通信工程外文翻译---蜂窝无线通信系统的仿真通信工程外文翻译---蜂窝无线通信系统的仿真

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1SIMULATIONOFACELLULARRADIOSYSTEMtakenfromPrenticeHallPrinciplesOfCommunicationSystemsSimulationWithWirelessAplicationspage6726761.IntroductionAwidevarietyofwirelesscommunicationsystemshavebeendevelopedtoprovideaccesstothecommunicationsinfrastructureformobileorfixedusersinamyriadofoperatingenvironments.Mostoftodayswirelesssystemsarebasedonthecellularradioconcept.CellularcommunicationsystemsallowalargenumberofmobileuserstoseamlesslyandsimultaneouslycommunicatetowirelessmodemsatfixedbasestationsusingalimitedamountofradiofrequencyRFspectrum.TheRFtransmissionsreceivedatthebasestationsfromeachmobilearetranslatedtobaseband,ortoawidebandmicrowavelink,andrelayedtomobileswitchingcentersMSC,whichconnectthemobiletransmissionswiththePublicSwitchedTelephoneNetworkPSTN.Similarly,communicationsfromthePSTNaresenttothebasestation,wheretheyaretransmittedtothemobile.CellularsystemsemployeitherfrequencydivisionmultipleaccessFDMA,timedivisionmultipleaccessTDMA,codedivisionmultipleaccessCDMA,orspatialdivisionmultipleaccessSDMA.Wirelesscommunicationlinksexperiencehostilephysicalchannelcharacteristics,suchastimevaryingmultipathandshadowingduetolargeobjectsinthepropagationpath.Inaddition,theperformanceofwirelesscellularsystemstendstobelimitedbyinterferencefromotherusers,andforthatreason,itisimportanttohaveaccuratetechniquesformodelinginterference.Thesecomplexchannelconditionsaredifficulttodescribewithasimpleanalyticalmodel,althoughseveralmodelsdoprovideanalyticaltractabilitywithreasonableagreementtomeasuredchanneldata.However,evenwhenthechannelismodeledinananalyticallyelegantmanner,inthevastmajorityofsituationsitisstilldifficultorimpossibletoconstructanalyticalsolutionsforlinkperformancewhenerrorcontrolcoding,equalization,diversity,andnetworkmodelsarefactoredintothelinkmodel.Simulationapproaches,therefore,areusuallyrequiredwhenanalyzingtheperformanceofcellularcommunicationlinks.2Likewirelesslinks,thesystemperformanceofacellularradiosystemismosteffectivelymodeledusingsimulation,duetothedifficultyinmodelingalargenumberofrandomeventsovertimeandspace.Theserandomevents,suchasthelocationofusers,thenumberofsimultaneoususersinthesystem,thepropagationconditions,interferenceandpowerlevelsettingsofeachuser,andthetrafficdemandsofeachuser,combinetogethertoimpacttheoverallperformanceseenbyatypicaluserinthecellularsystem.Theaforementionedvariablesarejustasmallsamplingofthemanykeyphysicalmechanismsthatdictatetheinstantaneousperformanceofaparticularuseratanytimewithinthesystem.Thetermcellularradiosystem,therefore,referstotheentirepopulationofmobileusersandbasestationsthroughoutthegeographicservicearea,asopposedtoasinglelinkthatconnectsasinglemobileusertoasinglebasestation.Todesignforaparticularsystemlevelperformance,suchasthelikelihoodofaparticularuserhavingacceptableservicethroughoutthesystem,itisnecessarytoconsiderthecomplexityofmultipleusersthataresimultaneouslyusingthesystemthroughoutthecoveragearea.Thus,simulationisneededtoconsiderthemultiusereffectsuponanyoftheindividuallinksbetweenthemobileandthebasestation.Thelinkperformanceisasmallscalephenomenon,whichdealswiththeinstantaneouschangesinthechanneloverasmalllocalarea,orsmalltimeduration,overwhichtheaveragereceivedpowerisassumedconstant.Suchassumptionsaresensibleinthedesignoferrorcontrolcodes,equalizers,andothercomponentsthatservetomitigatethetransienteffectscreatedbythechannel.However,inordertodeterminetheoverallsystemperformanceofalargenumberofusersspreadoverawidegeographicarea,itisnecessarytoincorporatelargescaleeffectssuchasthestatisticalbehaviorofinterferenceandsignallevelsexperiencedbyindividualusersoverlargedistances,whileignoringthetransientchannelcharacteristics.Onemaythinkoflinklevelsimulationasbeingavernieradjustmentontheperformanceofacommunicationsystem,andthesystemlevelsimulationasbeingacoarse,yetimportant,approximationoftheoveralllevelofqualitythatanyusercouldexpectat
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