中英文翻译无线通信.docx

中英文翻译无线通信.docx

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中英文翻译无线通信

RESEARCHOFCELLULARWIRELESSCOMMUNATIONSYSTEM

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）.Therefore,wecanimmediatelyseefromFigure2-2thatasmallclustersize（smallreusedistance

）,leadstohighinterferenceamongco-channelcells.

Thelevelofco-channelinterferencereceivedw