电子通信类专业英语论文

SimulationofacellularradiosystemYournameImageProcessingLabWuhanUniversityofTechnology,WHUTWuhan430070,[e-mail]-Awidevarietyofwirelesscommunicationsystemshavebeendevelopedtoprovideaccesstothecommunicationsinfrastructureformobileorfixedusersinamyriadofoperatingenvironments.Mostoftoday’swirelesssystemsarebasedonthecellularradioconcept.Cellularcommunicationsystemsallowalargenumberofmobileuserstoseamlesslyandsimultaneouslycommunicatetowirelessmodemsatfixedbasestationsusingalimitedamountofradiofrequency(RF)spectrum.TheRFtransmissionsreceivedatthebasestationsfromeachmobilearetranslatedtobaseband,ortoawidebandmicrowavelink,andrelayedtomobileswitchingcenters(MSC),whichconnectthemobiletransmissionswiththePublicSwitchedTelephoneNetwork(PSTN).Similarly,communicationsfromthePSTNaresenttothebasestation,wheretheyaretransmittedtothemobile.Keywords:Cellularcommunication,RF,MSC,PSTN1.INTRODUCTIONCellularsystemsemployeitherfrequencydivisionmultipleaccess(FDMA),timedivisionmultipleaccess(TDMA),codedivisionmultipleaccess(CDMA),orspatialdivisionmultipleaccess(SDMA).Wirelesscommunicationlinksexperiencehostilephysicalchannelcharacteristics,suchastime-varyingmultipathandshadowingduetolargeobjectsinthepropagationpath.Inaddition,theperformanceofwirelesscellularsystemstendstobelimitedbyinterferencefromotherusers,andforthatreason,itisimportanttohaveaccuratetechniquesformodelinginterference.Thesecomplexchannelconditionsaredifficulttodescribewithasimpleanalyticalmodel,althoughseveralmodelsdoprovideanalyticaltractabilitywithreasonableagreementtomeasuredchanneldata[1].However,evenwhenthechannelismodeledinananalyticallyelegantmanner,inthevastmajorityofsituationsitisstilldifficultorimpossibletoconstructanalyticalsolutionsforlinkperformancewhenerrorcontrolcoding,equalization,diversity,andnetworkmodelsarefactoredintothelinkmodel.Simulationapproaches,therefore,areusuallyrequiredwhenanalyzingtheperformanceofcellularcommunicationlinks.Likewirelesslinks,thesystemperformanceofacellularradiosystemismosteffectivelymodeledusingsimulation,duetothedifficultyinmodelingalargenumberofrandomeventsovertimeandspace.Theserandomevents,suchasthelocationofusers,thenumberofsimultaneoususersinthesystem,thepropagationconditions,interferenceandpowerlevelsettingsofeachuser,andthetrafficdemandsofeachuser,combinetogethertoimpacttheoverallperformanceseenbyatypicaluserinthecellularsystem.Theaforementionedvariablesarejustasmallsamplingofthemanykeyphysicalmechanismsthatdictatetheinstantaneousperformanceofaparticularuseratanytimewithinthesystem.Thetermcellularradiosystem,therefore,referstotheentirepopulationofmobileusersandbasestationsthroughoutthegeographicservicearea,asopposedtoasinglelinkthatconnectsasinglemobileusertoasinglebasestation.2.CELLULARRADIOSYSTEMSystem-LevelDescription：CellularsystemsprovidewirelesscoverageoverageographicserviceareabydividingthegeographicareaintosegmentscalledcellsasshowninFigure2-1.Theavailablefrequencyspectrumisalsodividedintoanumberofchannelswithagroupofchannelsassignedtoeachcell.Basestationslocatedineachcellareequippedwithwirelessmodemsthatcancommunicatewithmobileusers.Radiofrequencychannelsusedinthetransmissiondirectionfromthebasestationtothemobilearereferredtoasforwardchannels,whilechannelsusedinthedirectionfromthemobiletothebasestationarereferredtoasreversechannels.Theforwardandreversechannelstogetheridentifyaduplexcellularchannel.Whenfrequencydivisionduplex(FDD)isused,theforwardandreversechannelsaresplitinfrequency.Alternatively,whentimedivisionduplex(TDD)isused,theforwardandreversechannelsareonthesamefrequency,butusedifferenttimeslotsfortransmission.Figure1:BasicarchitectureofacellularcommunicationssystemHigh-capacitycellularsystemsemployfrequencyreuseamongcells.Thisrequiresthatco-channelcells(cellssharingthesamefrequency)aresufficientlyfarapartfromeachothertomitigateco-channelinterference.ChannelreuseisimplementedbycoveringthegeographicserviceareawithclustersofNcells,asshowninFigure2-2,whereNisknownastheclustersize.Figure2:Cellclustering:Depictionofathree-cellreusepatternTheRFspectrumavailableforthegeographicserviceareaisassignedtoeachcluster,suchthatcellswithinaclusterdonotshareanychannel.IfMchannelsmakeuptheentirespectrumavailablefortheservicearea,andifthedistributionofusersisuniformovertheservicearea,theneachcellisassignedM/Nchannels.Astheclustersarereplicatedovertheservicearea,thereuseofchannelsleadstotiersofco-channelcells,andco-channelinterferencewillresultfromthepropagationofRFenergybetweenco-channelbasestationsandmobileusers.Co-channelinterferenceinacellularsystemoccurswhen,forexample,amobilesimultaneouslyreceivessignalsfromthebasestationinitsowncell,aswellasfromco-channelbasestationsinnearbycellsfromadjacenttiers.Inthisinstance,oneco-channelforwardlink(basestationtomobiletransmission)isthedesiredsignal,andtheotherco-channelsignalsreceivedbythemobileformthetotalco-channelinterferenceatthereceiver.Thepowerleveloftheco-channelinterferenceiscloselyrelatedtotheseparationdistancesamongco-channelcells.Ifwemo