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1、OFDM基础--外文资料翻译附录A外文资料OFDMBasicsINTRODUCTIONThebasicprincipleofOFDMistosplitahigh-ratedatastreamintoanumberoflowerratestreamsthataretransmittedsimultaneouslyoveranumberofsubcarriers.Becausethesymboldurationincreasesforthelowerrateparallelsubcarriers,therelativeamountofdispersionintimecausedbymultip
2、athdelayspreadisdecreased.IntersymbolinterferenceiseliminatedalmostcompletelybyintroducingaguardtimeineveryOFDMsymbol.Intheguardtime,theOFDMsymboliscyclicallyextendedtoavoidintercarrierinterferenceInOFDMsystemdesign,anumberofparametersareupforconsideration,suchasthenumberofsubcarriers,guardtime,sy
3、mbolduration,subcarrierspacing,modulationtypepersubcarrier,andthetypeofforwarderrorcorrectioncoding.Thechoiceofparametersisinfluencedbysystemrequirementssuchasavailablebandwidth,requiredbitrate,tolerabledelayspread,andDopplervalues.Somerequirementsareconflicting.Forinstance,togetagooddelayspreadto
4、lerance,alargenumberofsubcarrierswithasmallsubcarrierspacingisdesirable,buttheoppositeistrueforagoodtoleranceagainstDopplerspreadandphasenoiseGENERATIONOFSUBCARRIERSUSINGTHEIFFTAnOFDMsignalconsistsofasumofsubcarriersthataremodulatedbyusingphaseshiftkeyingPSKorquadratureamplitudemodulationQAM.Ifdia
5、rethecomplexQAMsymbols,Nisthenumberofsubcarriers,Tisthesymbolduration,andfisthecarrierfrequency,thenoneOFDMsymbolstartingattt,canbewrittenas2.1Intheliterature,oftentheequivalentcomplexbasebandnotationisused,whichisgivenby2.2.Inthisrepresentation,therealandimaginarypartscorrespondtothein-phaseandqu
6、adraturepartsoftheOFDMsignal,whichhavetobemultipliedbyacosineandsineofthedesiredcarrierfrequencytoproducethefinalOFDMsignal.Figure2.1showstheoperationoftheOFDMmodulatorinablockdiagram.2.2Figure2.1OFDMmodulatorAsanexample,Figure2.2showsfoursubcarriersfromoneOFDMsignal.Inthisexample,allsubcarriersha
7、vethesamephaseandamplitude,butinpracticetheamplitudesandphasesmaybemodulateddifferentlyforeachsubcarrier.NotethateachsubcarrierhasexactlyanintegernumberofcyclesintheintervalT,andthenumberofcyclesbetweenadjacentsu
