第二章蜂窝概念系统设计基础

Chapter2

TheCellularConcept

–SystemDesignFundamentals

BasicConceptsforWirelessCommunicationsSystemBaseStation(BS)基站Afixedstationinamobilesystemusedforradiocommunicationwithmobilestations.MobileStation(MS)移动台Astationinthecellularradioserviceintendedforusewhileinmotionatunspecifiedlocations.Subscriber&TransceiverSubscriber用户Auserwhopayssubscriptionchargesforusingamobilecommunicationssystem.Transceiver(Transmitter+Receiver)收发信机AdeviceCapableofsimultaneoustransmittingandreceivingradiosignals.MSC&MTSOMTSO(MobileTelephoneSwitchingOffice)移动电话交换局MSC(MobileSwitchingCenter)移动交换中心Switchingcenterwhichcoordinatetheroutingofcallsinalargeservicearea.(交换中心用来在一个大的服务区协调呼叫路由)TheMSCcoordinatetheactivitiesofallofthebasestationsandconnectstheentirecellularsystemtothePSTN.(同时交换中心还用来协调所有基站的活动并连接公用电话网)AtypicalMSChandles100,000cellularsubscribersand5,000conversationsatatime,andaccommodatesallbillingandsystemmaintenancefunctions.(一个典型的移动交换中心可以管理100000个用户,并同时处理5000个通话.同时还提供所帐单和系统维护功能.)分类频段移动台发基站收基站发移动台收GSM900/1800频段900MHz频段890-915MHz935-960MHz1800MHz频段1710-1785MHz1805-1880MHzGSMSpectrum900MHz频段1800MHz频段频道间隔相邻频道间隔为200KHz每个频道采用时分多址接入(TDMA)方式分为8个时隙，即8个信道双工收发间隔45MHz95MHz等间隔频道配置频道序号1～124512～885频道数124374频道序号和频道标称中心频率的关系F1(n)=890.2+(n-1)*0.2上行

F2(n)=f1(n)+45MHz下行其中，n=1,….,124F1(n)=1710.2+(n-512)*0.2上行F2(n)=f1(n)+95MHz下行其中，n=512,513,…,885GSMChannelsControl/Forward/ReverseChannelControlChannel(控制信道)Radiochannelusedfortransmissionofcallsetup,callrequest,callinitiation,andotherbeaconorcontrolpurpose.ForwardChannel(DownLink）前向信道Radiochannelusedfortransmissionofinformationfrombasestationtomobilestation.ReverseChannel(UpLink）反向信道Radiochannelusedfortransmissionofinformationfrommobilestationtobasestation.CellularConceptHistory1947,Ideaforlargecapacitymobilecommunicationssystems.1940S~1950S,TVageforAmerica.1964,FCCrefuseagaintoallocatespectrumformobiletelephoneservice.1968~1970,assign40Mbandwidth.BellmobilesysteminNewYorkcityin1970scouldonlysupportamaximumoftwelvesimultaneouscallsoverathousandsquaremiles.FrequencyReuse/Planning

频率复用/频率规划FrequencyReuse(频率复用)Thedesignprocessofselectingandallocatingchannelsgroupsforallofthecellularbasestationswithasystem(基站信道分配)Basestationsinadjacentcellsareassignedchannelgroupswhichcontaincompletelydifferentchannelsthanneighboringcells.(为相邻小区基站分配完全不同的信道)Bylimitingthecoverageareatowithintheboundariesofacell,thesamegroupofchannelsmaybeusedtocoverdifferentcellsthatareseparatedfromoneanotherbydistancelargeenoughtokeepinterferencelevelswithintolerablelimits.(相同的信道分组可以分配给相隔足够远的基站)StripcoverageregionTheuserofthesytemisdistributedinastriparea,suchasriver,railway,highwayFacecoverageregionFigure3.1CommentsFigure3.1illustratestheconceptofcellularreuse,whichcellslabeledwiththesameletterusethesamegroupofchannels.(相同字母的小区使用相同的信道组)ThehexagonalcellshapeshowninFigure3.1isconceptualandasimplisticmodeloftheradiocoverageforeachbasestation,butithasbeenuniversallyadoptedsincethehexagonpermitseasyandmanageableanalysisofacellularsystem.(用六边形表示的小区是基站无线覆盖的简单模型)Theactualradiocoverageisknownasthefootprintanditisdeterminedfromfieldmeasurementorpropagationpredicationmodel.(实际的无线覆盖区域应从实际场地测量和传播预测模型中来)Whyhexagon?Whynotcircle?Whileitmightseemnaturaltochooseacircletopresentthecoverageareaofabasestation,adjacentcirclescannotbeoverlaidupononamapwithoutleavinggapsorcerateoverlappingregions.(圆型无法做到无缝且无重复覆盖)Whenconsideringgeometricshapeswhichcoveranentireregionwithoutoverlappingandwithequalarea,therearethreesensiblechoices:asquare,anequilateraltriangle,andahexagon.(要进行完全的无缝及无重复覆盖一相同区域可以有三种方式:正方形,等边三角形和六边形)Acellmustbedesignedtoservetheweakestmobileswithinthefootprint,andthesearetypicallocatedattheedgeofthecell.Foragivendistancebetweenthecenterofapolygonanditsfarthestperimeterpoints,thehexagonhasthelargestareaofthethree.(从中心位置到给定的最远距离,六边形是三种选择中具有最大覆盖面积的.)Cluster(簇)TheNcellswhichcollectivelyusethecompletesetofavailablefrequenciesiscalledCluster.(使用能够得到的完整频率集的N个小区叫一个簇)Ifeachcellisallocatedagroupkchannels,andifSchannelsaredividedamongNcellsintouniqueanddisjointchannelgroupswhichhavesamenumberofchannels,thetotalnumberofavailableradiochannelsScanbeexpressedas:S=kN(N个小区,每个小区分配k个信道,则共有S个信道)IfaclusterisreplicatedMtimeswithinthesystem,thetotalnumberofchannelsC,canbeusedasameasureofcapacityandisgivenbyC=MkN=MS(如果系统中一个簇重复M次,则总的信道数C=MS=MkN)frequencyreusefactor(频率复用因子)Thecapacityofcellularsystemisdirectlyproportionaltothenumberoftimesofaclusterisreplicatedinafixedservicearea.(系统容量与簇的复用次数成正比)ThefactorNiscalledtheclustersize

(簇大小)Iftheclustersizeisreducedwhilethecellsizeiskeptconstant,moreclustersarerequiredtocoveragivenarea,hencemorecapacity(alargervalueofC)isachieved.(小区大小不变簇大小减少,则覆盖一给定区域要求更多的簇,系统容量增加)Alargerclustersizeleadstoweakerco-channelinterference.Thefrequencyreusefactorisgivenby1/NCDMAsystemfrequencyreusefactoris1/1.ComputingthevalueofNClusterSizeN(1)ClusterSizeN(2)i=1,j=0,N=1i=1,j=1,N=3i=2,j=0,N=4i=2,j=1,N=7i=2,j=2,N=12i=3,j=0,N=9i=3,j=1,N=13i=3,j=2,N=19Someexamplesofcluster19-cellreuseexample(N=19)Figure3.2Methodoflocatingco-channelcellsinacellularsystem.Inthisexample,N=19(i.e.,I=3,j=2).(Adaptedfrom[Oet83]©IEEE.)ExampleP61Example3.1Ifatotalof33MHzofbandwidthisallocatedtoaparticularFDDcellulartelephonesystemwhichusestwo25kHzsimplexchannelstoprovidefullduplexvoiceandcontrolchannels,computethenumberofchannelsavailablepercellifasystemuses(a)four-cellreuse,(b)seven-cellreuse,and(c)12-cellreuse.If1MHzoftheallocatedspectrumisdedicatedtocontrolchannels,determineanequitabledistributionofcontrolchannelsandvoicechannelsineachcellforeachofthethreesystems.ChannelAssignmentstrategies(信道分配策略)Fixedchannelassignment(固定分配)Dynamicchannelassignment(动态分配)Dynamicchannelassignmentreducethelikelihoodofblocking,whichincreasethetrunkingcapacityofthesystem.DynamicchannelassignmentstrategiesrequiretheMSCtocollectreal-timedataonchanneloccupancy,trafficdistribution,andradio(receive)signalstrengthindications(RSSI).Handoff/Handover切换Handoff/HandoverTheprocessoftransferringamobilestationfromonechannelorbasestationtoanother(将移动台从一个信道或一个基站转换到另一个信道或基站的过程)Request:很多系统中，切换信道分配比初始呼叫信道分配具有更高优先级；切换应尽可能成功并尽可能少地发生；对用户透明。Inordertomeettherequirementslistedabove,anoptimumsignallevelatwhichtoinitiateahandoffisspecified.基站接收机可接受话音质量的最小可用信号强度：Prminimumusable启动切换的最恰当信号强度：PrhandoffDelta=Prhandoff-Prminimumusable当Delta太大，增加MSC的负担；当Delta太小，可能会掉话。Handoffs–thebasicsHandoffdecisionsMobileControlHandoff欧洲的DECT和北美的PACSNetworkControlHandoffTACS和AMPSMobileAssistedHandoffGSM和CDMAMethodsofHandoffHardHandoffSoftHandoffChannelassignmentinHandoffPrioritizingHandoffsAfractionofthetotalavailablechannelsinacellisreservedexclusivelyforhandoffrequestsfromongoingcallswhichmaybehandedoffintothecell.

Roamer(漫游)RoamerAmobilestationwhichoperateinaservicearea(market)otherthanthatfromwhichservicehasbeensubscribed.(当移动台不是在注册地使用服务时,移动台就成为漫游者)RoamingInterferenceandsystemcapacity

(干扰与系统容量)Interferenceisthemajorlimitingfactorintheperformanceofcellularradiosystem.Interference:Noise(噪声)MAI(MultipleAccessInterference)(多址干扰)ACI(AdjacentChannelInterference,AdjacentCell/sectorInterference)(邻道干扰)ISI(Inter-SymbolInterference)(符号间串扰)Co-channelInterference(同道干扰)Co-channelInterferenceandCapacity

(同道干扰与容量)Co-channelInterferencereuseratioQ(同道干扰复用比)WhereRisthecellradiusandDisthedistancebetweencentreofthenearestco-channelcellsCo-channelInterferenceandCapacityCo-channelInterferenceandCapacitySignal-to-interferenceratio(S/IorSIR)Signal-to-noiseratioSNRSINRCo-channelInterferenceandCapacityCo-channelInterferenceandCapacityCo-channelcellsfor7-cellreuseCo-channelInterferenceandCapacityTwousefulapproximateequation:SeePP.703.10and3.11SmallerNisgreatercapacityExampleP723.2Ifasignal-to-interferenceratioof15dBisrequiredforsatisfactoryforwardchannelperformanceofacellularsystem,whatisthefrequencyreusefactorandclustersizethatshouldbeusedformaximumcapacityifthepathlossexponentis(a)n=4,(b)n=3?Assumethattherearesixco-channelcellsinthefirsttier,andallofthemareatthesamedistanceformthemobile.Usesuitableapproximations.AdjacentChannelInterferenceInterferenceresultingfromsignalswhichareadjacentinfrequencytothedesiredsignaliscalledadjacentchannelinterference.See76TrunkingandGradeofService

中继和服务等级GOS:GradeofServiceQoS:QualityofServiceGosistypicallygivenasthelikelihoodthatacallisblocked,orthelikelihoodofacallexperiencingadelaygreaterthanacertainqueuingtime.Er