OFDM基础中英文对照外文翻译文献.docx

OFDM基础中英文对照外文翻译文献.docx

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OFDM基础中英文对照外文翻译文献

中英文对照外文翻译文献

（文档含英文原文和中文翻译）

外文:

OFDMBasics

INTRODUCTION

ThebasicprincipleofOFDMistosplitahigh-ratedatastreamintoanumberoflower

ratestreamsthataretransmittedsimultaneouslyoveranumberofsubcarriers.Becausethesymboldurationincreasesforthelowerrateparallelsubcarriers,therelativeamountofdispersionintimecausedbymultipathdelayspreadisdecreased.IntersymbolinterferenceiseliminatedalmostcompletelybyintroducingaguardtimeineveryOFDMsymbol.Intheguardtime,theOFDMsymboliscyclicallyextendedtoavoidintercarrierinterferenceInOFDMsystemdesign,anumberofparametersareupforconsideration,suchasthenumberofsubcarriers,guardtime,symbolduration,subcarrierspacing,modulationtypepersubcarrier,andthetypeofforwarderrorcorrectioncoding.Thechoiceofparametersisinfluencedbysystemrequirementssuchasavailablebandwidth,requiredbitrate,tolerabledelayspread,andDopplervalues.Somerequirementsareconflicting.Forinstance,togetagooddelayspreadtolerance,alargenumberofsubcarrierswithasmallsubcarrierspacingisdesirable,buttheoppositeistrueforagoodtoleranceagainstDopplerspreadandphasenoiseGENERATIONOFSUBCARRIERSUSINGTHEIFFT

AnOFDMsignalconsistsofasumofsubcarriersthataremodulatedbyusingphaseshiftkeyingPSKorquadratureamplitudemodulationQAM.IfdiarethecomplexQAMsymbols,Nisthenumberofsubcarriers,Tisthesymbolduration,andfisthecarrierfrequency,thenoneOFDMsymbolstartingattt,canbewrittenas2.1

Intheliterature,oftentheequivalentcomplexbasebandnotationisused,whichisgivenby2.2.Inthisrepresentation,therealandimaginarypartscorrespondtothein-phaseandquadraturepartsoftheOFDMsignal,whichhavetobemultipliedbyacosineandsineofthedesiredcarrierfrequencytoproducethefinalOFDMsignal.Figure2.1showstheoperationoftheOFDMmodulatorinablockdiagram.

2.2Figure2.1OFDMmodulator

Asanexample,Figure2.2showsfoursubcarriersfromoneOFDMsignal.Inthisexample,allsubcarriershavethesamephaseandamplitude,butinpracticetheamplitudesandphasesmaybemodulateddifferentlyforeachsubcarrier.NotethateachsubcarrierhasexactlyanintegernumberofcyclesintheintervalT,andthenumberofcyclesbetweenadjacentsubcarriersdiffersbyexactlyone.Thispropertyaccountsfortheorthogonalitybetweenthesubcarriers.Forinstance,ifthejthsubcarrierfrom2.2isdemodulatedbydownconvertingthesignalwithafrequencyofj/TandthenintegratingthesignaloverTseconds,theresultisaswrittenin2.3.Bylookingattheintermediateresult,itcanbeseenthatacomplexcarrierisintegratedoverTseconds.Forthedemodulatedsubcarrierj,thisintegrationgivesthedesiredoutputmultipliedbyaconstantfactorT,whichistheQAMvalueforthatparticularsubcarrier.Forallothersubcarriers,theintegrationiszero,becausethefrequencydifferenceproducesanintegernumberofcycleswithintheintegrationintervalT,suchthattheintegrationresultisalwayszero.

2.3

TheorthogonalityofthedifferentOFDMsubcarrierscanalsobedemonstratedinanotherway.Accordingto2.1,eachOFDMsymbolcontainssubcarriersthatarenonzerooveraT-secondinterval.Hence,thespectrumofasinglesymbolisaconvolutionofagroupofDiracpulseslocatedatthesubcarrierfrequencieswiththespectrumofasquarepulsethatisoneforaT-secondperiodandzerootherwise.TheamplitudespectrumofthesquarepulseisequaltosincnJT,whichhaszerosforallfrequenciesfthatareanintegermultipleof1IT.ThiseffectisshowninFigure2.2,whichshowstheoverlappingsincspectraofindividualsubcarriers.Attheimumofeachsubcarrierspectrum,allothersubcarrierspectraarezero.BecauseanOFDMreceiveressentiallycalculatesthespectrumvaluesatthosepointsthatcorrespondtotheimaofindividualsubcarriers,itcandemodulateeachsubcarrierfreefromanyinterferencefromtheothersubcarriers.Basically,Figure2.3showsthattheOFDMspectrumfulfillsNyquist'scriteriumforanintersymbolinterferencefreepulseshape.Noticethatthepulseshapeispresentinthefrequencydomain

andnotinthetimedomain,forwhichtheNyquistcriteriumusuallyisapplied.Therefore,insteadofintersymbolinterferenceISI,itisintercarrierinterferenceICIthatisavoidedbyhavingtheimumofonesubcarrierspectrumcorrespondtozerocrossingsofalltheothers.

Figure2.2ExampleoffoursubcarrierswithinoneOFDMsymbol

ThecomplexbasebandOFDMsignalasdefinedby2.2isinfactnothingmorethantheinverseFouriertransformofN,QAMinputsymbols.ThetimediscreteequivalentistheinversediscreteFouriertransformIDFT,whichisgivenby2.4,wherethetimetisreplacedbyasamplenumbern.Inpractice,thistransformcanbeimplementedveryefficientlybytheinversefastFouriertransformIFFT.AnNpointIDFTrequiresatotalofN~complexmultiplications-whichareactuallyonlyphaserotations.Ofcourse,therearealsoadditionsnecessarytodoanIDFT,butsincethehardwarecomplexityofanadderissignificantlylowerthanthatofamultiplierorphaserotator,onlythemultiplicationsareusedhereforcomparison.TheIFFTdrasticallyreducestheamountofcalculationsbyexploitingtheregularityoftheoperationsintheIDFT.Usingtheradix-2algorithm,anN-pointIFFTrequiresonlyN/2.log2Ncomplexmultiplications[I].Fora16-pointtransform,forinstance,thedifferenceis256multiplicationsfortheIDFTversus32fortheIFFT-areductionbyafactorof8!

Thisdifferencegrowsforlargernumbersofsubcarriers,astheIDFTcomplexitygrowsquadraticallywithN,whiletheIFFTcomplexityonlygrowsslightlyfasterthanlinear.

2.4

ThenumberofmultiplicationsintheJFFTcanbereducedevenfurtherbyusingaradix-4algorithm.Thistechniquemakesuseofthefactthatinafour-pointIFFT,thereareonlymultiplicationsby1,-1j,-j,whichactuallydonotneedtobeimplementedbyafullmultiplier,butratherbyasimpleaddorsubtractandaswitchofrealandimaginarypartsinthecaseofmultiplicationsbyjor-j.Intheradix-4algorithm,thetransformissplitintoanumberofthesetrivialfour-pointtransforms,andnon-trivialmultiplicationsonlyhavetobeperformedbetweenstagesofthesefour-pointtransforms.Inthisway,anN-pointFFTusingtheradix4algorithmrequiresonly3/8Nlog2N-2complexmultiplicationsorphaserotationsandMog2Ncomplexadditions[I]GUARDTIMEANDCYCLICEXTENSION

OneofthemostimportantreasonstodoOFDMistheefficientwayitdealswithmultipathdelayspread.BydividingtheinputdatastreaminNssubcarriers,thesymboldurationismadeNstimessmaller,whichalsoreducestherelativemultipathdelayspread,relativetothesymboltime;bythesamefactor.Toeliminateintersymbolinterferencealmostcompletely,aguardtimeisintroducedforeachOFDMsymbol.Theguardtimeischosenlargerthantheexpecteddelayspread,suchthatmultipathcomponentsfromonesymbolcannotinterferewiththenextsymbol.Theguardtimecouldconsistofnosignalatall.Inthatcase,however,theproblemofintercarrierinterferenceICIwouldarise.ICIiscrosstalkbetweendifferentsubcarriers,whichmeanstheyarenolongerorthogonal.ThiseffectisillustratedinFigure2.6.Inthisexample,asubcarrier1andadelayedsubcarrier2areshown.WhenanOFDMreceivertriestodemodulatethefirstsubcarrier,itwillencountersomeinterferencefromthesecondsubcarrier,becausewithintheFFTinterval,thereisnointegernumberofcyclesdifferencebetweensubcarrier1and2.Atthesametime,therewillbecrosstalkfromthefirsttothesecondsubcarrierforthesamereason.

Figure2.6Effectofmultipathwithzerosignalintheguardtime;thedelayedsubcarrier2causesICIonsubcarrier1andviceversa.

CHOICEOFOFDMPARAMETERS

ThechoiceofvariousOFDMparametersisatradeoffbetweenvarious,oftenconflictingrequirements.Usually,therearethreemainrequirementstostartwith:

bandwidth,bitrate,anddelayspread.Thedelayspreaddirectlydictatestheguardtime.Asarule,theguardtimeshouldbeabouttwotofourtimestheroot-mean-squareddelayspread.ThisvaluedependsonthetypeofcodingandQAMmodulation.HigherorderQAMlike64-QAMismoresensitivetoICIandIS1thanQPSK,whileheaviercodingobviouslyreducesthesensitivitytosuchinterference.

Nowthattheguardtimehasbeenset,thesymboldurationcanbefixed.Tominimizethesignal-to-noiseratioSNRlosscausedbytheguardtime,itisdesirabletohavethesymboldurationmuchlargerthantheguardtime.Itcannotbearbitrarilylarge,however,becausealargersymboldurationmeansmoresubcarrierswithasmallersubcarrierspacing,alargerimplementationcomplexity,andmoresensitivitytophasenoiseandfrequencyoffset[2],aswellasanincreasedpeak-to-averagepowerratio[3,4].Hence,apracticaldesignchoiceistomakethesymboldurationatleastfivetimestheguardtime,whichimpliesa1-dBSNRlossbecauseoftheguardtime.

译文：

OFDM基础

介绍

OFDM的基本原理是将一串高速数据流变成同时传输在一些副载波的低速率数据流。

由于低速率平行的副载波是符号持续时间增加,因而对多径效应引起的时延扩展有较强的抵抗力。

符号间干扰可以通过在每个OFDM符号前引入一个保护间隔来完全消除。

加入保护间隔后,OFDM符号通过周期性扩展来避免载波间干扰。

在OFDM系统设计中,大量的参数需要考虑,比如副载波的数量,保护间隔,持续时间,副载波间距,每一个副载波的调制类型,前项纠错编码的类型,参数的选择是受系统要求的如,可用带宽,需要的比特率,可容忍的延时时间和多普勒扩散值。

但是有些要求是相互矛盾的。

例如,为了得到一个好的延迟扩展公差,大部分副载波用一个小副载波的间距是可取的,但事实恰恰相反,一个好的公差不利于多普勒扩散和相位噪声。

用IFFT方法调制副载波

一个OFDM信号是利用相移键控调制相移键控或正交振幅调制组成的副载波之和,如果di是合成的QAM符号,N是子载波的数量,T是符号周期,f是载波频率,则一个OFDM符号从时刻开始,可以写成（2.1）

在文献中,等效的基带符号经常被写成式2.2。

在这个式子中,实部与虚部分别对应于OFDM信号的同相与正交部分,需要乘以一个余弦和正弦所需的载波频率生成最终的OFDM信号。

图2.1表示了OFDM调制器的运算框图。

2.2

图2.1OFDM调制