蜂窝无线通信系统的研究英文翻译.docx
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蜂窝无线通信系统的研究英文翻译.docx
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蜂窝无线通信系统的研究英文翻译
郑州轻工业学院
本科毕业设计(论文)
——英文翻译
题目蜂窝无线通信系统的研究
学生姓名耿永鹏
专业班级信息工程07-1班
学号200707070111
院(系)计算机与通信工程学院
指导教师(职称)张娜(讲师)
完成时间2011年5月30日
英文原文
RESEARCHOFCELLULARWIRELESSCOMMUNATIONSYSTEM
Abstract
Cellularcommunicationsystemsallowalargenumberofmobileuserstoseamlesslyandsimultaneouslycommunicatetowirelessmodemsatfixedbasestationsusingalimitedamountofradiofrequency(RF)spectrum.TheRFtransmissionsreceivedatthebasestationsfromeachmobilearetranslatedtobaseband,ortoawidebandmicrowavelink,andrelayedtomobileswitchingcenters(MSC),whichconnectthemobiletransmissionswiththePublicSwitchedTelephoneNetwork(PSTN).Similarly,communicationsfromthePSTNaresenttothebasestation,wheretheyaretransmittedtothemobile.Cellularsystemsemployeitherfrequencydivisionmultipleaccess(FDMA),timedivisionmultipleaccess(TDMA),codedivisionmultipleaccess(CDMA),orspatialdivisionmultipleaccess(SDMA).
1Introduction
Awidevarietyofwirelesscommunicationsystemshavebeendevelopedtoprovideaccesstothecommunicationsinfrastructureformobileorfixedusersinamyriadofoperatingenvironments.Mostoftoday’swirelesssystemsarebasedonthecellularradioconcept.Cellularcommunicationsystemsallowalargenumberofmobileuserstoseamlesslyandsimultaneouslycommunicatetowirelessmodemsatfixedbasestationsusingalimitedamountofradiofrequency(RF)spectrum.TheRFtransmissionsreceivedatthebasestationsfromeachmobilearetranslatedtobaseband,ortoawidebandmicrowavelink,andrelayedtomobileswitchingcenters(MSC),whichconnectthemobiletransmissionswiththePublicSwitchedTelephoneNetwork(PSTN).Similarly,communicationsfromthePSTNaresenttothebasestation,wheretheyaretransmittedtothemobile.Cellularsystemsemployeitherfrequencydivisionmultipleaccess(FDMA),timedivisionmultipleaccess(TDMA),codedivisionmultipleaccess(CDMA),orspatialdivisionmultipleaccess(SDMA).
Wirelesscommunicationlinksexperiencehostilephysicalchannelcharacteristics,suchastime-varyingmultipathandshadowingduetolargeobjectsinthepropagationpath.Inaddition,theperformanceofwirelesscellularsystemstendstobelimitedbyinterferencefromotherusers,andforthatreason,itisimportanttohaveaccuratetechniquesformodelinginterference.Thesecomplexchannelconditionsaredifficulttodescribewithasimpleanalyticalmodel,althoughseveralmodelsdoprovideanalyticaltractabilitywithreasonableagreementtomeasuredchanneldata.However,evenwhenthechannelismodeledinananalyticallyelegantmanner,inthevastmajorityofsituationsitisstilldifficultorimpossibletoconstructanalyticalsolutionsforlinkperformancewhenerrorcontrolcoding,equalization,diversity,andnetworkmodelsarefactoredintothelinkmodel.Simulationapproaches,therefore,areusuallyrequiredwhenanalyzingtheperformanceofcellularcommunicationlinks.
Likewirelesslinks,thesystemperformanceofacellularradiosystemismosteffectivelymodeledusingsimulation,duetothedifficultyinmodelingalargenumberofrandomeventsovertimeandspace.Theserandomevents,suchasthelocationofusers,thenumberofsimultaneoususersinthesystem,thepropagationconditions,interferenceandpowerlevelsettingsofeachuser,andthetrafficdemandsofeachuser,combinetogethertoimpacttheoverallperformanceseenbyatypicaluserinthecellularsystem.Theaforementionedvariablesarejustasmallsamplingofthemanykeyphysicalmechanismsthatdictatetheinstantaneousperformanceofaparticularuseratanytimewithinthesystem.Thetermcellularradiosystem,therefore,referstotheentirepopulationofmobileusersandbasestationsthroughoutthegeographicservicearea,asopposedtoasinglelinkthatconnectsasinglemobileusertoasinglebasestation.Todesignforaparticularsystem-levelperformance,suchasthelikelihoodofaparticularuserhavingacceptableservicethroughoutthesystem,itisnecessarytoconsiderthecomplexityofmultipleusersthataresimultaneouslyusingthesystemthroughoutthecoveragearea.Thus,simulationisneededtoconsiderthemulti-usereffectsuponanyoftheindividuallinksbetweenthemobileandthebasestation.
Thelinkperformanceisasmall-scalephenomenon,whichdealswiththeinstantaneouschangesinthechanneloverasmalllocalarea,orsmalltimeduration,overwhichtheaveragereceivedpowerisassumedconstant.Suchassumptionsaresensibleinthedesignoferrorcontrolcodes,equalizers,andothercomponentsthatservetomitigatethetransienteffectscreatedbythechannel.However,inordertodeterminetheoverallsystemperformanceofalargenumberofusersspreadoverawidegeographicarea,itisnecessarytoincorporatelarge-scaleeffectssuchasthestatisticalbehaviorofinterferenceandsignallevelsexperiencedbyindividualusersoverlargedistances,whileignoringthetransientchannelcharacteristics.Onemaythinkoflink-levelsimulationasbeingavernieradjustmentontheperformanceofacommunicationsystem,andthesystem-levelsimulationasbeingacoarse,yetimportant,approximationoftheoveralllevelofqualitythatanyusercouldexpectatanytime.
Cellularsystemsachievehighcapacity(e.g.,servealargenumberofusers)byallowingthemobilestationstoshare,orreuseacommunicationchannelindifferentregionsofthegeographicservicearea.Channelreuseleadstoco-channelinterferenceamonguserssharingthesamechannel,whichisrecognizedasoneofthemajorlimitingfactorsofperformanceandcapacityofacellularsystem.Anappropriateunderstandingoftheeffectsofco-channelinterferenceonthecapacityandperformanceisthereforerequiredwhendeployingcellularsystems,orwhenanalyzinganddesigningsystemmethodologiesthatmitigatetheundesiredeffectsofco-channelinterference.Theseeffectsarestronglydependentonsystemaspectsofthecommunicationsystem,suchasthenumberofuserssharingthechannelandtheirlocations.Otheraspects,morerelatedtothepropagationchannel,suchaspathloss,shadowfading(orshadowing),andantennaradiationpatternsarealsoimportantinthecontextofsystemperformance,sincetheseeffectsalsovarywiththelocationsofparticularusers.Inthischapter,wewilldiscusstheapplicationofsystem-levelsimulationintheanalysisoftheperformanceofacellularcommunicationsystemundertheeffectsofco-channelinterference.Wewillanalyzeasimplemultiple-usercellularsystem,includingtheantennaandpropagationeffectsofatypicalsystem.Despitethesimplicityoftheexamplesystemconsideredinthischapter,theanalysispresentedcaneasilybeextendedtoincludeotherfeaturesofacellularsystem.
2CellularRadioSystem
System-LevelDescription:
CellularsystemsprovidewirelesscoverageoverageographicserviceareabydividingthegeographicareaintosegmentscalledcellsasshowninFigure2-1.Theavailablefrequencyspectrumisalsodividedintoanumberofchannelswithagroupofchannelsassignedtoeachcell.Basestationslocatedineachcellareequippedwithwirelessmodemsthatcancommunicatewithmobileusers.Radiofrequencychannelsusedinthetransmissiondirectionfromthebasestationtothemobilearereferredtoasforwardchannels,whilechannelsusedinthedirectionfromthemobiletothebasestationarereferredtoasreversechannels.Theforwardandreversechannelstogetheridentifyaduplexcellularchannel.Whenfrequencydivisionduplex(FDD)isused,theforwardandreversechannelsaresplitinfrequency.Alternatively,whentimedivisionduplex(TDD)isused,theforwardandreversechannelsareonthesamefrequency,butusedifferenttimeslotsfortransmission.
Figure2-1Basicarchitectureofacellularcommunicationssystem
High-capacitycellularsystemsemployfrequencyreuseamongcells.Thisrequiresthatco-channelcells(cellssharingthesamefrequency)aresufficientlyfarapartfromeachothertomitigateco-channelinterference.ChannelreuseisimplementedbycoveringthegeographicserviceareawithclustersofNcells,asshowninFigure2-2,whereNisknownastheclustersize.
Figure2-2Cellclustering:
Depictionofathree-cellreusepattern
TheRFspectrumavailableforthegeographicserviceareaisassignedtoeachcluster,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.Ifwemodelthecellswithahexagonalshape,asinFigure2-2,theminimumdistancebetweenthecenteroftwoco-channelcells,calledthereusedistance
is
(2-1)
whereRisthemaximumradiusofthecell(thehexagonisinscribedwithintheradius
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