无线通信原理与应用英lecture2

Chapter2:TheCellularConcept-SystemDesignFundamentalsLecture22.1TheConcept:CellularNetworksgeometricshapesCircle:Equilateraltriangle:Square:Hexagon:CellularcoveragerepresentationMacrocelllarge,coveringawidearerangeofseveralhundredkilometres(km)totenkmmostlydeployedinruralandsparselypopulatedareasMicrocellmediumcell,coverageareasmallerthaninmacrocellsrangeofseveralhundredmeterstoacoupleofmetresdeployedmostlyincrowdedareas,stadiums,shoppingmallsPicocellsmall,coveringaverysmallarearangeofseveraltensofmetreslowpowerantennas,indoorThesizeofacellThreedifferentkindsofcellsBasestationtransmittersInhexagonalcells,basestationstransmittersareeither:centre-excited,basestationisatthecentreofthecelledge-excited,basestationat3ofthe6cellverticescentre-excitededge-excitedCcanbeusedasameasureofcapacity2.2FrequencyReuseandCapacityAmobilecommunicationsystemusesafrequencyreusefactorof1/7and416channelsavailable.If21channelsareallocatedascontrolchannels,computeitssystemcapacity.Assumeachannelsupports8usersChannelsavailableforallocation=416-21=395ChannelNumberofacell=395/7=57Numberofsimultaneoususerspercell=8x57=456Numberofsimultaneoususersinsystem=7x456=3192Example1Methodoflocatingco-channelcellsChannelAssignmentStrategies:TwoassignmentapproachesFixedandstatic(mostcommon)DynamicFixedchannelassignmentallchannelsinacellcouldbeinuseallthetimenewcallsarethenblocked(nochannelsleft)maybesolvedbyborrowingsparechannelsfromnearbycells2.3ChannelAssignmentandhandoffDynamicchannelassignmentMSCallocatesfrequencieswhenacallismadeProvideshighchannelutilizationTodothisitneedsreal-timeinformationonchanneloccupancytrafficdistributionradiosignalstrengthindication(RSSI)highcomputationalloadandincreasedstorageHandover(Handoff)ProvidescontinuityofcommunicationacrosscellsDifficultydroppingacallbeforereconnectingisunacceptabledifferentcellsusedifferentfrequenciesmobilephoneusersusuallymovefromplacetoplaceandveryquicklyHandoverProcessReceivedsignalweakensasmobilemovesoutofcellCellsiteatsomepointrequestshandovertocellwithstrongersignalstrengthMSCswitchescalltonewcellafterallocatingchannels.IllustrationofahandoffscenarioatcellboundaryThebasicconceptofhandoverstrategiesHandovermustnotbetoofrequentorsystemiskeptbusyservicinghandoverrequestshandoverthresholdissetslightlystrongerMinimumusablesignallevelisnormallysettobebetween-90dBmand-100dBmChoosingHandoverMarginsHandovermargin=Prhandover-PrminimumusableIfistoolargeunnecessaryhandoverwilloccur,burdeningtheMSCIfistoosmall,theremaybeinsufficienttimetocompletethehandoverbeforeacallislostduetoweaksignalsThereforeischosencarefullySettingHandoverThresholdsHandoffindifferentsystem1GsignalstrengthmeasurementsaremadebythebasestationsandsupervisedbytheMSCGSMHandoverbaseduponinformationprovidedtotheBTSbytheMSiscommonlyreferredtoasamobileassistedhandover(MAHO).CDMApilotchannel,softhandoffInterferenceandSystemCapacity:2.4Interference7-cellreusepatternSixEffectiveInterferingCellsofCell1Co-channelreuseratioQ12-cellreusepattern19-cellreusepatternCo-channelreuseratiovs.frequencyreusepatternTradeoffPropagationmodelCo-channelinterferenceratioExample2Moreexactgeometryfortheco-channelinterferencesS/Iisapproximatedas（3.10），Using，2.5TrunkingandGradeofService(GOS)Trunking:largenumberofuserssharearelativelysmallnumberofchannelsEachuserisallocatedachannelonpercallbasisAllchannelinuse:newuserisblockedorhastoTowaitinaqueueTrunkingtheory:A.K.Erlang(1917)GradeofService(GOS)GradeofService(GOS):measureoftheabilityofausertoaccessatrunkedsystemduringthebusiesthour.Prob.[callisblocked]orProb.[delay>T]Measureoftrafficintensity:Erlang1Erlang=amountoftrafficintensitycarriedbyachannelthatisCompletelyoccupiedAboutErlangOnaverage,duringthebusyhour,acompanymakes120outgoingcallsofaverageduration2minutes.Itreceives200incomingcallsofaverageduration3minutes.Find(1)theoutgoingtraffic,(2)theincomingtraffic,(3)thetotaltraffic.TheoutgoingtrafficisTheincomingtrafficisThetotaltrafficisDuringthebusyhour,onaverage,acustomerwithasingletelephonelinemakesthreecallsandreceivesthreecalls.Theaveragecalldurationis2minutes.Whatistheprobabilitythatacallfindsthelineengaged?OccupancyoflineErlangBformulaErlangs,thetotalofferedtrafficintensityandthetrafficintensityperchannel,respectively,isTheprobabilityofblockedcallis,Trunkedmobilenetworkprovidecellularserviceinthisarea.Systemhas394cellswith19channelseach.Findthenumberofusersthatcanbesupportedat2%blockingifeachuseraveragestwocallsperhouratanaveragecalldurationofthreeminutes.Computethetotalnumberofsubscribersthatcanbesupportedbythissystem.Probabilityofblocking=2%=0.02Numberofchannelspercellusedinthesystem,C=19Trafficintensityperuser,Au=λH=2x(3/60)=0.1ErlangsfromtheErlangBchart,thetotalcarriedtraffic,A,isobtainedas12Erlangs.Therefore,thenumberofusersthatcanbesupportedpercellisU=AIAu=12/0.1=120Sincethereare394cells,thetotalnumberofsubscribersthatcanbesupportedbySystemAisequalto120x394=47280Example3OftenlargecellsneedtobesplitintosmalleronesbecausethepopulationofusersinthebigcellhasincreasedbeyondwhatitcansupportCellsplittingincreasessystemcapacityIsusedinhighdensitysubscriberareasResultstoincreasedcosts(eg.newbasestations)2.6ImprovingCapacityinCellularSystemIllustrationofCellsplitting(tobecontinued)ScalableCellSplittingSectoringreducesinterferenceExample4Acellularsystem:anaveragecalllaststwominutes,andtheprobabilityofblockingistobenomorethan1%.Everysubscribermakesonecallperhour,onaverage.Ifthereareatotalof395trafficchannelsforaseven-cellreusesystem,therewillbeabout57trafficchannelspercell.AssumethatblockedcallsareclearedsotheblockingisdescribedbytheErlangBdistribution.FromtheErlangBdistribution,itcanbefoundthattheunsectoredsystemmayhandle44.2Erlangsor1326callsperhour.Ifemploying120°sectoring,thereareonly19channelsperantennasector(57/3antennas).Forthesameprobabilityofblockingandaveragecalllength,itcanbefoundfromtheErlangBdistributionthateachsectorcanhandle11.2Erlangsor336callsperhour.Sinceeachcellconsistsofthreesectors,thisprovidesacellcapacityof3x336=1008callsperhour.Thus,sectoringdecreasesthetrunkingefficiencywhileimprovingtheS/Iforeachuserinthesystem.Exercises1.If20MHzoftotalspectrumisallocatedforaduplexwirelesscellularsystemandeachsimplexchannelhas25kHzRFbandwidth,find:(a)thenumberofduplexchannels,(b)thetotalnumberofchannelspercellsite,ifN=4cellreuseisused.2.AcellularserviceproviderdecidestouseadigitalTDMAschemewhichcantolerateasignal-to-interferenceratioof15dBintheworstcase.FindtheoptimalvalueofNforomnidirectionalantennas(Assumeapathlossexponentofn=4.)