intelligent frequency hopping

HELSINKI UNIVERSITY OF TECHNOLOGY

Department of Electrical and Communications Engineering

Riku Ertimo

Planning and performance analysis of intelligent frequencyhopping GSM networks

Thesis submitted for fulfillment of the requirement for the degree of Master ofScience in Technology. Espoo _____________.

Supervisor:Prof Pertti Vainikainen

Instructor:Juhani Huttunen DTech

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Helsinki University of Technology Abstract of the Master's Thesis__________________________________________________________________________Author: Riku Ertimo

Name of the Thesis: Planning and performance analysis of intelligent frequency hoppingGSM networks

Date: 07. Jun. 99 Number of pages: 96__________________________________________________________________________

Faculty: Department of Electrical and Communications Engineering

Professorship: Radio Engineering Code: S-26__________________________________________________________________________

Supervisor: Prof Pertti Vainikainen

Instructor: Juhani Huttunen DTech__________________________________________________________________________

The main objective of this work was to study the capacity and the quality gains achieved bymeans of IFH solution. Another important objective was to find out the improvement in thequality that can be obtained by using computer aided network planning methods, and howwell the computerized network planning supports the actual implementation of the plan in areal network.

This work concentrates on the analysis of two GSM network capacity enhancement features:frequency hopping (FH) which is a standard GSM feature, and intelligent underlay-overlay(IUO) which is a feature proposed by Nokia. Combined FH and IUO is referred to asintelligent frequency hopping, IFH. In frequency hopping the frequency of the carrier waveis changed according to predefined spreading code known by the transmitter and receiver.IUO on the other hand is based on dividing the frequency band into two separate layershaving different reuse patterns. This way the spectral efficiency of network can be improved.

The analysis of the quality and capacity improvements achieved by means of IFH werestudied using simulations, and also a field test trial was conducted in co-operation with oneof Nokia’s customer to verify the gain achieved with IFH. According to simulations IFH canprovide a capacity gain of 35% when compared with pure frequency hopping networks.Based on the field test trial the capacity gain of IFH is around 39% over FH, which verifiesthe simulation results. In all these cases the frequency allocation was performed manually,thus the real interference was not taken into account in the allocation phase. When usingNokia’s network planning tool NPS/X, which tries to minimize the interference in thenetwork in the frequency allocation, the quality of the network was even better in terms ofdrop call rate.

In this thesis some guidelines are also given for how the networks utilizing IFH should beplanned. In addition, this work tries to outline how the future data services, HSCSD andGPRS, will interact with intelligent frequency hopping.__________________________________________________________________________Keywords: frequency hopping, IUO, IFH, GSM, radio network planning__________________________________________________________________________

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TEKNILLINEN KORKEAKOULU Diplomityön tiivistelmä__________________________________________________________________________Tekijä: Riku Ertimo

Työn nimi: Älykkäiden taajuushyppelyä käyttävien GSM-verkkojen suunnittelu jaevaluointi

Päivämäärä: 07.06.1999 Sivumäärä: 96__________________________________________________________________________Osasto: Sähkö- ja tietoliikennetekniikan osasto

Professuuri: Radiotekniikka Koodi: S-26__________________________________________________________________________Työn valvoja: Professori Pertti Vainikainen

Työn ohjaaja: TkT Juhani Huttunen__________________________________________________________________________

Työn päämääränä oli tutkia sitä kapasiteetti- ja toisaalta laatuparannusta, joka voidaansaavuttaa hyödyntäen älykästä taajuushyppelyä (IFH) GSM-verkossa. Toinen tärkeä tavoiteoli tutkia kuinka tietokonepohjaisia verkkosuunnittelumenetelmiä voidaan hyödyntää itseverkkosuunnittelussa, ja kuinka suuren laatuparannuksen tietokonepohjainen suunnittelutarjoaa.

Työssä tutkittiin kahta GSM-verkoissa käytettävää ominaisuutta, joilla voidaan parantaaverkon suorituskykyä. Toinen ominaisuuksista on taajuushyppely, joka on GSM-verkkojenstandardoitu ominaisuus. Toista puolestaan kutsutaan IUO:ksi, joka on vain Nokiankäyttämä ominaisuus GSM-verkoissa. Taajuushyppelyssä kantoaallon taajuutta vaihdellaanennalta määrätyssä järjestyksessä samaan tahtiin sekä lähetin- että vastaanotinpäässä. IUOpuolestaan perustuu käytettävissä olevan taajuusalueen kahtiajakoon, joista toisella alueellakäytetään perinteistä taajuustoistumaa, mutta toisella paljon perinteistä tiukempaataajuustoistumakuviota. Tällä tavoin spektritehokkuutta saadaan parannettua.

IFH:lla saavutettavaa kapasiteetti- ja laatuparannusta tutkittiin simuloimalla. Myöskenttätestejä suoritettiin erään Nokian asiakkaan verkossa, jotta IFH:sta saatava hyötyvoitaisiin varmemmin näyttää toteen. Simulointien mukaan IFH:n avulla voidaan parantaakapasiteettia noin 35% verrattuna vastaavaan taajuushyppelevään verkkoon. Kenttätesteissähavaittiin, että kapasiteetti todellisuudessa parani noin 39% verrattuna taajuushyppeleväänverkkoon. Taajuussuunnittelu tehtiin kaikissa em. tapauksissa manuaalisesti, jollointodellista muiden käyttäjien aiheuttamaa häiriötä verkossa ei voitu ottaa huomioontaajuusallokoinnissa. Kokeita suoritettiin myös käyttäen taajuussuunnitelussa Nokianverkkosuunnitteluohjelmaa, NPS/X:ää, jolloin häiriö verkossa pyritään minimoimaan.Tietokonepohjaisen taajuussuunnittelun antamat tulokset olivat vieläkin parempiaverrattaessa manuaalisesti tehtyyn taajuussuunnitteluun.

Tässä työssä hahmotellaan myös suuntaviivoja, joiden mukaan IFH-verkkoja pitäisisuunnitella. Samoin esitetään joitakin näkökohtia tulevaisuuden datapalveluiden, HSCSD:nja GPRS:n, sekä toisaalta IFH:n vuorovaikutuksista keskenään.__________________________________________________________________________Avainsanat: taajuushyppely, IUO, älykäs taajuushyppely, GSM, radioverkkosuunnittelu__________________________________________________________________________

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PREFACE

This thesis has been made for Nokia Telecommunications, Radio Network Planning Tools inEspoo. I would like to thank Nokia Telecommunications for providing the opportunity toprepare this work. Especially I would like to thank Juhani Huttunen who has been theinstructor of this thesis, and whose assistance and guidance in defining the subject andcontents of this work has been valuable. Also Jari Ryynänen, to whom I am very grateful,has given me many valuable comments concerning the contents of this thesis. I also owespecial thanks to Jaakko Melamies for co-operation in IUO blocking considerations, and forproviding support in Matlab programming.

Professor Pertti Vainikainen has been the supervisor of this thesis. To him I owe the greatestthanks for the advice and interest he has shown to this work.

And last, I would like to thank my parents and my fiancée Minna for the support andunderstanding during this project.

Espoo, 07. June 1999

Riku Ertimo

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ABSTRACT

LYHENNELMÄ

PREFACE

CONTENTS

TABLES AND FIGURES

LIST OF SYMBOLS

ACRONYMS

1 INTRODUCTION..............................................................................................14

2 GENERAL OVERVIEW OF GSM ....................................................................16

2.1 GSM system architecture ...................................................................................................................162.1.1 Basic network elements ...............................................................................................................162.1.2 Frequency band ...........................................................................................................................172.1.3 Access method.............................................................................................................................18

2.2 Transmission in air interface .............................................................................................................192.2.1 Source coding..............................................................................................................................192.2.2 Channel coding............................................................................................................................192.2.3 Interleaving..................................................................................................................................202.2.4 Ciphering.....................................................................................................................................202.2.5 Modulation method .....................................................................................................................20

2.3 Channel organization .........................................................................................................................212.3.1 Physical and logical channels ......................................................................................................212.3.2 Frame structure in GSM ..............................................................................................................21

2.4 Radio resource management..............................................................................................................222.4.1 Power control ..............................................................................................................................222.4.2 Discontinuous transmission .........................................................................................................232.4.3 Handover .....................................................................................................................................23

3 PRINCIPLES OF FH, IUO AND IFH ................................................................24

3.1 Properties of radio path .....................................................................................................................243.1.1 Large scale path loss....................................................................................................................243.1.2 Shadow fading.............................................................................................................................243.1.3 Multipath time delay spread ........................................................................................................243.1.4 Doppler spread ............................................................................................................................25

3.2 Properties of frequency hopping .......................................................................................................263.2.1 Frequency hopping theory ...........................................................................................................263.2.2 Frequency diversity .....................................................................................................................283.2.3 Interference diversity...................................................................................................................28

3.3 Frequency hopping in GSM...............................................................................................................293.3.1 Hopping modes ...........................................................................................................................293.3.2 MA lists and Hopping sequences ................................................................................................303.3.3 MAIO management .....................................................................................................................30

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3.3.4 Loading of FH system .................................................................................................................313.3.5 Reuse factor of frequency hopping network ................................................................................323.3.6 Frequency hopping gain ..............................................................................................................34

3.4 Intelligent underlay overlay ...............................................................................................................363.4.1 Principles of IUO ........................................................................................................................363.4.2 IUO parameters ...........................................................................................................................383.4.3 Intelligent frequency hopping......................................................................................................39

4 NETWORK PLANNING AND IFH SYSTEM SOLUTION.................................41

4.1 Principles of network planning..........................................................................................................41

4.2 Radio link measurements ...................................................................................................................424.2.1 Signal strength.............................................................................................................................424.2.2 Bit error rate and RXQUAL........................................................................................................434.2.3 Frame erasure ratio......................................................................................................................434.2.4 Drop call rate...............................................................................................................................444.2.5 Handover success rate .................................................................................................................444.2.6 Subjective voice quality measures...............................................................................................44

4.3 Nokia's system solution for IFH-networks........................................................................................454.3.1 Nokia's implementation in BSS...................................................................................................454.3.2 Nokia's network planning system (NPS/X) .................................................................................464.3.3 Network management system (NMS).........................................................................................47

5 IFH PLANNING STRATEGIES ........................................................................48

5.1 Planning concepts ...............................................................................................................................485.1.1 Frequency split between layers....................................................................................................485.1.2 TRX configurations.....................................................................................................................485.1.3 Effects of traffic distribution .......................................................................................................49

5.2 Blocking of IUO networks..................................................................................................................49

5.3 IFH-planning using NPS/X ................................................................................................................515.3.1 Allocation process .......................................................................................................................515.3.2 Automatic reference cell generation ............................................................................................535.3.3 Interference analysis ....................................................................................................................55

5.4 Simulations ..........................................................................................................................................565.4.1 Simulator .....................................................................................................................................565.4.2 Blocking probabilities .................................................................................................................585.4.3 Effect of direct access to super ....................................................................................................585.4.4 Simulated capacity gain of IFH ...................................................................................................59

6 FIELD TRIAL ...................................................................................................61

6.1 Trial environment ...............................................................................................................................61

6.2 Test cases .............................................................................................................................................616.2.1 Pure frequency hopping cases .....................................................................................................616.2.2 IFH cases .....................................................................................................................................62

6.3 Measurements .....................................................................................................................................636.3.1 Statistics collected in OMC.........................................................................................................636.3.2 Walk and drive tests ....................................................................................................................64

6.4 Transitions between regular and super layers .................................................................................646.4.1 C/I thresholds ..............................................................................................................................64

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6.4.2 Quality handovers........................................................................................................................656.4.3 Absorption...................................................................................................................................656.4.4 Direct access to the super layer ...................................................................................................67

6.5 Quality and capacity improvements..................................................................................................686.5.1 Traffic and Handovers.................................................................................................................686.5.2 Drop Call Rate.............................................................................................................................696.5.3 RXQUAL distributions ...............................................................................................................726.5.4 FER .............................................................................................................................................736.5.5 Quality gain of IFH .....................................................................................................................756.5.6 Quality estimated by NPS/X........................................................................................................766.5.7 Capacity gain of IFH ...................................................................................................................77

6.6 Network planning methods for IFH ..................................................................................................786.6.1 Manual planning..........................................................................................................................786.6.2 NPS/X planning...........................................................................................................................806.6.3 Combinations of manual and NPS/X planning ............................................................................816.6.4 MAIO management .....................................................................................................................816.6.5 Interference caused by the second adjacent channel....................................................................82

6.7 Performance of SDCCH and TCH ....................................................................................................83

7 PERFORMANCE OF IFH WITH OTHER FEATURES.....................................85

7.1 High speed circuit switch data (HSCSD) ..........................................................................................85

7.2 General packet radio system (GPRS)................................................................................................86

7.3 IFH in GSM900/GSM1800 networks ................................................................................................87

8 CONCLUSIONS...............................................................................................90

9 REFERENCES.................................................................................................92

10 APPENDICES..................................................................................................94

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TABLES AND FIGURES

Table 3.1 An example of MAIO allocation with synthesized RF hopping. ...........................................................................31Table 3.2 IUO handover parameters in BSC. ........................................................................................................................39Table 4.1 Mapping of RXLEV. .............................................................................................................................................42Table 4.2 Relation between BER and RXQUAL. .................................................................................................................43Table 4.3 Mapping between FER and subjective speech quality...........................................................................................44Table 4.4 Correspondences between SQI and MOS classes..................................................................................................45Table 5.1 Parameter settings used in the simulator................................................................................................................57Table 5.2 Summary of the simulation results.........................................................................................................................60Table 6.1 Frequency configurations in the trial. ....................................................................................................................63Table 6.2 Example of offset planning....................................................................................................................................82Table 6.3 Examples of manual planning with consecutive and punctured frequency groups................................................83Table 7.1 The data rates with different channel coding and different number of TSs. ..........................................................85Table 7.2 Data rates supported by GPRS. .............................................................................................................................87

Figure 2.1 Generic GSM system architecture. .......................................................................................................................17Figure 2.2 GSM bands...........................................................................................................................................................18Figure 2.3 Multiple access methods used in GSM. ...............................................................................................................18Figure 2.4 Principles of the burst forming in GSM (Speech/FS)...........................................................................................20Figure 2.5 Frame structure in GSM.......................................................................................................................................22Figure 3.1 Different signal multipaths. ..................................................................................................................................25Figure 3.2 Graphical presentation of rms delay spread..........................................................................................................27igure 3.3 Frequency correlation as a function of frequency spacing......................................................................................28Figure 3.4 Difference between BB- and RF-FH. ...................................................................................................................29Figure 3.5 An example of frequency load in an RF hopping cell. .........................................................................................32Figure 3.6 Frequency reuse of 7 and 3. .................................................................................................................................33Figure 3.7 Frequency diversity gain of frequency hopping link against co-channel interference compared to a non-hopping

link [Sal98]. ................................................................................................................................................................35Figure 3.8 Values of the averaged on a call C/I ratios, not exceeded at an outage probability equal to 10%, against the

carried traffic per cell (Erl) [Sal98].............................................................................................................................36Figure 3.9 Frequency reuse in IUO network. ........................................................................................................................37Figure 3.10 Handover hysteresiseris area in an IUO cell.......................................................................................................38Figure 4.1 Different hopping schemes...................................................................................................................................45Figure 4.2 NPS/X block diagram ..........................................................................................................................................47 Figure 5.1 Different schemes to share frequencies. ..............................................................................................................48Figure 5.2 Blocking probabilities of the regular layer as a function of offered load with different direct access to super (DA)

probability factors. ......................................................................................................................................................50Figure 5.3 Blocking probabilities of the super layer as a function of offered load with different direct access to super (DA)

probability factors. ......................................................................................................................................................51Figure 5.4 Relations between different hierarchical structures..............................................................................................52Figure 5.5 Actions prior to frequency allocation [Nok98f]. ..................................................................................................53Figure 5.6 Interference Calculation Area definition. .............................................................................................................54Figure 5.7 Simulated BER as a function of C/I. ....................................................................................................................56Figure 5.8 BER as a function of I/C. .....................................................................................................................................56Figure 5.9 Call dropping procedure in the simulator.............................................................................................................57Figure 5.10 Comparison of simulated and calculated blocking probabilities with 1+1 TRX configuration of IUO. ............58Figure 5.11 Dropped call versus direct access to super threshold .........................................................................................59Figure 6.1. Principle of the reuses in RF hopping cases........................................................................................................62Figure 6.2 Reuse patterns in easy IFH cases..........................................................................................................................62Figure 6.3 Reuse patterns in heuristic IFH cases. ..................................................................................................................63Figure 6.4. Absorption in different IFH cases. ......................................................................................................................66Figure 6.5. Minimum and maximum absorption. ..................................................................................................................67Figure 6.6 Failed and unsuccessful handovers due to lack of resources in all the test cases. ................................................68Figure 6.7 The number of call and handover attempts in all the test cases. ...........................................................................69Figure 6.8 Drop call rates with effective reuses in different FH cases...................................................................................70Figure 6.9 Drop call rates with effective reuses in different IFH cases. ................................................................................70Figure 6.10 Drop call rate as a function of effective frequency loading in IUO, FH and IFH cases. ....................................71Figure 6.11 The proportion of RXQUAL classes 6 and 7 with effective reuse in different FH cases. ..................................73Figure 6.12 The proportion of RXQUAL classes 6 and 7 with effective reuse in different IFH cases. .................................73Figure 6.13 FER distribution within each RXQUAL class for BCCH frequency layer.........................................................74Figure 6.14 FER distribution within each RXQUAL class for hopping regular layer. ..........................................................75

Riku Ertimo: Planning and performance analysis of intelligent frequency hopping GSM networks

Chapter 1: Introduction9

Figure 6.15 FER distribution within each RXQUAL class for the super layer......................................................................75Figure 6.16 Drop call rate as a function of effective frequency loading in modified IUO, FH and IFH cases. .....................76Figure 6.17 RXQUAL 1-7 distributions estimated using NPS/X interference analysis tool, and measured in the actual

network. ......................................................................................................................................................................77Figure 6.18 Capacity gain as a function of DCR. ..................................................................................................................78Figure 6.19 TCH and SDCCH success rates with different effective reuses. ........................................................................84



LIST OF SYMBOLS

α angle between MS and BSβ wave numberδ(t) impulse functionλ wavelengthλ call arrival probabilityµ call ending probabilityρ correlationτ mean excess delay∆ω difference of angular velocities

A offered traffica(i)n,m transition probability coefficientai reflection coefficient of the ith pathBc coherence bandwidthBER(C/I) BER as a function of carrier-to-interference ratio (C/I)C number of channels in systemc speed of lightD frequency reuse distanceDTX discontinuous transmission factords rms delay spreade(t) received resultant impulse signalErl(PC,NTCH) cell trafficf frequencyH average call holding timeIV interference valueJ0(⋅) Bessel function of first kind and zero orderK frequency reuse patternLeff effective frequency loadLfrac fractional loadLfreq frequency loadLHW hardware loadLMA1 length of the interfered MA listLMA2 length of the interfering MA listload frequency load factorm number of super layer channelsm number of burst over which the interleaving is performedNC number of common channels in MA list pairNcells number of cellsNf number of hopping frequencies in the serving cellNfreqs number of available frequenciesNMA number of frequencies in an MA listNTCH number of traffic channelsNTRX number of hopping TRXs in a celln absolute radio frequency channel numbern number of bits to be interleavedn integer in data speedsn number of regular layer channelsPbl blocking probability



Phit hit probabilityPI interference probabilityPn state probabilitypixD number of pixels in the dominance areapixI number of interfering pixelsp direct access probabilityR radius of a cellReff effective reuseRfa frequency allocation reuser distancer regular layer transition probabilitys super layer transition probabilitys0(t) impulse signalT time delayTErl traffic in a cellv speed



ACRONYMS

AFE Antenna Filter EquipmentAGCH Access Grant ChannelARFCN Absolute Radio Frequency Channel NumberAuC Authentication CenterBB-FH Baseband HoppingBCCH Broadcast Control CHannelBER Bit Error RateBSC Base Station ControllerBSIC Base Station Identity CodeBSS Base Station SubsystemBTS Base Transceiver StationCDMA Code Division Multiple AccessCSAC Cell Service Area ClassDA Direct Access to SuperDCR Drop Call RateDTX Discontinuous TransmissionEDGE Enhanced Data rates for GSM EvolutionEGSM extended GSMEIR Equipment Identity RegisterFACCH Fast Associated Control ChannelFCCH Frequency Correction ChannelFDMA Frequency Division Multiple AccessFER Frame Erasure RatioFG Frequency GroupFH Frequency HoppingGMSK Gaussian Minimum Shift KeyingGPRS General Packet Radio ServiceGSM Global System for Mobile communicationHCL Hierarchical Cell LayerHLR Home Location RegisterHO HandoverHSCSD High Speed Circuit Switched DataHSN Hopping Sequence NumberIFH Intelligent Frequency HoppingIUO Intelligent Underlay-OverlayLOS line of sightMA Mobile AllocationME Mobile EquipmentMOS Mean Opinion ScoreMRP Multiple Reuse PatternMSC Mobile Services Switching CenterNMS Network Management SystemNPS/X Nokia's network planning systemNSS Network SubsystemNT Non-TransparentOMC Operation and Maintenance CentrePC Power ControlPCH Paging ChannelPSTN Public Switched Telephone NetworkRACH Random Access CHannelRBER Residual Bit Error RateRELP-LPT Residually Excited Linear Predictive Coder-Long Term Predictor



RF-FH Radio Frequency HoppingRXLEV Received Signal StrengthRXQUAL Received Signal QualitySACCH Slow Associated Control ChannelSCH Synchronization ChannelSDCCH Stand-alone Dedicated Control ChannelSID Silence DescriptorSIM Subscriber Identity ModuleSQI Speech Quality IndicatorT TransparentTC TranscoderTCH Traffic ChannelsTDMA Time Division Multiple AccessTEMS Test Mobile SystemTRX TransceiverTS Time SlotVAD Voice Activity DetectorVLR Visitor Location RegisterWWW World Wide Web



1 INTRODUCTION

The history of mobile communication systems begins in the late 1940s when the first publicmobile telephone system was introduced in the US markets. The system used high poweramplifiers, and was able to cover distances of over 50 km. However, it took many decadesbefore the mobile systems became commercially important. In the late 1970s and in thebeginning of 1980 several analog cellular systems were introduced around the world. In theScandinavian countries a lot of effort was put on the development of one of the analogcellular systems (NMT). In the beginning of 90s digital cellular systems were introduced inaddition to the analog systems. The digital cellular standard developed in Europe (GSM) hasgained a worldwide acceptance as the first universal mobile system.

Wireless communication is enjoying its fastest growth in its history. This is due to the factthat the prices of the mobile terminals have decreased dramatically in the past five years.Because of this the tendency seems to be that the majority of the new subscribers is expectedto be private users. Typically private users have lower airtime demands than business users,thus the airtime usage per subscriber is expected to decrease slightly in the next few years.However, the total population of the users is expected rise significantly in many countries,thus the overall traffic to be supported by mobile networks will rise during the next fewyears. Also, the airtime usage per subscriber can be affected by, eg, tariff schemes andchanging user habits, and the effect of this fact can be difficult to foresee. To further increasethe airtime usage of private users the operators have approached individually differentmarket segments (ie business or private users) with a tariff package fitted to their needs.

There are many ways to increase the capacity of GSM network. In this thesis one possibilitythat will hopefully provide extra capacity without excessive hardware investments ispresented. The method can be divided into two separate GSM capacity enhancementfeatures. First one of these features is Frequency Hopping (FH), which is actually one of thestandard GSM features. The other one is referred to as Intelligent Underlay-Overlay (IUO),which is a capacity enhancement feature proposed by Nokia. The work is mainlyconcentrated on the capacity and quality analysis when introducing combined FH and IUO inthe network. The combined FH and IUO is referred to as Intelligent Frequency Hopping(IFH). Some suggestions how networks utilizing IFH should be planned are also given inthis thesis.

In Chapter 2 the general system architecture of GSM networks is described. The basicinformation prior to transmission, as well as the channel and frame structures in GSM areprovided. Basic cellular network concepts, such as handover and radio resource managementin general are discussed in this chapter.

Chapter 3 is concentrated on the basic functionality of frequency hopping and IUO. First, anoverview of the electromagnetic propagation is provided. Frequency hopping is discussed asone of the spread spectrum systems, and then the implementation of the frequency hoppingin GSM is described. Principles of IUO/IFH, and parameters related to IUO/IFH are alsoincluded in the chapter.

Chapter 4 outlines the problem related to the network planning process. Networkperformance indicators to be observed in IFH networks are described in this chapter. Nokia’ssystem solution for IFH is also presented here.



Chapter 5 deals with IFH planning strategies, ie how the frequencies should used in IFHnetworks, and which are the feasible hardware configurations for IFH. Some aspects of usingcomputerized planning of IFH networks are provided. In this chapter the problem related tothe dimensioning of the blocking probability in IUO/IFH network is discussed. Theperformance of IFH networks is analyzed by means of simulating the various factorsaffecting the ability of the network to absorb traffic.

The results of the field test trial conducted in co-operation with one of Nokia’s customer arepresented in Chapter 6. The suitable parameter sets, and their effect on the performance ofIFH networks are presented. The guidelines for the planning of IFH networks based on thetrial experiences are provided in this chapter.

Chapter 7 presents some other features available in GSM networks, and how these featuresand IFH interact with each other. The emphasis of the consideration is on the future dataservices, and their co-existence in the network with IFH.

The conclusions are provided in Chapter 8. Also the possible improvements to evolve theperformance of IFH networks are discussed. Some ideas related to IFH to be further testedare also given in the last chapter.


Chapter 2: General overview of GSM16

2 GENERAL OVERVIEW OF GSM

The acronym GSM stands for Global System for Mobile communication. The developmentof GSM started in early '80s in Europe, when it was realized that many European countriesused different incompatible mobile systems. Due to this, a special group was founded todevelop a new mobile system for Western Europe. Later GSM has been adopted also inmany countries outside Europe.

2.1 GSM system architecture

2.1.1 Basic network elements

A GSM network can be divided into three different subsystems, which are called NetworkSubsystem (NSS), Base Station Subsystem (BSS) and Network Management System (NMS).The actual network needed for call establishment consists of NSS and BSS. The latter isresponsible for radio path control, while NSS takes care of the call control functions. NMS isneeded for operational and maintenance purposes.

In NSS the call controlling is managed by Mobile Services Switching Center (MSC). Itidentifies the origin and the destination of the call, as well as the type of the call. In HomeLocation Register (HLR) subscriber related information is stored permanently, whereasVisitor Location Register (VLR) contains a copy of HLR. A VLR database is alwaystemporary, and it has more information eg about subscriber's location compared with HLR.Usually VLR is integrated with MSC. Authentication Center (AuC) is responsible forauthenticating the Subscriber Identity Module (SIM), and Equipment Identity Register (EIR)takes care of identifying the Mobile Equipment (ME).

In BSS Base Station Controller (BSC) is the central network element controlling the radionetwork. Base Transceiver Station (BTS) can be considered to be a slave of BSCmaintaining the air interface. In air interface the effective standard bit rate is 13 kbit/s, whileit in Public Switched Telephone Network (PSTN) is 64kbit/s. Thus a converter is needed tochange the data rates. In GSM this is called Transcoder (TC).

The purpose of NMS is to monitor various functions and elements of the network. Its majortasks can be divided into three parts: fault management, configuration management andperformance management. The generic GSM network architecture is depicted in Figure 2.1.



Figure 2.1 Generic GSM system architecture.

2.1.2 Frequency band

Normal GSM900 uses two frequency bands of 25 MHz. The positioning of these bands are890-915 MHz for subscriber-to-base station transmissions (uplink direction) and 935-960MHz for base station-to-subscriber transmissions (downlink direction) [ETS92a]. The usageof two different frequency bands allows simultaneous radio transmission and receptionbetween the mobile and the base station. This is also called full duplex. Correspondinguplink and downlink channels are always related to each other in a very simple manner:fixed frequency gap of 45 MHz, duplex separation, separates the channels. The channels arealso separated in time domain, ie transmission and reception do not happen at the sameinstant of time. If the Absolute Radio Frequency Channel Number (ARFCN) is known, thecorresponding frequency can be calculated for uplink direction using Equation (2.1) and fordownlink using Equation (2.2).

nnfUL 2.0890)( += MHz, n=1,2,… ,124 (2.1)45)()( += nfnf ULDL MHz (2.2)

In Equations (2.1) and (2.2) n corresponds the ARFCN. In some countries it may be possibleto allocate an extra frequency band of 10MHz below the normal GSM900 frequencies. Thenew frequency band is called extended GSM (EGSM), or also tri-band. This arrangementincreases the capacity. However, in order to utilize this extra capacity new kind of mobilessupporting this feature are needed. The ARFCNs between 880-890MHz are 975-1024, andthe frequency for uplink can be calculated using Equation (2.3). Equation (2.2) applies alsohere for downlink.

)1024(2.0890)( −+= nnfUL MHz, n=975,… .,1024 (2.3)

Frequency bands of 75 MHz have also been allocated for GSM around 1800MHz withduplex separation of 95MHz. All the above mentioned bands are depicted in Figure 2.2.Later in the future ETSI will specify frequency bands around 450MHz for GSM.

MSC

BSC

BSC OMCBTS

BTS

BTS

BTS

BTS

PSTNISDN

AuC

HLRVLR

EIR

BSS - Base Station Subsystem NSS - Network Subsystem

NMS - NetworkManagementSystem

AIR Abis A

TC

TC



Figure 2.2 GSM bands.

2.1.3 Access method

Different kinds of access methods are used to allow the users to share the finite frequencyresources. The most widely used multiple access methods are Frequency Division MultipleAccess (FDMA), Time Division Multiple Access (TDMA) and Code Division MultipleAccess (CDMA). In FDMA a unique frequency is assigned to an individual user. During thecall no other user is able to use that particular frequency, thus one FDMA channel canaccommodate one call at a time. In TDMA the resources between the users are shared intime domain, meaning that at the certain time only one user is able to either transmit orreceive. So in TDMA systems the transmission is based on bursts making it very attractive indigital communication systems, whereas FDMA is widely used in analog communication.GSM uses the combination of FDMA and TDMA [ETS92a]. The difference between FDMAand TDMA methods is depicted in Figure 2.3.

Figure 2.3 Multiple access methods used in GSM.

The carriers in GSM900 are positioned every 200kHz giving the total number ofindependent frequencies of 125 (25 MHz band for both up- and downlink). The duration ofone burst is 0.577 ms (more precisely 15/26 ms). One carrier contains always 8 time slots,thus one TDMA frame lasts for 60/13≈4.615 ms. There are thus 992 physical channelsavailable in GSM-band when we take also into account that at the both ends of the bandthere exists a guard band of 100kHz (124 times 8 = 992).

f1f2

f3f4

f1f2

f3f4

TS1 TS2TS0 TS3 TS4 TS5 TS6 TS7TS1 TS2TS0 TS3 TS4 TS5 TS6 TS7

FDMA TDMA FDMA and TDMAfrequency frequency frequency

time time time

code code code

EGSM900 (880-890) EGSM900 (925-935)

GSM900 (890-915) GSM900 (935-960)

GSM1800 (1710-1785)

GSM1800 (1805-1880)

UPLINK DOWNLINKDOWNLINK UPLINK

45MHz 95MHz



2.2 Transmission in air interface

2.2.1 Source coding

The speech coder in GSM is based on the Residually Excited Linear Predictive Coder-LongTerm Predictor (RELP-LPT) [ETS92b]. In the coding process the speech is first quantizedwith 8 bits using the A-law sampling rate being 8 kHz. The result is a digital signal of 64kbit/s. The data is then converted to 13 bit samples corresponding to linear representation ofthe signal amplitudes instead of A-law. The output of RELP-LPT coder provides 260 bitsevery 20ms yielding to a data rate of 13 kbit/s. In the air interface the total bit rate is about33.9 kbit/s, and if the speed rate after source coding and air interface are compared theefficiency is only 13/33.9≈38.4%.

2.2.2 Channel coding

The signal suffers from different kind of perturbations when transmitted through the airinterface. These distortions can be caused eg by noise when the received signal level is low,interference from other transmitters, Doppler shifts or multipath propagation delays. Thus,countermeasures are needed in order to avoid the perturbations.

Channel coding is a very useful method in digital communication systems to improve theperformance of the system. However, it is always trade-off between the data speed andsystem performance. In channel coding some redundant information calculated from thesource data is added to original data block. The decoding process takes advantages of theredundant bits allowing it to detect and even correct the errors occurred in the transmission.

In GSM many kind of coding schemes are used depending on the transmitted type of data.Here only one type of data, ie full speed speech, is handled in a bit more details. Moreinformation about the channel coding can be found eg in [ETS92c].

The source-coded data of 260 bits is first divided into 3 groups (type Ia, type Ib and type II)depending on the importance of the data: the more important the bits are the better they arecoded. The most important 50 bits (type Ia) have a parity check for the detection of the non-correctable errors. Type Ib bits and 4 tail bits are then concatenated to these 53 bits resultingto a data block of 189 bits. The tail bits are included to initiate the convolution encoder.Next the convolutional code is applied for error correction purposes. The rate of theconvolution encoder is ½ having constraint length K=5. The resulting data block has a lengthof 378 bits. The rest 78 bits (type II) are not coded at all, and they are appended to theexisting sequence as they are giving the total number of 456 bits. This corresponds to thedata rate of 22.8 kbits/s. The ratio between speeds of channel coded data and air interface is22.8/33.854≈67.3%. The graphical representation of error detection and correction for fullspeed speech in GSM can be seen is Figure 2.4.



Figure 2.4 Principles of the burst forming in GSM (Speech/FS).

2.2.3 Interleaving

The convolutional coding is not very efficient in the error protection when severalconsecutive bits are in error. On the other hand, the errors tend to occur in bursts, thusinterleaving is used to ease the error correction and so to improve the performance of thesystem.

Basically interleaving is based on spreading the n bits of a code word into the m bursts. Thebigger the value of n is, the more randomly the bits (also errors) are positioned in the burstafter de-interleaving. The gain of the interleaving increases when the value of n is increased.However, the bigger the value of m is the longer is the decoding delay time of the system,thus compromise between them have to be made. In order to simplify the implementation,the choice of n and m should be made so that n/m is an integer. The length of the code workin GSM is 456 bits for speech, thus allowing the value of m to be eg 4, 8, 24 or 76. Innormal speech a value of m=8 is used resulting to 57 bits, each filling up half a burst (oneburst being 144+2 bits, as will be explained later). As an example it can be mentioned thatthe interleaving depth is 19 for data services, 4 for General Packet Radio Service (GPRS),and 8 (ie the same as in speech) for High Speed Circuit Switched Data (HSCSD).

2.2.4 Ciphering

The encryption of the data is accomplished by performing an exclusive-or operation betweenthe burst and a pseudo-random sequence. The length of the sequence is 114 bits. Theciphering sequence is generated from the burst number and the encryption key transmitted inthe beginning of the session by means of signaling. The deciphering is done using the sameoperation, since performing an exclusive-or operation twice generates the original data flow[Mou92].

2.2.5 Modulation method

The modulation method used in the radio interface in GSM is Gaussian Minimum ShiftKeying (GMSK) with the normalized bandwidth product BT of 0.3. GMSK can beconsidered to be a derivative of MSK, where Gaussian pulse shaping smoothens the phasetrajectory of the MSK resulting to lower sidelobe levels in the transmitted power spectrum.This gives quite high spectrum efficiency while at the same time the demodulation is not toocomplicated. The modulation rate is about 270 kbit/s. More information about the used

Type Ia, 50 bits Type Ib, 132 bits Type II, 78 bits

50 3 132 4

Parity bits

378 78

Convolutional Code, rate 1/2, constraint length 5

456 bits/20ms speech frame

Tail bits



modulation method can be found in [ETS92d].

Enhanced Data Rates for GSM Evolution (EDGE) has been proposed by ETSI in order toevolve data services in GSM reusing as much of the physical layer as possible. EDGE isbased on a new modulation technique with the current working assumption for EDGE being8PSK. There three consecutive bits are mapped into one symbol on the I/Q diagram. With asymbol rate of 270 kbit/s data rates from 22.8 kbit/s to 69.2 kbit/s can be achieved dependingon the channel coding [ETS98].

2.3 Channel organization

2.3.1 Physical and logical channels

The combination of certain frequency and Time Slot (TS) form one physical channel. It cancontain a varying amount of logical channels. The logical channels, on the other hand, can bedivided to control and traffic channels.

There are six different Traffic Channels (TCH) in GSM. They vary depending on the speedand the transmitted data type (speech or user data). One traffic channel may carry eitherspeech or data, and it has the same functions and formats on the uplink and downlinkdirections.

The number of control channels is even greater. They carry signaling and synchronizinginformation between the base station and the mobile stations. Different kinds offunctionalities exist depending on the link direction. Frequency Correction Channel (FCCH)allows the mobile user to synchronize itself to the frequency of the base station.Synchronization Channel (SCH) gives the necessary synchronization information to themobile. Broadcast Control CHannel (BCCH) is used to provide the mobile station with theinformation such as network identity, current control channels, channel availability andcongestion situation. Paging Channel (PCH) carries paging signals from base station tomobile stations notifying a certain mobile of an incoming call. After responding to paging aphysical channel (ie ARFCN and TS number) is assigned to the mobile using Access GrantChannel (AGCH). All the above mentioned logical channels are transmitted in the downlinkdirection using TS0 at the BCCH carrier having certain ARFCN. In the uplink direction atthat same particular frequency only one channel called Random Access CHannel (RACH) istransmitted. RACH is assigned for acknowledging the paging from PCH, and mobiles tooriginate a call also use it.

In dedicated mode there are three different kind of dedicated control channels being all bi-directional. Stand-alone Dedicated Control Channel (SDCCH) is used to carry the necessarysignaling information before TCH assignment. Slow Associated Control Channel (SACCH)is used to transmit the supervisory data between the mobile and the base station during thecall. If the capacity of SACCH is insufficient more signaling capacity is arranged via FastAssociated Control Channel (FACCH). This is accomplished by means that framesoriginally allocated for TCH are now used by FACCH [ETS92c].

2.3.2 Frame structure in GSM

The smallest transmission quantum in GSM is called a burst. There are several differentburst types in GSM [ETS92a], but here only normal burst is dealt with in a bit more details



as an example. An actual normal burst begins and ends with 3 bits called tail bits. In themiddle of the burst is a 26 bit midamble, or training sequence, which is exploited in thesynchronizing and in determining the coefficients for channel correction device. The bitspositioned on both sides of the training sequence are called "stealing flag". Those bits are toindicate that instead of speech this frame is used for FACCH purposes. That kind ofsituation might happen eg in handover where more signaling is required. Next to the stealingflags are the actual 57 data bits giving the total number of 114 data bits in one burst. In thevery end of the burst is a guard period of 8.25 bits to make the practical implementation ofthe TDMA frame structure possible.

In Figure 2.5 is shown the way in which the different frames are connected to each other indedicated mode. One TDMA frame consists of 8 bursts containing 8*156.25=1250 bits. ATDMA frame forms a multiframe, where every 13th and 16th frame is used for signalinginstead of speech. A multiframe is then grouped into superframe and hyperframe; the laterforming the basis for frame numbering which is important in GSM since the encryptionalgorithm relies on particular frame number. Security can only be maintained by using alarge number of frames (26*51*2048=2 715 648 TDMA frames)

Figure 2.5 Frame structure in GSM.

2.4 Radio resource management

2.4.1 Power control

Power Control (PC) has two main functions: Firstly, its aim is to reduce the interferencelevel in the network by adjusting the transmitter power while still maintaining the acceptablequality level defined by the operator [ETS95]. Secondly, it is used in conserving the MSbattery power. This, of course, applies only power control in uplink direction and is widelyused by operators. On the other hand, DL PC is not widely used. This is due to the fact thatfrequencies have to be assigned to base stations in such a manner that sufficient C/I value isachieved in the highest interference situation usually occurring at cell borders. Since PC inGSM is relatively slow, response time being around 2 seconds, utilizing DL PC may in somecases endanger calls at cell borders. Also baseband frequency hopping (explained in Section3.3.1) may cause problems in DL PC because some mobiles have problems dealing with

0 1 2 4 5 6 7

3 31 157 57 8.25

Tail bits

Tail bits

Encrypteddata

Encrypteddata

Stealingflag

Stealingflag

Midamble Guardperiod

26

10 25

0 2 49 50

2047

48

20462045204420430 1 2 3 4 5

HYPERFRAME

SUPERFRAME

TDMA FRAME

15/26=0.577ms

8*15/26=4.615ms

26*8*15/26=120ms

120*51=6.12s

2048*6.12=3h 28min 53s 760ms

0

0 1

49 50

25

26 MULTIFRAME 51 MULTIFRAME

NORMAL BURST

2042

1

24

124

3



large changes in signal level.

Based on the previous trial is has been concluded that the average interference reductiongain achieved with PC is around 1.0-1.5 dB. However, if the majority of the mobiles arepositioned at cell edges the gain of PC is reduced.

2.4.2 Discontinuous transmission

Discontinuous Transmission (DTX) is another GSM specific optional feature to improve thequality or the capacity of the network and to increase the battery life. Basically DTX, whenactivated, will reduce the amount of transmission and consequently overall interference inthe network. Namely, during a normal conversation the participants alternate so that eachdirection is occupied about 50% (or less) of the time. When using DTX the transmitter isswitched on only for those frames containing useful data. [ETS92e]

The overall DTX mechanism requires the following functions. A Voice Activity Detector(VAD) on the transmitter side decides whether each speech frame of 20 ms contains speechor not. If VAD detects that there is no speech present the next step is the evaluation of theacoustic background noise on the transmitter side in order to transmit the characteristicparameters to the receiver side. After determining the parameters Silence Descriptor (SID)frame is encoded conveying information on the acoustic background noise. At the receiverside comfort noise must be generated based on the SID frames during those periods wherethe radio transmission is cut. The above mentioned procedure is due to the fact that when theconnection is cut the noise level drops to a very low level. The noise step would beperceived as very annoying by users and some countermeasures must be introduced. A SIDframe is sent at the beginning of every inactive period, and is then repeated at least twice asecond as long as the inactive period in speech lasts. In addition, the measurement done bythe mobiles have to be reported, and thus SACCH frames are always transmitted no matter ifDTX is used or not.

2.4.3 Handover

Making the traffic connection between BS and moving MS is possible with the help ofHandover (HO). The basic concept is quite simple: when the subscriber moves from thecoverage area of the cell in charge to another, a connection with the new cell has to be setup, and the connection with the old cell has to be released in order to avoid loosing the callin progress. In practice there are also other reasons than coverage area itself affecting thedecision whether the handover is to be performed or not.

Handover due to measurements occurs if the quality or the field strength of the radio signalfalls below certain level defined by specific parameters in BSC. The deterioration of thesignal is detected by constant signal measurements carried out by both the MS and BTS.Even if the handover is triggered the transmission quality can still be adequate. It can happenthat the global interference situation can be improved by means of performing a handover.

The second kind of handover is referred to as a traffic handover. It can occur that the trafficis unevenly distributed in the network one cell being in congestion while another cell is stillhaving free capacity. A traffic peak in one cell can be eg due to a sport event taking place inthat particular area. In such a case the mobile stations near the edges of the cell may behanded over to neighboring cells which have smaller traffic loads. However, this kind ofhandover has to be handled with great care, since usually the target cell is not the bestpossible cell if the quality of the connection is considered.


Chapter 3: Principles of FH, IUO and IFH24

3 PRINCIPLES OF FH, IUO AND IFH

In this chapter we concentrate on the basic functionality of frequency hopping, IUO and IFH.Frequency hopping is discussed as one of the spread spectrum systems, and then theimplementation of frequency hopping in GSM is described. Principles of IUO/IFH, andparameters related to IUO/IFH are also included in this chapter.

3.1 Properties of radio path

Contrary to wired communication systems where the behavior of the channel can becomparatively easy to predict, radio channel is usually not due to its random nature. Thetransmission path between transmitter and receiver can vary from line of sight (LOS) to thepath that is severely obstructed by eg buildings, hills and foliage. For that reason the analysisof radio channel is complicated, and a lot of effort has been put to develop propagationmodels that are accurate enough.

3.1.1 Large scale path loss

The propagation mechanisms of the electromagnetic waves can in general be attributed toreflection, diffraction, scattering, ducting and attenuation [Lin94]. The surrounding obstaclesand morphographic types, ie buildings, roads, lakes, foliage etc. affect the propagation of theradio wave causing severe distortion to the signal. The simplest case is the free spacepropagation in which the path loss can be calculated using Equation (3.1).

24

=

crfL π , (3.1)

where c is the speed of light, f frequency and r the distance between transmitter and receiverantennas.

In the environment where mobile phones traditionally are used the exponent of r can be onthe order of 3.5, or even higher, meaning that the signal attenuates much faster thanpredicted by Equation (3.1). Thereby, free space propagation model does not give veryaccurate predictions for mobile telecommunication systems, hence more sophisticatedmethods have to be used. Commonly used prediction methods are eg Okumura-Hata andWalfish-Ikegami models [Rap96].

3.1.2 Shadow fading

Shadow fading is caused by large obstacles, such as hills and buildings. The received signalstrength is attenuated by these obstacles resulting to fluctuations of the signal level as themobile moves. Shadow fading, also referred to as slow fading, can statistically be modeledwith log-normal distribution [Lee89]. The fluctuation of the signal can be compensated byusing adaptive power control. This means that the output power of the BS and/or MS ischanged based on the received power level at the other end.

3.1.3 Multipath time delay spread

The fading effects (also referred to as fast fading) due to the multipath time delay spread canbe classified either as a flat or frequency selective fading depending on the time dispersion



characteristic of the channel. If the amplitude response is constant and the phase response islinear over the bandwidth, which is greater than the bandwidth of the transmitted signal, thechannel is said to be a flat fading channel. In the flat fading channel the spectrum of thereceived signal is preserved. However, the gain of the channel is time dependent due tofluctuations in the amplitude response of the channel. Flat fading channels may also bereferred as narrowband channels. The amplitude fluctuations of a flat fading channel can bemodeled using Rayleigh distribution [Rap96].

If the amplitude response is constant and the phase response is linear over the bandwidthwhich is smaller than the bandwidth of the transmitted signal, frequency selective fadingoccurs in the channel. In that case the received signal consists of copies of the transmittedsignal, which are attenuated and delayed in time resulting to distortions in the received signalspectrum. The received electromagnetic field is the vector sum of all the signal copies withdifferent amplitudes and phase shifts, thus countermeasures are needed to overcome thisproblem. Frequency selective fading channels can also be referred to as wideband channelssince the bandwidth of the transmitted signal is wider than the channel impulse response.

3.1.4 Doppler spread

Doppler spread is due to the fact that the properties of the radio channel vary depending onthe motion of the receiver relative to the transmitter and obstacles [Rap96]. Differentmultipaths have different Doppler shifts, and the received signal consists of copies of thetransmitted signal each one having different Doppler shifts. If the transmitter is assumed tobe fixed, the maximum Doppler shift of the channel is

αλ

cos2v

fD = , (3.2)

where v is the speed of the MS, λ wavelength and α the angle between MS and BS in case ofLOS signal. The multipath propagation and the generation of Doppler shifts are illustrated inFigure 3.1.

Figure 3.1 Different signal multipaths.

A channel can be divided into fast or slow fading channels depending on rate the transmittedbaseband signal changes compared with the channel characteristics. If the impulse responseof the channel changes within a symbol duration the channel is classified as a fast fadingchannel. However, this occurs only for very low data rates. If the impulse response of thechannel, on the other hand, changes much slower than the transmitted baseband signal it isconsidered as a slow fading channel.

BS

MSv

vLOS



3.2 Properties of frequency hopping

3.2.1 Frequency hopping theory

Frequency hopping is one of the spread spectrum techniques. It means that the bits are firstmodulated using certain modulation scheme, eg GMSK in GSM, and then the frequency ofthe carrier wave is changed according to predefined spreading code known by the transmitterand receiver. In general, frequency hopping systems can be divided into two differentcategories: fast and slow frequency hopping. In slow frequency hopping the hopping rate issmaller than the symbol rate. On the other hand, in fast frequency hopping the frequency ischanged faster than the symbol rate. GSM utilizes slow frequency hopping.

When considering frequency hopping two concepts, rms delay spread and coherencebandwidth, must be introduced. Let s0(t)=aoδ(t) be the impulse signal where δ(t) is the Diracdelta function. Now the received resultant impulse signal e(t) is spread in time due tomultipath scattering and it can be expressed [Lee93] in

tjN

iii eTtaate ωδ −

=∑ −=

10 )()( , (3.3)

where Ti is the time delay and ai is the coefficient of the ith path. The rms delay spread isdefined as [Rap96]

2__

2 )(ττ −=ds , (3.4)

where the mean excess delay τ is the first moment of the power delay profile defined as

∑∑

=k

k

kkk

e

e

2

2ττ (3.5)

and __

2τ is the second moment of the power delay profile

∑∑

=k

k

kkk

e

e

2

22__

2τ

τ . (3.6)

The rms delay spread can thus be considered to be the standard deviation of the mean excessdelay time. In order to clarify the situation a principle of rms delay spread concept isdepicted in Figure 3.2.



S0(t)=a0δ(t)

e(t)=|a0ai|

t=0 t0 t[s]

A[V/m]

τ_

ds ds

Figure 3.2 Graphical presentation of rms delay spread.

Coherence bandwidth is a statistical measure of the range of the frequencies over which thechannel response remains the same, ie all the spectral components pass the channel withequal gain and linear phase. This means that the potential correlation between two signals atadjacent frequencies is strong. The coherence bandwidth depends on rms delay spread, andthe correlation coefficient between two received signals as a function of frequencyseparation and time separation can be calculated to be [Lee89]

22

20

)(1)(

),(δω

τβτωρ∆+

=∆ vJr , (3.7)

where ∆ω=2π∆f, τ is time separation, J0(⋅) the Bessel function of first kind and zero order,β=2π/λ, v velocity of the vehicle and δ rms delay spread. If the frequency correlationfunction is set to be 0.5 the coherence bandwidth Bc can be derived to be

πδ21=cB , (3.8)

if τ is supposed to be zero. However, it should be noted that any exact relationship betweentime delay spread and coherence bandwidth does not exist, and this is purely a theoreticalmodel. In Figure 3.3 is depicted the frequency correlation as a function of frequency spacingwith different time delay spread values in order to illustrate the Equation (3.7). The timeseparation τ is again supposed to be zero.



0%10%20%30%40%50%60%70%80%90%

100%

0 100 200 300 400 500 600 700 800 900 1000

Frequency spacing/kHz

Freq

uenc

y co

rrel

atio

n0.20.30.5123

rms time delay

spread / µs

Figure 3.3 Frequency correlation as a function of frequency spacing.

The coherence bandwidth is highly depended on the propagation environment since thesignal delay spread values vary case by case. In rural environment the time delay spread is inorder of 0.2 µs while in urban areas is can be as high as 3 µs. In Figure 3.3 it can be seen thata frequency spacing of 800 kHz corresponding 4 GSM frequencies will give adequatefrequency diversity in rural areas if the requirement for frequency correlation is set to 50%.However, in urban environment a frequency spacing of 50-150 kHz is enough with the samecriteria.

3.2.2 Frequency diversity

The received signal is a vector sum of number of copies of the initial signal having differentphases and amplitudes. The sum varies depending on the frequency and the location of thedisrupting obstacles relative to the receiver. Since the resultant signal is frequency dependedat some locations there may occur very low field strengths, called fading dips, while at someother frequency the field strength can be on its maximum value. However, it is most unlikelythat two fading dips occur at a certain location on another frequency supposing that thespacing between the two separate frequencies is big enough determined by the coherencebandwidth.

Frequency hopping takes advantage of the fading dips not occurring at two uncorrelatedfrequencies at the same location. The frequency is changed burst by burst, hence only someof the bursts are affected by the deep fade while most parts of the bursts are receivedproperly. This enables the reconstruction of the original signal by taking advantage ofinterleaving and utilizing error correction techniques.

3.2.3 Interference diversity

Due to the limited bandwidth assigned for GSM networks the same frequencies must beused several times in order to obtain the required coverage and grade of service. This causesdegradation in the quality of the network depending on how intensively the frequencies arereused. The longer the distance between two base stations transmitting on the samefrequency the better carrier to interference ratio (C/I) can be obtained.



However, the interfering signals are not uniformly distributed in the network, hence at somelocations C/I-value can be high while somewhere else it can be very low, ie the quality of theconnection is poor. In a conventional non-hopping network a frequency having low C/I-value can be assigned for a user resulting to degraded quality for that particular connection.If FH is utilized, the mobile will be using the severely interfered frequency only a smallportion of time, and probably the bursts on other frequencies will experience lowerinterference level so that the quality of the connection remains satisfactory. Thus theinterference is averaged among all the users in the network. In practice it means that thenumber of good and bad quality samples will decrease leading to better average quality.

3.3 Frequency hopping in GSM

3.3.1 Hopping modes

From BSS point of view frequency hopping feature can be implemented using eitherBaseband Hopping (BB-FH) or Radio Frequency Hopping (RF-FH). The choice of thehopping mode does not affect the functionality of the mobiles in frequency hoppingnetworks [Nok96].

In baseband hopping the Transceiver (TRX) transmits on a fixed frequency. The call isswitched burst by burst to be transmitted on a different TRX. Two different hopping groupshave to be generated in each cell. Time slots 1 to 7 belong to hopping group 1 and all thetime slot 0s excluding the BCCH timeslot belong to group 2. This is due to the fact that theinformation BCCH frequency contains must be transmitted continuously to allow themobiles to be able to attach to the base station. In other words TS0 of the BCCH frequencycannot hop, and although the other time slots are allowed to hop the power has to betransmitted continuously on every time slot on the BCCH frequency. In BB-FH the numberof frequencies to hop over is equal to the number of TRXs.

RF-FH differs in many ways from BB-FH. In RF-hopping mode the TRX contains afrequency synthesizer allowing rapid frequency changes. Also a wideband combiner isneeded. In theory the frequencies over which one TRX can hop is 63, so the number offrequencies over which to hop in one cell can be much bigger than the number of TRXs. Forthat reason, in RF-FH the BCCH TRX cannot hop because BCCH frequency must betransmitted continuously. The rest of the TRXs belong to the same hopping group. Thedifference between BB- and RF-FH is clarified in Figure 3.4.

Figure 3.4 Difference between BB- and RF-FH.

B BTRX-1 TRX-1

TRX-2

TRX-2

TRX-3

TRX-4

f

f

f

1

f

2

3

4

f1

ff

2

3

fn

.

.

.

ff

2

3

fn

.

.

.

TS0 of TRX-2, TRX-3 and TRX-4 hop over f2, f

3and f

4

B=BCCH timeslot. TRX-1 does not hop. B=BCCH timeslot. TS0 of TRX-1 does not hop.

TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7 TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7

TRX-2 is hopping over f1- f

n. .

TS1-TS7 are hopping over - .f1

f4

Baseband hoppingRadio frequency hopping



3.3.2 MA lists and Hopping sequences

In a non-hopping network the channel assignment is quite straightforward. The number ofdifferent cases is only 124 in GSM900, and the channel number to be used in that particularconnection can easily be encoded and transmitted form BS to MS by means of signaling[Mou92]. However, when frequency hopping is exploited, the situation becomes morecomplicated. The number of different frequency combinations explodes dramatically, andsome other means have to be introduced to overcome this problem.

MA stands for Mobile Allocation. It is a list containing the frequencies to be used during theconnection between MS and BS if frequency hopping is utilized. MA-list is a subset of thechannels allocated to a cell. The maximum number of frequencies in GSM900 in one MA-list is 63.

Hopping Sequence Number (HSN) defines in which order the frequencies assigned in MAlist to a mobile must be used in frequency hopping case. There are 64 different hoppingsequences, ie HSN can take a value between 0-63. HSN 0 is reserved for a sequentialsequence. This is also referred to as cyclic hopping. The rest of the hopping sequencenumbers are pseudo random sequences. They are usually referred to as random hoppingsince the frequencies appear randomly instead of cyclic order. The use of cyclic hopping isusually not preferred since random hopping gives in some cases better interference diversity.

3.3.3 MAIO management

There is no GSM system limitation on allocating the same MA-list to TRXs in differentsectors in sectorised cell. Because the sectors usually have the same HSN some means has tobe introduced to avoid the collisions between the frequencies of the different sectors. This isaccomplished with MAIO offset and MAIO step [Nie98]. MAIO offset gives each of thecells sharing the same MA-list a unique offset, or a starting point where the hopping isstarted. This ensures that the cells will not use the same frequencies simultaneously. Toavoid the collisions between TRXs, MAIO step will separate the frequencies within a cell. Ifthe MAIO step is 2, the MA list needs to include at least twice as much frequencies as thereare TRXs sharing the same MA list to avoid the usage of the same frequency simultaneouslywithin the site. Generally, if NTRX is the number of the frequencies sharing the same MA listand MAIOSTEP the size of MAIO step the minimum number of frequencies NMIN can becalculated to be

.STEPTRXMIN MAIONN = (3.9)

MAIO concept can be used provided that synthesized RF-hopping is used, the sectorssharing the same MA list are synchronized and the MA list contains at least as manyfrequencies as given by Equation (3.9). An example of MAIO concept is presented in Table3.1. For simplicity reason the HSN is N=0, ie cyclic hopping is used. The MA list consists ofconsecutive frequencies MA={1,2,3,4,5,6,7,8,9,10,11,12}. With MAIO step 2 it is possibleto avoid adjacent channel interference.



Table 3.1 An example of MAIO allocation with synthesized RF hopping.

3.3.4 Loading of FH system

When contemplating the factors limiting the capacity in a frequency hopping network twodifferent phenomena can be found, ie hard blocking and soft blocking. By hard blocking ismeant that the whole radio resource offered by the base station is in use. At that instant oftime no more calls can be established. If the capacity is limited by hard blocking themaximum traffic the cell can support can be found using the Erlang B table. The Erlang Btable gives the maximum offered load that the system can tolerate if the number of availablechannels and blocking probability are known. A typical design criteria is 2 % blocking,meaning that 2 % of the incoming calls will be blocked due to lack of hardware resources ofthe base station. If the cell becomes soft blocked the capacity of an individual cell is notlimited by lack of hardware resources but interference. Soft blocking is usually dominating ifthe way that the frequencies are reused is very aggressive, ie that the same frequencies areused in adjacent cells. In case the network becomes soft blocking limited there has to be amechanism by which the interference of the network can be controlled. If a certain threshold,soft blocking limit, has been exceeded the establishment of a new call will result toincreasing number of dropped calls or bad quality. In practice only RF-hopping networks canbe interference limited.

When considering the load concept in a frequency hopping network two different loadfactors can be distinguished. First of them is called fractional loading. A cell is said to befractionally loaded when the number of frequencies assigned to a hopping cell exceeds thenumber of TRXs equipped into the cell. The fractional load is given by Equation (3.10)[Sal98]

MA

TRXfrac N

NL = , (3.10)

where NTRX is the number of hopping TRXs in a cell, and NMA is the number of frequenciesin the MA list. Fractional loading has two benefits compared with the conventionalimplementation of frequency hopping. The interference diversity achieved by means offractional loading is bigger since the bigger is the number of frequencies over which to hop

HSN MAIO offset MAIO step MAIO Hopping sequenciesSector 1 N 0 2TRX 1 BCCH frequency 1TRX 2 0 1 2 3 4 5 6 7 8 9 10 11 12TRX 3 2 3 4 5 6 7 8 9 10 11 12 1 2Sector 2 N 4 2TRX 1 BCCH frequency 2TRX 2 4 5 6 7 8 9 10 11 12 1 2 3 4TRX 3 6 7 8 9 10 11 12 1 2 3 4 5 6Sector 3 N 8 2TRX 1 BCCH frequency 3TRX 2 8 9 10 11 12 1 2 3 4 5 6 7 8TRX 3 10 11 12 1 2 3 4 5 6 7 8 9 10



the better is the interference averaging. This actually yields also the second advantage, sinceproper hopping gain can be achieved even with small number of TRXs as more frequenciescan be allocated to one cell. In practice, the implementation of fractional loading requiresRF-hopping. In BB-hopping each frequency needs its own TRX which makes theimplementation of fractional loading uneconomical. For that reason the fractional load is inBB-hopping case Lfrac=1.

Hardware load, on the other hand, is determined as a relation of the Erlangs carried by thecell, TErl , to the number of the traffic channels, NTCH :

TCH

ErlHW N

TL = , (3.11)

Frequency load Lfreq is determined as a product of hardware and fractional load, Equation(3.12)

TCHMA

ErlTRXHWfracfreq NN

TNLLL == (3.12)

and it actually tells the degree of utilization of the hopping frequencies. Frequency load isillustrated in Figure 3.5. Since 3 TRXs are hopping over 5 frequencies the fractional load isLfrac= 3/5=0.6. HW load is LHW=18/24=0.75 leading to the frequency load of Lfreq=0.45.

Figure 3.5 An example of frequency load in an RF hopping cell.

If the frequencies are reused very intensively some of the bursts are necessarily lost due tocollisions occurring between the bursts. However, by lowering the frequency load the hitprobability that two bursts collide can be made so small that when exploiting the errorcorrection techniques available in GSM the lost bits can be recovered in spite of collisions.In practice it means that the length of the MA list must be long enough.

3.3.5 Reuse factor of frequency hopping network

The internationally allocated frequency band to GSM is limited and further it is nationallyshared between operators by local authorities. For that reason an individual frequency cannotbe assigned to every cell, thus in order to fulfil the required Grade-of-Service (GoS) samefrequencies must be reused in distant cells. A given radio channel can be reused if the twobase stations sharing the same frequency are so far away from each other that the twooccurring signals do not cause too severe co-channel interference in the cell border areas.

In order to obtain the full coverage in a certain area, in the theoretical analysis the area maybe substituted with hexagonals. For hexagonal cells the Equation (3.13) holds [Lee89]

TRX-1

TRX-2

TRX-3

TRX-4

TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7BCCH f1

f2 f3 f4 f5 f6

f2 f3 f4 f5 f6

f2 f3 f4 f5 f6

Active slots Empty slots



KRD 3= , (3.13)

where R is the radius of the cell, D is the frequency reuse distance and K is the frequencyreuse pattern defined by shift parameters i and j. K must satisfy the condition K=i2+j2+ijwhere i and j are integers. The situation is depicted in Figure 3.6. By increasing K thefrequency reuse distance D also increases resulting to the reduction of the co-channelinterference.

Figure 3.6 Frequency reuse of 7 and 3.

Defining the frequency reuse factor in a frequency hopping network is a bit morecomplicated than in a conventional non-hopping network. When the frequency reusedistance in a conventional non-hopping network becomes too small the, quality of thenetwork is not satisfactory at the cell border areas due to the severe interference caused bythe neighboring base stations. The same applies to BB-hopping networks, but because offrequency hopping gain a bit smaller reuse distance can be used. However, in case of RF-hopping the reuse distance can be set as small as wanted. This is due to the fact that an RF-hopping cell can accommodate more frequencies than there are TRXs. Thus, the frequenciesare only fractionally loaded as presented in Figure 3.5 leading to two different reusedefinitions: effective reuse and frequency allocation reuse [Sal98].

Effective reuse is determined as

aveTRX

freqs

cellsTRX

cells

freqseff N

N

NN

NR

,1

==∑

, (3.14)

where Nfreqs is the number of available frequencies, Ncells is the number of cells and NTRX isthe number of TRXs in cells. Frequency allocation reuse, on the other hand, can becalculated as

aveMA

freqs

cellsMA

cells

freqsfa N

N

NN

NR

,1

==∑

, (3.15)

1

2

3

4

5

6

7

1

2

3

1

2

3

4

5

6

7

1

2

3

4

5

6

7

1

2

3

1

2

3

1

2

3

1

2

3

K=7 K=3

D

R

i

j



where Nfreqs is the number of available frequencies, Ncells is the number of cells and NMA isthe number of frequencies in MA lists. In non-hopping and BB-hopping cases NTRX,ave=NMA,ave.

3.3.6 Frequency hopping gain

Frequency hopping gain is achieved by two means, which are interference and frequencydiversity gain. If frequency hopping is implemented in a non-hopping network withoutchanging frequency reuse pattern better quality can be obtained. However, usually it is notworth having the quality in the network above certain minimum threshold, and the improvedquality can thus be transferred to capacity improvement by tightening the reuse factor of thenetwork. It is up to operators to decide whether better quality or more capacity is to bedesired.

Both frequency and interference diversity gain has been studied in [Sal98]. In Figure 3.7 thefrequency diversity gain is presented in case of co-channel interference as a function offrequencies over which to hop over. The gain was studied with 1,2,3,4,5,6,8 and infinitenumber of frequencies, 1 being the non-hopping case. The simulations were performed fortwo different kind of propagation environment, TU3 and FLAT3, and also two differentquality measures were used. TU3 stands for Typical Urban with mobile speed of 3 km/h.TU3 model considers six propagation paths and in the simulations six statisticallyindependent fading processes were generated for each carrier in the hopping sequence forwanted and interfering signals. FLAT3 model is a time-dependent one path Rayleigh fadingmodel mobile speed being 3km/h. The frequency hopping was simulated so that the numberof statistically independent fading processes was equal to the number of frequencies in thehopping group. It was also generated for wanted and interfering signals. The quality criteriawas set so that Residual Bit Error Rate (RBER) was not allowed to exceed 0.2% for class 1bbits, see Figure 2.4, and Frame Erasure Ratio (FER) had to be below 3%. One framecontains a 20ms speech sample transmitted in 8 consecutive bursts. More information aboutFER will be provided in Section 4.2.3. In Figure 3.7 it can be seen that according to thesimulations the maximum frequency diversity gain achieved using cyclic FH isapproximately 9dB. It can also be seen that the gain saturates when the number offrequencies increases. In these simulations interference diversity gain can be neglected dueto cyclic hopping and synchronized bursts.



0

1

2

3

4

5

6

7

8

9

No hop 2 3 4 5 6 8 Infinite

Number of carriers

∆C/Ic

(dB

)FLAT 3FER = 3%

FLAT3RBERCl 1b = 0,2%TU3FER = 3%

TU3RBERCl 1b = 0,2%

Figure 3.7 Frequency diversity gain of frequency hopping link against co-channelinterference compared to a non-hopping link [Sal98].

Interference diversity gain was simulated separately from frequency diversity gain. In thiscase the simulated network consisted of 175 cells each having three sectors. The availablebandwidth was 5.4 MHz. Three different frequency allocation reuses were simulated, namely9,6 and 3. A non-hopping case with Rfa=9 was also simulated for a reference. The results arepresented inFigure 3.8. It can be seen that the interference diversity gain is around 4-5 dB for all theabove mentioned configurations. For configurations having Rfa=9 and 6 the network becamehard blocking limited, ie in this case 2% blocking level was achieved with certain load.However, when considering Rfa=3 it is possible to provide 9 frequencies for each cellmeaning that maximum 9 TRXs can be allocated to every cell. Now it is not feasible to loadthe cells up to hard blocking limit since C/I value falls below acceptable level before eg 2%blocking level is reached. The network is said to be soft blocking limited. According to thesesimulations nearly 100% capacity gain is achieved using Rfa=3 compared with a non-hopping network.

Interference diversity gain depends also on the order in which the frequencies are used. Inrandom hopping the source of interference varies from burst to burst, and the interference ismore or less averaged over the entire network. On the other hand, if cyclic mode is usedwhere the frequencies are used in a consecutive order the whole interference diversity gaincan be lost especially if grouped frequency planning is used, ie the same number offrequencies and TRXs is allocated to every cell. Namely, in this case the bursts belonging toa certain connection still collide in spite of changing the frequency burst by burst. Whenconsidering frequency diversity gain the situation is different. When using random hoppingthe frequency for a burst is selected from MA-list according to predefined pseudo randomsequence, and it is possible that the same frequency is used in consecutive bursts. That iswhy cyclic hopping can be more feasible in small configurations, where the probability ofusing the same frequency in consecutive bursts is big if utilizing random mode.



Figure 3.8 Values of the averaged on a call C/I ratios, not exceeded at an outageprobability equal to 10%, against the carried traffic per cell (Erl) [Sal98].

The speed of the mobile is a dominant factor in frequency hopping gain. The best frequencyhopping gain is obtained by slow moving mobiles. The faster the subscriber is moving thesmaller is the frequency hopping gain. However, the quality of the connection is notremarkably decreased since the lost frequency hopping gain is compensated with the gainobtained from moving.

3.4 Intelligent underlay overlay

Unlike frequency hopping, Intelligent Underlay-Overlay (IUO) is a Nokia specific GSMnetwork capacity enhancement feature. IUO can offer to the operator better spectralefficiency and thus more capacity without making any major hardware investments orextensive network modifications. The improved capacity is achieved by using new softwarein base stations. The mobile stations are not affected by IUO at any level.

3.4.1 Principles of IUO

IUO is based on dividing the frequency band into two separate layers having different reusepatterns. The frequency allocation of the upper layer, referred to as a regular layer, whichprovides the continuous coverage in the network is based on conventional frequency reuse.In order to achieve more capacity, the frequencies are allocated in the lower layer veryaggressively. The lower layer is referred to as a super layer. The different reuse patterns areillustrated in Figure 3.9, where the effective reuse factor of the regular layer is Reff,reg=12,and for the super layer Reff,sup=3.

13

14

15

16

17

18

19

20

14 16 18 20 22 24 26 28 30 32

Carried traffic per cell (Erl)

C/I

(dB

) reuse 3 FHreuse 6 FHreuse 9 FHreuse 9 No FH

soft blocking limitedblocking probability

Frequency hopping

No frequency hopping

2%

2%

2%



f1

f2 f3

f4

f5 f6

f10

f11 f12

f13

f13f13

f14

f14f14 f15f15

f15

f 7

f 8 f 9

f7

f8 f9

f15

f13

f14

f 1 3

f 1 5f 1 4

High customer density

Loeer customer densityRegular reuse frequenciesSuper reuse frequncies

Figure 3.9 Frequency reuse in IUO network.

Due to the tighter frequency reuse leading to higher interference in the cell border areas theservice area of the super layer is smaller compared with the regular layer. In order to avoidthe degradation of the quality caused by the increased interference, BSC directs the mobilestation to those frequencies that are good enough to sustain the required radio connectionquality. In practice this means that the super frequencies can only be used if C/I value isabove certain predefined threshold. BSC calculates the C/I value based on the measurementsreported by the mobiles. The C/I ratio is calculated by comparing the downlink signal levelof the serving cell and the downlink signal level of the six nearest neighbor cell sharing thesame super reuse frequencies [Wig97] using Equation (3.16),

[]∑=

= 6

1iin

c

P

PIC , (3.16)

where Pc is the measured power of serving channel, and Pn[i] is the measured power ofBCCH (channel) in neighboring cells.

In an IUO network the call is always started on a regular frequency, because during the callsetup on SDCCH the quality of the connection is not yet known for sure. After the callestablishment BSC starts calculating the C/I ratio for that particular connection, and if theC/I ratio is above certain predefined good C/I threshold the mobile is handed over to one ofthe super frequencies. BSC continues the evaluation of the C/I ratio after the mobile hasentered the super frequency. If the ratio is below certain predefined bad C/I threshold themobile is handed back to one of the regular frequencies. If the regular layer becomescongested it is clear that the mobiles are not able to enter super frequencies even if there stillwere free capacity on the super frequencies. However, a new feature called direct access tosuper has been introduced that allows the mobiles to enter the super frequencies directlyfrom SDCCH provided that the received signal strength exceeds certain predefined level. Inthat case the quality is assumed to be good enough. Since this is only an estimation of thequality the actual quality is not known for sure. That is why the criterion must be set highenough to ensure adequate quality. This feature should only be used if congestion occurs onthe regular layer.



3.4.2 IUO parameters

Intelligent underlay overlay brings several new parameters to BSC. These parameters do notplace any special requirements to the other parameter settings but rather provide new meansto adjust C/I calculation and handover evaluation process of the IUO traffic control. Thehandover control parameters are controlled on a cell by cell basis [Laa96].

SuperReuseGoodCiThreshold and SuperReuseBadCiThreshold are the parameters triggeringthe handovers between the super and the regular layers. The first one is the threshold valuefor downlink C/I ratio to trigger a handover from the regular TRX to the super TRX.Correspondingly, the second one is the threshold value for triggering a handover from superto regular TRX. Of course, SuperReuseGoodCiThreshold must be higher thanSuperReuseBadCiThreshold. Lowering these values will obviously increase the potentialservice area of a super TRX. This is highly desirable since the balance of the traffic movestowards the super layer where frequency efficiency is better. However, decreased quality andan increased number of handovers back to the regular layer indicates that the thresholds areset to be too low. There are also two other parameters closely linked to the SuperReusethresholds. In both cases, Px is the number of comparisons out of total comparisons wherethe downlink C/I value has to be greater than or equal the threshold before the handover ispossible. The change in C/I has to be long-standing before handover is triggered. If areaction to the rapid changes in C/I is needed the value should be lowered. Nx is the totalnumber of comparisons to be taken into account before the handover is possible. Thehysteresis area defined by SuperReuseGoodCiThreshold and SuperReuseBadCiThreshold isdepicted in Figure 3.10. The difference between those two parameters should be set highenough to avoid useless handovers between layers.

Figure 3.10 Handover hysteresis area in an IUO cell.

SuperReuseEstMethod defines the method to be used in the C/I evaluation procedure tocalculate the downlink C/I ratio of the super TRX. The two alternative methods are averagetaking method and maximum taking method. If the value is set 'not in use' intelligentunderlay overlay procedure is not employed in the BTS. IntfCellAveragingWindowSizedetermines the number of successive downlink signal strength measurement samples of aninterfering cell to be used in the averaging process. The value must be big enough to ensurethat rapid variations in received signal level are eliminated. IntfCellNumberOfZeroResultindicates the number of zero results, which can be omitted when the measurement results ofthe interfering cells are being averaged for the C/I evaluation process. By using thisparameter the distortion of measurement results caused by unheard interfering neighbors canbe diminished. AllInterferingCellsAveraged defines whether measurement results will beaveraged for all the interfering cells or only for those interfering cells that are among the sixbest neighbor cells in the latest measurement report.

Base station

Super layer

IUO HO margin

Regular layer



A very important IUO parameter is MinBsicDecodeTime. It determines the period startingfrom the call set up or handover during which a handover to a super TRX is not possiblebecause the decoding of Base Station Identity Code (BSIC) of the neighbors is not finished.BSIC is referred to as a "color code" which allows the mobiles to distinguish the cellssharing the same beacon frequency. In the IUO capacity point of view it is desirable toreduce the time before a handover to the super layer is performed to as short as possible. Ahandover attempt can be performed immediately after C/I evaluation, handover decision andtarget evaluation procedures are finished, provided that the MinBsicDecodeTime hasexpired. Transition to the super layer can be speeded up by adjusting the parameters relatedthis procedure. However, if eg BSIC decoding time is reduced there is a risk that C/Iestimate is unreliable.

The last IUO related parameter introduced here is EnableIntraHoInterUL, which indicateswhether an intra cell handover within a super reuse frequency group caused by uplinkinterference is enabled. An intra cell handover is safe to perform if the other super TRX hasthe same interferers. All the IUO handover parameters are summarized in Table 3.2. Also thedefault value and the range of each parameter is provided.

Table 3.2 IUO handover parameters in BSC.

3.4.3 Intelligent frequency hopping

Combined IUO and FH is called Intelligent Frequency Hopping (IFH), which is a Nokiaspecific feature. The capacity gain achieved by IFH is due to the fact that because offrequency hopping gain the frequencies can be reused more aggressively leading to betterspectral efficiency. This applies to both regular and super frequencies. However, the tighterreuse factor cannot be used with BCCH frequencies since they are not allowed hop and thusfrequency hopping gain is lost. Actually, in BB hopping only TS 0 is not allowed to hop, butsince TS 0 is the restrictive factor when determining the frequency reuse pattern it cannot betightened.

As in IUO, also in IFH BSS is capable of managing two sets of frequencies: one for theregular layer and one for the super layer. Both layers have their own MA-lists, and also thehopping sequences can be assigned independently for both layers. It is possible to have bothlayers hopping, or only either one of the layers hopping the other one being at non-hopping

PARAMETER RANGE DEFAULTSuperReuseEstMethod - MaximumSuperReuseGoodCiThreshold -127dB to 127 dB 17Px (for good C/I threshold) 1 to 32 8Nx (for good C/I threshold) 1 to 32SuperReuseBadCiThreshold -127dB to 127 dB 12Px (for bad C/I threshold) 1 to 32 10Nx (for bad C/I threshold) 1 to 32 8IntfCellAveragingWindowSize 1 to 32 SACCH 8 SACCHsIntfCellNumberOfZeroResults 0 to 31 SACCH 7 SACCHsAllInterferingCellsAveraged - YesMinBsicDecodeTimo 0 to 128 SACCH 10 SACCHsEnableIntraHoInterUL - Yes



mode. However, the hopping mode has to be the same for both layers, ie hopping mode canbe either BB hopping or RF hopping but not a mixture of those.


Chapter 4: Network planning and IFH system solution41

4 NETWORK PLANNING AND IFH SYSTEM SOLUTION

In this chapter the problem related to the network planning process is outlined. Networkperformance indicators important for IFH networks are described, as well as Nokia’s systemsolution for IFH.

4.1 Principles of network planning

The radio network planning process can be defined as a sequence of planning actions inwhich a given radio system is configured to satisfy the offered traffic demand and fulfillingthe given quality criteria in a certain geographical area. The main targets of the networkplanning are to achieve the required radio coverage, to maximize the network capacity withthe available frequency spectrum still maintaining the certain quality of service, and mostimportant from the operator’s point of view to minimize the network infrastructure cost. Thenetwork planning process can be divided into five main phases: network dimensioning,coverage planning, transmission planning, frequency planning, and parameter planning[Nok98a].

The aim of dimensioning is to give an estimation of the amount of hardware, eg the numberof cells and carriers, needed to fulfill the needs of the offered traffic. Other outputs of thedimensioning phase are transmitter powers and cell ranges acquired from link budgetcalculations. The most valuable input for the capacity assessment in the dimensioning phasewould be a traffic density map which can be based eg on the statistics of population, incomelevel, land usage, and mobile phone penetration and distribution. However, usually a trafficdensity map is not available, and often guesses based on eg data collected from an existingnetwork have to be made. Other input values for dimensioning are eg time frame of theproject, system and frequency band available for the operator, and economical factors.

In an IFH network the TRXs must be divided between regular and super layers which makesthe capacity estimation even more difficult. The placement of IFH cells is also of greatimportance: it is feasible to place an IFH cell close to the high traffic density area. Of course,the traffic efficiency of the super layer is limited by its ability to absorb traffic. If the superlayer becomes congested, new calls cannot be transferred there. Also, if the regular layerbecomes congested, it is possible that a new connection cannot be established althoughhardware resources were available on the super layer. What makes the capacity assessmenteven more difficult is that the capacity an IFH network can carry depends also on theassigned parameters, especially good and bad C/I thresholds. By increasing the thresholdsmore traffic can be carried by the super layer.

Coverage and transmission planning are closely related to each other. They should always becarried out simultaneously. The aim of coverage planning is to meet the desired coverageprobability requirements in a specified area. Coverage probability means that field strengthexceeds a given threshold value in a certain number of locations (eg 90%). This is due to thefact that it would be too expensive, even impossible, to implement such a plan that thecoverage is provided in all locations. Digital maps including topography, morphography, andbuilding information are an essential part of coverage predictions since the accuracy of thepredictions depends partly on the accuracy of the map. Another issue of great importanceaffecting the predictions is the chosen propagation model. It must be suitable for thepropagation environment (eg rural or dense city) in question. Also measurements are veryimportant when tuning the several parameters that are included in the propagation models.The coverage area of a given cell can be extended by increasing the power of TRX, choosing



an antenna with higher gain, using feeder cables having lower loss or increasing the towerheight. In transmission planning it must be guaranteed that enough transmission capacitybetween sites exists. When selecting the site leased lines must be available, or then it mustbe checked that line of sight (LOS) exists between sites for microwave links.

The purpose of frequency planning is to maximize the efficiency of spectrum usage withinthe given minimum quality requirements and bandwidth. Because frequency assignmentrequirements are quite difficult to consider in the capacity calculations, capacity estimationand frequency planning form an iterative process. This frequency assignment process isbased on the interference analysis of the network. Interference analysis is based on coverageanalysis, because the aim is to calculate carrier-to-interference ratio, ie what is the relationbetween the field strengths of the serving channel and interfering channel(s). This is a verylimiting factor when designing a cellular network since the available bandwidth is verylimited. The aim of frequency allocation is to plan the frequencies in such a way that the co-channel C/I as well as the adjacent channel C/I exceed the given system and the qualitydependent thresholds in the serving area of each cell. In general the frequency assignmentproblem is en extremely complicated task, and usually computer-based systems have to beused to overcome this problem.

The performance of the designed network is optimized by selecting suitable BSS parametersstored in the BSC radio network database. The parameter planning can begin with a defaultparameter set. An important part of this phase are the field test measurements, in which callsare generated in the test routes driven in the real environment. Parameters can then beadjusted according to the measurements. It is possible that all the problems, eg handoverfailures, cannot be solved by tuning the parameters, which requires returning to previousplanning phases. Thus the whole planning forms an iterative process.

4.2 Radio link measurements

4.2.1 Signal strength

The Received Signal Strength (RXLEV) is measured by both MS and BSS, and the receivedinput shall be reported by MS to BSS every 480ms (corresponding 104 TDMA frames). Therange of the measurements is from –110 dBm to –48 dBm, and these power levels aremapped to RXLEV values according to Table 4.1. The are 64 different RXLEV categories,thus 6 bits are required to transmit the received power level.

Table 4.1 Mapping of RXLEV.

RXLEV Received power P [dBm]0 P<-1101 -110<P<-1092 -109<P<-108… …61 -50<P<-4962 -49<P<-4863 P>-48



4.2.2 Bit error rate and RXQUAL

The Received Signal Quality measure (RXQUAL) is used as a criterion in the RF powercontrol and handover decision, and it is one of the most important and used quality measuresin GSM networks. RXQUAL is measured by both MS and BS and reported every SACCH,ie every 480 ms, further to BSC which makes the actual decisions concerning PC and HO.RXQUAL is actually an estimation of Bit Error Rate (BER), but instead of BER RXQUALis reported to BSC [ETS95]. BER is estimated before channel decoding and the calculatedBER is then mapped to eight RXQUAL classes according to Table 4.2. As can be seen theBER values increase exponentially as RXQUAL increases. The method to be used in BERestimation is not specified in the GSM specifications, and it is up to manufacturers to selectthe method.

Table 4.2 Relation between BER and RXQUAL.

4.2.3 Frame erasure ratio

One speech frame would fit into four bursts. The bits are however transmitted in eightsuccessive bursts meaning that the bits belonging to a certain speech frame fill always onlyhalf of the capacity carried out by one burst. In one SACCH of 480 ms 24 frames aretransmitted if DTX is not utilized 2 frames being reserved for signaling purposes. FrameErasure Ratio (FER) is the proportion of the speech frames that the speech decoder discards.A frame is considered to be discarded if any of the bits belonging to class 1a bits is changedbased on the three parity bits, see Figure 2.4. The evaluation of the frame is accomplishedafter decoding process and exploiting error correction techniques.

Since FER is calculated after the decoding process it gives the information how successfullythe speech frame was received. It is supposed to better indicate the subjective quality of theconnection compared with RXQUAL, since even if the RXQUAL class is high the qualitycan still be adequate due to error correction techniques [Haa97]. This applies especially infrequency hopping networks because the quality in such a network is more or less averaged.The distribution of the RXQUAL samples compared to non-hopping network changes sothat the amount of values 2-5 tends to increase and the amount of samples belonging toclasses 1,6 and 7 will decrease. By looking at the RXQUAL values it can appear that thequality of the network has deteriorated after utilizing frequency hopping while the number ofbad speech frames has actually decreased. In Table 4.3 the correspondences between thedifferent FER values and subjective speech quality are presented [Haa97]. The results arebased on listening tests. It is clear that since the mapping is based on the subjective tests alsoother kind of categories could be created.

RXQUAL BER value [%] Assumed BER [%]RXQ0 <0.2 0.14RXQ1 0.2-0.4 0.28RXQ2 0.4-0.8 0.57RXQ3 0.8-1.6 1.13RXQ4 1.6-3.2 2.26RXQ5 3.2-6.4 4.53RXQ6 6.4-12.8 9.05RXQ7 >12.8 18.1



Table 4.3 Mapping between FER and subjective speech quality.

4.2.4 Drop call rate

Drop Call Rate (DCR) tells the share of the calls in which the radio link was not releasedsuccessfully. Drop Call Rate is a good network performance indicator, and in a goodnetwork it should be between 2-4% depending on network planning. Call dropping can becaused by many reasons, eg bad frequency or parameter planning, or as well it can happendue to the transmission link failure. It gives a good picture of the overall networkperformance. Especially when studying the capacity of soft blocking limited hoppingschemes, often the DCR that is affected by the increased interference, is used as a softblocking criteria. For that purpose the DCR needs to be set to a fixed value (eg to 2%), andthe capacity limit will then be met, when either the DCR exceeds the set value or the shareof the bad quality becomes too high. However, no matter how well the networks are plannedthere will always be some background DCR. This is due to the fact that eg the battery of amobile can run out during an ongoing call resulting to a dropped call. This is independent ofthe network quality, of course.

4.2.5 Handover success rate

Handover (HO) is a basic functionality of cellular networks, and it can be performed formany reasons. HOs can be distinguished roughly as either intra cell or inter cell handovers.Intra cell handover is performed within a single cell meaning that the timeslot or carrierfrequency is changed in the same cell. Intercell handover happens between two separatecells, and they can be categorised further. Handover Success Rate parameter describes thehandover performance. It is a ratio of the successful HO attempts (intra-cell and inter-cell) toall HO attempts. An attempt is unsuccessful in case the mobile has to return to the oldchannel [Nok98b].

4.2.6 Subjective voice quality measures

Test Mobile System (TEMS) is a device developed by Ericsson for measuring the subjectivevoice quality. The TEMS test telephone has the Speech Quality Indicator (SQI) whichshould be even more accurate than the analysis of just FER. The SQI scale is from –15 dB to+ 40 dB and the effect of HO's muting effect is included. The TEMS/FICS postprocessedstatistical results are obtained in 5 dB steps. There is correspondence to the Mean OpinionScore (MOS) scaling with difference that the SQI is more accurate especially in badconditions. The following correspondence, Table 4.4, can be used the focus being in classespoor and fair:

FER % Speech Quality0-4 Excellent4-8 Good

8-10 Fair10-15 Poor> 15 Not Usable



Table 4.4 Correspondences between SQI and MOS classes.

4.3 Nokia's system solution for IFH-networks

4.3.1 Nokia's implementation in BSS

The new Intelligent Frequency Hopping feature will be introduced in BSS7 software release(S7), and will provide the combined gain of IUO and FH. The hopping mode in IFH can beeither baseband (BB-FH) or radio frequency (RF-FH) hopping. As with conventional FH, inBB-FH mode the BCCH frequency is included into hopping sequence, but not in RF-FHmode. In Figure 4.1 the difference of the hopping modes in IFH case is illustrated.

Regular layer

Super reuselayer

f1BCCHTCH

TRX-1TRX-2 f2

f3f4

RF hopping cell

TCHTCH

TRX-3TRX-4

f5f6f7

BCCHTCH

TRX-1TRX-2TRX-3

f1f2f3

BB hopping cell

TCH

TCHTCHTRX-5

f4f5f6TCHTRX-6

TRX-4

f5f6f7

Figure 4.1 Different hopping schemes.

The unique C/I evaluation principle described in Section 3.4.1 is used in IFH networks in thesame manner as with IUO. Based on the profile of interference each mobile is exposed to,the BSC determines whether handover between the layers is performed.

By comparing the downlink signal level of the serving cell and the downlink signal levels ofthe neighbouring cells which use the same super reuse frequencies as the serving cell, theBSC can calculate the C/I ratio on the super reuse frequencies at the location of each activemobile station. Based on that a decision is made whether the super layer is able to serve thecall or should the regular layer serve the call.

TEMS SQI [dB] MOS Classes+30 to 40 5 Excellent+20 to 30 4 Good +10 to 20 3 Fair- 5 to +10 2 Poor- 15 to -5 1 Bad



As the IFH has FH element included, the signal in IFH system is more robust towards theinterference than in non-hopping case. That has two consequences: the frequency reusefactors on any layer of an IFH cell can be reduced, and the C/I criteria can be altered bymodifying the C/I thresholds. When the C/I thresholds are modified the super layer will beable to serve the call with lower C/I, hence both the 'good C/I threshold' and the 'bad C/Ithreshold' can be brought down by some decibels. This in fact means that better absorptioncan be achieved. Absorption is the number of mobiles located on super layer divided by thetotal number of mobiles in the cell. In theory the thresholds could be lowered by at least avalue that equals the FH gain, ie according to simulations potentially by 5-6dB (in city area).Practical values may vary from those but through first implementations they can be verified.

Both hopping modes, BB and RF hopping, have their advantages and disadvantages. BBhopping has less hardware restrictions, and it is supported by all BTS generations. Also allantenna combining methods are feasible. However, operators with small bandwidth andsmall configurations may experience problems. If operator is having eg 2+2 configuration, ie2 TRXs are allocated to both layers, BB hopping is not feasible since hopping over twofrequencies does not provide much hopping gain. In that case 3+1 might be better solution,where 3 TRXs on regular layer are hopping over three frequencies and the only TRX superlayer is not hopping at all. This way some hopping gain can be achieved on the regular layer.Still, BB hopping sets limitations for those operators having small configurations since fullhopping gain cannot be obtained.

RF hopping is much more flexible compared with BB hopping. Full frequency hopping gaincan be achieved with small configurations and small bandwidth. With MAIO managementhopping is possible over large number of frequencies even if the number of TRXs is small,as explained is Section 3.3.3. However, RF hopping is not supported by old BTSgenerations, and before RF hopping is possible all the old generation BTSs should bereplaced with new Nokia 3rd generation GSM base stations. Also wideband antenna couplingequipment, Antenna Filter Equipment (AFEs), are needed.

4.3.2 Nokia's network planning system (NPS/X)

NPS/X is an integrated software package for cellular network design. It provides tools foractual planning and documentation, from rough sketches to accurate design. The differentplanning phases are based mainly on predicted field strength data. The available modelsgiving these predictions in NPS/X are Okumura-Hata, Juul-Nyholm, Walfish-Ikegami andray tracing model, which has been developed at Nokia. The predicted data can be verifiedand tuned using measurements. All the predictions are based on digital maps. Of course,certain parameters like BS transmit power, antenna direction, and antenna tilt have to bedefined prior to any calculations.

Automatic frequency allocation tool is used to optimize the available radio resources in abest possible way. The frequency allocation is based on the generation of interference andseparation matrixes, which on the other hand are based on the predicted coverages. Theinterference matrix can also be calibrated using measurements performed by BSC.Allocation can be performed for entire network, or for a smaller specified area. Thefrequency allocator supports planning in both hopping and non-hopping networks, as well asin multilayer networks. The allocation results can be verified using interference analysis tool,which also supports frequency hopping and multilayer planning. In a non-hopping case theinterference is reported in terms of C/I ratio, while in a hopping case RXQUAL qualitymeasure is used. In IUO and IFH planning reference cells, ie the cells that are used in the C/I



estimation whether MS is handed over from the regular layer to the super layer or vice versa,can be generated automatically. Figure 4.2 presents the basic structure of NPS/X [Nok98e].

Figure 4.2 NPS/X block diagram

4.3.3 Network management system (NMS)

NMS is the operation and maintenance related part of the network. It is also needed for thewhole network control. The network operator observes and maintains the network qualityand service offered through NMS.

According to its name, the purpose of NMS is to monitor various functions and elements ofthe network. These tasks are carried out by the Operation and Maintenance Centre (OMC),which consist of a number of work stations, a server and a front end acting as an interfacebetween the network element and OMC.

Cell Doctor is a reporting package in NMS which provides effective tools to cover all thefunctional areas of the NMS/2000: configuration management, fault management, andperformance management, with a special focus on the needs of network planning, operationand maintenance.

Separate CellDoctor software has to be installed to operator’s NMS/2000. Cell Doctorscripts are then executed from the NMS/2000 workstation. They can be programmedbeforehand to be executed at certain times and days. During a trial, which takes a long timeto execute, a lot of information will be collected. It is therefore important to try to decreasethe number of scripts run in order to keep the amount of information within reasonablelimits. This could otherwise cause problems in storage and readability of the obtained data.

NPS/XNMS/X

Databases

System Data

Base Station DataPropagation Model

Antenna DBDigital MapMap Editor

Measured Data


Chapter 5: IFH planning strategies48

5 IFH PLANNING STRATEGIES

The problem related to the division of the frequencies between the layers in IFH networks,and which are the feasible hardware configurations for IFH are discussed in this chapter.Some aspects of using computerized planning for IFH networks is provided. A method fordimensioning the blocking probability of IUO/IFH networks is also presented. Theperformance of IFH networks is analyzed by means of simulating the various factorsaffecting the ability of the network to absorb traffic.

5.1 Planning concepts

5.1.1 Frequency split between layers

There are three main strategies to consider when determining the frequency allocationschemes to be used in IUO networks. First, common band can be used for all the TRXs. Itmeans the frequencies are assigned for the BCCH-, regular- and super layers from onecommon band, which is the whole band available for the operator. Another possibility is thatthe super frequencies are separated from the BCCH and regular frequencies, and only theBCCH and regular layers share the same frequency pool. The third, and most commonlyused approach is to separate the frequencies for every layer, ie BCCH, regular and superfrequencies are assigned from dedicated frequency pools for each TRX. The differentdivision schemes are illustrated in Figure 5.1.

Figure 5.1 Different schemes to share frequencies.

5.1.2 TRX configurations

One problem in IFH planning is how to divide the TRXs between the super and regularlayers. All the TRX configurations are not equally favorable. On one hand, it is desirable tohave as many super TRXs as possible in terms of frequency efficiency, since on the superlayer the frequencies are reused in a more efficient way than on the regular layer. On theother hand, if too many of the cell’s TRXs are allocated to the super layer it is possible thatthe regular layer becomes congested due to lack of resources on the regular layer. If directaccess to super feature is not available, the call set up must always take place on the regularlayer. Now, if there are no resources available on the regular layer, the call cannot beinitiated and thus never handed over to the super layer. Actually, here we have quite aserious drawback present in TDMA based systems: in GSM the channels must be allocatedin blocks of 8 channels, and finding the best possible and optimum distribution of thechannels between the super and regular layers can be virtually impossible.

BCCH+regular+super

BCCH+regular super

BCCH regular super



5.1.3 Effects of traffic distribution

Traffic distribution inside the cell greatly affects the performance of an IFH cell. If the trafficis concentrated in the areas where the super layer is not able to serve because of theinterference conditions, the traffic pressure in the regular TRXs increases and theperformance can decline. On the other hand, the larger proportion of the cell traffic is in theso called ‘interference free area’, the larger proportion of the total traffic can be transferredto the super layer. In this case the regular layer has less meaning as an independent trafficlayer, but it is rather feeding the super layer.

5.2 Blocking of IUO networks

In a conventional network the blocking probability Pbl can be calculated using Erlang Bformula [Rap96], Equation (5.1),

∑=

=C

n

n

C

bl

n

CP

0 !1

!1

µλ

µλ

, (5.1)

where λ is the call arrival probability, H=1/µ is the average call holding time and C is thenumber of channels in the system. With the given offered traffic A=λ/µ and the number ofchannels C the probability that a call is blocked due to lack of hardware resources can thenbe calculated. A common design criterion for blocking is 2%.

In IUO and IFH networks the situation is not so straightforward, and Equation (5.1) cannotbe applied as it is. The hardware resources are split between the super and regular layers andthe offered traffic is divided between these layers. Also transitions from the regular to thesuper layer and vice versa are possible. A method for estimating the blocking probability oftwo layer networks is proposed in Appendix A. It is based on two dimensional Markovchains and is therefore quite similar to derivation of Erlang B formula. However, analyticalsolution cannot be found (or anyway not with reasonable work effort), and a numericalmethod to solve the system of equations has to be considered. In this case Matlab was usedto obtain the results.

The input values for estimating the blocking probability of IUO/IFH networks are the offeredload, the number of channels on both layers, the transition probabilities from the regular tothe super layer and from the super to the regular layer, and the BCCH decoding time. It isalso possible to estimate the effect of direct access to super feature.

In Figure 5.2 is presented one example of the results when applying the above-describedmethod. The configuration was 2+2, ie that on both layers there were 2 TRXs in each cell.For simplicity the BCCH and signaling channels are omitted in the calculations. Theblocking probability is presented as a function of offered load with several Direct Access toSuper (DA) parameter values. Also the blocking probability of the super layer is presented inFigure 5.3. However, the blocking of the super layer has no interest from user’s point ofview, since the user experiences only the overall blocking on the regular layer. Oneobservation concerning Figure 5.2 and Figure 5.3 can be made: the blocking probability of



the super layer is much higher compared with the regular layer. This was as expected, sincein IUO/IFH networks the traffic is guided to the super layer due to its better spectralefficiency.

Let’s now suppose that we have a homogeneous network of 24 three sectorised sites (72cells). By using Erlang B table it can seen that each cell can carry 23.7 Erlangs if all theTRXs in a cell are in one pool and 2% blocking criteria is set. If we further suppose Rfa being12, it can be calculated that the traffic this kind of network can carry is 35.6 Erlangs/MHz. InFigure 5.2 it can be seen that if the cells in the previously mentioned network were IUO cellseach of them could absorb 18.1 Erlangs traffic with 2% blocking supposing that directaccess to super feature is not used. Assuming the reuse factor being on the regular layer 12and on the super layer 3 it can be calculated that the network can handle in this case 43.4Erlangs/MHz. This is about 22% more than in the conventional network. Using frequencyhopping the reuse factor of the regular layer could be lowered to e.g. 9. Hence thecorresponding values is 54.3 Erlangs/MHz, capacity improvement being about 54%. Havingthe direct access to super parameter value of 0.3 the capacity improvements wouldcorrespondingly be 27% and 58%. However, this model is purely theoretical and is givingonly a very rough assessment about the gain achieved with IFH.

Figure 5.2 Blocking probabilities of the regular layer as a function of offered load withdifferent direct access to super (DA) probability factors.

12 14 16 18 20 22 24 260

1

2

3

4

5

6

7

8

9

ErlB

Regular blocking

Traffic [Erl]

Blo

ckin

g [%

]

ErlB ErlB ErlB ErlB ErlB

DA=0.3

DA=0.5

DA=0.7

DA=1

DA=0.1DA=0



Figure 5.3 Blocking probabilities of the super layer as a function of offered load withdifferent direct access to super (DA) probability factors.

5.3 IFH-planning using NPS/X

5.3.1 Allocation process

The whole frequency allocation process has been revised in NPS/X 3.3. It is now possible tomodel a number of different networks and system features, which produces more accuratedata for automatic frequency allocation. The frequency allocation tool has been designedkeeping usability and flexibility in mind. Verified default values have been assigned to allthe parameters, which together with the intuitive user interface make allocation easy, also fora novice network planner.

The frequency allocation procedure in NPS/X 3.3 starts with Hierarchical Cell Layer (HCL)definitions. With HCLs it is possible to divide the cells into logical layers, such as micro ormacro cell layer. Each hierarchical cell layer can have different parameters for the cellservice area definition, eg own parameter set for rural area and urban areas. Theseparameters are defined in the Cell Service Area Class (CSAC). The CSACs contain allparameters related to the definition of cell service area (coverage margin, dominance margin,and umbrella HO threshold for micro cells, hierarchical cell layer membership and HOmethods used in the cell). Therefore it is possible to define different parameter sets fordifferent areas (rural, urban, etc.) and for different HCLs. One HCL can have several CSACswhile one CSAC can belong only to one HCL. When calculating interference matrices it isalso possible to have different interference matrix parameters for each cell service area class.[Nok98c]

There can be three types of TRXs: BCCH, regular layer TCH and super layer TCH. Each ofthese can have different sub-bands and different separation matrix parameters. When usingMultiple Reuse Pattern (MRP) method, each TRX has a different sub-band and differentseparation matrix parameters. Therefore there is a TRX specific concept called FrequencyGroup (FG) defining the sub-band and separation matrix parameters. Each TRX must belong

12 14 16 18 20 22 24 260

2

4

6

8

10

12

14

16

18Super blocking

Traffic [Erl]

Blo

ckin

g [%

]

DA=0.3DA=0.5DA=0.7DA=1

DA=0.1DA=0



to one of these Frequency Groups. Furthermore, each MA list belongs always to one FG. InFigure 5.4 the relations between HCLs, CSACs and FGs are illustrated.

MA lists:

macro

micro

pico

Hierarchical Cell Layers (HCL):

Frequency Layers:

BCCHOther RegularsSuper

Frequency Sub-bands:

Cell Service Area Classes (CSAC):

macro-ruralmacro-urban

micro-1micro-2

pico

1, 3, 5, 7, 9, 11,13, 15

Figure 5.4 Relations between different hierarchical structures.

Of course, the coverage data has to be calculated and verified prior to any actions in thefrequency planning. After calculating coverages that are tuned using measurements,interference matrix can be calculated. Due to inaccuracies in digital maps and propagationmodels interference matrix can be calibrated in order to obtain better results in frequencyallocation phase. Calibration is based on the measurements carried out by BSC, and thosemeasurements can then be transferred to NPS/X in order to calibrate the interference matrix.

In frequency allocation phase the user can in RF hopping and IFH cases first calculate thelengths of the MA lists. The MA list calculation in NPS/X is based on traffic (measured orpredicted using Erlang B formula) and fractional loading given by the user. Of course, theuser can define the MA list lengths by him/her self, too. When the lengths of the MA listsare determined, frequency allocation tool allocates the frequencies to the MA lists accordingto Frequency Group (FG) definitions (ie if the frequency group for TRX in question isdefined to be regular, the frequencies are allocated from regular frequency pool)

To make the processes that are related to frequency allocation more concrete Figure 5.5clarifies the situation.



Capacityestimation,cell basis

Capacityestimation,cell basis

Planningconceptdecision

Planningconceptdecision

Estimation ofneeded numberof frequencies

Definition ofFrequency groupsand CSACs

Coverage dataCoverage data

Neighbour cellmeasurements with

GPA tool

Neighbour cellmeasurements with

GPA tool

Interference

Calibration Tool

Interference matrixgeneration

Interference matrixgeneration

Automatic interferergeneration for IUO

Automatic interferergeneration for IUO

FrequencyrequirementsFrequency

requirements

FrequencyAllocation

FrequencyAllocation

Spectrumand HWconstraints

Spectrumand HWconstraints

NPS/X 3.3

Planning of otherparameters

Planning of otherparameters

OMC / CDW/ NDW

Quality AnalysisAutomaticParameter tuning

Quality AnalysisAutomaticParameter tuning

NetDim /NPS/X 3.3

Figure 5.5 Actions prior to frequency allocation [Nok98f].

5.3.2 Automatic reference cell generation

NPS/X 3.3 contains a new tool, reference cell generation tool, which automatically generatesthe reference cells. Reference cells are used in IUO/IFH networks in the C/I estimation inBSC to decide whether MS is handed over from the regular layer to the super layer or viceversa. Reference cell generation is probably the biggest bottleneck of IUO/IFH planning, anddetermination of the reference cells is a very time consuming process. That is why there is abig need for automatic reference cell generation.

Interference for interfered TRXs is calculated so that co-channel and/or adjacent-channelinterference caused by interfering TRX is taken into account. The following aspects must beconsidered: interference type, ie is the interference caused by co- or adjacent channel orboth, co-channel C/I threshold, adjacent channel C/I threshold and area percentage to beexceeded before the cell is accepted as a reference cell. [Nok98d]

The interference is calculated so that if the C/I value in the pixel is below the pre-defined C/Ivalue, that pixel is considered to be an interfering pixel. Now, if the number of interferingpixels pixI divided by the number of pixels in the dominance area pixD, ie the interferenceprobability PI defined by Equation (5.2), exceeds the area percentage the cell is added to areference cell list. The bigger is the calculated interference value, the bigger interferer theinterfering cell is. Interfering cells, which have 10 biggest interference values, are includedinto the reference cell list. The situation is depicted in Figure 5.6.

D

II pix

pixP = (5.2)



Cell 1"Interfered cell"

Cell 2"Interfering cell"

calculation region border

cell coverage area border with given threshold

cell dominance area border with given margin

interfered area

Figure 5.6 Interference Calculation Area definition.

This tool can also handle interference calculations in IFH networks. The interferers in IFHnetworks are generated so that an interference value is calculated for every cell pair(interfered -> interfering) in a target area. The interference value represents a probability thata certain number of TRXs in the interfering cell is interfering the TRXs of the interfered cell.The interference value is a function of the interference probability, the number of TRXs andthe length of the MA lists.

The hit probability for the MA list pair can be calculated

TRXMAMA

Chit N

LLN

P ⋅⋅

=21

, (5.3)

where NC is the number of common channels in MA list pair, LMA1 the length of theinterfered MA list, LMA2 the length of the interfering MA list, and NTRX the number ofhopping TRXs in the interfering layer.

The function for calculating the interference value IV can be presented to be

hitIV PPI ⋅= . (5.4)

By inserting Equations (5.2) and (5.3) into (5.4) we obtain

TRXMAMA

C

D

Iv N

LLN

pixpix

I ⋅⋅

⋅=21

. (5.5)

It should be noted that if MAIO Management is enabled, it affects to C/I calculation resultsof MAIO managed sites immediately. The affect depends on the cell specific MAIOparameters.



5.3.3 Interference analysis

In a non-hopping network the interference is based on C/I ratio which is calculated by usingthe power levels of the serving carrier and the interfering carriers. The analysis can beperformed for all the frequency channels, or for the selected channels. It is also possible toanalyze both co-channel and adjacent channel interference.

In frequency hopping case the situation is a bit more complicated. It is difficult to use C/I asa quality measure since the frequency is changed every 4.615 ms. That is the reason for usingRXQUAL to estimate the quality of frequency hopping networks. The RXQUAL qualityestimation is performed in downlink direction for every pixel in the work area. A specialRXQUAL evaluation algorithm presented in Equation (5.6) has been developed at Nokia[Ter97]. RXQUAL is calculated as

))(1

(∑ ∑ ⋅⋅=cells freqs f

loadDTXIC

BERN

RXQUALRXQUAL , (5.6)

where Nf is the number of hopping frequencies in the serving cell, BER(C/I) is BER as afunction of C/I, DTX is the discontinuous transmission factor and load is the frequency loadfactor. The load factor in Equation (5.6) for each cell can be calculated using Equation (5.7):

f

TRXbl

TCH

TCHbl

NN

PN

NPErlload )1(

),( −= , (5.7)

where Erl(Pbl,NTCH) is the cell traffic in Erlangs as a function of required blockingprobability Pbl and the number of traffic channels NTCH, NTRX is the number of TRXs in thecell and Nf is as defined above.

In the quality estimation the C/I value is first mapped into BER. The mapping is based onsimulations [Ter97], and the relation between BER and C/I is presented in Figure 5.7. TheRXQUAL algorithm determines separately the BER corresponding to the C/I value causedby each interfering cell. In a frequency hopping case each call is experiencing interferencecausing BER only when it is transmitted on that particular frequency. That is why the BERscaused by each individual frequency are summed and then divided by the number offrequencies in the serving cell. This approach is justified if BER is a linear function of C/Iand infinite C/I results to zero BER. In Figure 5.8 BER is presented as a function of I/C. Itcan be seen that the dependence between C/I and BER is not linear. It is linear after aboutI/C value of 0.14 corresponging C/I=8.5dB [Sal98]. However, the line does not intercept they-axis at 0 but at about 0.08. This causes some excess BER of maximum 0.08 in case thatC/I value is below 8.5 dB.



Figure 5.7 Simulated BER as a function of C/I.

0

0.05

0.1

0.15

0.2

0.25

0 0.2 0.4 0.6 0.8 1 1.2 1.4

I/C

BE

R

Simulated BER

Linear BER

Figure 5.8 BER as a function of I/C.

Furthermore, in Equation (5.2) the average BER is multiplied by the DTX and load factors.If the interfering carrier is BCCH carrier, DTX=load=1. This is due to the fact that BCCHcarrier must be transmitted constantly corresponding to a fully loaded TRX.

5.4 Simulations

5.4.1 Simulator

The IFH simulations were performed using a dynamic simulation tool, Capacity, which hasbeen developed in co-operation with Nokia Telecommunications and University of Aalborg.With Capacity it is possible to measure both the performance and the capacity of GSMnetworks. The simulation tool is able to simulate the factors affecting the performance of theGSM network, and returns the quality experienced by each individual mobile station at a

0

0.05

0.1

0.15

0.2

0.25

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

C/I [dB]

BE

R



given system load. The quality is measured in terms of C/I, percentage of dropped calls andRXQUAL distribution.

The network structure was a regular grid cell radius being 3 km. Totally 24 three sectorizedsites were placed on the network area resulting to 72 cells. Calls were initiated randomly andthey were supposed to arrive using Poisson distribution average call length being 80seconds. Each call was supposed to move with constant speed (3km/h or 50 km/h) torandomly chosen direction. Handovers had higher priority than new calls meaning that it wasfirst checked if there are mobiles requesting handover to that cell before a new connectioncould be established. In the simulator all handovers succeed supposing that free channels areavailable in the target cell. This is however not the case in real life since call can be droppedbecause of handover failure. The call dropping procedure is depicted in Figure 5.9. In thesimulator the user can determine certain RXQUAL value which defines the threshold afterwhich the quality of the connection is considered poor. Every time (ie every SACCHmultiframe) the reported RXQUAL value exceeds this threshold a specific mobile countervalue is increased by one. On the other hand, if the RXQUAL value is below this limit thecounter is decreased by two. When the counter exceeds user definable dropped call thresholdthe call is dropped. In the simulations the RXQUAL threshold was set to 5 (ie thatRXQUAL of 5 was still considered adequate) and dropped call threshold to 19. With theseconfigurations it is means that it takes at least 20*480 ms=9.6 s from the beginning of theconnections before the call can be dropped.

Figure 5.9 Call dropping procedure in the simulator.

Both Rayleigh and shadow fading are considered in the simulations. The log-normal shadowfading is correlated over 110m, and standard deviation of 6 dB is used as a reference[Wig97]. Measurements are reported every SACCH multiframe, but are performed for every104 bursts. Table 5.1 summarizes the general parameters applied in the simulations.

Table 5.1 Parameter settings used in the simulator.

Number of sites 24Number of cells 72Path Loss L=35log(d)Shadow fading standard deviation 6dBShadow fading correlation distance 1/e at 110mAverage call length 80sMobile speed 3 km/h and 50km/hCell radius 3kmMax BS output power 43dBmMax MS output power 13dBmAntennas 65° sectorisedDropped call RXQUAL threshold 5Dropped call threshold 19

5 67 47 7 7 7 7 7 7 7 7 7 76 6 6 6 6 60 1 2 3 4 5 6 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

7 6QualityCounter Call dropped

time



The capacity gain achieved when using IFH feature was compared with conventionalnetwork. It means that a configuration consisting of one regular and one super TRX wascompared with 2 TRX conventional configuration, for example. The comparison was madein traffic per MHz, ie, how much traffic this particular system can handle without exceedingthe predefined quality criteria. In the simulations this maximum load was determined whenthe DCR was less that 2%, and the percentage of blocked calls was less that 2%.

5.4.2 Blocking probabilities

The blocking probability results obtained when exploiting the method described in Section5.2 were verified with simulations. In the simulator good- and bad-C/I thresholds were set to17dB and 12dB. TRX configuration was 1+1, ie one TRX was allocated to both layers. Thenumber of blocked calls was then calculated in several points having different offered loads.In the simulator also error margins for the blocking probabilities are provided. Thesimulation results were then compared with calculated values. In the calculations good- andbad-C/I probabilities of 0.5 and 0.55 were used. The results are presented in Figure 5.10. Itcan be seen that the calculated values match quite well with the simulations and almost allthe values are within the error limits.

0

1

2

3

4

5

6

7

3 4 5 6 7 8 9

Off.load[Erl]

Blo

ckin

g[%

]

Upper limitSimulatedLower limitCalculated

Figure 5.10 Comparison of simulated and calculated blocking probabilities with 1+1TRX configuration of IUO.

5.4.3 Effect of direct access to super

When IUO was first introduced a call had to be first set up on the regular layer and after C/Ievaluation handed over to the super layer when C/I allowed that. This principle reduced thepotential gain of IUO and could also lead to congestion on the regular layer under certainconditions.



In the BSC software release S7 this problem will be solved by allowing direct access to thesuper layer without any adverse impact from the degree of regular layer loading. The accesscriterion is based on the received DL signal level reported by the MS. But since the actualC/I is not known, it is possible that the call is dropped due to wrong estimation in quality.

The effect of the direct access to super feature was simulated using Capacity simulator. Thestarting value can be found from RXLEV measurement by studying the minimum values ofthe super-reuse TRXs and maximum values of regular-reuse TRXs correspondingly. Afterthe starting point is found the parameter can be optimised until the still acceptable call droprate is achieved. When the optimum value for direct access threshold is found it can bereused over the network.

In Figure 5.11 the results are presented [Wig97]. The upper curve represents the DCR as afunction of direct access level. The figures in the lower curve, on the other hand, describe theproportion of the mobiles that have accessed the super layer directly from SDCCH. It can beclearly seen that after certain power level threshold, in this case –85 dBm, the dropped callratio starts to increase significantly. However, this is not any absolute value that could beused in every network. The power levels can vary a lot in different networks, and eg in microcell networks the value of the direct access to super parameter should be higher.

8%

46%

35%

24%17%

12%

0

0.2

0.4

0.6

0.8

1

1.2

-100-95-90-85-80-75-70-65

Direct access threshold [dBm]

Sim

ulat

ed D

rop

Cal

l Per

cent

age

[%] Drop (%)

Dir.Acc (%)

Figure 5.11 Dropped call versus direct access to super threshold

5.4.4 Simulated capacity gain of IFH

The capacity gain of IFH was calculated as follows [Nie97]. Whenever IUO element wasincluded in the baseband frequency hopping network, the used configuration was 4+3 (iefour TRXs on the regular layer one being a BCCH TRX, and three TRXs on the super layer).The frequency reuses factors were 3/9 and 1/3 respectively. Thus, together 45 frequencieswere required. 3/9 reuse means that nine different frequency groups are rotated within threesites (ie each site contains three cells). 1/3 reuse, on the other hand, means the frequencypool is split into three parts, and these groups are then used within one site. In the purefrequency hopping cases the used reuse patterns were 3/9 with 5 TRXs per cell, and 1/3 with12 TRXs per cell plus one BCCH TRX having a frequency reuse of 3/9. Also both frequencyhopping cases require a total number of 45 frequencies. The simulations were carried out



with two separate speeds of mobiles (3 km/h and 50 km/h) since the frequency diversity gainis quite dependent on the speed of the mobiles.

The offered traffic was increased until the hard blocking or the soft blocking limit wasreached. The hard blocking criterion was set to 2% blocking, and the maximum amount ofacceptable dropped calls was fixed to 5%. The simulation results are summarized in Table5.2. From the results it can be seen that with the mobile speeds of 3 km/h the pure frequencyhopping 1/3 reuse case and the IFH case provide the highest capacity. However, the drop callrate is much higher in 1/3 FH case compared with the IFH case. In respect with 3/9frequency hopping case IFH gives almost 35% more capacity. In comparison with those twocases it can be seen that the drop call rates are at the same level in both cases, and thenetwork was hard blocking limited. When the speed to the mobile is increased to 50 km/h,the IFH network becomes soft blocking limited. The capacity gain of IFH in respect withpure 3/9 hopping case decreases to 26%. The more positive results with the mobile speed of3 km/h are partly caused by the fact that the power control can better regulate thetransmission power, when the speed is low. Also, the number of handovers decreases withlower speeds.

Table 5.2 Summary of the simulation results.

Reuse Speed [km/h] Load [%] DCR [%] Blocking type Capacity [Erl/Cell]BB-FH 1/3 3 40 5 soft 41.6

1/3 50 33 5 soft 34.3BB-FH 3/9 3 78 <1 hard 31.2

3/9 50 78 <1 hard 31.2BB-IFH 3/9&1/3 3 75 <1 hard 42

3/9&1/3 50 70 5 soft 39.2


Chapter 6: Field trial61

6 FIELD TRIAL

The field test trial was conducted in co-operation with one of Nokia’s customer. The maingoal of the trial was to verify the quality and capacity gain achieved by means of IFH. Thetrial arrangements from network planning to the actual implementation were mostly carriedout by Nokia. The traffic for the trial was generated by operator’s normal customers.

6.1 Trial environment

The trial took place in urban environment. Mostly the terrain type was quite flat, and onlyone big hill located in the trial area.

The trial area consisted of 11 sites having together 31 cells, ie two sites contained twosectorised cells the rest nine sites having three cells each. All the base stations were Nokia3rd generation GSM base stations, and also AFEs were used in every cell. The AFEs wereneeded to perform the tests in RF hopping cases. The number of TRXs was four in almostevery cell the average number of TRXs being 3.74. An area referred to as a buffer area wasgenerated for the trial purposes. The buffer area consisted of cells that were close to the cellslocated in the trial area it self. The buffer area was generated because the trial area located inthe middle of the city, and interference outside the trial area could not be prevented becausea separate frequency band for the trial cells was not available, of course. Also whenperforming the frequency allocation with NPS/X the overall interference condition has to betaken into account, and thus the interference caused by the buffer area cells was calculated.Also some reference cells were included in the reference cell lists from the buffer area. Theperformance of the buffer area was monitored to ensure that the quality of the trial area is notimproved at the expense of the buffer area.

6.2 Test cases

The test cases can be divided into two parts: pure frequency hopping cases and IFH cases.The pure frequency hopping cases were needed to be able to compare the results with IFHtests. The trial consisted of 8 different main test cases. In all test cases the existing BCCHplan with 18 frequencies was used. Thus, only the number of frequencies in the hoppingchannels was changed.

6.2.1 Pure frequency hopping cases

The frequency allocation schemes in the RF hopping cases can be roughly divided into twoparts according to the planning methods: easy planning where the planning was performedmanually, and computerized NPS/X planning where the frequencies were planned usingNokia’s network planning tool. The first tested frequency allocation scheme was so called1/3. The basic idea of 1/3 easy allocation is to divide the hopping frequencies into threedifferent groups and reuse the groups over all the sites (group A for sector 1, group B forsector 2 and group C for sector 3). As mentioned, the easy allocation can be done manually,ie no planning tools are needed. That is the biggest advantage of it. The 1/3 easy planningscheme was tested with 30 (6 MHz), 18 (3.6 MHz) and 15 (3 MHz) frequencies resulting toMA list lengths of 10, 6 and 5 frequencies.



The other tested easy planning method was 1/1 scheme. The easy 1/1 frequency allocationconsists of only one single hopping group, ie all frequencies (except BCCH frequency) areallocated to one MA list and co- and adjacent channel interference between the sectors aretaken care by MAIO management.

The heuristic allocation is based on the frequency allocation algorithm implemented inNPS/X 3.3. NPS/X can allocate the frequencies according to the current interferencesituation and therefore it gives better allocation than the 'easy' methods. In the heuristicallocation the frequency pool consisted of 18 frequencies. Thus, the frequency allocationreuse was Rfa=3. In Figure 6.1 the reuse patterns are presented to better illustrate thedifferent reuse concepts. First figure represents the easy allocation 1/3 case the secondillustrating the heuristic 1/3 case. In heuristic allocation every MA list can be different tominimize the interference. In the easy allocation cases all the sectors having frequency groupA contain exactly the same frequencies. However, in the heuristic cases most of thefrequencies in the frequency groups A1, A2, A3 and A4 are the same, but some variationexists between the different ‘A’ groups. The frequency groups having almost the samefrequencies in their heuristically allocated MA lists are separated using the subscripts. Thelast figure depicts the single MA list case.

A

BC

A

BC

A

BC

A

BC

A1

B1

C1

A3

B3

C3

A4

B4

C4

A2

B2

C2

A

AA

A

AA

A

AA

A

AA

Figure 6.1. Principle of the reuses in RF hopping cases.

6.2.2 IFH cases

Also in IFH cases the frequency planning methods used can be divided between manual easyplanning, Figure 6.2, and heuristic NPS/X planning, Figure 6.3. Of course, different reuseschemes can be used for both regular and super layers. The same notation applies as in thepure RF hopping cases. Table 6.1 summarizes the used frequency allocation methods in thetrial, also for RF hopping cases. The number of frequencies used is presented in every case,and also the effective reuses calculated using the Equation (3.14) are provided in the table.

A

A

A

D

D

D

BD

D

D

A

A

A

BB

C

CC

B

BB

C

CC A

A

A

A

A

A

BA

A

A

A

A

A

BB

C

CC

B

BB

C

CC A

A

A

B

B

B

CB

B

B

A

A

A

CC

C

CC

C

CC

C

CC

Regular: 2/2 Super: 2/2 Regular:1/1 Super: 2/2 Regular: 2/2 Super 1/1

Figure 6.2 Reuse patterns in easy IFH cases.



D1

D1

D1

E2

E2

E2

A1

E1

E1

E1

D2

D2

D2

B1

C1

A3

B3

C3

A4

B4

C4

A2

B2

C2 F1

E

D1

F3

D3

E3

A1

F2

D2

E2

F4

D4

E4

B1

C1

A3

B3

C3

A4

B4

C4

A2

B2

C2

Regular: 2/2 Super: 1/3 Regular: 1/3 Super: 1/3

Figure 6.3 Reuse patterns in heuristic IFH cases.

Table 6.1 Frequency configurations in the trial.

6.3 Measurements

The performance of the network was monitored by using NMS/2000 performancemeasurements (see Section 4.3.3) and drive/walk tests with measuring equipment.NMS/2000 scripts were used for collecting the results, and drive/walk tests were mainlyperformed to support and verify the results collected with NMS/2000. Drive/walk tests werealso performed for trouble shooting purposes.

6.3.1 Statistics collected in OMC

Automated script running was defined in the NMS/2000 for most scripts and some scriptswere executed manually according to occasional requirements. The most importantperformance figures to observe were resource availability, DCR, RXQUAL qualitydistributions, C/I statistics, traffic on TCH and SDCCH channels, blocking figures, and callsuccess rates. For IUO and IFH purposes super layer absorption and super layer usage

Case Reg. Reuse Sup. Reuse Reff,Tot #Reg. Freqs #Sup. FreqsIUO - - 11.7 20 12IFH Easy pl 1/1 Easy pl 2/2 11.3 19 12IFH Easy pl 1/1 Easy pl 2/2 8.0 10 12

RF FH Easy pl 1/3 - 10.9 30RF FH Easy pl 1/3 - 6.6 18RF FH Easy pl 1/3 - 5.5 15RF FH NPS/X 1/3 - 6.6 18RF FH Easy pl 1/1 - 6.6 18

IFH NPS/X 1/3 NPS/X 1/3 8.0 10 12IFH NPS/X 2/2 NPS/X 1/3 6.6 9 9IFH Easy pl 2/2 Easy pl 1/1 5.1 8 6IFH Easy pl 2/2 Easy pl 1/1 6.6 9 9IFH Easy pl 2/2 Easy 2/2 11.7 20 12



figures were observed. Also handover performance, especially reasons for the super layer tothe regular layer handovers, was seen very important.

6.3.2 Walk and drive tests

In addition to collecting valuable performance data in the OMC, the walk and drive testswere performed to support the planning and implementation process. The CellDoctorindicators are not able to see all the fine details of eg HO parameters or MA-list frequencies.The functionality and quality of the test area can be seen from the end user point of view.

The walk and drive tests were performed by using both Nokia FER-tool and Erisoft TEMSmeasurement equipment. The FER-tool is a tailor-made tool and implemented into a Nokia8110 GSM phone with a PC application. It gives the layer specific FER and RXQUAL.TEMS is a commercial tool for measuring and testing digital air interface and is the onlycommercial tool which gives also FER; not layer specific, though.

6.4 Transitions between regular and super layers

6.4.1 C/I thresholds

By adjusting the good and bad C/I thresholds it is possible to change the traffic distributionbetween the layers. By lowering these thresholds more traffic can be transferred from theregular layer to the super layer. However, if the good and bad C/I thresholds are set too low,the quality of the network may be sacrificed. This especially concerns the good C/Ithreshold. This is due to the fact that if the mobile is handed over from the regular layer tothe super layer and the quality is not good enough, the call can be dropped on the superlayer. This is because the mobile is not able to make a handover back to the regular layersince the signaling is not going through either.

In an IUO case the default value for bad C/I threshold is usually 12dB. This is achieved byadding the flat fading margin of 3 dB to the minimum C/I requirement in GSMspecifications. This minimum value is 9 dB. The fading margin is of course not fixed to 3dB. However, in IUO networks the fading margin of 3 dB for bad C/I thresholds is seen to beenough. Good C/I value is derived from the bad C/I threshold so that a hysteresis margin isadded to the bad C/I value. If the margin is set too low it can cause unnecessary handoversbetween the layers. This is because the call can be handed over back to the regular layer rightafter it was handed over from the regular layer to the super layer. The hysteresis margin isusually set to 3-5 dB resulting to good C/I value of 15-17 dB.

When frequency hopping is introduced in an IUO network, the C/I thresholds can belowered. However, to determine how much they can be lowered is not necessarily verystraightforward. At least the thresholds can be lowered by the amount of frequency diversitygain described in Section 0. Eg if the hopping sequence consists of 6 frequencies thefrequency diversity gain is around 5 dB, thus the C/I thresholds can be lowered by 5 dB. Butto determine the effect of interference diversity gain is more difficult. Of course, the morefrequencies are included in the hopping sequence the better the interference is averaged overthose frequencies. But, to have long MA lists means also that the source of the inference iscloser to the carrier cell. An extreme case is the so called 1/1 reuse case, where one singleMA list is allocated in every cell. Now the length of the MA list is on its maximum value,and the interference is averaged most efficiently. However, also the interference experienced



in case of collisions is very high. How the actual gain must be determined is not known, butit is assumed that eg in 1/1 case the C/I thresholds could be lowered by 2-3 dB.

6.4.2 Quality handovers

Quality handover from the super layer to the regular layer is triggered if the quality on thesuper layer is not acceptable. It is highly desirable that the transition form the super layer tothe regular layer is based on the bad C/I threshold but, eg, if a mistake has occurred in thereference definition phase the C/I evaluation is not reliable. The RXQUAL experienced bythe user can be degraded too much even though the C/I estimation would show acceptableinterference level. In that case a handover based on the quality is triggered to avoid the calldrop.

In the initial IUO network the quality handover margin was set to RXQUAL 4. When IFHwas introduced, the margin was lowered to RXQUAL 6 based on the fact that a frequencyhopping network is more robust against interference and bad quality. Also the window sizesdetermining the number of samples that have to belong to the bad quality classes before thehandover is triggered were reduced. This way the call was transferred faster back to theregular layer if the quality remained at the unacceptable level for a long time.

6.4.3 Absorption

In Figure 6.4 the absorption in different cases is presented. The absorption figure is theaverage absorption over the whole trial area. It has also been averaged over all themeasurement days for each test case. In the original IUO plan the absorption was slightlybelow 56 %. Then, frequency hopping was introduced in the network and the absorptionfigure dropped to 51%. However, this was expected to happen, since the interferers are muchcloser in IFH as explained in Section 6.4.1, and all the thresholds remained untouched. Then,good and bad C/I thresholds were lowered to 10 and 7 dB, ie they were lowered by 5 dB(IFH 1/1&2/2 optimized case). Based on Figure 3.7 this is approximately the frequencyhopping gain that was expected to be acquirable when the MA lists consisted of 6frequencies. Also the quality handover threshold was set from 4 to 6 at this point. Afteroptimizing the parameters the absorption was over 60 %, which is over 4 % more than inIUO case.



46.048.050.052.054.056.058.060.062.0

IUO (32 freqs.) IFH 1/1 & 2/2 (22freqs.)

IFH 1/1 & 2/2 (C/Ioptimization)

Heuristic 1/3&1/3(22 freqs.)

Heuristic 2/2&1/3(18 freqs.)

Easy 2/2&1/1 (14freqs.)

Easy 2/2&1/1 (18freqs.)

Easy 2/2&2/2 (32freqs.)

Case

Abs

orpt

ion

[%]

Figure 6.4. Absorption in different IFH cases.

In heuristic cases (heuristic 1/3&1/3 and heuristic 2/2&1/3) the reference cells were createdusing NPS/X. But since it was not known how well the new interferer generation tool worksthe number of reference cells was quite excessive. In many cases the maximum number of10 reference cells were included in the reference cell list in BSC. That is the reason why theabsorption figures are 3-4 percentage units lower on those days when heuristic frequencyplanning and reference cells generated by NPS/X were used.

In ‘easy 2/2&1/1’ phase when easy frequency planning was used again the absorptions seemto be even lower than in heuristic cases. The reason for that is the used frequency reusescheme: in this phase 1/1 reuse scheme was used on the super layer. Now the interferers areeven closer compared with the situation in 2/2 reuse. In this case the C/I thresholds shouldhave been lowered again and still we would not have endangered the quality since in 1/1case the hopping sequence is longer and more frequency diversity gain can be achieved. Alsointerference diversity gain is bigger. Thus, C/I thresholds could have been decreased bysome decibels, eg 2-3 dB. However, the absorptions were still around 55% which is aboutthe same as the initial absorption figure in the IUO case.

In the last case 2/2 reuse scheme on the super layer was used again, and now the absorptionfigure is again 1-2 percentage units higher than in ‘easy 2/2&1/1’ cases. However, the samelevel as in the ‘optimized IFH 1/1&2/2’ case was not obtained.

In Figure 6.5 the minimum and maximum absorptions are presented for each case. Thismeans that in each case the cells having the smallest and biggest absorption figure are takeninto account. The good absorption level is dependent on the TRX division of an IUO cell,and in 2+2 configuration an absorption figure of 60% is considered good. Also, it isdesirable to have a small variation in the absorption figure, since low absorption means thatnot much traffic is served by the super layer. This can be due to eg errors in the definition ofthe reference cells.

In Figure 6.5 it can be clearly seen that in IFH cases the variations between the best and theworst cell in terms of absorption are smaller than with pure IUO. In IUO network theabsorption varies between 20 and 85 %, while in IFH cases after optimizing C/I and qualityhandover thresholds it varies between 30 and 80 %. For some reason in the last IFH case thevariation is as big as in the initial IUO case. One explanation can be the errors in thereference cell definition. Namely, in the last case the reference cells were again createdmanually.



Figure 6.5. Minimum and maximum absorption.

6.4.4 Direct access to the super layer

Direct Access to the Super Layer Feature was used in some cells to improve the absorptionrate. The direct access threshold was set to –50 dBm.

Originally this feature is meant for preventing blocking in the regular layer in theconfigurations where the regular layer has smaller or the same number of TRXs than thesuper layer. In the trial area there was no blocking in the regular layer. Therefore, it wasdifficult to see any difference in the absorption rate after implementing the feature.

Direct access threshold of –50 dBm was found adequate and a safe level without sacrificinggood quality. However, in another kind of environment the level of –50 dBm might be a fartoo high value to bring any benefit for the blocking problem. The field strength levels werevery high in the trial area: 60 % of the level samples were higher than RXLEV 63 (-47dBm), which is the highest RXLEV value reported by the mobile.

The direct access level should be optimized to correspond the average level with which themobiles go to the super layer. If the direct access level is set too low, the mobile can becomeimposed by too high interference after reaching the super layer and the call can easily bedropped. By implementing a too high threshold, the mobiles cannot get to the super layer bythis feature at all and the blocking situation in the regular layer remains.

0102030405060708090

100

IUO (32 freqs.) IFH 1/1 & 2/2 (22freqs.)

IFH 1/1 & 2/2 (C/Ioptimization)

Heuristic 1/3&1/3 (22freqs.)

Heuristic 2/2&1/3 (18freqs.)

Easy 2/2&1/1 (14freqs.)

Easy 2/2&1/1 (18freqs.)

Easy 2/2&2/2 (32freqs.)

Case

Abs

orpt

ion

[%]

MAXMIN



6.5 Quality and capacity improvements

6.5.1 Traffic and Handovers

In Figure 6.6 are presented the failed handover rate, and unsuccessful handovers due to lackof resources rate in all the test cases. Failed handover figure describes the number ofhandovers after which the mobile has to return to the old channel due to signaling failure inthe handover procedure. The handover is unsuccessful is the target cell does not have freehardware resources available. When comparing the failed handover rates between purefrequency hopping and IFH cases, it seems to be that IFH gives superior performance inhandover failure rate compared with FH. However, this is not the case. In IFH (and IUO)network the call is usually always established on the regular layer. Now, if the C/I conditionis good enough in the place where the mobile is located, the mobile is transferred from theregular layer to the super layer causing an extra handover. Also, if the mobile is alreadytransferred to the super layer and the C/I condition worsens the mobile is handed over backto the safe regular layer. As depicted in Figure 6.7, in frequency hopping cases the number ofhandovers equals the number of call attempts. In IFH network this is not the case, but thenumber of handovers is almost 3 times the number of call attempts due to the transitionsbetween the layers. An IUO/IFH handover is usually safe to perform, and only very few ofthe IUO/IFH handovers are failed. Thus, in percentages the IUO/IFH network seems to givea very good handover performance.

0.00%

2.00%

4.00%

6.00%

8.00%

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12.00%

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IUO_2

0//6_

32f

IFH_1

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2_31

f

IFH_1

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2_22

f

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/3 eas

y_30f

FH_1

/3 eas

y_18f_

partial

FH_1

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y_15f

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/3_heu

r_18f

FH_1

/1_18

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/3heu/

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/2heu/

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f

IFH_2

/2ea//1

/1_18

f

IFH_2

/2ea_3

2f

Failedhandovers

Unsuccessfulhandovers dueto lack ofresources

Figure 6.6 Failed and unsuccessful handovers due to lack of resources in all the testcases.

Another interesting observation in Figure 6.6 is the number of unsuccessful handovers due tolack of resources in IFH cases. As estimated in Figure 5.2 and Figure 5.3 the blocking of thesuper layer is bigger than the blocking of the regular layer. Due to congestion on the superlayer the number of unsuccessful handovers due to lack of resources also increases. Ofcourse, from the user’s point of view this has no effect, since the user experiences only the



overall blocking situation in the network. If the call becomes blocked when trying to performa handover from the regular layer to the super layer, the call can remain on the regular layer.

0

10000

20000

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50000

60000

IUO_2

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y_30f

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y_15f

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/3_heu

r_18f

FH_1

/1_18

f

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/3heu/

/1/3h

eu_22

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/2heu/

/1/3h

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/2ea//1

/1_14

f

IFH_2

/2ea//1

/1_18

f

IFH_2

/2ea_3

2f

Call attempts

Handover attempts

Figure 6.7 The number of call and handover attempts in all the test cases.

6.5.2 Drop Call Rate

Drop call rates are depicted in Figure 6.8 for pure frequency hopping cases and in Figure 6.9for IFH cases. Also effective reuses calculated using Equation (3.14) are provided in thefigures. The cases in the figures are arranged in an ascending order according to DCR.

In the frequency hopping cases the drop call rate improves when the effective reuse factor isincreased, as was expected to happen. As can be seen in Figure 6.8, the heuristic frequencyplanning seems to give very good results compared with the manual planning. The DCR of2% was obtained with NPS/X frequency planning when using 18 frequencies for theallocation. The DCR was higher in both manual planning cases with 18 frequencies. Theachieved DCR in manual planning cases is better only when using as much as 30 frequencies(Reff=11) for the allocation.

In the IFH cases the situation is a bit more complicated. The effective reuses and drop callrates do not follow each other as nicely as in FH cases. When using IFH, heuristic frequencyplanning gives even better results compared with the FH cases as can be observed in Figure6.9. For example, the IFH case having 14 frequencies resulted to DCR of 2.1%, while theDCR in the FH case having 15 frequencies was 3%. Thus, an improvement of 0.9%percentage units was found for the IFH with even one frequency less compared with the FHcase. The two best DCR figures as a whole were achieved by means of heuristic frequencyplanning and IFH. Also the reference cells were generated using NPS/X in those cases.However, as can be seen in Figure 6.9 the difference in DCRs between heuristic frequencyplanning and the manual planning cases having the whole available bandwidth (31 and 32frequencies) is very small. Since the number of measurement days for each case was toosmall, the statistical fluctuation is bigger than the difference between those cases. Of course,



the number of frequencies is much bigger in the manual frequency planning cases andrelatively heuristic allocation gave much better results.

0.0

2.0

4.0

6.0

8.0

10.0

12.0

FH_1/3 easy_30f FH_1/3_heur_18f FH_1/3easy_18f_partial

FH_1/1_18f FH_1/3_easy_15f

Call Drop Rate (%)

Effective reuse

Figure 6.8 Drop call rates with effective reuses in different FH cases.

0.0

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/3heu/

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/1_18

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/2ea//1

/1_14

f

Call Drop Rate (%)

Effective reuse

Figure 6.9 Drop call rates with effective reuses in different IFH cases.



A new concept, effective frequency load Leff, is defined as

freqsTCH

erlTRX

fa

freqeff NN

TNRL

L == , (6.1)

were NTRX is the average number of TRXs in the network, Terl average traffic in the cells,NTCH the average number of TCHs and Nfreqs the number of available frequencies.

In Figure 6.10 the drop call rate is presented as a function of effective frequency load Leff inIUO, FH and IFH cases. Also the one measurement point which is available in heuristicfrequency hopping case is provided in the figure. The effective frequency loads arecalculated in each tested case using the Equation (6.1). In Figure 6.10 each calculated Leff ismapped to the corresponding DCR value. Then, a polynomial of 2nd degree has been fitted tothe measured data in FH and IFH cases. In IUO case the number of measurements is too lowfor any appropriate curve fitting, but for comparison an exponential function as been fittedalso in IUO case.

The coefficient of determination (R2-value), which is a method of determining the accuracyof the best-fit line [Mur80], is also included in Figure 6.10. The coefficients of determinationof 0.25 to 0.45 are mildly significant statistically, 0.50 to 0.70 are moderately significant,and a value over 0.75 shows a high degree of significance. In all cases the function givingthe best R2-value has been fitted to the measurement data.

As can be seen, at low frequency loads (Leff<5%) the drop call rates are very close to eachother in FH and IFH cases. However, when the frequency load increases the DCR increasesmuch faster in the pure FH case, while in the IFH network the drop call rate remains at theacceptable level. In other words, IFH network is not so sensitive to the increments in traffic.Thus, in terms of DCR the quality of the network degrades much faster when using onlyfrequency hopping.

y = 53.646x2 + 6.9638x + 1.1219R2 = 0.9244

y = 462.2x2 - 23.205x + 1.82R2 = 0.9777

y = 1.4052e11.798x

R2 = 0.8112

0

0.5

1

1.5

2

2.5

3

3.5

4

0% 2% 4% 6% 8% 10%

Effective Frequency Loading [%]

DC

R [%

]

HeuristicRF-FHIFH

RF-FH

IUO

Poly. (IFH)

Poly. (RF-FH)Expon.(IUO)

Figure 6.10 Drop call rate as a function of effective frequency loading in IUO, FH andIFH cases.



6.5.3 RXQUAL distributions

Figure 6.11 presents the percentages of the RXQUAL samples belonging to the qualityclasses of 6 and 7 in frequency hopping cases. Frequency hopping networks are more robustagainst the interference, and thus the selection of monitoring only the quality classes 6 and 7is justified. In a conventional non-hopping network the quality criterion to be monitored isusually set to 5-7. Also the effective reuses calculated using Equation (3.14) are provided foreach hopping case.

It can be clearly seen that an increment in effective reuse factor results in better quality interms of RXQUAL. This was of course as expected since the bigger the effective reusefactor is the smaller is the interference experienced by the users. No difference could be seenbetween manual or heuristic NPS/X planning. In Figure 6.11 there are three cases eachhaving the same number of frequencies, but the frequency allocation scheme differs in everycase. In two of those cases manual planning approach has been used, and in one case thefrequency allocation is performed with NPS/X. In each of those cases the quality remains atthe same level.

In Figure 6.12 the measured RXQUAL values are presented for the IFH cases. The analysisis also made for RXQUAL classes 6 and 7. Also here the quality is improved when looserfrequency reuse is used. In comparison between FH and IFH figures it can be stated that inthe uplink direction no big difference in RXQUAL values can be seen. However, downlinkseems to be better in all IFH cases if the comparison is made between FH and IFH caseshaving equal effective reuses.

It is worth noticing that in FH cases uplink quality is always better than downlink. This isdue to the fact that in the uplink direction power control is utilized resulting to lowerinterference level, which can then be seen as improved quality. However, in IFH cases theorder of the qualities swaps, ie, now the quality of the downlink direction is better than foruplink. The reason for that is not known for sure, but it can be due to the fact that the C/Iestimation in BSC is based on the measurements performed by the mobiles in the downlinkdirection. Because the decisions of the transitions between the layers are based on thedownlink measurements, the downlink quality is preferred in the decision making at theexpense of the uplink direction. For example, if the mobile is located on the super layer andC/I estimation shows that the quality is satisfactory to remain on the super layer, this isnecessarily not the case in the uplink quality. In uplink the quality can decrease resulting toworse quality distribution in the uplink RXQUAL.



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FH_1/3_easy_15f FH_1/1_18f FH_1/3_heur_18f FH_1/3easy_18f_partial

FH_1/3 easy_30f

RXQual (%) UL

RXQual (%) DL

Effective reuse

Figure 6.11 The proportion of RXQUAL classes 6 and 7 with effective reuse indifferent FH cases.

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/2ea_3

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RXQual (%) UL

RXQual (%) DL

Effective reuse

Figure 6.12 The proportion of RXQUAL classes 6 and 7 with effective reuse indifferent IFH cases.

6.5.4 FER

The intention in the trial was to perform the FER measurements with Nokia’s tailor-madeFER tool, and with TEMS. However, due to hardware failures the measurements were lostfor many days, and with the collected statistics any conclusions concerning FER between thetested cases cannot be made.



Despite the lack of data some conclusions about the performance of the non-hopping andhopping network can be made. In Figure 6.13 - Figure 6.15 the FER distributions arepresented in every RXQUAL class for the non-hopping BCCH frequency layer, for thehopping regular layer, and for the super layer. Especially, when comparing the FERdistributions in RXQUAL classes 5 and 6 the advantage of frequency hopping can be seen.In the non-hopping case the number of FER samples of the highest FER class in theRXQUAL class 6 is over 90%. However, the FER distributions of the hopping regular andsuper layers differ from the non-hopping case. Now the number of FER samples belongingto the highest FER class in the RXQUAL class 6 is under 70% on the both hopping layers.This is in spite of the fact that the effective frequency allocation reuse Reff is higher on theBCCH frequency layer than on the other layers (around Reff =18). The same tendency in theFER distributions can be seen in RXQUAL class 5 also. The FER samples belonging to thehighest FER class is smaller in the hopping layers than in the non-hopping BCCH layer. Inthe RXQUAL class 7 no difference between the hopping and non-hopping layers can beseen: all the samples belong the FER class of >15%. The results support the observationsthat have been made in the previous frequency hopping trials [Sal98]. RXQUAL is not thebest possible quality measure in frequency hopping networks. The actual speech qualityperceived by the end user is more dependent on the discarded frames, and with the sameRXQUAL distribution FER is smaller in frequency hopping networks than in non-hoppingnetworks.

Figure 6.13 FER distribution within each RXQUAL class for BCCH frequency layer.

7 6 5 4 3 2 1 0

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Figure 6.14 FER distribution within each RXQUAL class for hopping regular layer.

Figure 6.15 FER distribution within each RXQUAL class for the super layer.

6.5.5 Quality gain of IFH

In the calculation of the quality and the capacity gains of IFH the result in Figure 6.10 isexploited. What makes the situation a bit difficult is that the existing IUO network was notoptimized, thus it can be unfair to compare the optimized FH and IFH cases with IUOnetwork. Another problem in calculating the quality gain of IFH is the small number ofmeasurement samples available in IUO case making the calculated results unreliable. To

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make the comparison with IUO fairer it is assumed that the number of used frequencies onthe regular layer (BCCH frequencies excluded) is 12 instead of 20. Even though 8frequencies have been removed from the frequency pool of the regular layer it is assumedthat the quality would not have been degraded considerably, especially if the frequency planhad been optimized at the same time. The modified IUO case is presented in Figure 6.16,and the result is used as a basis for the quality gain calculations.

The quality gain of IFH is calculated as follows. In frequency hopping case the qualitycriterion was set to 2 % DCR. Now, the effective frequency load Leff corresponding to DCRof 2 % is calculated by solving the polynomial equation, the result being Leff=5.7%. Theacquired effective frequency load is then substituted to the corresponding DCR equations ofIUO and IFH cases. In IUO case the DCR is 2.33% and in IFH case 1.69% with Leff=5.7%.Thus, FH gives 0.33 percentage units better quality over IUO in terms of DCR, IFH gives0.31 percentage units better quality over FH, and IFH gives 0.64 percentage units betterquality over IUO. It seems to be so that pure FH gives better quality compared with IUO.However, IUO concept provides also considerable quality gain, as can be seen in thecomparison between pure FH and IFH case.

Figure 6.16 Drop call rate as a function of effective frequency loading in modified IUO,FH and IFH cases.

6.5.6 Quality estimated by NPS/X

The quality of the network was also estimated with NPS/X using the method described inSection 5.3.3. Two main targets for the verification of the interference analysis tool were set.First, it was to be confirmed how well the RXQUAL estimated with NPS/X corresponds tothe actual quality measured in the real network. And second, the difference between manualand computerized frequency planning according to the interference analysis tool was also ofgreat interest.

y = 53.646x2 + 6.9638x + 1.1219R2 = 0.9244

y = 462.2x2 - 23.205x + 1.82R2 = 0.9777

y = 1.4052e8.8488x

R2 = 0.8112

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The comparison is made between the two 1/3 pure frequency hopping cases having 18frequencies in the frequency pool for the allocation. According to the interference analysistool the NPS/X heuristic frequency planning gave better quality compared with the manualplanning. The percentages of the quality classes 6 and 7 together were lower in heuristicfrequency planning as can be seen in Figure 6.17. However, the amount of quality classes 6and 7 is quite high compared with the real statistics collected from the OMC. This isprobably because of the high mountains in the trial area. According to NPS/X, and possiblyalso in the real network, these mountain areas seem to have very high interference level.However, not much traffic is generated in that area, and since the quality analysis tool cannottake the traffic distribution into account, the results are too pessimistic compared with thestatictics acquired from the real network.

Figure 6.17 RXQUAL 1-7 distributions estimated using NPS/X interference analysistool, and measured in the actual network.

6.5.7 Capacity gain of IFH

Figure 6.16 has also been exploited in the calculation of the IFH capacity gain. The qualitycriterion was set to DCR of 2 %, and then in each case the effective frequency loadcorresponding to the DCR of 2% was calculated. The calculated frequency loads wereLeff=4.0% for IUO, Leff=5.7% for FH and Leff=7.9% for IFH. Thus, the acquired capacitygains are 42.5% for FH over IUO, 38.6% for IFH over FH, and 97.5% for IFH over IUO. Inaddition, Figure 6.18 shows the capacity gain of IFH over FH as a function of DCR. As canbe seen, the capacity gain increases as a function of DCR, ie by allowing higher drop callrate in the network more capacity gain can be achieved by means of IFH.

02468

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Figure 6.18 Capacity gain as a function of DCR.

6.6 Network planning methods for IFH

6.6.1 Manual planning

Even if the manual planning approach does not give the best possible quality, it has certainadvantages. In some cases, especially if the maximum capacity is not needed, the manualplanning approach can be tempting for the operators. However, this concerns only thefrequency planning phase. To define the interfering cells manually is always a very laborioustask. In IFH there can be up to 10 interfering cells in BSC for every cell. In the manualinterfering cell definition the network planner has to examine every cell pair using a map ifthe same set of frequencies is used on the super layer. Especially in IFH this can be difficult,since the interference caused by the interfering cells is proportional to the number of TRXsin both cells, to the lengths of the MA lists, and to the number of common channels in theMA lists. For a large group of cells the definition of the interfering cells can be virtuallyimpossible without computer based planning.

The simplest way to plan the frequencies is the single MA list case, ie 1/1 reuse case on bothlayers. From the network planning point of view this is the easiest way to allocate thefrequencies, since actually no frequency planning is needed. The whole frequency bandavailable for the operator can be assigned for every cell, divided of course between theregular and super layers. With single MA list case MAIO planning is needed, but it is not abig issue.

However, 1/1 case has certain limitations. According to the trial results the best possiblequality is not achieved by means of single MA list case. If the capacity is the limiting factorin the network, 1/1 reuse is not necessarily the best planning method. Also, 1/1 reuse hassome hardware restrictions. To be able to use MAIO management the site has to besynchronized. The current BSC software release can support up to 12 synchronized TRXsper site. If one site contains three cells, the maximum configuration is 4+4+4. This meansthat every cell can have 4TRXs. In high traffic density areas this is not necessarily enough,thus single MA list case is not feasible. With 1/1 reuse case the feasible TRX divisionsbetween the layers are 2+2 (2 TRXs on the both layers), and 3+1 (3 TRXs on the regular



layer and 1 on the super layer). Thus RF hopping capability is required to obtain thesufficient frequency hopping gain.

In 1/1 case the definition of the reference cells is manually as easy as possible. Since all thecells are sharing the same set of frequencies, also the most interfering cells are the closestcells to the carrier cell supposing that all the cells are having nearly equal number of TRXs.

Another possible frequency allocation reuse scheme is so called 2/2, in which the frequencypool is divided into two frequency groups. One group is then allocated in every second sitein the most rational way, meaning that the interference is minimized between the sites. Inthis case the interfering cells are further away from each other, thus the caused interferenceis smaller to other cells. But at the same time the obtained frequency and interferencediversity gains are smaller due to the fact that the frequencies are divided into two groups. Inthe trial no evidence was found that the 2/2 reuse scheme would exceed the performance of1/1 case. The definition of reference cells requires more effort since the frequency groups inthe cells must be checked: the cells not sharing the same frequency group must not bedefined as reference cells, of course.

The third tested easy frequency allocation case was 1/3 reuse scheme, in which the availablefrequency pool is divided into three subgroups. Because in many cases the sites contain threecells, the frequency groups can be allocated in such a way that all the subgroups are usedwithin one site. Then, when allocating the frequencies to the next site, the subgroups areallocated so that the interference is minimized in respect with the other sites. With 1/3 reusescheme offset planning between the sectors is not required, since every sector is sharing adifferent frequency set within the site.

However, 1/3 allocation scheme has one major drawback with small configurations: it isdifficult to obtain a low effective frequency allocation reuse factor. Let's suppose that theconfiguration in the cell is 2+2, one TRX being the BCCH TRX. Thus, the RF hoppingregular layer contains one TRX, and the super layer 2 TRXs. To obtain any frequencyhopping gain at least 3, rather 4, frequencies are required in the hopping sequence. With fourfrequencies in the hopping sequence a total number of 12 frequencies must used for eachsite, thus on the regular layer Reff=12, and on the super layer Reff=6. With big enoughconfigurations this is not a problem. For example, with 4+3 configuration the correspondingreuse figures would be Reff=4 (BCCH TRX excluded in the calculations). This would bealready too low for the regular layer, but very feasible for the super layer. With 4+3configuration also BB-hopping mode could be used.

Usually random frequency hopping is preferred to cyclic hopping. This is because randomhopping gives better interference diversity gain. Let's now consider two cells with an equalnumber of TRXs, cyclic hopping being used in both cells, and the same set of frequenciesallocated in both cells. Now, it is possible that two calls are using the same time slot, and ifthe calls are hopping over the exactly same frequencies the interference diversity gain is lost.The bursts collide constantly since the frequencies are used in the same sequential order. Ifthe number of TRXs is different in those cells, this does not apply.

On the other hand, with short MA lists and random hopping being used it is possible that thesame frequency appears in the hopping sequence several times consecutively. In this case, ofcourse, the frequency diversity gain may be lost. However, when using cyclic hopping thefrequencies appear in the hopping sequence in consecutive order, and one frequency is neverrepeated twice in the sequence.



The number of subscribers is typically growing rapidly in GSM networks. This leads to aneed to add sites, cells and TRXs in the network. How easily the network can be expanded isof course a major issue in the network planning. In general it can be concluded that whenperforming the IFH network planning manually, the expansion of the network causes lesschanges in the network plan than when using computerized planning methods. For example,when adding TRXs (eg in single MA list case) no frequency changes are required if thefrequency load does not increase too much. Also the reference cells remain the same. SomeMAIO planning is required, and the changes in the MAIO are dependent on the relationbetween the number of frequencies in the MA list, and the number of TRXs.

When adding a cell on the site some more changes are required. The addition of the cell mayhappen in a way that the number of cells on the site is increased from eg two to three. In 1/1and 2/2 cases the same MA list already used in the site in question must be attached to thenew cell. The MAIO planning must be re-performed on that site. Also some changes areneeded in reference cell definition. The new cell has to be added to the reference cell list onthose sites that are sharing the same MA list, and are close enough to become interfered bythe new cell. For the new cell the reference cell list has to be created, of course. In 1/3 reusethe frequency group (see Figure 6.2) not used on the site must be provided for the new cell.The reference cells must be re-defined in the same manner as in 1/1 and 2/2 cases.

When adding a totally new site, the frequency planning is very easy: the frequency groupsare assigned to every cell according to Figure 6.1 and Figure 6.2. The reference celldefinition is affected in the same way as adding a cell.

6.6.2 NPS/X planning

The performance of the network which is achieved by means of using NPS/X in the planningprocess was better than with manual planning approach as was stated in Section 6.5.2.Although the performance of the network was good when using NPS/X, the planningprocedure itself must be improved. Frequency allocation with NPS/X was easy and fast.Also the generation of the reference cells with NPS/X resulted to time saving compared withmanual reference cell definition. However, the biggest bottleneck was to transfer the plangenerated with NPS/X to the real network. All the parameters had to be transferredmanually, and due to this interworking problem between NPS/X and the actual network notime saving was achieved with NPS/X planning compared with manual planning.

Heuristic frequency plan is more sensitive to the changes in the network than manualfrequency plan. When adding a TRX, or especially a cell or a site in the network, theinterference situation changes. And because the frequency allocation tool in NPS/X tries tominimize the interference in the network, the expansion of the network may result to bigchanges in the frequency plan. Necessarily this is not a problem, if the changed frequencyplan can be transferred to the network without manual work. Also, some operators anywaychange their frequency plan, eg, four times a year. Expansions in the network may affect thereference cell definition: when the frequency plan is changed the interfering cells changealso. Thus, new reference cell lists must be generated. As in frequency planning, this is not aproblem if the reference cell lists can be transferred to the network without manual work.The only concern for heuristic IFH planning is then the full functionality of necessaryparameter transfer between NPS/X and the network.

BSC might restrict in some cases the usage of heuristic frequency planning. Namely,nowadays 256 TRXs can be attached to one BSC. The number of MA lists that can be



generated under one BSC is 128. Now, lets suppose that each cell under one BSC has 4TRXs, which leads to 64 cells under one BSC. When using NPS/X in the frequency planningphase it is possible, that every cell has its own unique MA list. In this case this would meanthat a total number of 64 MA lists must generated under the BSC. In IFH both hoppinglayers must have an MA list of their own, thus the number of required MA lists is 2*64=128MA lists, which equals to the number of available MA lists in BSC. Now, if the frequencyplan is to be changed eg due to addition of new hardware, 64 new MA lists must begenerated for both layers. Of course, the old MA lists must not be deleted before the newones are created. Thus, the situation is virtually impossible since all the MA list locations arein use, and new ones cannot be generated.

In the trial the frequency band was divided into three parts: the frequencies were allocatedfor the BCCH, hopping regular, and hopping super layers from a layer-dedicated sub-band.This way it can be ensured, that no interference between the layers occurs. However, a veryinteresting approach to test would be to allocate the frequencies for all the layers from onefrequency pool, ie, from the whole band available for the operator. This way the frequenciesminimizing the overall interference in the network could always be assigned to the TRXs orMA list in question. But the risk that the quality of the hopping regular, or especially BCCHlayer is endangered is probably too high. The frequency plan should be accomplished withcomputer aided methods, of course. To achieve an optimized frequency plan, the fieldstrength prediction of the propagation model should also be accurate enough.

6.6.3 Combinations of manual and NPS/X planning

Both manual and NPS/X planning have their advantages and disadvantages. With NPS/Xplanning a small change in the network might lead to a totally different frequency andreference cell plan, which can make the follow-up of the network evolution difficult.Especially, if the whole process from the planning made with NPS/X to the actualimplementation in the real network is not thoroughly considered, this could lead to hugeamount of manual work (c.f., parameter transfer problem). On the other hand, with manualplanning the maximum capacity cannot be achieved in the network. Also, to define thereference cells manually requires a lot of work. Thus, in some cases a combination ofmanual and NPS/X planning might be a feasible solution.

If the maximum amount of capacity is not required in the network, the frequency planningcan be performed manually. For example, by using 1/1 reuse concept the expansion of thenetwork is very easy until a certain limit has been reached, and with this reuse concept noactual frequency planning is needed. However, since the definition of reference cells is thebiggest bottleneck in IFH planning, the reference cell lists could be generated using NPS/X.Now, when changes are taking place in the network, the frequency plan is not necessarilyaffected much, and the small changes in the reference cell lists could be done manually tothe original plan. This ‘easy to maintain’ method can reduce the number of planning actionsneeded in evolving networks. If major changes in the network take place constantly, thisapproach cannot be used.

6.6.4 MAIO management

Mobile Allocation steps and offsets have to be planned manually since the current version ofNPS/X does not support that kind of activity. Especially with shorter MA lists, i.e., whenNfreqs<2NTRX, and the frequencies in the MA list are in consecutive order, the steps and the



offsets have to be planned carefully to avoid adjacent channel interference as much aspossible. NPS/X takes into account automatically, however, that there is no co-siteinterference , if MAIO management is enabled.

Table 6.2 shows one example of the difference in MAIO planning between the heuristic andmanual planning. The frequency pool of which the allocation was performed consisted offrequencies 83,84,85,87,88,89,90,91 and 94. The frequency allocation reuse scheme was 1/3.First, frequencies were allocated using NPS/X. As can be seen in the Table 6.2, the offsetshad to be planned separately for every site to minimize the adjacent channel interference. Ifthe number of cells to be planned is large, this can be a very time consuming task, and it isalso very easy to make mistakes in this phase. However, if frequencies were plannedmanually (‘stupid’ planning), offset planning was very easy or it was not required at all.Because the same frequencies are reused in the very same way in every site, it is enoughwhen the offsets are planned once for one site.

Table 6.2 Example of offset planning.

NPS/X: Easy planning:MA lists MA offset MA lists MA offset

Site1 83,85,87 0 83,84,85 088,89,90 0 87,88,89 084,91,94 1 90,91,94 0

Site2 87,90,94 0 83,84,85 083,89,91 0 87,88,89 084,85,88 1 90,91,94 0

Site3 85,89,91 0 83,84,85 083,87,90 0 87,88,89 084,88,94 1 90,91,94 0

Site4 85,87,90 1 83,84,85 088,91,94 1 87,88,89 083,84,89 0 90,91,94 0

Site5 85,90,94 0 83,84,85 083,87,89 0 87,88,89 0

Site6 84,85,90 1 83,84,85 083,88,94 0 87,88,89 087,89,91 1 90,91,94 0

6.6.5 Interference caused by the second adjacent channel

There are a few ways how the frequencies can be divided in manual frequency planningwithin a site. Let's first suppose that the frequency pool consists of 12 consecutivefrequencies, and also that a 1/3 reuse network with 2 TRXs per cell is to be planned. Thefrequencies can be assigned so that always 4 frequencies are assigned to every cell inconsecutive order, ie that frequencies 1-4 are in the first cell, frequencies 5-8 in the secondcell, and frequencies 9-12 in the third cell (see Table 6.3). Or then, the frequencies can beassigned so that starting from frequency 1 every 3rd frequency is always assigned to the cell1, (see Table 6.3), the second case. The benefit of the second approach is that we avoid theinterference caused by the second adjacent frequency inside the cell that we experienceconstantly in the first case. In the case were the mobile is located very close to the base



station and downlink power control is enabled, the base station is transmitting on a lowpower. Now, if another mobile is located on the border area of the cell, the base station islikely to transmit with high power level causing high interference to the mobile close to thebase station. This is because the HSN is the same inside the cell.

However, when using the second approach MA offsets have to be planned. This is due to thefact that if MA offsets are not used, as can be done in the first case, first adjacent channelinterference occurs between sectors. The same applies if a mistake is made in MAIOplanning. Since the effect of the second adjacent channel interference is very small ornegligible, it is recommended that the first approach is used. This way it is much easier tocontrol the adjacent channel interference between the sectors, and mistakes in offsetplanning cannot be made since MAIO planning is not needed.

Table 6.3 Examples of manual planning with consecutive and punctured frequencygroups.

In the trial both manual planning approaches described above were tested, and no differencecould be found between those cases.

6.7 Performance of SDCCH and TCH

SDCCH channel is one of the three dedicated control channels used in GSM (see Section2.3.1). It is used in the call set-up phase to carry the necessary signaling information beforeTCH assignment. Also in transmission of Short Messages an SDCCH channel is assigned tothe mobile if the mobile is in the idle mode.

In Figure 6.19 the success rates of TCH and SDCCH channels are depicted as a function ofeffective reuse. The success rate is the proportion of the channel requests in which thechannel assignment is succeeded. SDCCH success rate describes how well the mobile canaccess SDCCH from AGCH, and correspondingly TCH success rate how well the mobilecan access TCH from SDCCH. In Figure 6.19 it can be clearly seen that in tight reuses, ie,when the effective reuse factor Reff is low, the performance of the SDCCH channel isdegraded when compared with the TCH channels. With high effective reuses the successrates are between 98 % and 99 %. The success rate of SDCCH is a bit worse than TCH. Butwhen the effective reuse reaches a value of Reff=8, the performance of SDCCH becomesworse much faster than the performance of TCH. The biggest gap between the success ratesis over three percentage units with the tightest reuse. Thus, the performance of SDCCH isnot as good as the performance of TCH in interference limited networks.

One explanation for the worse performance of SDCCH can be its interleaving depth. Trafficchannel, which is used for transmission of speech, is interleaved over eight bursts, whileSDCCH is interleaved only over 4 bursts. The observation, that in an interference limitednetwork the performance of a channel interleaved only over 4 bursts seems to be degraded, is

Freqs= {1,2,3,4,5,6,7,8,9,10,11,12}MA lists MA steps MA offsets MA lists MA steps MA offsets

Cell1 1,2,3,4 2 0 Cell1 1,4,7,10 2 0Cell2 5,6,7,8 2 0 Cell2 2,5,8,11 2 1Cell3 9,10,11,12 2 0 Cell3 3,6,9,12 2 0



of great importance since the future data service, General Packet Radio Service (GPRS), isusing the same interleaving depth. This has to be taken into account in the planning of GRPSnetworks. This observation may also have effect on the planning of frequency hoppingnetworks. In the trial two SDCCH channels were allocated to every cell: one was on a non-hopping TRX (in RF-FH the BCCH TRX is not allowed to hop), and one on a hoppingTRXs. It might be worth considering that the SDCCH channels should be allocated on theBCCH TRX, which is not allowed to hop, and is also having a higher reuse factor providingbetter interference conditions.

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Figure 6.19 TCH and SDCCH success rates with different effective reuses.


Chapter 7: Performance of IFH with other features85

7 PERFORMANCE OF IFH WITH OTHER FEATURES

It is not very clear how the future data services affect the performance of IFH network. Inthis chapter the co-existence of the data services and IFH is considered in a general level.Also some aspects related to dual band networks and IFH are presented here.

7.1 High speed circuit switch data (HSCSD)

High speed circuit-switched data feature provides accelerated data rates for end-userapplications. Current trend is for increased demand for high data rate applications like WorldWide Web (WWW), file transfer, and facsimile which have so far been fairly impractical touse in mobile environment due to slow data connections.

In HSCSD the data rate of a single TCH/F can vary. Till nowadays the maximum rate fordata connection has been 9.6 kbit/s but quite lately 14.4 kbit/s data rates have beenintroduced to be used over one TCH/F [ETS97a]. 14.4 kbit/s radio interface is achieved bychanging the puncturing scheme. By puncturing the number of channel coding bits in 22.8kbit/s frame is reduced by a pre-determined rule. Due to this, 14.4 kbit/s connection is morevulnerable for bad quality, and also the cell service area for 14.4 kbit/s service will reduce.According to the study [Saa99] the 14.4 kbit/s connections require 3-4 dB better C/I valuethan 9.6 kbit/s service. When the error rate for 14.4 kbit/s connection is unacceptable, theperformance can significantly be improved by automatic link adaptation function. Inautomatic link adaptation the data rate is changed back to 9.6 kbit/s.

In HSCSD even higher data rates can be offered by using several TCH/F channels for oneconnection. New functionality is needed in MS and MSC to split the data to be carried inseveral radio interface TCH/Fs, and to be combined in the other end. For cellular operationHSCSD channels in the same connection are controlled as one radio link, eg inter cellhandover is made simultaneously for all the channels in one HSCSD connection. In Table7.1 the available data rates up to four time slots (TS) are listed.

Table 7.1 The data rates with different channel coding and different number of TSs.

The services in HSCSD can be divided between Transparent (T) and Non-Transparent (NT)services. In transparent data service the throughput of the connection is constant. In HSCSDthis means that the requested data rate have to be fulfilled from the call setup to the releaseof the call, including possible handovers during the call. NT service makes it possible to useradio interface data rate flexibly. For HSCSD this means, that the reserved radio resourcescan also vary during a call. Depending on the available resources the HSCSD connection canoccupy channels from one to user defined maximum number of channels.

TS 7.2 kbit/s 9.6 kbit/s 14.4 kbit/s1 7.2 kbit/s 9.6 kbit/s 14.4 kbit/s2 14.4 kbit/s 19.2 kbit/s 28.8 kbit/s3 21.6 kbit/s 28.8 kbit/s 43.2 kbit/s4 28.8 kbit/s 38.4 kbit/s 57.6 kbit/s



In frequency hopping networks the same frequency hopping sequence must be used for allthe channels in the HSCSD connection. When baseband frequency hopping is in use, theHSCSD channels in the configuration must be allocated from the same hopping group. Thismeans that TS0 (see Figure 3.4) cannot be used for connections having several TSs in use,but the call must be allocated to TS1-TS7.

In IUO the HSCSD radio resource parameters are defined separately for the regular layer. Inthe super layer the interference band recommendation defined by BSC must be fulfilled byall the channels in HSCSD configuration. Because of the lower error correction level n*14.4kbit/s (n an integer) connections are more vulnerable to the bad quality. By raising the C/Ithresholds in IUO/IFH it is possible to further improve the quality of the super layer. Thisway it could be guaranteed that 14.4 kbit/s connections would not be downgraded to lowerdata speed connections due to bad quality.

If a big portion of the HSCSD calls using several time slots is located at the cell borderareas, it can lead to congestion on the regular layer. It is possible that there is free capacityon the super layer, and without direct access to super layer feature all the capacity of the cellcannot be utilized. On the other hand, let’s suppose that a transparent HSCSD call having 4time slots in its use is served by the super layer and the quality of the connection starts todecrease. If the call cannot be transferred from the super layer to the regular layer due to lackof resources, it can be dropped. Of course, this is always a risk when returning to the regularlayer from the super layer. But it is more likely that four free time slots does not exist on theregular layer compared with the transition of a single TS call. With NT calls this does notcause problems, since the speed of the connection can be downgraded according to theavailable hardware resources.

7.2 General packet radio system (GPRS)

GPRS is a GSM Phase 2+ service that requires many changes in the network infrastructure.This is due to the fact that GSM is based on a circuit-switched transmission mode while theGPRS uses packet-switched connections.

BSS provides radio path for GSM traffic as well as for GPRS service. The basic principle ofGPRS is that circuit-switched traffic shall not suffer from the launch of GPRS services.GPRS uses dynamically the air interface capacity left free from the circuit-switched traffic.However, dedicated capacity can also be allocated for GPRS.

BTS shall provide the GPRS support so that only software upgrade is needed in the firstrelease. It shall be possible to configure all the BTS generations and all the TRXs withinthem to support GPRS traffic. In Table 7.2 the different coding schemes (CS1-CS4) andtheir data rates are presented [ETS97c].



Table 7.2 Data rates supported by GPRS.

In the beginning of GPRS era the most common GPRS mobile types will have typicallymaximum 2-4 timeslot capabilities, and also 1 timeslot GPRS mobiles are available. Some5-8 timeslot mobiles may also appear into the market in the early phase of GPRS service.Anyway as long as 5-8 timeslot mobiles make a very small share of GPRS mobilepopulation the cost to offer 5-8 timeslot bit rates in the networks will make lower bit ratesservices more attractive for operators.

A new mobility management concept is introduced allowing the mobiles primarily select thecell by themselves. Network controlled cell re-selection to allow GPRS intra cell handoversdue to capacity or interference reasons is not supported in the GPRS Release 1. In thenetwork GPRS traffic causes higher interference level than the normal circuit-switchedtraffic, since in the downlink direction the power control is not supported, and the mobiles aswell as the base stations must transmit on the maximum power. The increased interferencecan be avoided by allocating the GPRS mobiles on the BCCH frequencies, which mustanyway be transmitted at the full power level.

A new release of Radio Network planning tool NPS/X introducing GPRS support is neededfor planning radio networks with GPRS service. The new features include coverage planningfor different coding schema, capacity planning to take into account the packet nature of thetraffic and frequency planning to take into consideration the interference generated byGPRS.

GPRS has similar restrictions concerning frequency hopping than HSCSD. When basebandhopping is used, the timeslots for multislot mobiles have to be allocated from the samehopping group, ie TS0 cannot be used with the mobiles in multislot connection.

Because network controlled cell re-selection is not supported in GPRS Release 1, the mobilecan not make a handover from the regular layer to the super layer. MS can only use regularTRXs, and it can not be ordered to use super reuse TRXs. However, if there is capacitydedicated for GPRS on the regular layer, this must be taken into account in the capacityplanning to avoid blocking on this layer. On the other hand, if the regular layer is incongestion, GPRS traffic cannot be served at all, if dedicated time slots are not reserved forGPRS.

7.3 IFH in GSM900/GSM1800 networks

Dual band GSM900/1800 network operation is specified in the ETSI multiband operationspecification [ETS97b]. From user’s point of view there is no difference between a singleband GSM900 or GSM1800 network, and a dual band network. The dual band network of a

Coding scheme SpeedCS-1 9.05 kbit/sCS-2 13.4 kbit/sCS-3 15.6 kbit/sCS-4 21.4 kbit/s



single operator appears as a single PLMN to the subscribers using dual band mobiles.However, support for dual band operation in the MSC and the BSS is required.

It has been discovered by measurements, that the higher frequency of the GSM1800 systemmeans that the attenuation of the signal with distance is higher, and hence the range of theGSM1800 cell is lower than that of GSM900. In addition, the building penetration loss ishigher for GSM1800. Thus, the indoor coverage from outdoors is worse than with theGSM900 system. However, there may be some special cases where due to buildingconstruction, for example window sizes, GSM1800 may have a better penetration.

In order to get the maximum capacity, Nokia IUO, frequency hopping and IntelligentFrequency Hopping can be used together with dual band (in both layers of a dual bandnetwork if necessary). Still, some limitations must be noted when dual band is used togetherwith IUO feature.

As IUO is a single band feature, it can be disturbed by the following dual bandfunctionalities. IUO relies on the same band interferer measurements. The moremeasurements are sent to the BSC, the more accurate is the C/I calculation to ensure areliable handover between the regular and the super layers. As a dual band MS is measuringneighbours in both bands, less interferers are analyzed, and IUO efficiency could then beaffected.

When IUO or IFH is used in a dual band network it will generally be preferable that dualband mobiles are directed to the 1800 layer, rather than to the 900 super layer assuming 900is the network on which IUO or IFH are implemented. This is to maximise the capacity forsingle 900 band users. To ensure that this takes place the BSS7 software has a feature whereit can be selected that dual band mobiles are not allowed to access the super layer of one orboth bands. By preventing dual band mobiles to use the super layer the network plan withIFH can be made exactly as in the single band case, i.e., dual band will have no impact onthe capacity for the single band mobiles.

When it is required that dual band MS can access the super layer a new feature "MulticellReporting" can be used. A GSM mobile can only report the 6 strongest cells it measures tothe network. In a dual band network these would be a mixture of cells from both bands,which has to be taken into account when planning IFH to ensure that sufficient interferingcell measurements are reported. The "Multicell Reporting" feature allows the operator todefine how many measurement reports are made from each band. For example, the 6reported cells can be set so that 5 cells are reported from the serving band (in which thehandover attempt to super layer can be initiated) and only the strongest cell from the otherband in included into reported set.

In later BSC software releases a feature called single BCCH is included in the softwarepackage. With this feature the BCCH frequency does not have to be allocated on bothGSM900 and GSM1800 bands. It means that we may have cells without BCCH. This allowsthe optimal use of IFH in the network. One possibility is to allocate all the frequencies fromGSM1800 band on the super layer, while the frequencies on the regular layer are allocatedfrom GSM900 band. By using the so called 1/1 reuse scheme on the super layer theallocation process can be made very easy. And since the number of available frequencies onthe GSM1800 band is big, the hit probability of the bursts is so small that the quality wouldbe excellent. The single BCCH feature reduces also the number of required neighbourdefinitions in the network, since the neighbours do not need to be generated separately forboth bands.



The usage of single BCCH feature has however some disadvantages. The service area of theGSM1800 band TRXs is smaller due to RXLEV approximation, because only GSM900frequencies are measured by the mobiles. Single BCCH solution does not provide supportfor the GSM1800 mobiles, since the BCCH frequency on GSM1800 does not exist. It is alsorequired that the cells on the both layers have to be located on the same site.


Chapter 8: Conclusions90

8 CONCLUSIONS

The main objective of this work was to study the capacity and the quality gains achieved bymeans of IFH solution. Another important objective was to find out the improvement in thequality that can be obtained by using computer aided network planning methods. Thesupport of NPS/X given for the network planning of IFH networks, and which kind ofparameter sets must be used in the planning were also subjects in this study. All theseobjectives were achieved.

One objective of this work was to provide means for dimensioning the blocking probabilityof IUO/IFH network as a function of given offered traffic. The results of this method, whichwas presented in Appendix A, were very promising. The blocking figures given by thismodel were verified with simulations. The calculated and simulated blocking results werepresented in Figure 5.10, which shows that the model gives a good estimation of theIUO/IFH blocking probabilities.

When considering the results of the IFH trial it is quite clear that IFH solution providesquality and capacity gain when compared with IUO or pure frequency hopping solutions.According to the field test trial the achieved capacity gain of IFH was around 40% over FH.Based on the simulations the expected capacity gain was about 35%, thus the trial verifiedthe gain of IFH predicted by the simulations. However, it is very important that the resultspresented in this work are confirmed in further trials. Namely, in many test cases the numberof measurement days was not high enough to make any accurate conclusions. For example,it is not known which reuse concept provides the best possible capacity gain. For that reasontesting of different reuse approaches is a subject to further trials. It is also very importantthat the existing network, no matter whether it is a conventional or IUO network, isoptimized before the actual trial cases are tested. Without good enough benchmarking thetest cases may seem to provide superior quality and capacity gains without solid enoughbackground.

The trial also showed that the network planning with NPS/X gives better results than manualplanning approach. Also some default parameter sets were verified for later use. Despite thegood performance of the computerized network planning some bottlenecks were also foundin the planning process. It was realized that it is very important that the frequency allocationresults, as well as the reference cell lists, can be transferred directly from NPS/X to theactual network without manual work. If the interface between the planning tool and thenetwork management system does not work well, the required amount of manual work is toobig to plan IFH networks for large areas.

It would have been possible to even further improve the frequency allocation resultsobtained with NPS/X. Namely, the calculation of the interference matrix which is the basisfor the frequency allocation was based on the predicted field strengths. Of course, themodels were tuned in the trial, but the traffic distribution was not taken into account in theallocation. Real traffic data could have been imported from the network. After this processthe traffic could have been weighted according to the morphographic types. The weighting isbased on the fact that, e.g., on the water or cultivation morphography areas not much trafficis generated, while on the other hand urban and suburban areas may have a very high trafficdensity. With NPS/X it is possible to weight the traffic according to these morphographyfactors. With a proper traffic density layer, the allocator tries to minimize the interferedtraffic, not the interfered area, that is the case if the uniform traffic distribution is used in the


Chapter 8: Conclusions91

generation of the interference matrix. By utilizing the traffic density layer it is possible toguide the interference to the areas where the traffic density is low, and thus less traffic willbe interfered. It is of great interest to test this functionality of NPS/X in later trials. Mostlikely the performance of the network can be further improved.

Another interesting functionality to be tested in NPS/X would be a frequency allocation,which is based on the tuned interference matrix. The interference data is imported to NPS/Xfrom a real network, and then the predicted interference values in the matrix are replacedwith the measured ones for those cell pairs for which the measurements are available. Nowthe frequency allocation would be partly based on the measured data. This approach shouldbe able to provide the best possible allocation result.

In the trial it was not possible to test the gains achieved with PC and DTX standard GSMenhancement features. Uplink PC was used during the whole trial, whereas downlink PC andDTX were disabled. However, in later trials the functionality of DL-PC and DTX with IFHmust be tested. Also the effect of the direct access to super -feature must be further tested toensure its functionality in the real network.

The quality information provided by FER was almost entirely lost in the trial due tohardware failures in the equipment. It would be very valuable information, if the FERmeasurements were available for all the test cases in the future trials. This is because FER issupposed to better indicate the subjective voice quality of the connection that is perceived bythe users [Haa97].

In this thesis the difficulty of determining the good and bad C/I values in frequency hoppingnetworks was discussed. It was stated that those thresholds could be lowered at least by theamount of frequency diversity gain. The frequency diversity gain can be determined at someaccuracy. However, it is not known how the interference diversity gain should be calculated,and how much the thresholds could be lowered accordingly. For that reason it might beworth considering the change of the estimation method so that decision concerning thehandovers between the regular and super layers could be based on BER. The C/I valuesreported by the mobile could be mapped to the BER. The mapping could be based on thesystem level simulations in the same manner as in quality analysis tool in NPS/X, in whichthe quality of frequency hopping networks is presented in terms of RXQUAL. Then, theoverall BER caused by all the frequencies could be calculated, and according to this BERvalue the handovers between the layers could be controlled in the BSC.

Some ideas were also given about how IFH and future data services might interact with eachother. Without proper network planning the IUO/IFH network can become congested whenintroducing the new data services. GPRS traffic can also lead to higher interference level inthe network, which possibly must be taken into account in the frequency allocation.However, the question how the new data services and IFH can co-exist in a real network willbe seen in the near future.


Chapter 9: References92

9 REFERENCES

[ETS92a] ETSI GSM 05.01, Physical Layer on the Radio Path: General Description,European Telecommunication Standards Institute, 1992, 11p.

[ETS92b] ETSI GSM 06.10, GSM Full Rate Speech Transcending, EuropeanTelecommunication Standards Institute, 1992, 93p.

[ETS92c] ETSI GSM 05.03, Channel Coding, European Telecommunication StandardsInstitute, 1992, 22p.

[ETS92d] ETSI GSM 05.04, Modulation, European Telecommunication StandardsInstitute, 1992, 3p.

[ETS92e] ETSI GSM 06.01, Speech Processing Functions: General Description,European Telecommunication Standards Institute, 1992, 8p.

[ETS95] ETSI GSM 05.08, Radio Sub-System Link Control, EuropeanTelecommunication Standards Institute, 1995, 37p.

[ETS97a] ETSI GSM 02.34, High Speed Circuit Switched Data; Stage 1, EuropeanTelecommunication Standards Institute, 1997, 15p.

[ETS97b] ETSI GSM 03.26, Multiband Operation of GSM/DCS 1800 by a SingleOperator, European Telecommunication Standards Institute, 1997, 17p.

[ETS97c] ETSI GSM 03.64, General Packet Radio Service; Overall Description of theGPRS Radio Interface, European Telecommunication Standards Institute,1997, 42p.

[ETS98] ETSI GSM 10.59, Enhanced Data Rates for GSM Evolution (EDGE),European Telecommunication Standards Institute, 1998, 18p.

[Haa97] J. Haataja, Taajuushyppelyn vaikutus DCS1800/1900-järjestelmän laatuun,Master Thesis, Helsinki University of Technology, Radio Laboratory, 1997,63p.

[Laa96] J.P. Laakso, Work Instruction, Generic IUO planning for GSM/DCS 1800,Nokia Telecommunications, 1996, 49p.

[Lee89] W. C. Y. Lee, Mobile Cellular Telecommunications Systems, New York,McGraw-Hill, 1989, 449p.

[Lee93] W. C. Y. Lee, Mobile Communications Design Fundamentals, New York,John Wiley, 1993, 372p.

[Lin94] I. Lindell, Radioaaltojen eteneminen (Propagation of Radio Waves, inFinnish), 3rd corrected edition, Espoo, Otatieto Oy, 1994 261p.


Chapter 9: References93

[Mou92] M.Mouly, M.B. Pautet, The GSM System for Mobile Communications, Cell &Sys, 1992, 701 p.

[Mur80] R. Murray, Probability and Statistics, McGraw-Hill Inc., 1980, 672p.

[Nie97] T.T. Nielsen, J. Wigard and P. Mogensen, “On the Capacity of a GSMFrequency Hopping network with Intelligent Underlayer-Overlayer”, IEEEVTS 47th Vehicular Technology Conference, Phoenix, 1997, pp. 1867-1871

[Nie98] T.T. Nielsen, J. Wigard, P.H. Michaelsen, P. Mogensen, “Slow FrequencyHopping Solutions for GSM Network of Small Bandwidth”, IEEE VTS 48thVehicular Technology Conference, Ottawa, 1998, pp. 1321-1325

[Nok96] Frequency Hopping BSS Implementation, Nokia Telecommunications, 1995,28p.

[Nok98a] Extended Network Planning Introduction, Nokia Telecommunications, 1998,171p.

[Nok98b] System Training for GSM, Nokia Telecommunications, 1998, 134p.

[Nok98c] NPS/X 3.3 Support for Frequency Allocation with FH, IUO and IFH,Requirement Specification, Nokia Telecommunications, 1998, 77p.

[Nok98d] NPS/X 3.3 IFH Interferer Tool for Multilayered Networks, RequirementSpecification, Nokia Telecommunications, 1998, 21p.

[Nok98e] NPS/X 3.2 Network Planning System, User Manual, NokiaTelecommunications, 1998, 680p.

[Nok98f] Frequency Hopping Planning Guide, Nokia Telecommunications, 1998, 80p.

[Rap96] T.S. Rappaport, Wireless Communications: Principles and Practice, NewJersey, Prentice-Hall, 641 p.

[Saa99] A. Saarimäki, Radio Network Aspects in GSM Data Evolution, Master Thesis,Helsinki University of Technology, Radio Laboratory, 1999, 92 p.

[Sal98] M. Salmenkaita, Planning Methodology for Frequency Hopping Solutions inGSM Networks, Master Thesis, Helsinki University of Technology, RadioLaboratory, 1998, 106 p.

[Ter97] K. Terävä, Analysis and Implementation of an Algorithm for Estimating theQuality of Frequency Hopping GSM, Master Thesis, Helsinki University ofTechnology, Communications Laboratory, 1997, 96p.

[Wig97] Jeroen Wigard, Thomas Toftegård Nielsen, Preben Mogensen, “ImprovedIntelligent Underlay-Overlay Combined with Frequency Hopping in GSM”,Proc. of PIMRC’97, Helsinki, 1997, pp 376-380


Chapter 10: Appenxies94

10 APPENDICES

APPENDIX A

A method for estimating the blocking probability of IUO networks is presented here. It isbased on Markov chains, and the method is therefore quite similar to derivation of Erlang Bformula, see Equation (5.1). In the derivation of Erlang B one-dimensional Markov chain isused to obtain the formula, whereas here in order to be able to determine the blockingprobability of IUO networks two-dimensional Markov chain has been exploited. However,so far analytical solution to the problem is not available, hence numerical method (Matlab)has been used to calculate the blocking probabilities.

The transition intensities are presented as a Markov chain state diagram in Figure 1.1. On thehorizontal axis are the states of a regular layer, whereas on the vertical axis are presented thestates of a super layer. In other words, when moving from left to right in Figure 1 the numberof calls on a regular layer increases, and in the same way when moving downwards thenumber of calls on a super layer increases.

Figure 1. State diagram.

The transition probabilities are defined and calculated as follows:

1. The probability that there will be a change from n channels to n+1 channels is λ

2. The probability of call directed to super layer from SDCCH (direct access to super) isdenoted by p. So the call arrival probability to super is pλ and the call arrival probability

0,0 1,0 2,0 n,0

0,2

0,m

1,1

1,2

1,m

2,1

2,2

2,m

n,1

n,2

n,m

µ 2µ 3µ nµ

µ 2µ 3µnµ

µ 2µ 3µ nµ

µ 2µ 3µ nµ

Regular

Super

1-(1-s)1

1-(1-s)2 1-(1-s)3

1-(1-s)n

1-(1-s)1

1-(1-s)1

1-(1-s)1

1-(1-s)2 1-(1-s)3

1-(1-s)n

1-r1

1-r2

1-r3

1-rn

1-r2

1-(1-s)2

1-(1-s)2 1-(1-s)3

1-(1-s)3

1-(1-s)n

1-(1-s)n

1-r3

1-rn

1-λ 1-(1-(1-s)1+ λ+ µ) 1-(1-(1-s)2+ λ+ 2µ) 1-(1-(1-s)n+ pλ+ nµ)

1-(1-r1+λ+ µ)

0,1

1-(1-(1-s)1+1-r1+λ+ µ+ µ) 1-(1-(1-s)2+1-r1+λ+ 2µ+ µ)

1-r1 1-r1 1-r1

1-r2 1-r2

1-r31-r3

1-rn 1-rn

1-(1-r2+λ+ 2µ) 1-(1-(1-s)1+1-r2+λ+ µ+ 2µ) 1-(1-(1-s)2+1-r2+λ+ 2µ+ 2µ)

1-(1-rn+λ+ mµ) 1-(1-rn+λ+ µ+ mµ) 1-(1-rn+λ+ 2µ+ mµ) 1-nµ-mµ

(1-p)λ

pλµ

pλ3µ

pλmµ

pλµ

pλµ

pλµ

pλµ

pλ2µ

pλ2µpλ

2µ

pλ2µ

pλ2µ

pλ3µ

pλmµ

pλmµ pλ

mµ

pλmµ

pλ3µ

pλ3µ

pλ3µ

(1-p)λ (1-p)λ (1-p)λ

(1-p)λ(1-p)λ(1-p)λ(1-p)λ

(1-p)λ (1-p)λ (1-p)λ (1-p)λ

(1-p)λ(1-p)λ(1-p)λ(1-p)λ

1-(1-(1-s)n+ pλ+ nµ+ µ)

1-(1-(1-s)n+ pλ+ nµ+ 2µ)



to regular is (1-p) λ.

3. The outgoing probability from n regular channels to n-1 is nµ, and from m superchannels to m-1 is mµ.

4. Let s be the share of the traffic that can be transferred from regular to super layer. ("goodC/I threshold"). Now the probability that one call is transferred to super is s. If there are nongoing calls on the regular layer, the probability that none of the calls is transferred tosuper is (1-s)n, so the probability that one or more calls can be transferred to super is 1-(1-s)n.

5. If r is the share of traffic that can stay on super layer ("bad C/I threshold"), then (1-r) isthe proportion of the traffic on the regular layer of the whole cell service area. So theprobability that one call is transferred from super to regular is (1-r). Correspondingly, theprobability that none of the m calls located in super layer is transferred from super toregular layer is (1-(1-r))m=rm, and so the probability that one or more calls can betransferred to regular is 1-rm.

6. In general, the likelihood that the system will remain at state (n,m) is1-λ-nµ-mµ-(1-(1-s)n)-(1-rm).

The probability of call being at state Pn,m (n is state number for regular and m for super) isthe product of those state and transition probabilities from which the transition to state Pn,mis possible added with the probability of being at state Pn,m multiplied by the likelihoodfactor 1-λa-nµ-mµ-(1-(1-s)n)-(1-rm).

.))1)1(1(1()1(

)1()1())1(1(

,1,11

1,,11,11

,1,

mnmn

mnm

mnmnmnn

mnmn

PmnrsPr

PmPnPsPP

µµλµµλ+++−+−−−+−+

++++−−+=

+−+

++−++

− (A1)

Equation (A1) does not take into account the possibility to have direct access to the superlayer. However, the situation is very similar that in the Equation (A1), only the first termλPn-1,m on the right side is divided into λ(1-p)Pn-1,m and λpPn,m-1. Simplifying and movingall the terms to the left side in Equation (A1) we now have

.0)1()1()1(

))1(1())1)1(1(

1,11

1,,11,1

1,1,

=−−+−+−−−−−+++−+−−

+−+

++−+

+−

mnm

mnmnmn

nmnmn

mn

PrPmPnP

sPPmnrs

µµλµµλ

(A2)

Writing the equations for all the states yields (n+1)(m+1) times (n+1)(m+1)+1 system ofequations,

=

10

0

0

111)()()(

)1()1()1(0

,0,0,0

,0,0,0

M

M

M

M

LLLL

MOMMOMMOM

LL

C

n

mnn

mnn

P

P

P

nmanmanma

aaa

, (A3)



where a(i)n,m is the transition probability coefficient and Pn the state probability(C=(n+1)(m+1)). The last row comes from the condition that the sum of all the stateprobabilities must be one, ie

10

=∑=

C

nnP , (A4)

where C is the number of states, in this case again C=(n+1)(m+1).

The state probabilities Pn are then calculated by solving the system presented in Equation(A3) using Matlab. The calculation of regular blocking, which is actually the overallblocking experienced by MX, is very straightforward. The call is blocked if there is no moreavailable channels,

∑=

⋅=M

iiNbl PP

1

, (A5)

where N is the maximum number of regular channels (ie N=number of regular TCHs andM=number of super TCHs).

intelligent frequency hopping

Documents

prof pertti

rms delay

rms delay

received signal

synthesized

offrequency

received power

flat fading