OFDM正交频分复用.docx
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OFDM正交频分复用
OFDM正交频分复用
原文载于维基百科,由zslcn周生烈编译摘注评,西邮学子整理用于《通信与电子信息工程专业英语》Unit3ExpandingreadingBOFDM
Orthogonalfrequency-divisionmultiplexing(OFDM),essentiallyidenticaltocodedOFDM(COFDM)anddiscretemulti-tonemodulation(DMT),isa frequency-divisionmultiplexing (FDM)schemeusedasadigitalmulti-carrier modulation method.Alargenumberofcloselyspaced orthogonal sub-carriersignals areusedtocarry data.Thedataisdividedintoseveralparalleldatastreamsorchannels,oneforeachsub-carrier.Eachsub-carrierismodulatedwithaconventionalmodulationscheme(suchas quadratureamplitudemodulation or phase-shiftkeying)atalow symbolrate,maintainingtotaldataratessimilartoconventionalsingle-carriermodulationschemesinthesamebandwidth.
OFDM本质上与编码OFDM(COFDM)和离散多音调制(DMT)是一样的。
这是一种频分复用(FDM)方案,用来作为数字多载波调制方法。
大量紧挨的正交子载波信号用来承载数据;数据分成若干个并行的数据流或信道,每个信道一个子载波;每个子载波使用通常的调制方案(如正交幅度调制或相移键控),以低的信符速率(symbolrate)对其进行调制;从而维持整个数据速率,以类似于相同带宽的常规单载波数据速率,进行传输。
Orthogonalfrequency-divisionmultiplexing(OFDM)isamethodofencodingdigitaldataonmultiplecarrierfrequencies.OFDMhasdevelopedintoapopularschemeforwideband digitalcommunication,whether wireless orover copper wires,usedinapplicationssuchasdigitaltelevisionandaudiobroadcasting, DSL broadbandinternetaccess.
正交频分复用(OFDM)是一种在多个载波频率上对数字数据编码的方法。
OFDM已经发展成为宽带数字通信的一种流行方案;无论是无线还是在有线上,这种方案广泛应用于如数字电视和音频广播、DSL宽带互联网接入.
TheprimaryadvantageofOFDMoversingle-carrierschemesisitsabilitytocopewithsevere channel conditions(forexample, attenuation ofhighfrequenciesinalongcopperwire,narrowband interference andfrequency-selective fading dueto multipath)withoutcomplexequalizationfilters.Channel equalization issimplifiedbecauseOFDMmaybeviewedasusingmanyslowlymodulated narrowband signalsratherthanonerapidlymodulated wideband signal.Thelowsymbolratemakestheuseofa guardinterval betweensymbolsaffordable,makingitpossibletoeliminate intersymbolinterference (ISI)andutilizeechoesandtime-spreading(thatshowsupas ghosting onanalogueTV)toachievea diversitygain,i.e.a signal-to-noiseratio improvement.Thismechanismalsofacilitatesthedesignof singlefrequencynetworks (SFNs),whereseveraladjacenttransmitterssendthesamesignalsimultaneouslyatthesamefrequency,asthesignalsfrommultipledistanttransmittersmaybecombinedconstructively,ratherthaninterferingaswouldtypicallyoccurinatraditionalsingle-carriersystem.
OFDM比单载波方案优越,主要在于OFDM有能力应付恶劣的信道条件(例如,在长铜线中的高频衰减、窄带干扰、和由于多径引起的频率选择性衰落),免去了对均衡滤波器过于复杂的需求。
之所以能简化信道均衡,是因为OFDM可以看作为使用许多被慢调制的窄带信号,而不是一个被快调制的宽带信号。
低信符速率使有可能不会花费太大的代价,在信符间使用保护间隔,来避免信符间干扰(ISI)、消除回声和延时扩展(在模拟电视上显示为重影),以达到分-集增益,也就是改善信号信噪比,而不会太多降低频谱效率。
这种机制也有利于单一频率网络(SFN)的设计。
SFN是一种这样的网络,其中几个相邻发信机,同时发送相同频率的同样信号,好像这些发自多个远隔的发信机的信号,在结构上组合在一起。
而通常在一个传统的单载波系统中却是很易发生干扰的。
InOFDM,thesub-carrierfrequenciesarechosensothatthesub-carriersareorthogonal toeachother,meaningthat cross-talk betweenthesub-channelsiseliminatedandinter-carrierguardbandsarenotrequired.Thisgreatlysimplifiesthedesignofboththe transmitter andthe receiver;unlikeconventional FDM,aseparatefilterforeachsub-channelisnotrequired.
在OFDM中,子载波频率是这样选择的,即各子载波彼此频率正交,意味着子信道之间的串扰是可以被抵消的,而且也不需要载波间的保护频段。
这就大大简化了发信机和收信机的设计;不再像常规的FDM那样,已没有必要为每一个子信道安排一个分开的滤波器。
OFDMrequiresveryaccuratefrequencysynchronizationbetweenthereceiverandthetransmitter;withfrequencydeviationthesub-carrierswillnolongerbeorthogonal,causinginter-carrierinterference(ICI)(i.e.,cross-talkbetweenthesub-carriers).Frequencyoffsetsaretypicallycausedbymismatchedtransmitterandreceiveroscillators,orby Dopplershift duetomovement.WhileDopplershiftalonemaybecompensatedforbythereceiver,thesituationisworsenedwhencombinedwith multipath,asreflectionswillappearatvariousfrequencyoffsets,whichismuchhardertocorrect.Thiseffecttypicallyworsensasspeedincreases, andisanimportantfactorlimitingtheuseofOFDMinhigh-speedvehicles.SeveraltechniquesforICIsuppressionaresuggested,buttheymayincreasethereceivercomplexity.
OFDM需要在收信机和发信机之间有非常准确的的频率同步;子载波的频率偏差将导致不再正交,造成载波间干扰(ICI)(即子载波之间的串扰)。
频率偏移通常是由于发信机和收信机的振荡器不匹配所引起的,或者,由于移动而引起多普勒频移。
虽然单独的多普勒频移可以在收信机中得到补偿,但当同时有多径结合时,情况就会恶化;因为各种频率偏移都会出现反射,这就更难校正了。
这种效应通常随速度增加而恶化, 成为限制OFDM在高速行驶的车辆中使用的一个重要因素。
对于ICI的抑制有若干种技术建议,但他们可能会增加收信机的复杂性。
orthogonalityallowsforefficientmodulatoranddemodulatorimplementationusingtheFFTalgorithmonthereceiverside,andinverseFFTonthesenderside.Althoughtheprinciplesandsomeofthebenefitshavebeenknownsincethe1960s,OFDMispopularforwidebandcommunicationstodaybywayoflow-cost digitalsignalprocessing componentsthatcanefficientlycalculatetheFFT.
正交允许在接收端采用FFT算法,并在发送端使用反向FFT,以高效地执行解调器和调制器功能。
虽然其原理及其得益早在20世纪60年代以来就已众所周知,然而,直到今天才普及OFDM宽带通信,是由于采用了低成本的数字信号处理组件;使用它,才可以有效地计算FFT。
OnekeyprincipleofOFDMisthatsincelowsymbolratemodulationschemes(i.e.,wherethesymbolsarerelativelylongcomparedtothe channel timecharacteristics)sufferlessfrom intersymbolinterference causedby multipathpropagation,itisadvantageoustotransmitanumberoflow-ratestreamsinparallelinsteadofasinglehigh-ratestream.Sincethedurationofeachsymbolislong,itisfeasibletoinserta guardinterval betweentheOFDMsymbols,thuseliminatingtheintersymbolinterference.
OFDM的关键原理之一,是因为低信符速率的调制方案(即信符时间间隔与信道时间特性相比,相对较长),从而由于多径传播引起的信符间干扰少了,因此,用多个并行发送的低速率流,来取代一个单一的高速率流,是有利的。
由于每个信符的持续时间很长,在OFDM信符之间插入保护间隔也就可行,从而消除了信符间干扰。
Theguardintervalalsoeliminatestheneedfora pulse-shapingfilter,anditreducesthesensitivitytotimesynchronizationproblems.
保护间隔也消除了脉冲整形滤波器的需要,并减少了时间同步问题的敏感性。
Asimpleexample:
Ifonesendsamillionsymbolspersecondusingconventionalsingle-carriermodulationoverawirelesschannel,thenthedurationofeachsymbolwouldbeonemicrosecondorless.Thisimposessevereconstraintsonsynchronizationandnecessitatestheremovalofmultipathinterference.Ifthesamemillionsymbolspersecondarespreadamongonethousandsub-channels,thedurationofeachsymbolcanbelongerbyafactorofathousand(i.e.,onemillisecond)fororthogonalitywithapproximatelythesamebandwidth.Assumethataguardintervalof1/8ofthesymbollengthisinsertedbetweeneachsymbol.Intersymbolinterferencecanbeavoidedifthemultipathtime-spreading(thetimebetweenthereceptionofthefirstandthelastecho)isshorterthantheguardinterval(i.e.,125microseconds).Thiscorrespondstoamaximumdifferenceof37.5kilometersbetweenthelengthsofthepaths.
一个简单的例子:
如果某站在无线信道上,使用传统的单载波调制,发送每秒一兆个信符,则每个信符的持续时间是1微秒或更少。
这严重制约了同步,同时还必须消除多径干扰。
如果将该每秒一兆个信符扩展到一千个正交子信道中,每个信符的时间间隔就延长1000倍(即1毫秒),而总带宽近似相同。
假定,在每个信符之间插入1/8信符长度的保护间隔。
如果多径时间展延(接收第一个信符和最后一个回声信符之间的时间)小于保护间隔(即125微秒,这相当于路径长度最大差值为37.5公里),信符间干扰就可以避免。
The cyclicprefix,whichistransmittedduringtheguardinterval,consistsoftheendoftheOFDMsymbolcopiedintotheguardinterval,andtheguardintervalistransmittedfollowedbytheOFDMsymbol.ThereasonthattheguardintervalconsistsofacopyoftheendoftheOFDMsymbolissothatthereceiverwillintegrateoveranintegernumberofsinusoidcyclesforeachofthemultipathswhenitperformsOFDMdemodulationwiththeFFT.
在保护间隔期间发送循环前缀。
它是由表示OFDM信符结束的符号拷贝组成,并跟在OFDM信符之后被发送。
之所以要这样做,是因为由OFDM信符结束部分的副本组成的保护间隔,能在收信机使用FFT执行OFDM解调时,对于每个多径,将会在整数个正弦周期上进行整合
The cyclicprefix,whichistransmittedduringtheguardinterval,consistsoftheendoftheOFDMsymbolcopiedintotheguardinterval,andtheguardintervalistransmittedfollowedbytheOFDMsymbol.ThereasonthattheguardintervalconsistsofacopyoftheendoftheOFDMsymbolissothatthereceiverwillintegrateoveranintegernumberofsinusoidcyclesforeachofthemultipathswhenitperformsOFDMdemodulationwiththeFFT.
在保护间隔期间发送循环前缀。
它是由表示OFDM信符结束的符号拷贝组成,并跟在OFDM信符之后被发送。
之所以要这样做,是因为由OFDM信符结束部分的副本组成的保护间隔,能在收信机使用FFT执行OFDM解调时,对于每个多径,将会在整数个正弦周期上进行整合(保证对所需信号的完整提取)。
Theeffectsoffrequency-selectivechannelconditions,forexamplefadingcausedbymultipathpropagation,canbeconsideredasconstant(flat)overanOFDMsub-channelifthesub-channelissufficientlynarrow-banded(i.e.,ifthenumberofsub-channelsissufficientlylarge).Thismakesfrequencydomain equalization possibleatthe receiver,whichisfarsimplerthanthetime-domainequalizationusedinconventionalsingle-carriermodulation.InOFDM,theequalizeronlyhastomultiplyeachdetectedsub-carrier(eachFouriercoefficient)ineachOFDMsymbolbyaconstantcomplexnumber,orararelychangedvalue.
关于频率选择性对信道环境的影响,例如由多径传播引起的衰落,如果子信道的带宽足够窄(也就是子信道的数目足够大),可以认为,在OFDM子信道上是不变的,于是就有可能在接收端实现频域均衡,这比用于传统单一载波调制的时域均衡要简单得多。
在OFDM中,均衡器只要对检测到的每个OFDM信符的子载波(每个傅里叶系数)乘以一个复常数,或一个更改很少的值就可以了。
Ourexample:
TheOFDMequalizationintheabovenumericalexamplewouldrequireonecomplexvaluedmultiplicationpersubcarrierandsymbol(i.e., complexmultiplicationsperOFDMsymbol;i.e.,onemillionmultiplicationspersecond,atthereceiver).TheFFTalgorithmrequires [thisisimprecise:
overhalfofthesecomplexmultiplicationsaretrivial,i.e.=to1andarenotimplementedinsoftwareorHW].complex-valuedmultiplicationsperOFDMsymbol(i.e.,10millionmultiplicationspersecond),atboththereceiverandtransmitterside.Thisshouldbecomparedwiththecorrespondingonemillionsymbols/secondsingle-carriermodulationcasementionedintheexample,wheretheequalizationof125microsecondstime-spreadingusinga FIRfilter wouldrequire,inanaiveimplementation,125multiplicationspersymbol(i.e.,125millionmultiplicationspersecond).FFTtechniquescanbeusedtoreducethenumberofmultiplicationsforan FIRfilter basedtime-domainequalizertoanumbercomparablewithOFDM,atthecostofdelaybetweenreception