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毕业设计专业外文翻译BasebandDemodulation/DetectionInthecaseofbasebandsignalingthere__ivedw__eformsarealreadyinapulse-likeform.Onemightaskwhythenisademodulatorneededtorecoverthepulsew__eformsTheansweristhatthearrivingbasebandpulsesarenotintheformofidealpulseshapeseachoneoccupyingitsownsymbolinterval.Thefilteringtosufferfrominter-symbolinterferen__ISIandthusappearasanamorphous“__eared”signalnotquitereadyforsamplinganddetection.Thegoalofthedemodulatorre__ivingfilteristore__iverabasebandpulsewiththebestpossiblesignal-to-noiserationSNRfreeofanyISI.Equalizationcoveredinthischapterisatechniqueusedtohelpaccomplishthisgoal.Theequalizationpro__ssisnotrequiredforeverytypeofcommunicationchannel.Howeversin__equalizationembo___sasophisticatedsetofsignal-pro__ssingtechniques__kingitpossibletocompensateforchannel-indu__dinterferen__itisanimportantareafor__nysystems.Theband-passmodelofthefourthchapterdealswiththedetectionpro__ssanddescribedinthischapteressentiallythesameasthebasebandmodel.Beforetestingyoumustfirstbandpassconversiontothebasebandsignal.Forsignaldetectioninlinearsystems__the__ticalexpressionsdonotac__ptthefrequencymoveandh__ethefollowingequivalen__theoremequivalen__theoremstates:thecurrentpro__ssingisfirstbandpasssignalisconvertedtothebaseandthenusingtheheterodynemethodbandsignal;theresultswiththefirstheterodynesignalisconvertedtothebasebandsignalthecorrespondinglinearpro__ssingonthebasebandsignal.Heterodyneheterodyingreferstoatechniquecalledfrequencyconverterfrequencyconversionormixingofthefrequencyashamethatsoftwaremixingissignalpro__ssingherealizedthespectrumofthesignalmove.Acorollaryoftheequivalen__theoremandlinearsimulationpro__sstheChengDuijibandsignalgenerallytheresultisthesameastheresultofbandpassroleThisindicatesthatmostofdigitalcommunicationsystemsasabasebandsystemtodescribeand____yze.1SIGNALSANDNOISE
1.1Error-perfor__n__DegradationinCommunicationSystemsThepurposeofthedetectorisbasedonasfewerrorsaspossibletorestoretheoriginalsignalstreamfromthere__ivedw__eformdistortion.Therearetwo__inreasonsfortheerrorperfor__n__degradation.Thefirstreasonisthat
3.3willintrodu__asenderchannelandre__iverfilternon-idealsystemtransferfunctionwillcausethesymboltailarisingfrominter-symbolinterferen__ISI.Anotherreasonisthattheelectronicnoiseandothernoisesour__ssuchasatmosphericnoiseoftheuniversetheswitchingtransientnoiseinter-modulationnoiseandinterferen__signalsfromothernoisesour__sthesewillbediscussedinChapter
5.Appropriatepreventivemeasurescaneffectivelyredu__oreveneliminatethenumberofre__ivernoiseandinterferen__.Howeverthereisanoisecannotbeeliminatedistheelectronicther__lmotionoftheconductorther__lnoisethenoiseisanadditivenoiseexistintheamplifiercircuit.Theuseofquantummechanicshasbeeninformedofthestatisticalpropertiesofthether__lnoise.
1.2TheBasicSNRParameterforDigitalCommunicationSystemsLearned____ogcommunicationreadersarefamiliarwithanindicatorthatthesignal__eragepowerandnoise__eragepowerratioreferredtoasthesignaltonoiseratioorSNRofIndigitalcommunicationsystemsusuallyusingthenor__lizedformofsignaltonoiseratioastheperfor__n__index.istheenergyperbitisequaltotheproductofthesignalenergywitheachbitdurationisthenoisepowerspectraldensityequaltotheratioofthenoisepowerandbandwidth;reciprocalandbecauseeachbitdurationandthebitratecanbeusedinsteadofthereforethefollowingexpressionsetup:==
1.1datarateisoneofthemostcommonlyusedindicatorofthedigitalcommunicationstosimplifythedescriptionthebookwillrememberthebitrateRisR.Tounderlinethenor__lizedbandwidthandbitrateformofequation
1.1into
1.2Digitalcommunicationsystemsthemostimportantperfor__n__measureisoneofthebiterrorrateandcurveseeFigure
1.1theof.Dimensionlessratioisastandarddigitalcommunicationssystemperfor__n__indicatorsthatcanbeasameasureofcomparativeperfor__n__advantagesanddisadvantagesofthetwocommunicationsystems:theconditionsforagivenerrorprobabilitytherequiredis__allerthehigheraccuracyofdetection.Figure
1.1GeneralshapeofthePversusE/Ncurve
1.3WhyE/NisaNaturalFigureofMeritBeginnerdigitalcommunicationthereader__ydoubttheusefulnessoftheparameter.In____ogcommunicationisaveryusefulindicatorofthemolecularexpectto__intainthetran__issionofpowerthedenominatorrepresentsthesizeofthenoise.ButwhyyouwanttouseindigitalcommunicationwiththedifferentindicatorsperbitenergytonoisepowerspectraldensityratioTheexplanationisgivenbelow.Insection
1.
2.4wedefinethepowersignal__eragepowerislimitedwhiletheinfiniteenergyofthelargesignalandtheenergysignalisdefinedasthe__eragepowerisequaltozerowhiletheenergy-limitedsignal.Thisclassificationisveryusefultocomparethe____oganddigitalsignals.The____ogsignalisclassifiedasapowersignal.Whatisthesignifican__Usuallythedurationofthesimulationw__eformofinfinitelengthdonotneedtopartitionorincreasethetimewindow.Infinitetimedo__insignalw__eformitsenergyisinfiniteandthereforecannotusetheenergytodescribethesignal.For____ogsignalsthepowerorenergytransferrateisamoreusefulparameter.Howeverinthedigitalcommunicationsystemusingthelengthoftimeforthew__eformofthesymbolintervaltosendandre__ivecode.The__eragepowerforeachsymbol__erageisequaltozerothroughoutthetimelinesothepowercannotbeusedtodescribeadigitalsignal.Thereforethedigitalsignalshouldbeusedwithinthetimewindowtomeasurethesignalmeasure.Inotherwordsthesymbolenergypowerpointsinisamoresuitabledescriptionofthedigitalsignalw__eformparameters.Re__ivedenergycanbewelldescribedbythedigitalsignalbutitdidnotsaywhytheisagoodindicatorofthedigitalsystem.Thedigitalw__eformisamediumforrepresentingdigitalinfor__tiontheinfor__tion__ycontainabitbinarybitquaternary...10-bit1024hex.Thisdiscreteinfor__tionstructureiscompletelydifferentsour__ofinfor__tionofthe____ogcommunicationsystemofunlimitedquantifiedcontinuousw__e.Thedigitalsystemmetricsmustbemorethanthepremiumontheperfor__n__ofbothsystems.Becausethedigitalsignalw__eform__ycontainonly1bit2bits...10bitsetc.sotheofthedigitalsignalcannotbedescribed.Forexampleifagivenerrorprobabilityabinarydigitalsignalrequiredfortheis
20.Notethatthemeaningofthefigurescontainedinitsequivalentofthedigitalsignalw__eform.Binaryw__eformcontainsabitofinfor__tionperbitrequiredfortheis
20.Ifthesignalis1024hextherequiredisstill
20.Thew__eformcontains10bitsofinfor__tionperbitrequiredforthe
2.Thusraisesthequestionofwhyisitnotmoresuitableparameters-thebitlevelontheenergyparameterstodescribethisindicatorWiththesameisadimensionlessratiothefollowingexpressiontoprovethispoint:==2INTERSYMBOLINTERGEREN__Figureadescribesthefilteringproblemofatypicaldigitalcommunicationsystem.Theentiresystemtran__itterre__iverandchannelisdifferenttypesoffiltersaswellasinertcircuitcomponentssuchasinductorsandcapacitors.Inthetran__itterthemessagesymbolsoftheformofpulsesorleveldebuggingfilteredintopulsesthatmeetthebandwidthrequirements.Thebasebandhassystemthechannelcabledistributionreactan__pulsesignaldistortion.Band-passsystemsuchaswirelesssystemsisusuallyfadingchannelseeChapter15thistypeofchannelisequivalenttodonotexpectthefilterresultinginsignaldistortion.Tocompensateforthetran__itterandchannelisdistortioncausedbytheac__ptan__filterequalizationfilterorac__pt/equalizationfilterre__iving/equalizingfilter.Figurebshowsa______modelofsuchsystemsitwillbeallthefilteringeffectisequivalenttoasystemtransferfunction:
2.1tran__itfilterchannelfilterac__ptan__/equalizationfilter.onbehalfoftheentiresystemtransferfunctionacombinationofallofthefilteringeffectofthetran__itterchannelandre__iverlink.InabinaryPCMcommunicationsystemsuchasNRZ-Lthedetectoraccordingtothere__ivedsignalsamplingandthresholddecision.ForexampleinFigureaofthedetectorifac__ptedsignalisgreaterthanzerothenthejudgmentsentbitis1;iflessthanzerothenthejudgmentbitis
0.Duetothefilteringeffectofthesystemtore__ivethepulseoverlapshowninFigureb.Trailingpulseappearstooccupytheadja__ntsymbolintervalthusinterferingwiththesignaldetectionpro__sstherebyresultinginthereductionoftheerrorperfor__n__;thistypeofinterferen__calledISIinter-symbolinterferen__ISI.Evenwithoutthedistortioncausedbythenoisefilteringandchannelwillalsoresultinintersymbolinterferen__.InsomecasesisfixedtheproblembecomeshowtodetermineandsothattheoutputsignaloftoobtaintheminimumISI.TheNyquiststudythere__ivingenddoesnotprodu__theISIre__ivedpulseshape.Heproved:__kethecoderatecode/ssignalisnotpresenttheISIthetheoreticallyrequiredminimumsystembandwidth/2Hz.MinimumconditionssetupbythesystembandwidththesystemtransferfunctionistherectangularfunctionasshowninFigurea.BasebandsystemTheunilateralbandwidth1/2TrectangularfunctionidealNyquistfilterthesystemsimpulseresponsetheinverseFouriertransformcalledtheidealNyquistpulse=sinct/Tsinct/Tmultiplelobeincludinga__inlobeandsidelobesidelobealsoknownasthe__invalvebeforeandafterthetrailingpulsetobothsidesoftheinfiniteextension.Nyquistprovedthatifthere__ivedsequen__ofeachpulseissinct/Tshapethepulsesequen__freefromtheeffectsofintersymbolinterferen__isdetected.Figurebillustratesthereasonsto__oidintersymbolinterferen__.Therearetwoadja__ntpulsesandhasalongtrailingpulsebutthesamplingpointsint=T-time=0Similarlyinthepulsek=1,2,….thesamplingofhtsidelobevaluesarezero.Thereforethesamplingtimeaccurateandnotexistinter-symbolinterferen__.BasebandsystemISIdetectthesymbolrate1/Tpulsesymboltherequiredbandwidthof1/2T;Inotherwordsthebandwidth/2Hz.Thesysteminensuringnointercrosstalkconditionscansupportthe__ximumtran__issionrateofcode/sNyquistlimit.ThustheidealNyquistfiltersystemtoensurenointercrosstalkeveryHertzthe__ximumpossibletran__issionratecalledthecoderatecompressionfor2symbol/s/Hz.IdealNyquistfiltertransferfunctionofarectangularshapethecorrespondingimpulseresponseforaninfinitelylongitisclearthatthefiltercannotbeachievedcanonlyapproxi__tetoachieve.
2.1PulseShapingtoRedu__ISIMorenarrowspectrumofthesignalallowsthedataratehigherwhilethenumberofusersre__ivingservi__sandmore.Communicationsservi__providerwhichhasgreatsignifican__becausethehighertheincomethemoreyoucanusethebandwidthutilization.MostofthecommunicationsystemChapter12ex__ptforSpreadSpectrumSystemsgoalistominimizetherequiredbandwidth.Nyquistredu__systembandwidthlimit.IfthesystembandwidthiswhatwillhappenPulsesintimedo__inwillbeextendedtheresultingcodewillredu__thecrosstalkbetweentheerrorperfor__n__ofthesystem.ThereforeareasonabletargetthecompresseddatapulsesothatithasNyquistminimumbandwidthslightlylargerbandwidth.ThiscanbeaNyquistfilterpulsefor__tion.IfthebandedgeofthefilterisrelativelysteepclosetotherectangleinFigure
3.16bthesignalspectrumofthenarrowest.HoweverthedurationoftheimpulseresponseofthisfilterisclosetoinfinityseeFigurebtheentiresequen__ofpulseoverlap.Timedo__inwidthoftheimpulseresponsecorrespondingtothecharacteristicsinthefrequencydo__inistheamplitudeofeachnearthe__inlobesidelobe.ThesesidelobeisundesirablebecausefromFigurebsamplingonlyinthecorrectsamplingtimedoesnotexistinISI;largesidelobelevela__allsamplingofthetimingdeviationwillleadtointer-symbolcrosstalk.Sowhilethenarrow-bandspectrumsignalscanprovidethebestbandwidthutilizationbutitstimingerrorscausedbyinter-symbolinterferen__isverysensitive.
2.2TwoTypesofError-Perfor__n__DegradationRedu__theerrorperfor__n__ofdigitalcommunicationsystemstherearetwocases.Thefirstduetoincreaseintheenergyofthere__ivedsignalenergytoredu__ornoiseinterferen__signalthesignaltonoiseratioreduction__kingtheperfor__n__degradation;thesecondisattributedtothesignaldistortionsuchasbytheinter-symbolcrosstalkcausedbydistortion.Thefollowingdiscussthedifferen__ofthesetwocases.AssumethattheneedtodesignthecommethattheconstmunicationsystemWu-bitrateBERofcurveinFigureapracti__.Assuructedsystemtestsystemperfor__n__canbefoundinWubitrateofcurveisnotatheoreticalcurvebutthedottedlineinFigureBecausethesignalislessbadandnoiseinterferen__toimprovethelevelcausingthesignaltonoiseratioofislessbad.Assumethatexpectationsofthebiterrorrateof10thenintheorythe10dB.Thebiterrorrateofthesamecaseduetoadeclineinsystemperfor__n__requiredforroseto12dBknownbythedashedlines.IfyoucannotsolvelessbadneedtoachievethesamebiterrorrateofthenumberofTheansweris2dB.Thisisaveryseriousproblemespeciallyinthesystemisapowerlimitmulti-2dBsignaltonoiseratioisverydifficult.Howeverthesignaldistortioncausedbyperfor__n__degradationcomparedtothereductionofsignaltonoiseratioisnottooterrible.Stillassumethatthesystemdoesnotmeetexpectationsofperfor__n__showninpracti__asshowninFigure
3.18b.Atthispointisnota______SNRimpairmentbutthereissystemperfor__n__degradationcausedbyISIshownindottedline.IfyoucannotresolvethisprobleminordertoachievethedesiredbiterrorratebutalsoprovidemoresignaltonoiseratioitTheanswerisinfinitymeaningthatthereisnowaytoachieve.Whenthecurvetoreachcannotredu__thepointassumingthatthelowestpointabovethesystemrequirementstoincreasethesignaltonoiseratiodoesnotimprovetheerrorperfor__n__.Thereisnodoubtthatthelowestpointofeachofandcurvecanfallanywhereifthepointdoesnotcauseanyeffectintheregionunderconsider.Increasedcrosstalkbetweensignaltonoiseratiocannotsolvethecodeproblemcurvereachesthelowestpointincreasingthesignaltonoiseratiocannotsolvetheproblem.ObserveinFigureboverlappingpulsescaninferthisconclusion:ifweincreasethetheratiodoesnotredu__theoverlappingpulsesthew__eformisreallythesame.Sohowtosolvetheproblemofinter-symbolinterferen__Inbalan__dsee
3.
4.Sin__theinter-symbolinterferen__causedbythefilteringeffectofthetran__itterandchannelthentheequilibriumcanbeseenastheinversepro__ssofthenon-opti__lfiltering.3CHANNELCHARACTERIZATIONTran__issioncharacteristicsofthecommunicationchannelsuchastelephonewirelesschannelisequivalenttotheimpulseresponseasband-limitedlinearfilterthefrequencyresponse
3.1andistheFouriertransformtheamplitudeofthechannelfrequencyresponseisthephase-frequencyresponse.
1.
6.3Thesectionhasprovedtoachievetheidealofthechannellosslesstran__issionmustmeettheWwithinthesignalbandwidthisconstantfisalinearfunctionoffrequencyieallfrequenciesofthesignalcomponentthetimedelayconstant.Wwithinthescopeisnotconstantitwillcauseamplitudedistortion;intherangeofthelinearfunctionofthefrequencywillcausephasedistortionthenumberofchannelssuchasfadingchanneltheamplitudeandphasedistortionisusuallyatthesametimeexist.Inthetran__issionofthepulsesequen__thisdistortionperfor__n__forsignaldispersionor__earingw__eformdemodulationsequen__ofdefor__tion.W__eformoverlappingortailingknownasinter-symbolinterferen__anditexistsinmostofthemodulationsystemisoneofthe__inobstaclestoreliablehigh-speedtran__issioninbandwidth-limitedchannel.Broadlyspeakingtheequilibriumreferstoallsignalpro__ssingorfilteringtechniquestoeliminateorredu__theISI.Thebalan__canbedividedintotwocategoriesshowninFigure
2.
1.Thefirstcategoryisthe__ximumlikelihoodesti__tion__ximum-likelihoodesti__tion.Thesequen__MLSEneedtoobtainesti__tesofadjustthere__ivertoadaptthetran__issionenviro__ent.Thepurposeofthisadjustmentisto__keabetteresti__teofthedetectorhadreallydemodulationpulsesequen__.MLSEre__iverre-shapingorotherdirectcompensationisnotadistortionofsamplingbuttoadjustitselftobetterdealwiththetruew__eform.Anexampleofthisapproachistosection
15.
7.1tointrodu__Bitebalan__d.Thesecondcategoryistheequalizationfilterfiltercompensationsignaldistortion.Kindofequalizerdemodulatorsampledsequen__bytheequalizertoeliminateinter-symbolinterferen__ISIandenterthedetector.Thissectiondescribesthecommonequalizationfiltersuchequalizercanbefurtherdivided.AclassificationisbasedonthestructureofthefiltercanbedividedintothelinearsystemtransversalfilterwithfeedforwardunitandbothbeforethecollapseunitandfeedbackunitofthenonlinearsystemDecisionFeedbackEqualizer;canalsobedividedintopresetstylebasedontheiradaptabilityandadaptive;canalsobeclassifiedaccordingtotheresolutionandupdaterateofthefilter.Detectionsignalcanbesampledvalueofthesymbolboundarythatiseachsymbolisonlysampledon__thissamplingknownasthesymbolintervalsampling;canalsobesampled__nytimeseachsymbolthissamplingcalledsamplingofpartofthecodeinterval.Re__iver/equalizerfilterintheseparationofthere__iverfilterandtheequalizerinsteadofequation
3.77thetransferfunctionofthewholesystemisraisedcosinefunctiondenotedasso
3.2Intherealsystemcannotbefullyknownchannelfrequencytransferfunctionanditsimpactresponseandthusdesignednottocrosstalkatanymomenth__enointerconveyor.Usuallyselectedto__tchthetran__itfilterandre__ivefilterthefollowingexpression:
3.3Inthiswayandthefrequencytransferfunctionareboththesquarerootofthecosinefunctionsquarerootraisedcosinefunction.Usedtocompensateforchanneldistortionequalizertransferfunctionistheinverseofthechanneltransferfunction:
3.4Sometimesdeliberatelychosentointer-symbolinterferen__inthefrequencytransferfunctionsuchasGaussianfiltertransferfunctionthesamplingpointanditspurposeistoimprovethebandwidthutilizationusingtheraisedcosinefilter.Thetaskofequalizationfilterisnotonlycompensateforthechannelbutalsotocompensateforthetran__itfilterandre__ivefilterduetointer-symbolinterferen__.基带__解调与检测基带__传输中的接受波形已经是脉冲形式,___还需要解调器来恢复脉冲波形呢?原因是到达的基带波形不是理想脉冲(每个码元只占据自己的码元间隔)由于发送端滤波器和信道的原因,接受脉冲序列存在着码间串扰(ISI)从而产生拖尾__,这些__不利于采样检测解调器(接收滤波器)的目的是消除码间干扰,恢复具有大信噪比(SNR)的基带__本章讲述的均衡技术就能够实现这个目的虽然不是所有的通信信道都需要均衡,但由于它是一种补偿信道干扰的综合__处理技术,因此成为许多系统非常重要的组成部分第四章论述的检测过程的带通模型,本质上与本章介绍的基带模型相同因为在检测之前,必须先将带通__转换为基带__对于线性系统来说__检测的数学表达式不收频率搬移的影响,并有下面的等价定理quivalen__theorem先对带通__做现行处理,然后用外差法将__转换为基带__;其结果与先用外差法将__转换为基带__,然后对基带__做相应的线性处理相同“外差”(heterodying)是指一种称为“频率转换”(frequencyconversion)或“混频”(frequencymixing)的__处理过程,他实现了__的频谱搬移该等价定理的一个推论是,所有的线性仿真处理过程对基带__(一般比较简单)作用的结果与对带通__作用的结果都是相同的,这表明可以把大部分数字通信系统当做基带系统进行描述和分析1__与噪声
1.1通信系统中差错性能的劣化检测器的目的,是以尽可能少的差错从已经产生失真的接收波形中恢复原__流引起差错性能下降的主要原因有两个第一个原因是
3.3节将介绍发送端、信道和接收端的滤波的影响,非理想的系统传递函数会引起码元“拖尾”而产生码间串扰(ISI)另一个原因是电子噪声以及其他各种噪声源的干扰,如宇宙大气噪声、开关瞬态噪声、互调噪声和来自其他噪声源的干扰__(这些将在第5章讨论)采用适当的预防措施可以有效的减少甚至消除许多___中的噪声和干扰但是,有一种噪声是无法消除的,就是导体中电子热运动产生的热噪声,该噪声是一种加性噪声,存在于放大器和电路中运用量子力学已获知了热噪声的统计特性
1.2数字通信系统中的信噪比参数学过模拟通信的读者都很熟悉一个指标,即__平均功率和噪声平均功率的比值,简称为信噪比(或称为SNR)在数字通信系统中,通常用信噪比的归一化形式作为性能指标为每比特能量,等于__能量与每比特持续时间的乘积;是噪声功率谱密度,等于噪声功率与带宽之比;又因为每比特持续时间与比特速率互为倒数,可用代替,因此有下列表达式成立==
1.1以不为单位的数据速率是数字通信中最常用的指标之一,为简化描述,本书将比特速率简记为为强调是的归一化带宽和比特率形式,将式(
1.1)转化为
1.2图
1.1与的关系曲线数字通信系统性能最重要的度量之一是误码率与的关系曲线(见图
1.1),若,则无量纲比值是数字通信系统性能的一个标准指标,可以将系统所需的作为比较两个通信系统性能优劣的量度在给定差错概率的条件下,所需的越小,检测的准确性就越高.
1.3作为度量指标的原因初学数字通信的读者可能会怀疑参数的有用性在模拟通信中,是一个非常有用的指标,其分子表示期望保持的传输功率大小,分母表示噪音干扰的大小但___在数字通信中要使用与之不同的指标(每比特能量与噪声功率谱密度之比值)呢?下面给出对这个问题的解释在
1.
2.4节中,我们将功率__定义为平均功率有限而能量无穷大的__,而将能量__定义为平均功率等于零而能量有限的__这样的分类在对模拟__和数字__做比较时是非常有用的我们将模拟__归类为功率__这有什么意义呢?通常模拟波形的持续时间为无限长,不需要做分割或加时间窗对时域无限的电__波形而言,其能量为无穷大,因此不能用能量来描述该__对模拟__而言,功率(或能量传输速率)是一个更有用的参数然而,数字通信系统采用时间长度为码元间隔的波形来发送和接收码元每个码元的平均功率(在整个时间轴上取平均)等于零,所以功率不能用于描述数字__因此对于数字__应该采用能在时间窗内度量__的测度换而言之,码元能量(功率在上的积分)是一个更适合于描述数字__波形的参数接收能量可以很好地描述数字__,但这还没有说明___是数字系统的一个很好的指标数字波形是代表数字信息的媒介,信息可能包含1比特(二进制)、2比特(四进制)、……、10比特(1024进制)等与这种离散信息结构完全不同,模拟通信系统的信息源是无__化的连续波数字系统的衡量指标必须在比特级上比较两个系统的性能因为数字__波形只可能包含1比特、2比特、……、10比特等的信息,所以用无法对数字__进行描述例如,若给定差错概率,某二进制数字__所需的是20注意,数字__波形与其包含的数字含义等价因为二进制波形包含1比特信息,所以每比特所需的是20若__是1024进制,所需的仍为20由于该波形包含10比特信息,所以每比特所需的为2由此产生一个问题,___不用更适合的参数——比特级别上的能量相关参数来描述这个指标呢?与相同,也是一个无量纲比值下列表达式证明了这点=焦耳/瓦每赫兹=瓦.秒/瓦.秒2码间干扰如图描述了典型数字通信系统的滤波问题整个系统(发送机、接收机和信道)中有各种类型的滤波器(以及惰性电路元件,如电感和电容)在发送端,将脉冲或电平形式的消息符号进行调试,经滤波后变成符合带宽要求的脉冲对于基带系统,信道(电缆)中存在的分布电抗使脉冲__发生失真一些带通系统(如无线系统)通常是衰减信道,这类信道相当于不期望的滤波器致使__发生失真为了补偿发射机和信道引起的失真,接受滤波器通常是均衡滤波器或接受/均衡滤波器图给出了这类系统的一个简单模型,它将所有的滤波作用等效为一个系统传输函数:
2.1其中,表示发送滤波器,表示信道内的滤波器,表示接受/均衡滤波器代表整个系统的传输函数,综合了发射机、信道和接收机链路中所有的滤波作用在一个二进制PCM通信系统中(如NRZ-L),检测器根据接收__的采样与门限值的比较做出判决例如图中的检测器,若接受__大于零,则判决发送比特为1;若小于零,则判决比特为0由于系统的滤波作用,接收脉冲之间会发生交迭,见图脉冲出现拖尾占据了相邻码元间隔,从而干扰了__检测过程,进而造成误差性能的降低;这类干扰称为码间串扰(ISI)即便没有噪声,滤波和信道引起的失真也会导致码间串扰在某些情况下,是固定的,问题就变为如何确定和,从而使的输出__获得最小的码间串扰奈奎斯特研究了接收端不产生码间串扰的接收脉冲形状问题他证明要使码元速率为码元/s的__不存在码间串扰,理论上所需的最小系统带宽为最小系统带宽成立的条件是,系统传输函数是如图所示的矩形函数对于基带系统,是单边带宽为的矩形函数(理想奈奎斯特滤波器)时,系统的冲击响应即的傅里叶逆变换为称为理想奈奎斯特脉冲,它的多重波瓣包括一个主瓣和多个旁瓣,旁瓣又称为主瓣的前后尾随脉冲,向两边无限延伸奈奎斯特证明,若接收序列的每个脉冲都是形状,则脉冲序列的检测不受码间串扰的影响图说明了避免码间串扰的原因图中有两个相邻脉冲和,具有很长的尾随脉冲,但在的采样点即t=T时刻,同样地,在脉冲=1,2,….的采样时刻的所有旁瓣取值都为零由此可知,若采样时刻准确就不存在码间串扰对于基带系统,__间串扰地检测码元速率为的脉冲(码元)所需带宽为;换而言之,带宽的系统在保证__间串扰的条件下能够支持的最大传输速率为码元/s(奈奎斯特带宽限制)由此可见,理想奈奎斯特滤波系统(保证__间串扰)毎赫兹的最大可能传输速率(称之为码速压缩)为2码元/s/Hz理想奈奎斯特滤波器的传输函数形状为矩形,其相应的冲激响应为无限长,显然该滤波器是不可实现的,只能近似实现
2.1目标和权衡__的频谱越窄,允许的数据速率就越高,同时接受服务的用户数也越多这对通信服务提供商具有非常重大的意义,因为可以用带宽的利用率越高收益就越多大部分的通信系统(第12章的扩频系统除外)的目标是尽可能的减少系统所需带宽奈奎斯特给出了降低系统带宽的极限如果系统带宽会出现什么状况呢?脉冲的时域范围会扩展,产生的码间串扰会降低系统的误差性能因此合理的目标是,压缩数据脉冲使之具有比奈奎斯特最小带宽稍大的带宽这可以用奈奎斯特滤波器进行脉冲形成来实现如果滤波器的频带边缘比较陡峭,接近图中的矩形,则__频谱达到最窄但是,这种滤波器冲击响应的持续时间接近无穷大(见图),整个序列的脉冲相互交迭时域宽的冲击响应,对应在频域中的特性是每一个主瓣附近都有大振幅的旁瓣这些旁瓣是不受欢迎的,因为由图可知,只有在正确的采样时刻采样才不存在码间串扰;当旁瓣电平很大时,很小的采样定时偏差就会导致码间串扰所以,虽然窄带频谱__能够提供最佳的带宽利用率,但是它对定时误差引起的码间串扰非常敏感
2.2差错性能劣化的两种类型数字通信系统中差错性能的降低有两种情况第一种,由于接收__能量的降低或者噪声(干扰__)能量的增加导致信噪比的减少,从而使得性能下降;第二种归因于__失真,例如由码间串扰引起的失真下面讨论这两种情况的差别假定需要设计的通信系统,其吴比特率(误码率)与的关系曲线是图中的实践假设已构造好系统,测试系统性能可以发现吴比特率与的关系曲线并不是理论曲线,而是图中的虚线由于__减衰和噪声(干扰)电平的提高,造成信噪比的减衰假定期望误比特率为10,那么理论上所需的为10dB误比特率相同的情况下,由于系统性能的下降所需的升至12dB(由图中虚线可知)若无法解决减衰问题,为达到相同的误比特率需要多提供多少的呢?答案是2dB这是一个很严重的问题,特别在系统是功率限制时,多提供2dB的信噪比是很困难的然而,与__失真引起性能下降相比,信噪比的降低还不算十分可怕仍假定系统不满足实践所示的期望性能,如图所示此时不是简单的信噪比减损,而是存在由码间串扰引起的系统性能下降(如图中虚线所示)若无法解决这个问题,为了达到期望的误比特率,又要多提供多少信噪比呢?答案是无穷大,也就是说没有办法实现当曲线达到不能再减小点时(假定最低点高于系统要求的),增大信噪比也不能改善差错性能毫无疑问,每条与关系曲线的最低点可以落在任何地方,该点若在考虑的区域之下就不会造成什么影响增大信噪比不能解决码间串扰问题(若曲线达到了最低点,增大信噪比不可能解决问题)观察图中的交叠脉冲可以推断出这个结论若增大,交叠脉冲的比例不会减少,波形是真情况相同那么如何解决码间串扰问题呢?通常是采用均衡(见
3.4节)既然码间串扰是发送机和信道的滤波作用引起的,那么可以将均衡看做非最佳滤波的逆过程3信道特性许多通信信道(如__、无线信道)的传输特性相当于冲激响应为的带限线性滤波器,其频率响应为
3.1和是傅里叶变换对,是信道的幅频响应,是相频响应
1.2节已证明,要实现信道的理想(无失真)传输,必须满足在__带宽W范围内是常数,是频率的线性函数(即对__的所有频率分量,时间延迟为常数)如果在W范围内不是常数,则会引起振幅失真;如果在W范围内不是频率的线性函数,则会引起相位失真,许多信道(如衰落信道),振幅和相位失真通常会同时存在在传输脉冲序列时,这种失真表现为__弥散或拖尾,因此解调序列中的波形有明显的形变波形重叠或拖尾称为码间串扰,它存在于大部分调制系统中,是在有带宽限制的信道中实现可靠高速传输的主要障碍之一均衡可分为两大类,如图所示第一大类是最大似然估计需要获得对的估计,调整接收机使之适应传输环境这种调整的目的是使检测器根据已失真的解调脉冲序列做出更好的估计采用MLSE接收机,并不是对失真采样进行重新整形或其他的直接补偿,而是调整其本身以更好地处理是真波形第二类是均衡滤波器,即用滤波器补偿__失真在这类均衡器中,解调样值序列经均衡器消除码间串扰(ISI)后输入检测器本节将介绍比较常用的均衡滤波器,这类均衡器还可以进一步的划分一种划分方法是根据滤波器的结构,可分为只含前馈单元的线性系统(横向滤波器)和既有前溃单元又有反馈单元的非线性系统(判决反馈均衡器);也可以根据他们的自适应性分为预置式和自适应式;还可以根据滤波器的分辨率和更新速率进行分类被检测__可以是码元边界的采样值(即每个码元只采样一次),这种采样称为按码元间隔采样;也可以对每个码元做多次采样,这种采样称为按部分码元间隔采样用分离的接收滤波器和均衡器代替式(
3.2)中的接收/均衡滤波器,整个系统的传输函数是升余弦函数,记为,于是有
3.2在实际系统中,不可能充分已知信道的频率传输函数及其冲击响应,因而设计不出在任意时刻都有__间串扰的传输机通常选择相匹配的发送滤波器和接收滤波器,使之满足下列表达式
3.3这样,和的频率传输函数都是生余弦函数的平方根(平方根升余弦函数)用于补偿信道失真的均衡器的传输函数就是信道传输函数的倒数
3.4有时根据需要特意选择在采样点存在码间串扰的频率传输函数(如高斯滤波器传输函数),其目的是提高带宽利用率此时均衡滤波器的任务不只是补偿信道,还要补偿发送滤波器和接收滤波器引起的码间串扰E/NxPForPPxE/N=ForE/NEMBEDEquation.3xPEMBEDEquation.3P10。