无线通信系统的研究

蜂窝无线系统的研究PAGEPAGE1文献翻译题目蜂窝无线通信系统的研究学生姓名党勇专业班级通信工程12-01班学号541207040104院（系）计算机与通信工程学院指导教师（职称）黄立勋（讲师）完成时间2016年5月30日蜂窝无线通信系统的研究摘要蜂窝通信系统允许大量移动用户无缝地、同时地利用有限的射频(radiofrequency,RF)频谱与固定基站中的无线调制解调器通信。基站接收每一个移动台发送来的射频信号，并把他们转换到基带或者带宽微波链路，然后传送到移动交换中心(MSC)，再由移动交换中心连入公用交换电话网(PSTN)。同样的，通信信号也可以从PSTN传送到基站，再从这里发送个移动台。蜂窝系统可以采用频分多址(FDMA)、时分多址(TDMA)、码分多址(CDMA)或者空分多址(SDMA)中的任何一种技术。1概述人们开发出了许多无线通信系统，为不同的运行环境中的固定用户或移动用户提供了接入到通信基础设施的手段。当今大多数无线通信系统都是基于蜂窝无线电概念之上的。蜂窝通信系统允许大量移动用户无缝地、同时地利用有限的射频（radiofrequency，RF）频谱与固定基站中的无线调制解调器通信。基站接收每一个移动台发送来的射频信号，并把他们转换到基带或者带宽微波链路，然后传送到移动交换中心（MSC），再由移动交换中心连入公用交换电话网（PSTN）。同样的，通信信号也可以从PSTN传送到基站，再从这里发送个移动台。蜂窝系统可以采用频分多址（FDMA）、时分多址（TDMA）、码分多址（CDMA）或者空分多址（SDMA）中的任何一种技术。无线通信链路具有恶劣的物理信道特征，比如由于传播途径中有再大的障碍物，会产生时变多径和阴影。此外，无线蜂窝系统的性能还会受限于来自其他用户的干扰，因此，对干扰进行准确的建模就很重要。很难用简单的解析模型来描述复杂的信道条件，虽然有集中模型确实易于解析求解并与信道实测数据比较相符，不过，即使建立了完美的信道解析模型，再把差错控制编码、均衡器、分集及网络模型等因素都考虑再链路中之后，要得出链路性能的解析在绝大多数情况下任然是很困难的甚至是不可能的。因此，在分析蜂窝通信链路的性能时，常常需要进行仿真。跟无线链路一样，对蜂窝无线系统的性能分析使用仿真建模时很有效的，这是由于在时间和空间上对大量的随机事件进行建模非常困难。这些随机事件包括用户的位图2-2小区簇：三小区复用模式的描述在一个指定小区中接收到的同频干扰的电平，还取决于任一时刻活跃的同频小区的数量。如前所述，在我们感兴趣的那个特定小区周围，同频小区组成一个个的层。在一个给定层中，同频小区的数量取决于层的阶次和用来表示小区的几何形状（如一个基站覆盖的面积）。对于典型的六边形，最近的同频小区在第一层，有六个同频小区，第二层有12个，第三层有18个，以此类推。因此，总的同频干扰时从所有层的全部同频小区发送出的同频干扰信号的总和。但是第一层的同频小区对总的干扰时从所有层的全部同频小区发送出的同频干扰信号的总和。但是第一层的同频小区对总的干扰有较强的影响，因为它们更靠近测量干扰的小区。人们认识到同频干扰时制约无线通信系统的容量和链路质量的主要因素之一。在系统容量（大尺度系统问题）和链路质量（小尺度系统问题）之间作折中时，它起到举足轻重的作用。例如，在不增加分配给系统的无线频谱带宽的前提下，得到高容量（大量的用户）的一种措施是，通过减小蜂窝系统簇的大小N，来缩短信道复用距离。然而，减少簇大小又增加了同频干扰，这会降低链路质量。蜂窝系统中的干扰电平在任何时候都是随机的，必须通过对蜂窝之间的射频传播环境和移动用户的位置进行建模才能仿真。另外，每个用户话务量的统计特性以及基站中信道分配方案的类型决定了瞬时干扰电平和系统的容量。同频干扰的影响可以用通信链路的信干比（SIR）来估计，这里信干比定义为有用信号的功率S与总干扰信号的功率I之比。由于无线传播影响，用户移动性以及话务量的变化，功率级S和I都是随机变量，SIR也是一个随机变量。因此，同频干扰对系统性能产生影响的严重程度，通常用系统的中断概率来进行分析。在这个特定场合下，中断概率定义为SIR低于给定阈值的概率，即（2-2）其中是SIR的概率密度函数。要注意链路中断概率和系统中断概率之间的区别，前者是根据可接受的声音性能所需的特定误比特率（BER）或者Eb/N0阈值，确定是否为中断，而后者考虑的是一个典型用户可接受的移动性能所需的SIR阈值。如前所述，用来估计蜂窝系统中断概率的解析方法，需要已知射频传播影响、用户移动性和话务量变化等随机量的易于处理的模型，以求得的解析表达式。然而，由于这些影响和接受信号电平间的复杂关系，很难对这些影响采用解析模型。因此，主要靠仿真来估计蜂窝系统的中断概率，仿真还为分析提供了灵活性。本章我们给出了蜂窝通信系统的简单仿真示例，着重考虑通信系统的一些系统方面的问题，包括多用户性能、话务量工程和信道复用。为了进行系统级仿真，要考虑单个通信链路的许多方面，包括信道模型、天线辐射模式，以及Eb/N0（如SIR）和可接受性能之间的关系。

RESEARCHOFCELLULARWIRELESSCOMMUNATIONSYSTEMAbstractCellularcommunicationsystemsallowalargenumberofmobileuserstoseamlesslyandsimultaneouslycommunicatetowirelessmodemsatfixedbasestationsusingalimitedamountofradiofrequency(RF)spectrum.TheRFtransmissionsreceivedatthebasestationsfromeachmobilearetranslatedtobaseband,ortoawidebandmicrowavelink,andrelayedtomobileswitchingcenters(MSC),whichconnectthemobiletransmissionswiththePublicSwitchedTelephoneNetwork(PSTN).Similarly,communicationsfromthePSTNaresenttothebasestation,wheretheyaretransmittedtothemobile.Cellularsystemsemployeitherfrequencydivisionmultipleaccess(FDMA),timedivisionmultipleaccess(TDMA),codedivisionmultipleaccess(CDMA),orspatialdivisionmultipleaccess(SDMA).1IntroductionAwidevarietyofwirelesscommunicationsystemshavebeendevelopedtoprovideaccesstothecommunicationsinfrastructureformobileorfixedusersinamyriadofoperatingenvironments.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.2CellularRadioSystemSystem-LevelDescription：CellularsystemsprovidewirelesscoverageoverageographicserviceareabydividingthegeographicareaintosegmentscalledcellsasshowninFigure2-1.Theavailablefrequencyspectrumisalsodividedintoanumberofchannelswithagroupofchannelsassignedtoeachcell.Basestationslocatedineachcellareequippedwithwirelessmodemsthatcancommunicatewithmobileusers.Radiofrequencychannelsusedinthetransmissiondirectionfromthebasestationtothemobilearereferredtoasforwardchannels,whilechannelsusedinthedirectionfromthemobiletothebasestationarereferredtoasreversechannels.Theforwardandreversechannelstogetheridentifyaduplexcellularchannel.Whenfrequencydivisionduplex(FDD)isused,theforwardandreversechannelsaresplitinfrequency.Alternatively,whentimedivisionduplex(TDD)isused,theforwardandreversechannelsareonthesamefrequency,butusedifferenttimeslotsfortransmission.Figure2-1BasicarchitectureofacellularcommunicationssystemHigh-capacitycellularsystemsemployfrequencyreuseamongcells.Thisrequiresthatco-channelcells(cellssharingthesamefrequency)aresufficientlyfarapartfromeachothertomitigateco-channelinterference.ChannelreuseisimplementedbycoveringthegeographicserviceareawithclustersofNcells,asshowninFigure2-2,whereNisknownastheclustersize.Figure2-2Cellclustering: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.Ifwemodelthecellswithahexagonalshape,asinFigure2-2,theminimumdistancebetweenthecenteroftwoco-channelcells,calledthereusedistance,is(2-1)whereRisthemaximumradiusofthecell(thehexagonisinscribedwithintheradius).Therefore,wecanimmediatelyseefromFigure2-2thatasmallclustersize(smallreusedistance),leadstohighinterferenceamongco-channelcells.Thelevelofco-channelinterferencereceivedwithinagivencellisalsodependentonthenumberofactiveco-channelcellsatanyinstantoftime.Asmentionedbefore,co-channelcellsaregroupedintotierswithrespecttoaparticularcellofinterest.Thenumberofco-channelcellsinagiventierdependsonthetierorderandthegeometryadoptedtorepresenttheshapeofacell(e.g.,thecoverageareaofanindividualbasestation).Fortheclassichexagonalshape,theclosestco-channelcellsarelocatedinthefirsttierandtherearesixco-channelcells.Thesecondtierconsistsof12co-channelcells,thethird,18,andsoon.Thetotalco-channelinterferenceis,therefore,thesumoftheco-channelinterferencesignalstransmittedfromallco-channelcellsofalltiers.However,co-channelcellsbelongingtothefirsttierhaveastrongerinfluenceonthetotalinterference,sincetheyareclosertothecellwheretheinterferenceismeasured.Co-channelinterferenceisrecognizedasoneofthemajorfactorsthatlimitsthecapacityandlinkqualityofawirelesscommunicationssystemandplaysanimportantroleinthetradeoffbetweensystemcapacity(large-scalesystemissue)andlinkquality(small-scaleissue).Forexample,oneapproachforachievinghighcapacity(largenumberofusers),withoutincreasingthebandwidthoftheRFspectrumallocatedtothesystem,istoreducethechannelreusedistancebyreducingtheclustersizeNofacellularsystem.However,reductionintheclustersizeincreasesco-channelinterference,whichdegradesthelinkquality.ThelevelofinterferencewithinacellularsystematanytimeisrandomandmustbesimulatedbymodelingboththeRFpropagationenvironmentbetweencellsandthepositionlocationofthemobileusers.Inaddition,thetrafficstatisticsofeachuserandthetypeofchannelallocationschemeatthebasestationsdeterminetheinstantaneousinterferencelevelandthecapacityofthesystem.Theeffectsofco-channelinterferencecanbeestimatedbythesignal-tointerferenceratio(SIR)ofthecommunicationlink,definedastheratioofthepowerofthedesiredsignalS,tothepowerofthetotalinterferencesignal,I.SincebothpowerlevelsSandIarerandomvariablesduetoRFpropagationeffects,usermobilityandtrafficvariation,theSIRisalsoarandomvariable.Consequently,theseverityoftheeffectsofco-channelinterferenceonsystempe