. This will be the ground for thechoice of PGS. Knowing the number of optimal sets in the net, three vectorsof different permutation orders can be constructed. For the 7 frequency/7-setnetwork studied in the simulations of this thesis, these 3 vectors of sets will be.

S 1 = { [1 3 2] [6 1 7] [2 6 4] [7 2 5] [4 7 3] [5 4 1] [3 5 6] }

S 2 = { [1 3 2] [4 7 3] [6 1 7] [5 4 1] [2 6 4] [3 5 6] [7 2 5] }

S 3 = { [1 3 2] [2 6 4] [4 7 3] [3 5 6] [6 1 7] [7 2 5] [5 4 1] }

When a BS calculates their frequency choice, it checks the data base for knowl-

edge of two sub-CGI numbers from one station. The antenna that the data isreceived on will determine which collection of sets to choose from. i.e. if data of a BS using frequency set [1 3 2] is heard on antenna 1, the base station shouldchoose set [6 1 7] if possible. In this way the station checks what possible setsit can choose. When 3 different sets are proposed, none of them will be chosen.If a majority of base stations chooses a particular set, that set will be chosen.This feature is called the Majority Rules. In the small example in picture 5.5all the 3 BSs are pushing for the same set, which is [3 5 6].

A base station can choose a possible set if the frequencies for each antennado not interfere with the database of over-heard frequencies. This means thatthe choice of an PGS is only based on what you directly hear, and the weightsor 1% terms as in LIFS are not included. If there is no possible PGS to chose,or if the PGS collide with what you know, the base station will choose a LIFS.

In order to get the optimal sets started the DBS will choose one of the optimalsets, from this the planning will spread. The basic thought is for all stationsto try and choose an optimal set, and the few stations that cant due do somepropagation effects will choose an LIFS.




Cell 1

Cell 2

Figure 5.6: No antenna tuning

5.4.6 Antenna tuningThis section discusses the basic antenna tuning algorithm. It is not proposedto be used in a commercial product, but it shows quite good performance en-hancements in the simulations made for this thesis. Please keep in mind thatthe lobe patterns in the pictures are only used as visualization aid, and do notsymbolize the maximum coverage area from one antenna.

Background

In the perfect hexagonal grid, the antennas with their directions in 90◦, 210◦

and 330◦ will cover as much of the physical area as possible, see Figure 4.1. Butwhen the stations are moved from the center location of the cell, the perfect

plan is disrupted and we will in many cases have double the amount of coveragein some areas, while almost no coverage in other. This is depicted in Figure 5.6,where we can see overlapping in the transmissions from cell 1 and 2.

As we can see the there is a large area to the left of the antenna lobe fromcell 1 that is not covered. By tuning the lobe of cell 1 some degrees to the rightwe would make the uncovered area smaller on the cost of the overlapped area.In picture 5.7 we can see this when it is done.

The antenna tuning algorithm featured in this thesis is built on this idea,as shown in Figure 5.7. In phase 1 of the algorithm, the base station transmittheir location as well as their CGI and sub-CGI numbers. By knowing its ownlocation and the location of the base station to tune to, the change in directioncan be calculated through basic trigonometry. The criterion used in decidingwhere to tune the antennas to, is based on how many sub-CGI numbers that

can be heard from the other base stations. The stations to the north, southwest and south east are the stations that the BS should consider tuning theantennas towards, and the same time they are the stations most likely beingable to transmit to the BS of interest from to different sending antennas. By onlyusing the number of sub-CGI numbers heard, the tuning may become unstablesince it is possible to hear of multiple sub-CGI numbers from basically any otherstation in the net. This necessitated the addition of using reception power. Of




Cell 1

Cell 2

Figure 5.7: Using antenna tuning

the antennas that is possible to tune in to, the correct one is the most likely oneto have stronger reception power from. On top of this, the differences betweenthe reception powers from the two separate antennas will be smaller comparingto the differences between antennas on other BSs.

Restrictions

This tuning has positive results in networks with moderate scattering of the basestations. If the scattering is large this kind of tuning will not be of much use.With this information an extra stabilizing restriction of limiting the antennas totune the direction to a maximum of 30◦ is added. The reason for not allowingthe BSs to tune their antenna more than 30◦ is that if they were allowedto tune the directions arbitrary we could experience stations overlapping their

own transmission areas with two antennas. This results in less coverage in someareas.



Chapter 6

Evaluation of the

simulations

After the frequency assignment has settled it needs to be tested. This section isstructured in the following way. First a description to the attenuation formulaand its differences from the attenuation in BS to BS communication is given.

In section 6.1 the method for how the testing of the net is described, and insection 6.2 the assumptions and simplifications for the evaluation is explained.The simulation test is only based on the downlink in the communication. Inthe uplink case the mobile phones transmit at much lower power and withoutbeam-forming on the antenna, which is why this interference will be smaller. Iwill here only consider the worst case; witch will be in the downlink. The path

loss model used, for the evaluation of the frequency plan, in the transmissionbetween base stations to mobile stations is the ETSI model for urban usagedescribed in the chapter on simulation environment.

L = 40(1 − 4 × 10−3∆hb)log10(R) − 18log10(∆hb) + 21 log10(f ) + 80dB

During all the test of the reception power BS to MS I assume, as recommendedin the model, the MS to be at the average height of 1.5 m above ground.

6.1 Method

A large number of mobile stations are randomly positioned within the net.

For each of the MSs the received power from all the antennas on all the basestations within the net is calculated. Out of all these received powers the largestis assumed to be the received signal power from the serving base station. Whenwe know the serving base station it is simple to retrieve the frequency thecommunication is operating at. Then the received powers from all the other basestations operating at the same frequency are summed together and assumed tobe interference noise. When we know both the received signal power and the

45



CHAPTER 6. EVALUATION OF THE SIMULATIONS 46

noise power, the signal to interference ratio is calculated and compared to avalue which is specified as the lowest tolerable threshold for the service in mind.In the report, the lowest received SIR tolerated is set to be 5 dB. If the SIRshould be below the specified value, the position of the MS is saved, and laterit can be plotted among the others to show of weaknesses within the planning.By using a great number of MSs and not adding any fading terms and keepingthe lowest tolerable SINR relatively high, we will be able to detect areas withinthe net, that are most likely to contain dead spots in deployment.

6.2 Assumptions and simplifications

I have only considered evaluation in the downlink, since the uplink will besimilar. In the uplink case the transmitters, which will be the MSs, transmits

at lower power and does not use beam-forming and therefore the receiving basestations will receive a lower interference ratio. For this simulation we will besatisfied with examining the worst case only.

The background noise in the system is neglected since it is much lower thanthe interference created by the other bases stations. It can be added in thesimulations but since the transmission powers from all the base stations are somuch greater, no remarkable differences has been seen.

The amount of interference within the net is assumed to be from fully loadedcells, and will of course be less if not all channels within the other cells areoccupied. The SINR is also calculated with the approach of that the strongestreceived signal will be from the serving BS. Within a real GSM net, a handovermight have been done when the SINR value decreases, since these are basedupon signal quality and not just received signal strength.

The interference within the net is simulated at worst case possible. In anactual network the stations would benefit from what was earlier described asDTX, which was discontinuous transmissions, i.e. lower data rates when userswere quiet. This function is not added to the simulated network, and thereforeall the evaluation simulations carried out will suffer higher SINR because of unordinary high transmission power from all other within the whole network.

Since evaluation is done for one MS at a time and with the simulator thatdoes not support power control for downlink, all the measurements are done asif all the other BSs are transmitting to all their users at maximum power. Thiswill render much more noise than the BS usually would see in a GSM network.Combined with not using DTX and VAD, as described above, we will experiencea co-channel interference of such a level that it will cover the interference by the

adjacent frequencies as well. This is the reason for only studying interferenceform one frequency band.

No fast time fading has been considered in the simulations. In real situationa random attenuation resulting from the fast time fading should be added.The reason for neglecting this fading, which is relatively high, is that in theevaluations made here it is of interest to find the weak spots that are mostlikely to contain errors. In most cases the high interference that is assumed in



CHAPTER 6. EVALUATION OF THE SIMULATIONS 47

the net will cover even the fast time rayleigh-fading.



CHAPTER 7. SIMULATIONS AND EVALUATION 50

0 1000 2000 3000 4000 5000 6000 7000 8000

1000

0

1000

2000

3000

4000

5000

15

4 1

24

7 3

37

2 5

42

6 4

52

6 4

66

1 7

71

3 2

8

3

5 6

91

3 2

103

5 6

115

4 1

124

7 3

134

7 3

147

2 5

152

6 4

16

6

1 7

172

6 4

186

1 7

191

3 2

203

5 6

213

5 6

225

4 1

234

7 3

24

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2 5

254

7 3

267

2 5

272

6 4

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1 7

Figure 7.1: Frequency plan in perfect grid simulation

Lowest measured SINR 9.92 dBMS below SINR / total MS simulated 0 / 100000

Data messages sent for convergence 186Frequency messages sent for convergence 84

Total number of messages sent for convergence 270

Table 7.2: Perfect grid simulation results

described in the background section. The reason for this is that the algorithmchooses the PGSs if possible as explained in section 5.4.5. The implementationof PGS combined with the feature of ”majority rules” makes the BSs help eachother in creating the best frequency planning for all stations in the network.

In Figure 7.2 we can see the location of the base station which is marked bya *, and its antenna directions. When the stations are positioned in the centerof each cell the optimal antenna direction is 90◦, 210◦ and 330◦, and thereforethe antenna tuning algorithm will not affect the directions of the lobes. Asseen in Figure 7.1 and Table 7.2, no MSs reported a SINR below 5 dB. Theevaluation results show that the lowest measured SINR in the network was

9.92 dB. Another way to further show that the SINR is sufficiently high is thefollowing: The signal powers and the interference power were measured at allpositions in the network. The serving base station is chosen as the one stationwith the highest received power, as explained earlier. This will render a receivedpower plot as shown in Figure 7.3.

Given the serving base station, the interference is measured as the sum of the received powers from all other stations in the net using the same frequency




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Figure 7.2: Antenna direction in perfect grid simulation

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Figure 7.3: Signal power from serving BS in perfect grid simulation




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Figure 7.4: Interference power in perfect grid simulation

band. The interference plot is show in Figure 7.4.The reason for the sharp edges between the small areas in the interference

power plot is that it is here that the handover occurs. When a handover is madethe interference levels will be calculated from different frequency bands, hencethese rapid changes. Remember that this is slightly different from how decisionon how handover is made in a real GSM network. In an actual GSM networkthe handovers are based upon SINR and not based on the received power fromthe serving base station. But in our simulations this criterion for serving basestation is sufficient for testing our algorithm.

By subtracting the signal power by 5 dB and showing both the signal powerand the interference power in the same plot, we could see if the SINRs are below5 dB in any location in the net. A SINR value of less than 5 dB would in thisplot be seen as an intersection between the two surfaces. This joint plot is shownin Figure 7.5 Since the interference levels never intersect the signal power levels,it is clearly seen that all locations within the network have a SINR level above

5 dB.

7.2 Semi-random grid

In this section we examine a semi-random network with high center offset of the base stations to show how the algorithm works for a non perfect setup.




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Figure 7.5: Signal to interference power in perfect grid simulation

Tabulated in Table 7.3 are the input parameters used for the simulation.In Figure 7.6 we see the result of the antenna planning and the evaluation

of the system. The circles shown in the picture are cases where the SINR values

for the mobile stations are below the threshold for the service. To be able to seewhy these areas occur we can not only look at the frequency assignment for thesectors, but we also need to look at the BS position and the antenna direction.The positions and antenna directions are shown in Figure 7.7.

This frequency plan is a good example of how we combine the use of LIFSand PGS, as explained in section 5.4.5. Recall from 5.4.5, that the BSs choose aPGS if possible, and in the cases where this is not possible; they use the LIFS.

Number of stations 28Available frequencies in the net 8Threshold for message reception 5 dB

Center offset 90% of the cell radiusBase station heights Random 0-50m

Cell size Micro cells 700m radiusMaximum transmit power at BS 20 W

Spreading Gain 5 dB

Table 7.3: Semi-random grid simulation parameters




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Figure 7.6: Frequency plan in perfect grid simulation

Lowest measured SINR 2.98 dBMS below SINR / total MS simulated 8 / 100000

Data messages sent for convergence 193Frequency messages sent for convergence 112

Total number of messages sent for convergence 305

Table 7.4: Semi-random simulation results

The reason that makes a station pick a set outside the collection of PGSs is thatwhen the station, due to offset in the center, hears interfering stations that itwould not hear if they were perfectly centered. In the right part of the networkwe also see a station that has made use of the extra frequency that was allowed.

As we can see, we now encounter locations in the net where the SINR is toolow. The reasons that these spots occur can be seen if we look at the antennapositions in Figure 7.7. The interference in the network is almost the same,but in some areas the received signal power from the serving base station is tolow due to the large distance between BS and MS. In the upper right cornerof the network the received signal is strong but then we have high interference

instead due to the short distance between the BSs in the area. In the sameway as the SINR values were evaluated in section 7.1 (perfect hexagonal grid)Figure 7.5, the signal powers and the interference power for this network aremeasured. These values are shown in Figure 7.8. In this Figure we can see threesmall areas where the SINR is to low. They areas are located roughly around[X,Y] = [6500, 900], [X,Y] = [7000, 3000] and [X,Y] = [4000, 3500]. The threeareas coincide with the areas where we found MSs with too high interference




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Figure 7.7: Semi-random antenna direction

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Figure 7.8: Signal to interference power in semi-random grid simulation.




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Figure 7.9: Plot of the average number of messages per base station.

ratios in Figure 7.6. The reason that these areas occur is a combination of lowreceived power and high interference from other stations, which is a result fromthe poorly chosen positions of the antennas.

7.3 Messages per station

Besides knowing when the algorithm works it is of interest to know the amount of communication needed for convergence. The more data needed to be transmit-ted, the more interference to the existing GSM communications will be added,see [2].

The amount of data that was transmitted before the had algorithm convergedwas simulated by counting the average number of transmissions needed for alarge number of network simulations. Figure 7.9 displays plots showing thenumber of messages per BS needed for the frequency assignment to converge.These two plots are made in simulation environments where the antennas wereallowed a offset of about 40%.

As can be seen in the plots in Figure 7.9, more messages per BS are sentin the location update section than in the actual frequency assignment phase.The reason for this is that if we allow the information about the correct DBSto spread through the whole network before starting the frequency assignment,all the stations will wait for the DBS to make its choice of frequency. Whena BS receives frequency information, it will be a higher probability that the

stations between DBS and the BS-in-mind have a correct frequency assignment,thus making it easier for it to choose a correct set. This leads to the point thatthe more reliable the location information is, the easier it is for the stations tochoose frequency set. So ensuring the reliability of the information minimizesthe number of required iterations. Both the location update and the frequencyassignment phase are needed in the algorithm. The total amount of informa-tion needed for the autonomous frequency assignment to work is therefore the




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P ( % )

Figure 7.10: PDF of simulation success

sum of all types of messages sent in the network. If the simulations are madewith a lower amount of location information messages, the number of frequencymessages grows and results in a higher amount of messages sent in total.

As we can see in Figure 7.9, the number of messages (in small networks,0-50 stations) increases as the number of cells in the network increases. As thenet gets larger the average number of messages per base station will reach athreshold value. The shape of the curve can be described as follows. Smaller

networks will have a larger percentage of border cells (cells lying at the borderof the network area). Border cell base station have fewer neighboring stations,hence they will not receive as many messages that would require a change infrequency. As the network grows larger, the percentage of border cells willdecrease and the amount of messages per BS will reach its threshold.

7.4 Reliability of the frequency plan

To test the reliability of the algorithm two kinds of simulations were made. Inthe first simulation we will look at the number of MSs within the net that expe-rienced SINR values below the specified threshold. This simulation was made ina semi-hexagonal net structure where the base stations were allowed a random

offset up to 50% away from the center of the cell radius. The frequency planningand evaluation were made for 1000 different networks with these parameters. InFigure 7.10 we see the probabilities of a network having x% of mobile stationsthat experience too low SINR values plotted against the percentage of the totalnumber of MSs experiencing these values. The largest outage found in all sim-ulation was 1.4%. By looking at the block diagram, the percentage of outagesin the networks seems to have an exponential distribution. If we would allow




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BS offset from the cell center (% of cell radius)

Figure 7.11: Outage as a function of center offset

the network to have an outage of coverage up to 1%, this would mean that thealgorithm works with satisfaction in 98.9% of the networks tested.

The networks where the outage is above the desired level does not necessarilymean that the algorithm has failed but could appear in networks where theantennas has been positioned in unfavorable positions, as in the example inFigure 4.4.

In the second simulation we look at how the percentage of outage at the MSs

in the network varied according to the random offset of the location of the BSs.In the simulations, base stations’ locations within the hexagon’s varied from 0to 100% of the cell radius. How the percentage of outages over a large numberof network simulations increased as a function of the randomization of the BSlocation. This could be seen in Figure 7.11.

As we can see in Figure 7.11 the percentage of outage stays moderate whenthe base station locations are allowed to vary up to 50% of the cell radius. Byletting the center offset go above 60% we will encounter larger areas of deadspotsthat will lead to higher outage in the network. This is due to larger areas wherethere is no serving base station. The plot only goes up to 100% of the cell radius,because higher values would probably only make the stations swap positions,and no operator would consider positioning the base station in this manner.



Chapter 8

Conclusions and future

Work

This section summarizes the results of the simulations of the proposed algo-rithm. Section 8.1 discusses some of the weaknesses of the algorithm and someproposed solutions. In section 8.2 a number of ideas on future work and ideasfor implementing implement them are presented.

8.1 Conclusions of the results

In this thesis an algorithm for autonomous frequency planning for base stationshas been developed and examined. The goal was to design the algorithm so thatit could be implemented for an existing network with minimal change to the ex-isting units. For the development and evaluation of the proposed algorithmGSM was used as an example network. By having the base stations trans-mit small amounts of information using CDMA modulation a way for the basestations to find out about their surroundings and choose a frequency set thatmitigates the intercell interference was developed. The training times shouldtake place at times when the traffic is normally low in the networks, for exam-ple late night/early morning. This will affect the existing communication in thenetwork less, and therefore most of the mobile users will be unaware of thatthe frequency planning is taking place. The algorithm was tested in two typesof network environments: a perfect hexagonal- and a semi-random grid setup.These two network setup types have their limitations, but are sufficient for the

testing needed to prove the advantages of this algorithm. The algorithm pro-posed for the frequency assignment works with satisfactorily in environmentswith moderate scattering of the positions of the base stations. This algorithmworks well in simulations and demonstrates the feasibility of autonomous fre-quency planning. However before it could be implemented in a real system,it would need some improvements, especially for stabilizing the assignment. Insection 8.3, Future Work, some examples of improvements are presented, as well

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CHAPTER 8. CONCLUSIONS AND FUTURE WORK 60

as short discussions of ways to implement similar functions.

8.2 Ideas on improvements

The simulator constructed for testing and evaluation of this algorithm couldbe improved. More realistic simulations could be done if the simulator werechanged so that it could simulate a real time network. This would allow oneto find out exactly how long the frequency assignment would take. It couldalso test the optimal time for the base stations to wait and verify frequencyinformation before transmitting.

8.3 Future work

In the future , extra functions could be added to the existing algorithm. Thesewould include both functions for better stabilization of the existing algorithmand extra functions for more reliable testing and frequency assignment. Belowsome of the ideas that have been considered during the development of thealgorithm so far are listed.

Better Antenna tuning

When deploying a cellular network, people are sent out to measure the receivedpower at different positions to calculate how the antennas on each base stationshould be tuned for optimal coverage. The antenna tuning feature that haveimplemented in the simulations shown in this report is basic and it turns theantennas towards other stations that are in the vicinity to better cover more of the physical area. This antenna tuning function can be expanded to resemblethat which is done by hand at the moment. At all times during a mobilecommunication (MS-BS), the MS periodically transmits informational data backto the BS concerning the signal quality and the SINR. An idea to get a moreself-calibrating network of base stations would be to deploy the a network thathas implemented the autonomous frequency planning algorithm proposed in thisthesis. When the net is up and running you could have all the base stationsregistering and saving the received SINRs during a period of time. Using thisdata and a more sophisticated algorithm that tunes the antennas in a similarmanner to that what is done by hand nowadays would make the network closeto self operating.

Frequency decision with help from mobile stations

Explained in this thesis is how to automatically choose frequencies from receiv-ing messages from other base stations. In the antenna tuning section above itis also explained that the MS periodically transmits data of the quality of thereceived signal. Combining these two could probably make the algorithm morestable and reliable. After the frequency assignment is made the base stations



CHAPTER 8. CONCLUSIONS AND FUTURE WORK 61

could save the data received from the MSs and use these as an extra criterionin how to pick the LIFS.

Time-stamps on messages and information

When deploying this algorithm in an existing net of base stations it could berecommended that the base stations add time-stamps to the information keptin their database. With this time information the base stations can delete oldinformation. This is a way to keep their saved information up to date, and bythis way making their choice of frequency more reliable.

Levels of reliability of data

As described in the section of data reliability, the stations will wait a while toverify some data before transmitting. If we had a real time simulator, the timeto wait could b e evaluated. With this knowledge the stations would transmitmore reliable information and this would result in fewer messages sent in total.Even with the stations waiting before transmissions, the time for convergencecould be cut. This is because the more reliable the information is, the morelikely the stations will be to choose a correct set directly without numerousiterations.



Abbreviations

BS Base StationBSC The Base Station ControllerBTS Base Transceiver Station

CDMA Code Division Multiple AccessCGI Cell Global IdentityDBS Designated Base StationFDMA Frequency Division Multiple AccessGSM Groupe Spcial MobileLIFS Least Interfering FrequencyMS Mobile StationMSC The Mobile Switching CenterPGS Perfect Grid SetsSINR Signal to Interference Noise RatioTDMA Time Division Multiple Access

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Bibliography

[1] Umts 30.03 v3.2.0 tr 101 112, 1998-4. Appendix B.

[2] P. Koorevaar and J. Ruprecht. Frequency overlay of GSM and cellular B-

CDMA. 48(3), 1999.

[3] J F. Kurose and K W. Ross. Computer Networking, a top-down approach

featuring the internet . Pearson Education, Inc., 2:nd edition, 2003.

[4] M. Mouly and M-B. Pautet. The GSM System for mobile communications.1992.

[5] J G. Proakis and M Salehi. Communication systems engineering . Prentice-Hall, Inc, 2:nd edition, 2002.

[6] S. M. Redl, M. K. Webster, and M. W. Oliphant. An Introduction to GSM .Artech House Publishers, 1995.

[7] B. Sklar. Rayleigh fading channels in mobile digital communication sustemspart 1: Charaacterization. june, 1997.

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