Cellular Network PlanningandOptimizationPart V:GSMJyriHmlinen,

Communications andNetworking Department,TKK,18.1.2008

2GSMBriefly

3General

GSMwasthefirstdigitalcellularsystem. GSMwaslaunchedin1992 Over2billionsubscribersworldwide Over600millionsubscribersinEurope

ConventionalGSMvoiceisCircuitSwitched(CS) Calloccupiesthechannelduringthedurationofth ecall

Thelateststandardphaseincludes GeneralPacketRadioService(GPRS) EnhancedDataRatesforGSM(EDGE) GPRSandEDGEare packet switched systems

4GSMSpectrum

OriginallyGSMoperatedonlyin900MHzband Laterextendedto1800MHzand1900MHzbands MostGSMnetworksoperateinthe900MHzor1800

MHzbands Also 450MHzband isused insome countries

Spectrumexample Inso-calledprimaryGSM900MHztheuplink

frequencybandis890915 MHz,andthedownlinkfrequencybandis935960 MHz.

This25MHzbandwidthissubdividedinto124carrie rfrequencychannels,eachspaced200 kHzapart.

5GSMSystemarchitectureOurmainfocusarea

6BaseStationSubsystem(BSS) BaseTransceiverStation(BTS)

RadiointerfacecontrolbetweenBTSandMS Transmissionexecution,channelencryption,diversi ty,

frequencyhopping BaseStationController(BSC)

Handovercontrol,channelassignments,collectiono fcellconfigurationdataetc

7NetworkSwitchingSystem(NSS) MobileSwitchingCenter(MSC)

Callswitching;MSCcontrolcallsbetweenBSSando thernetworks(PSTN,PLMN)

GatewayMSC(GMSC) ExecutesgatewayfunctionsforMSC

HomeLocationRegister(HLR) Databasethatcontainsinformationofoperatorsown subscribers.

VisitorLocationRegister(VLR) Databasethatcontainsinformationofvisitingsubs cribers.Thereis

oneVLRperMSC ServingGPRSSupportNode(SGSN)

HandlespacketdatafromGPRS/EDGE GatewayGPRSSupportNode(GGSN)

GatewayfunctionsforSGSN

8GSMFramestructure

TimeDivisionMultiplexing(TDMA)isused 8full-rateor16half-ratespeechchannelsper200 kHz

channel.Thereareeightradiotimeslotswithinar adioframe.

Halfratechannelsusealternateframesinthesametimeslot.Thechanneldatarateis270.833 kbit/s,a ndtheframedurationis4.615 msTDMA/FDMAscheme

Ontopofthis frame structure there ismultiframe,superframe andhyperframe structures.

0 1 2 3 4 5 6 7

4.615ms

200kHz

9LogicalchannelstructureGSMchannels

Traffic channels (TCH) Signaling channels

Full rate Half rate

TCH/F TCH/H

BCCH(broadcast ch)

DCCH(dedicated ctrlch)

CCCH(commonctrlch)

DLDL UL slow fast

FCCH SCH PCH AGCH RACH SACCH

SDCCH FACCH

10

Channelstructure

Traffic channels (our focus isinspeech service) Full rate:9.6kbps forspeech (1time slot/radioframe ) Half rate:4.8kbps forspeech (1time slot in

alternating radioframes) BCCH

Provides generalcell specific information MScan register tocell only if it can detect BCCH

From our perspective TCHandBCCHare most important.

11

Channelstructure

FCCH=Frequency correction channel SCH=Synchronization channel PCH=Paging channel AGCH=Accessgrant channel RACH=Random access channel SACCH=Slow assosiated control channel SDCCH=Stand-alone dedicate control channel FACCH=Fast assosiated control channel

12

Network planning issues inGSM

13

Frequency reuse

Cluster sizes up toK=12or even more are used insome extreme cases. Due toincreasing traffic cluster sizes tend todecrease = >

replanning andoptimization ofpresent networks isongo ingactivity

Cluster size K=1isnot used due tohigh co-channelinterference

BCCHandTCHmay have different cluster sizes BCCHiscrucial forconnection =>larger cluster sizes

14

Frequency reuse Frequencyreuserate

Measureforeffectivenessoffrequencyplan Trade-off:Effectivenessvs interference

Multiplere-useratesincreaseeffectivenessoffre quencyplan Compromisebetweensafe,interferencefreeplanning and

effectiveresourceusage1 3 6 9 12 15 18 21

safeplanning(BCCHlayer)

normalplanning(TCHmacrolayer)

tightre-useplanningsamefrequencyineverycell

Cluster size

15

TRX

Atransmission/receptionfacilityiscalledasatr ansceiver(TRX). Typically,acellhasseveralTRXs,andonefrequen cyisallocatedto

eachTRX. Thecapacityofacellcanthereforebemeasuredin thenumberofTRXs.

Atimeslotthatcarriesusertrafficisalsocalle dasTrafficChannel(TCH).

Foruser speech service 1time slot/radioframe isreser ved inTCH/Fand1time slot inalternating radioframes inTCH/H

Foradministrativepurposes,e.g.providingmobile terminalswithcontrolinformation,thereisalsooneBCCHineach cell.

Thischannelconsumesonetimeslot(slot#0),and isoperatedbyoneTRXineachcell.

InphysicalBCCHthereareatleastFCCHandSCH+ basicsystemrelatedinformation

16

SINRandTXpowers

According toGSMspecifications C/I>9dBfornominal performance. C/I=carrier tointerference power ratio Here interfere nce term

contains also AWGN. MStransmissionpower

InGSM850/900TXpowerisbetween0.8Wand8W.Usua lvalueis2W=33dBm

InGSM1800/1900TXpowerisbetween0.25Wand4W BStransmissionpower

InGSM-900and1800TXnormal BTS(=macro BTS)power isusually between 5Wand40W=46dBm.

Outputpowers inmicro BTSs are between 14dBmand32dBm FormacroBTSpowerismeasuredincombinerinput FormicroBTSpowerismeasuredinantennaconnecto r

17

Example

Assume that Operator has 5MHzavailable forGSM Operator makes theTCHfrequency plan according to

specifications (SINR>9dB)but add 6dBmarginal forBCCHSINR.

Propagation exponent is4,system isinterferencelimited

Problems What are thecluster sizes forBCCHandTCH? How many TRXs are needed What isthenumber ofTCH/FandTCH/Hspeech

channels percell?

18

Solution

5MHz/200kHz=25subcarriers,8time slots ineach =>200time slots intotal

SINRforTCH=9dB=> =7.943 SINRforBCCH=15dB=> =7.943 Propagationexponent =4=>C( )~7 Interference limited system =>we can useequation

After solving KforTCHwe find that K=2.4855 Similarly we find forBCCHthat K=4.959

=7.943 (TCH)

19

Solution

Closest cluster sizes from ForTCHcluster size =3 ForBCCHcluster size =7

One TRXcan handle 1subcarrier (200kHz) We need 1BCCH-TRX/cell K=7forBCCH=>7subcarriers needed forBCCH There are 18subcarriers left forTCH.Since K=3forTCH we need 6

TRX/cell inaddition totheone that iscarrying BCCH Intotal we need 7TRX/cell

Number ofspeech channels TCHTRXs:7x8speech channels forfull rate and7x16f orhalf rate BCCH-TRX:7speech channels forfull rate and14speech channels

forhalf rate Intotal there will be 63speech channels forfull rate and 126forhalf

rate.

This isabout themaximum capacity configuration forthe operator

20

Sectorizing andrange

Three-sector sites are most commoninGSM Sectorization gives antenna gain inBS

Maximum cell size isround 35km Cell size depends onthetiming advance which is232.47 s

inbasic GSMsystem Usually cell sizes are between few hundreds meters tofew

kilometers.Very large cells may occur e.g.insea coastswhere bunch ofislands are covered by asingleBS.

21

Cell categories

Commonlyusedcellcategories Macrocell=cellwherethebasestationantennais

installedonamastorabuildingaboveaverageroo ftoplevel.Macrocellsarecommoninalloutdoorenvironments

Microcell=cell whereantennaisplacedunderrooftoplevel.Microcellsareusedinurbanareastoc overfewblocksofbuildings

Picocell=cell thatadmitcoverageofsometensofmeters(coveragecanvarydependingontheenvironment).Usedindoors.

22

Indoor coverage

Usually cheapest andmost commonway toprovide indoor co verageistouse outdoor macro base stations forthis purpose

=outdoor toindoor coverage. Indoor users onthecell edge usually define thecell size since

penetration loss due tobuiding walls can be tens ofdecibe ls (usualvalue forpenetration loss is20dB)

Indoorcoveragemaybeprovidedalsobyindoorpico basestations. Suitable inoffice buildings;each floor can be covered by few pico

base stations Thelarge number ofbase stations may increase thenetwor k costs

Radiorepeaters andRFheads provide another solution Inrepeater distributed antennas (e.g.antenna/floor) are fed through

power splitters Repeater system applies anoutdoor antenna that receives t he

outdoor BSsignal after which thesystem repeates thesig nal fromindoor antennas.

23

Frequencyhopping(FH)inGSM FHmeansthataTRXshiftsfrequencyateverynew

radioframe =>hoppingisperformedapproximately1/(4.615*0.00 1)=

217timespersecond. Foraspecificuser,anewfrequencyisusedfromo netime

slottothenext. TheadvantagesofFHaretwofold

Frequencydiversity Interferencediversity.

ThedrawbackwithFHisthatthenetworkcomponentsbecomemorecomplexandexpensive.

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FHdiversity gain

Transmittedsignalissubjecttofastfadingduetomultipathnatureofthepropagation.

Especially,ifthereceiverismoving,theuserwil lexperiencefadingdipsfrequently,andthedistancebetweenthefadingdipsdependsonthefrequencyus ed.

Changingfrequencycontinuouslyisonewaytoreduc etheinfluenceoffadingdipsandtheprobabilityof goodlinkqualityisincreased.TheadvantageofusingF Histhatthechannelwillusuallynotsufferseverefad ingdipsunderlongertimeperiods.

Thenumberofhoppingfrequenciesisanimportantfactorthataffectsthefrequencydiversitygain.H ighergainisachievedwithmorehoppingfrequencies,butmorethan8hoppingfrequenciesgivelessimproveme nt

25

FHdiversity gain

0 1 2 3 4 5 6 7

0 1 2

Channelpower response isgood Channelpower response isbad

Channelbits from consecutive slots are mixed (interleavi ng inTXunitanddeinterleaving inRXunit)=>fading isaveraged a nddetection hasbetter probability tobe successfull

Frequency 1

Frequency 2after ahop

26

FHinterference diversity gain WithoutFH,neighboringcellswithco-channelfrequ enciestothe

carrierfrequencywillinterferecontinuously. Theinterferencecanbesevereforcertainfrequenc iesandmaylead

toinformationloss. FHmeansthatdifferentfrequencieswillinterfere withthecarrierat

differenttimeslots. Insteadofhavingaconstantlevelofinterference betweentwocells,

theinterferenceisspreadamongseveralfrequencie s. Foraspecificcarrier,onlysometimeslotswills ufferfromhigh

interference.Fromasystempointofview,strongi nterferersaresharedbetweenusersandsocalledinterferenceave ragingisachieved.

AGSMnetworkcanbeplannedforaverageinterferen ceinsteadofworstcaseinterference .

Theinterferencediversitygainisdependentonthe numberofhoppingfrequenciesandinterferencelevelsfromtheinterf eringcells.Asignificantinterferencediversitygainisobtained withaslittleas3hoppingfrequencies.

27

FHStrategies CyclicFH

Incyclichopping,allinterferingcellsusethesa mehoppingsequence,andachannelwillbeaffectedbythesameinterfer ersallthetime.Nointerferencediversityisobtainedwithcyclichopp ing.

RandomFH Randomhoppingusessequencesthatarepseudo-rando mly

generated=>interferencediversityisobtained. ThephysicalwaystoperformFHinacellarebaseb andhoppingand

synthesizedhopping. Inbasebandhopping,everyTRXisassignedtoafix edfrequency,

whilethechannelisshiftedbetweentheTRXs.The numberoffrequencycarriersmustbeequaltothenumberofT RXs.

Insynthesizedhopping,channelstaysatacertain TRX,whiletheTRXshiftsamongtheavailablefrequencies.Inthelatt ermethod,thenumberofcarriersisgreaterthanorequaltothe numberofTRXs.

Sincethenumberoffrequenciescanbemorethanth enumberoftransceiversinacellwhensynthesizedhoppingis used,thereusefactorcanbechangedwhilethenumberofTRXs isfi xed.

28

FHStrategies

Therearetwostrategiesforusingthefrequencyba ndfortheTCHandBCCHcarriers Incommonbandstrategy bothTCHandBCCHcarriersuse

theentireband,andaTCHcanreceiveinterference frombothTCHandBCCHcarriers.

Indedicatedbandstrategy thefrequencybandissplitintotwoparts,onefortheTCHfrequenciesandonefor theBCCHfrequencies.InthisstrategyaTCHcarrierca nonlybeinterferedbyanotherTCH.FHisnotperformeda ttheTRXs thatoperatetheBCCH.

Simulationshaveshownthatdedicatedspectrumband sgivelessinterferencethancommonbandstrategywh ichsufferedfromseveredisturbanceinthedownlink.I tiseasiertoputextraTCH-TRXinanexistingcellwhendedic atedbandsareusedsincetheBCCHfrequencyplandoesn othavetobechanged.

29

Frequency assignment problem

Classicalfrequencyassignmentproblem(FAP)belong stotheclassof NP-completeproblems,whichmeansthattheproblemprobablycannotbesolvedinpolynomia ltime.

ThereexisttwomajorapproachestodealwiththeF APinwirelessnetworks FixedChannelAssignment(FCA). InFCA,achannelis

assignedtoaconnectionbeforehandandcannotbechangedon-line.

DynamicChannelAssignment(DCA). InDCAthechannelsarechangedon-linewhenevertheradioconnectionsuffersfrominterferenceandthequalit yrequirementsarenotfulfilled.

30

FCAvs DCA

FixedChannelAllocation(FCA) Thisisconventionalapproachwhereeachcellisal locatedtoa

predeterminedsetofchannels.Channelsareallocat edaccordingtofrequencyplan

Radioresources(channels)cantbetransferredbet weencellsandnetworkplanningisusuallydoneonworstcase basis=>trunking losssincetrafficloadchanges.

DynamicChannelAllocation(DCA) Channelsarenotallocatedpermanentlytodifferent cellsbut

BSallocateschannelsdynamicallytocomingcalls. DCAappliesalgorithmthattakeintoaccounte.g.

Likelihoodofblocking,co-channelinterference,ot hercostfunctions

Inextremecaseallchannelsareavailableineach cellandDCAeliminatestheneedforfrequencyplanning

Radioresourcereuseischangingdynamically=>hig hertrunking efficiency

31

Frequency assignment problem inFCA

ThereexistseveralversionsofFCAapproach MinimumspanFAP .IntheminimumspanFAP,certainsoft

and/orhardrestrictionsaregivenforthequality ofthenetwork.Theserestrictionscanbeinterferenceandseparati onrequirements.Thesoftrestrictionscanbeviolated atsomepenaltycost,butthehardrestrictionsmustbemet .Theobjectiveistominimizethedifferencebetweenthe minimumandmaximumfrequenciesassigned,thespan.

MinimumorderFAP .Insteadofminimizingthespanoffrequencies,thenumberoffrequenciesisminimized inminimumorderFAP.

MinimumblockingFAP .TheminimumblockingandminimuminterferenceFAP(seebelow)useafixedspanoffr equencies.IntheminimumblockingFAP,thegoalistoassign frequenciesinsuchawaythattheoverallblockingprobability ofthenetworkisminimized.

MinimuminterferenceFAP .Theobjectiveistominimizethetotalsumofinterferenceinthenetwork.

32

Frequency assignment problem inDCA

All or almost all frequencies are available ineach cell Very generaloptimization problem faced Like inFCArelated FPAdifferent approaches can be

adopted (starting from blocking,interference or othermeasures)

Theinformation change between base stations isslow=>fast interference mitigation isdifficult

Slow interference avoidance methods can be designedbased oninterference matrices.

Different companies have their own solutions

33

Simplified link budgets

34

Backgroundinformation

RXsensitivity( 3GPPTS05.05V8.20.0(2005-11)) GSM900MS:

forGSM 900smallMS -102 dBm GSM900BTS

fornormalBTS-104 dBm formicroBTSfrom-87downto-97 dBm dependingont heBTS

class Forpico BTS-88dBm

DCS1 800MS forDCS 1 800MS-102 dBm (forsomeclassesalso-100d Bm)

DCS1 800BTS fornormalBTS-104 dBm formicroBTSfrom-92dBmdownto-102 dBm depending onthe

class forpico BTS-95dBm

Note:RXsensitivityisthethe minimumreceivedpow erforwhichthepredefinedSNRrequirementisachieved.

35

Recall:SINRandTXpowers

According toGSMspecifications C/I>9dBfornominal performance. C/I=carrier tointerference power ratio Here interfere nce term

contains also AWGN. MStransmissionpower

InGSM850/900TXpowerisbetween0.8Wand8W.Usua lvalueis2W=33dBm

InGSM1800/1900TXpowerisbetween0.25Wand4W BStransmissionpower

InGSM-900and1800TXnormal BTS(=macro BTS)power isusually between 5Wand40W=46dBm.

Outputpowers inmicro BTSs are between 14dBmand32dBm FormacroBTSpowerismeasuredincombinerinput FormicroBTSpowerismeasuredinantennaconnecto r

36

Simple ULlink budget:Illustrative example

Pathloss=152dBFeeder/cableloss=4dB

TXPower33dBm(2W)

Antennagain=16dBi

Diversitygain(2antennas)=3dB

33dBm

- 116dBm- 100dBm

- 104dBm

RXSensitivity-104dB Antenna gain isadesignissue,usually

16-18dBiwhen 3sectors (cells)insite.RXsensitivity isalso designissue butphysics andcosts put limits toit.Neither interference nor shadow fadingistaken intoaccount here

37

Simple DLlink budget:Illustrative example

Pathloss=152dB

Combinerloss=5dB

Feeder/cableloss=4dB

RXSensitivity- 102dBm

TXPower43dBm(20W)

Antennagain=16dBi(nodiversity)

- 102dBm

50dBm34dBm

38dBm

Recall:Combiner isadevice thatcombines feedsfromseveralTRXs sothattheycouldbesentoutthroughasingleantenna

38

Cell range

Afterthemaximumallowablepathlosshasbeendetermined,thecellsizecanbeevaluated

Determinationisdonebyusingbasicpropagationpredictionformulas Okumura-Hata Walfish-Ikegami

Wetypicallywanttohave90%locationprobabilityoverthecellarea=>shadowfadingmarginneedstobeaddedaccordingly.

39

Linkbudgettestexample

WehavemadeasimpleexceltoolforGSMlinkbudgetcomputation

Inthefollowingwegothroughanexample. Itissuggestedthatallparticipantsplaywiththetool(itcanbeloadedfromcoursepages) Learnhowdifferentparametersarecomputed Examinetheimpactofparameteres likeantennahight

etc

40

Linkbudgetexample/TXcharacteristics

41

Linkbudgetexample/RXcharacteristics

Downlink unit Uplink unit

Systemgaindefinestheattenuationthatsystemcan tolerate

42

Linkbudgetexample/margins

Downlink unit Uplink unit

0 0.5 1 1.5 2 2.5 3 3.5 410-5

10-4

10-3

10-2

10-1

100

x

Q

(

x

)

Note:thisissimplifiedapproach.Usuallyweneedtocomputeshadowfadingmarginbasedonpredefinedcellcoverageprobability

43

AllowedpropagationlossAllowedpropagationloss=Systemgain- margins

Allowedpropagationlossdefineshowmuchsystemca nstandpropagationloss.Hence,wecannextcalculatethe cellrange

ThereisslightimbalancebetweenULandDL

Okumura-Hata modelhasbeenused

44

Exampleexercise

AssumeGSMsystemsuchthat TXpowerinBSis10Wand1WinMS ForBSantenna

Horizontal3dBbeamwidthis60degrees Fourdipolesareusedtoformverticalantennabeam

MSantennagainis0dBi Cablelossis3dB,combinerlossis3.5dB Bodylossis2.5dB

ComputetransmitterEIRPinbothBSandMS BSantennagainis16dBiandTXpoweris40dBm(fin dout

details).ThenBSEIRP=40dBm+16dBi-3dB-3.5dB=49 .5dBm MSTXpoweris30dBmandEIRP=30dBm+0dBi-2.5dB=

27.5dBm Itisusefultospendsometimewithlinkbudget!

45

Howtofinddetailedtechnicalinformation

GSM(andGPRS,EDGE,WCDMA,LTE)specificationsareavailableon3GPPpages 3GPP=3rdGenerationPartnershipProject Theoriginalscopeof3GPPwastoproduceglobally

applicableTechnicalSpecificationsandTechnicalReportsfora3rdGenerationMobileSystembasedonevolvedGSMcorenetworksandtheradioaccesstechnologiesthattheysupport(e.g.WCDMA,HSPA,etc).

ThescopewassubsequentlyamendedtoincludethemaintenanceanddevelopmentoftheGSMTechnicalSpecificationsandTechnicalReportsincludingevol vedradioaccesstechnologies(e.g.GPRSandEDGE).

Specificationsdefinepreciselythefunctionalities andrequirementsofMS,BSothernetworkelementsof3GPPsystemslikeGSM, WCDMA,

46

Standardization Inadditiontoreadyspecifications3GPPdatabases contain

technicaldiscussionconcerningtoongoingstandard izationofvarioustopics

Relateddocumentsarenoteasyreading buttheyco ntainlatesttechnicalideas.

CompanieslikeNokia,NSN,Ericsson,Motorola,Sams ung,Huaweiandmanyothersproposecompetingsolutions(about whichtheyhavepatentapplications)instandardizationdocuments( calledcontributes).

Oncestandardreleaseisreadycompaniesnegotiate ontheirshareofso-calledessentialpatents (patentsrelatedtost andardizedsolutions).

Thesharesofpatentsgiveguidelinesforcross-lic ensingbetweencompanies.Licensingcontractsdefinehowmuchmanu facturerpayorreceivefeeswhenusingthestandardizedtechnology initsproducts.

Somecompanies(thosewhoarenotdoingstandardiza tionresearch/arenotsuccessfulinit)mayneedtopay tensorevenhundredsofmillionsofeuros peryearinordertoh avearighttousecertaintechnologiesinproductstheymake.

47

Standardizationdocuments

3GPPdatabasesaremostlypublic Easieswaytofindtechnicaldocuments

Google3GPP andgoto3GPPmainpage ClickTechnicalbodies

YoufindTSG(TechnicalSpecificationGroup)organization

UnderTSGGERAN youfindGSManditsevolution(GPRS,EDGE)radiospecifications

UnderTSGRAN youfindWCDMA,(+HSDPA,HSUPA,LTE)radiospecifications

Also,if youknownumberofthespecification/standardizationdocumentyoucangoogle itdirectly