第二章蜂窩概念系統(tǒng)設(shè)計基礎(chǔ)

Chapter2

TheCellularConcept

–SystemDesignFundamentals

BasicConceptsforWirelessCommunicationsSystemBaseStation(BS)基站Afixedstationinamobilesystemusedforradiocommunicationwithmobilestations.MobileStation(MS)移動臺Astationinthecellularradioserviceintendedforusewhileinmotionatunspecifiedlocations.Subscriber&TransceiverSubscriber用戶Auserwhopayssubscriptionchargesforusingamobilecommunicationssystem.Transceiver(Transmitter+Receiver)收發(fā)信機(jī)AdeviceCapableofsimultaneoustransmittingandreceivingradiosignals.MSC&MTSOMTSO(MobileTelephoneSwitchingOffice)移動電話交換局MSC(MobileSwitchingCenter)移動交換中心Switchingcenterwhichcoordinatetheroutingofcallsinalargeservicearea.(交換中心用來在一個大的服務(wù)區(qū)協(xié)調(diào)呼叫路由)TheMSCcoordinatetheactivitiesofallofthebasestationsandconnectstheentirecellularsystemtothePSTN.(同時交換中心還用來協(xié)調(diào)所有基站的活動并連接公用電話網(wǎng))AtypicalMSChandles100,000cellularsubscribersand5,000conversationsatatime,andaccommodatesallbillingandsystemmaintenancefunctions.(一個典型的移動交換中心可以管理100000個用戶,并同時處理5000個通話.同時還提供所帳單和系統(tǒng)維護(hù)功能.)分類頻段移動臺發(fā)基站收基站發(fā)移動臺收GSM900/1800頻段900MHz頻段890-915MHz935-960MHz1800MHz頻段1710-1785MHz1805-1880MHzGSMSpectrum900MHz頻段1800MHz頻段頻道間隔相鄰頻道間隔為200KHz每個頻道采用時分多址接入(TDMA)方式分為8個時隙，即8個信道雙工收發(fā)間隔45MHz95MHz等間隔頻道配置頻道序號1～124512～885頻道數(shù)124374頻道序號和頻道標(biāo)稱中心頻率的關(guān)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

頻率復(fù)用/頻率規(guī)劃FrequencyReuse(頻率復(fù)用)Thedesignprocessofselectingandallocatingchannelsgroupsforallofthecellularbasestationswithasystem(基站信道分配)Basestationsinadjacentcellsareassignedchannelgroupswhichcontaincompletelydifferentchannelsthanneighboringcells.(為相鄰小區(qū)基站分配完全不同的信道)Bylimitingthecoverageareatowithintheboundariesofacell,thesamegroupofchannelsmaybeusedtocoverdifferentcellsthatareseparatedfromoneanotherbydistancelargeenoughtokeepinterferencelevelswithintolerablelimits.(相同的信道分組可以分配給相隔足夠遠(yuǎn)的基站)StripcoverageregionTheuserofthesytemisdistributedinastriparea,suchasriver,railway,highwayFacecoverageregionFigure3.1CommentsFigure3.1illustratestheconceptofcellularreuse,whichcellslabeledwiththesameletterusethesamegroupofchannels.(相同字母的小區(qū)使用相同的信道組)ThehexagonalcellshapeshowninFigure3.1isconceptualandasimplisticmodeloftheradiocoverageforeachbasestation,butithasbeenuniversallyadoptedsincethehexagonpermitseasyandmanageableanalysisofacellularsystem.(用六邊形表示的小區(qū)是基站無線覆蓋的簡單模型)Theactualradiocoverageisknownasthefootprintanditisdeterminedfromfieldmeasurementorpropagationpredicationmodel.(實(shí)際的無線覆蓋區(qū)域應(yīng)從實(shí)際場地測量和傳播預(yù)測模型中來)Whyhexagon?Whynotcircle?Whileitmightseemnaturaltochooseacircletopresentthecoverageareaofabasestation,adjacentcirclescannotbeoverlaidupononamapwithoutleavinggapsorcerateoverlappingregions.(圓型無法做到無縫且無重復(fù)覆蓋)Whenconsideringgeometricshapeswhichcoveranentireregionwithoutoverlappingandwithequalarea,therearethreesensiblechoices:asquare,anequilateraltriangle,andahexagon.(要進(jìn)行完全的無縫及無重復(fù)覆蓋一相同區(qū)域可以有三種方式:正方形,等邊三角形和六邊形)Acellmustbedesignedtoservetheweakestmobileswithinthefootprint,andthesearetypicallocatedattheedgeofthecell.Foragivendistancebetweenthecenterofapolygonanditsfarthestperimeterpoints,thehexagonhasthelargestareaofthethree.(從中心位置到給定的最遠(yuǎn)距離,六邊形是三種選擇中具有最大覆蓋面積的.)Cluster(簇)TheNcellswhichcollectivelyusethecompletesetofavailablefrequenciesiscalledCluster.(使用能夠得到的完整頻率集的N個小區(qū)叫一個簇)Ifeachcellisallocatedagroupkchannels,andifSchannelsaredividedamongNcellsintouniqueanddisjointchannelgroupswhichhavesamenumberofchannels,thetotalnumberofavailableradiochannelsScanbeexpressedas:S=kN(N個小區(qū),每個小區(qū)分配k個信道,則共有S個信道)IfaclusterisreplicatedMtimeswithinthesystem,thetotalnumberofchannelsC,canbeusedasameasureofcapacityandisgivenbyC=MkN=MS(如果系統(tǒng)中一個簇重復(fù)M次,則總的信道數(shù)C=MS=MkN)frequencyreusefactor(頻率復(fù)用因子)Thecapacityofcellularsystemisdirectlyproportionaltothenumberoftimesofaclusterisreplicatedinafixedservicearea.(系統(tǒng)容量與簇的復(fù)用次數(shù)成正比)ThefactorNiscalledtheclustersize

(簇大小)Iftheclustersizeisreducedwhilethecellsizeiskeptconstant,moreclustersarerequiredtocoveragivenarea,hencemorecapacity(alargervalueofC)isachieved.(小區(qū)大小不變簇大小減少,則覆蓋一給定區(qū)域要求更多的簇,系統(tǒng)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(動態(tài)分配)Dynamicchannelassignmentreducethelikelihoodofblocking,whichincreasethetrunkingcapacityofthesystem.DynamicchannelassignmentstrategiesrequiretheMSCtocollectreal-timedataonchanneloccupancy,trafficdistribution,andradio(receive)signalstrengthindications(RSSI).Handoff/Handover切換Handoff/HandoverTheprocessoftransferringamobilestationfromonechannelorbasestationtoanother(將移動臺從一個信道或一個基站轉(zhuǎn)換到另一個信道或基站的過程)Request:很多系統(tǒng)中，切換信道分配比初始呼叫信道分配具有更高優(yōu)先級；切換應(yīng)盡可能成功并盡可能少地發(fā)生；對用戶透明。Inordertomeettherequirementslistedabove,anoptimumsignallevelatwhichtoinitiateahandoffisspecified.基站接收機(jī)可接受話音質(zhì)量的最小可用信號強(qiáng)度：Prminimumusable啟動切換的最恰當(dāng)信號強(qiáng)度：PrhandoffDelta=Prhandoff-Prminimumusable當(dāng)Delta太大，增加MSC的負(fù)擔(dān)；當(dāng)Delta太小，可能會掉話。Handoffs–thebasicsHandoffdecisionsMobileControlHandoff歐洲的DECT和北美的PACSNetworkControlHandoffTACS和AMPSMobileAssistedHandoffGSM和CDMAMethodsofHandoffHardHandoffSoftHandoffChannelassignmentinHandoffPrioritizingHandoffsAfractionofthetotalavailablechannelsinacellisreservedexclusivelyforhandoffrequestsfromongoingcallswhichmaybehandedoffintothecell.

Roamer(漫游)RoamerAmobilestationwhichoperateinaservicearea(market)otherthanthatfromwhichservicehasbeensubscribed.(當(dāng)移動臺不是在注冊地使用服務(wù)時,移動臺就成為漫游者)RoamingInterferenceandsystemcapacity

(干擾與系統(tǒng)容量)Interferenceisthemajorlimitingfactorintheperformanceofcellularradiosystem.Interference:Noise(噪聲)MAI(MultipleAccessInterference)(多址干擾)ACI(AdjacentChannelInterference,AdjacentCell/sectorInterference)(鄰道干擾)ISI(Inter-SymbolInterference)(符號間串?dāng)_)Co-channelInterference(同道干擾)Co-channelInterferenceandCapacity

(同道干擾與容量)Co-channelInterferencereuseratioQ(同道干擾復(fù)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

中繼和服務(wù)等級GOS:GradeofServiceQoS:QualityofServiceGosistypicallygivenasthelikelihoodthatacallisblocked,orthelikelihoodofacallexperiencingadelaygreaterthanacertainqueuingtime.Er