《电子信息类专业英语 》课件-第7章

Unit7Digital

ModulationPassageADigitalModulationPassageBBinaryModulatedModulationPassageCMultiplexing

PassageADigitalModulation

1.Introduction

Moderncommunicationsystemsusedigitalmodulationtechniques.Advancementsinverylargeintegration(VLSI)anddigitalsignalprocessing(DSP)technologyhavemadedigitalmodulationlesscostthananalogtransmissionsystems.Digitalmodulationoffersmanyadvantagesoveranalogmodulation.Someadvantagesincludegreaternoiseimmunityandrobustnesstochannelimpairments,easiermultiplexingofvariousformsofinformation(e.g.,voice,data,andvideo),andgreatersecurity.

Furthermore,digitaltransmissionsaccommodatedigitalerror-controlcodesthatdetectand/orcorrecttransmission

errors,andsupportcomplexsignalconditioningandprocessingtechniquessuchassourcecoding,encryption,andequalizationtoimprovetheperformanceoftheoverallcommunicationlink.Newmultipurposeprogrammabledigitalsignalprocessorshavemadeitpossibletoimplementdigitalmodulatorsanddemodulatorscompletelyinsoftware.Insteadofhavingaparticularmoderndesignpermanentlyfrozenashardware,embeddedsoftwareimplementationsnowallowalterationsandimprovementswithouthavingtoredesignorreplacethemodem.

Indigitalcommunicationsystems,themodulatingsignal(e.g.,message)maybe

representedasatimesequenceofsymbolsorpulses,whereeachsymbolhasmfinitestates.Eachsymbolrepresentsnbitsofinformation,wheren=log2m

bits/symbol.［1］

Manydigitalmodulationschemesareusedinmoderncommunicationsystems,andmoreandmorearesuretobeintroduced.Someofthesetechniqueshavesubtledifferences

betweenone

another,andeachtechniquebelongstoafamilyofrelatedmodulationmethods.

2.FactorsThatInfluencetheChoiceofDigitalModulation

Severalfactorsinfluencethechoiceofadigitalmodulationscheme.Adesirablemodulationschemeprovideslowbiterrorratesatlowreceivedsignal-to-noiseratios,performswellinmultipathandfadingconditions,occupiesaminimumofbandwidth,andiseasyandcost-effectivetoimplement.Existingmodulationschemesdonotsimultaneouslysatisfyalloftheserequirements.Somemodulationschemesarebetterintermsofthebiterrorrateperformance,whileothersarebetterintermsofbandwidthefficiency.Dependingonthedemandsoftheparticularapplication,tradeoffsaremadewhenselectionadigitalmodulation.

Theperformanceofamodulationschemesisoftenmeasuredintermsofitspowerefficiencyandbandwidthefficiency.Powerefficiencydescribestheabilityofamodulationtechniquetopreservethefidelityofthedigitalmessageatlowpowerlevels.Inadigitalcommunicationsystem,inordertoincreasenoiseimmunity,itisnecessarytoincreasethesignalpower.However,theamountbywhichthesignalpowershouldbeincreasedto

obtainacertainleveloffidelity(i.e.,anacceptablebiterrorprobability)dependsontheparticulartypeofmodulationemployed.

Thepowerefficiency,

ηp(sometimescalledenergyefficiency)ofadigitalmodulationschemeisameasureofhowfavorablythistradeoff

betweenfidelityandsignalpowerismade,andisoftenexpressedastheratioofthesignalenergyperbittonoisepowerspectraldensity(Eb/No)requiredatthereceiverinputforacertainprobabilityoferror(say10-5).［2］

Bandwidthefficiencydescribestheabilityofamodulationschemetoaccommodate

datawithinalimitedbandwidth.Ingeneral,increasingthedatarateimpliesdecreasingthepulse-widthofadigitalsymbol,whichincreasesthebandwidthofthesignal.Thus,thereisanunavoidablerelationshipbetweendatarateandbandwidthoccupancy.However,somemodulationschemesperformbetterthantheothersinmakingthistrade-off.Bandwidthefficiencyreflectshowefficiencytheallocatedbandwidthisutilizedandisdefinedastheratioofthethroughputdatarateperhertzinagivenbandwidth.

IfRisthedatarateinbitspersecond,andBisthebandwidthoccupiedbythemodulationRFsignal,thenbandwidthefficiencyηBisexpressedas

(7.1)

Thesystemcapacityofadigitalcommunicationsystemisdirectlyrelatedtothebandwidthefficiencyofthemodulationschemes,sinceamodulationwithagreatervalueofηBwilltransmitmoredatainagivenspectrumallocation.

Thereisafundamentalupperboundonachievablebandwidthefficiency.Shannon’schannelcodingtheoremstatesthatforanarbitrarilysmallprobabilityoferror,themaximumpossiblebandwidthefficiencyislimitedbythenoiseinthechannel,andisgivenbythechannelcapacityformula.NotethatShannon’sboundappliesforAWGNnon-fadingchannels.

(7.2)

WhereCisthechannelcapacity(inbps),BistheRFbandwidth,andS/Nisthesignal-to-noiseratio.

Inthedesignofadigitalcommunicationsystem,veryoftenthereisatradeoffbetweenbandwidthefficiencyandpowerefficiency.

Whilepowerandbandwidthefficiencyconsiderationsareveryimportant,otherfactorsalsoaffectthechoiceofadigitalmodulationschemes.Forexample,forallpersonalcommunicationsystemswhichservealargeusercommunity,thecostandcomplexityofthesubscriberreceivermustbeminimized,andamodulationwhichissimpletodetectismostattractive.Theperformanceofthemodulationschemeundervarioustypesofchannel

impairmentssuchasRayleighandRiceanfadingandmulitpathtimedispersion,givenaparticulardemodulatorimplementation,isanotherkeyfactorinselectingamodulation.

Notes

[1]Instead

of

having

aparticula

rmodern

design

permanently

frozen

as

hardware,theembedded

software

implementations

nowallowalterations

and

improvementswithouthaving

to

redesign

or

replace

the

Modem.调制解调器用软件实现,而不是专门设计定制好的硬件,在更新和升级的时候就不需要重新设计或者更换Modem。

·调制解调器的功能就是将数字信号和模拟信号这两者进行互相转换。

［2］Indigitalcommunicationsystems,themodulatingsignal(e.g.,message)mayberepresentedasatimesequenceofsymbolsorpulses,whereeachsymbolhasmfinitestates.Eachsymbolrepresentsnbitsofinformation,wheren=log2mbits/symbol.

在数字通信系统中,调制信号(比如消息)用符号或者脉冲的时间序列来表示,每一个符号有m个有限状态。每个符号表示n比特的信息,这里n＝log2m比特/符号。

·在数字通信中,经常会提到波特率(baud/symbolrate)和比特率(bitrate)两个术语。波特率又称为符号率,即每秒传输的符号数,而比特率是每秒传输的比特数,两者之间即是这里描述的n倍关系。

［3］Thepowerefficiency,ηp(sometimescalledenergyefficiency)ofadigitalmodulationschemeisameasureofhowfavorablythistradeoffbetweenfidelityandsignalpowerismade,andisoftenexpressedastheratioofthesignalenergyperbittonoisepowerspectraldensity(Eb/No)requiredatthereceiverinputforacertainprobabilityoferror(say10-5).

数字调制方法的功率效率ηp(有时也称为能量效率)用来衡量精确度和信号功率之间平衡的好坏程度,通常以接收机的输入满足给定误码率(比如10-5)时,单位比特信号能量与噪声的功率谱密度之间的比值(Eb/No)来表示功率效率。

·biterrorrates的意思是误码率,简记为BER。signal-to-noiseratio的意思是信噪比,常记作SNR。BER一般用来评价一个数字通信系统的好坏,而SNR常用来评价模拟通信系统。

·signalenergyperbittonoisepowerspectraldensity(Eb/No)指的是每比特的信号能量与噪声的功率谱密度之间的比值,Eb/No是计算误比特率时的关键参数,通常误比特率曲线以Eb/No为横坐标画出。

[4]Ingeneral,increasingthedatarateimpliesdecreasingthepulse-widthofadigitalsymbol,whichincreasesthebandwidthof

thesignal.Thus,thereisanunavoidablerelationshipbetweendatarateandbandwidthoccupancy.

通常,提高数据速率意味着降低数字符号的脉宽,也就增加了信号的带宽。因此,在数据速率和所占带宽之间存在不可分割的联系。

·Ingeneral的意思是“通常;大体上;一般而言,总的来说

”。

·unavoidablerelationship指的是不可分割的联系。

[5]TheperformanceofmodulationschemeundervarioustypesofchannelimpairmentssuchasRayleigh,Riceanfadingandmultipathtimedispersion,givenaparticulardemodulatorimplementation,isanotherkeyfactorinselectingamodulation.

在具有瑞利衰落、莱斯衰落和多径时延扩展的不同信道损伤下,调制方法的性能不同将决定解调器的实现,这是影响调制方法选择的另一个关键因素。

·Demodulator的意思是“解调器;检波器”。

·variousof指的是各种。

·所谓瑞利衰落,是指在无线通信信道中,由于信号进行径传播达到接收点处的场强来自不同传播的路径,各条路径延时时间是不同的,而各个方向分量波的叠加,又产生了驻波场强,从而形成了信号快衰落。

Exercises

1.PleasetranslatethefollowingphrasesintoEnglish.

(1)超大规模集成电路(2)数字信号处理

(3)数字通信系统(4)误码率

(5)信道编码理论(6)个人通信系统

(7)多径时延扩展

2.PleasetranslatethefollowingphrasesintoChinese.

(1)digitalsignalprocessing(DSP)

(2)digitalmodulation

(3)analogmodulation

(4)error-controlcodes

(5)pulse-width

(6)AWGN

(7)non-fadingchannel

(8)RadioFrequency

3.Answerthequestions.

(1)Listtheadvantagesofdigitalmodulationmentionedinthispassage.

(2)Howmanyfactorsinfluencethechoiceofadigitalmodulationscheme?

(3)Howcanwemeasuretheperformanceofamodulationschemes？

PassageBBinaryandMultilevelModulation

DigitallymodulatedbandpasssignalsaregeneratedbyusingthecomplexenvelopesforAM,PM,orQM(quadraturemodulation)signalingthatwerestudiedinprevioussections.Fordigitalmodulatedsignals,themodulatingsignalm(t)isadigitalsignalgivenbythebinaryormultilevellinecodes.Inthissection,detailsofbinarymodulatedsignalsaregiven.Andmultilevelandminimum-shift-keyed(MSK)digitallymodulatedsignalsare

described.

Themostcommonbinarybandpasssignalingtechniques,illustratedinFigure7.1,areasfollows:

Figure7.1BandpassDigitallyModulatedSignals

On-offkeying(OOK),alsocalledamplitudeshiftkeying(ASK),whichconsists

ofkeying(switching)acarriersinusoidonandoffwithaunipolarbinarysignal.OOKisidenticaltounipolarbinarymodulationonaDSB-SCsignal.Morsecoderadiotransmissionisanexampleofthistechnique.［1］

OOKwasoneofthefirstmodulationtechniquestobeusedandprecedesanalogcommunicationsystem.

Binaryphase-shiftkeying(BPSK),whichconsistsofshiftingthephaseofasinusoidalcarrier0°or180°withaunipolarbinarysignal.BPSKisequivalenttoPMsignalingwithadigitalwaveformandisalsoequivalenttomodulatingaDSB-SCsignalwithapolardigitalwaveform.

Frequency-shiftkeying(FSK),whichconsistsofshiftingthefrequencyofasinusoidalcarrierfromamarkfrequency(corresponding,forexample,tosendingabinary1)toaspacefrequency(correspondingtosendingabinary0),accordingtothebasebanddigitalsignal.FSKisidenticaltomodulatinganFMcarrierwithabinarydigitalsignal.

Thebandwidthofthedigitalsignalneedstobeminimizedtoachievespectralconservation.Thismaybeaccomplishedbyusingapre-modulationraisedcosine-rollofffiltertominimizethebandwidthofthedigitalsignalandyetnotintroduceISI［2］.Thus,theBPSKsignalofFigure7.1(d)becomesaDSB-SCsignal(Figure7.1(f))whenpre-modulationfilterisused.

BinaryPhase-ShiftKeying(BPSK)

TheBPSKsignalisrepresentedby

s(t)=Accos［ωct+Dpm(t)］(7.3)wherem(t)isapolarbasebanddatasignal.Forconvenience,letm(t)havepeakvalueof±1andarectangularpulseshape.

WenowshowthatBPSKisalsoaformAM-typesignaling.Weget

s(t)=Accos[Dpm(t)]cosωct-Acsin[Dpm(t)]sinωct

(7.4)

Recallingthatm(t)hasvaluesof±1andthatcosxandsinxareevenandoddfunctionsofx,weseethattherepresentationoftheBPSKsignalreducesto

(7.5)Thelevelofthepilotcarriertermissetbythevaluethepeakdeviation,Δθ=Dp.

Fordigitalangle-modulatedsignals,thedigitalmodulationindexhisdefinedby

(7.6)where2Δθisthemaximumpeak-to-peakphasedeviation(radians)duringthetime

requiredtosendonesymbol,Ts.Forbinarysignaling,thesymboltimeisequaltothebittime(Ts=Tb).

Thelevelofthepilotcarriertermissetbythevalueofthepeakdeviation,whichisΔθ=Dpform(t)=±1.IfDpissmall,thepilotcarriertermhasarelativelylargeamplitudecomparedtothedataterm;consequently,thereisverylittlepowerinthedataterm(whichcontainsthesourceinformation).Tomaximizethesignalingefficiency(sothatthereisalowprobabilityoferror),thepowerinthedatatermneedstobemaximized.ThisisaccomplishedbylettingΔθ=Dp=90°=π/2radians,whichcorrespondstoadigitalmodulationindexofh=1.Forthisoptimumcaseofh=1,theBPSKsignalbecomes

s(t)=-Acm(t)sinωct(7.7)

Throughoutthetext,weassumethatΔθ=90°,h=1,isusedforBPSKsignaling(unlessotherwisestated).Equation(7.7)showsthatBPSKisequivalenttoDSB-SCsignalingwithapolarbasebanddatawaveform.ThecomplexenvelopeforthisBPSKsignalis

g(t)=jAcm(t)

(7.8)WeobtainthePSDforthecomplexenvelope,viz.

(7.9)

wherem(t)hasvaluesof±1,sothats(t)hasanaveragenormalizedpowerof

.ThePSDforthecorrespondingBPSKsignalisreadilyevaluatedbytranslatingthebasebandspectrumtothecarrierfrequency.TheresultingBPSKspectrumisshowninFigure7.2.Thenull-to-nullbandwidthforBPSKinalso2R,thesameasthatfoundforOOK.

Figure7.2PSDofBPSKSignal

TodetectBPSK,synchronousdetectionmustbeused,asillustratedinFigure7.3(a).SincethereisnodiscretecarriertermintheBPSKsignal,aPLLmaybeusedtoextractthecarrierreferenceonlyifalow-levelpilotcarrieristransmittedtogetherwiththeBPSKsignal.Otherwise,aCostaslooporasquaringloopmaybeusedtosynthesizethecarrierreferencefromthisDSB-SC(i.e.,BPSK)signalandtoprovidecoherentdetection.

Figure7.3DetectionofBPSKandDPSK

ForoptimumdetectionofBPSK(i.e.,thelowestpossibleBERforthecaseofAWGN),thelow-passfilterinFigure

7.3(a)isreplacedbyanintegrate-and-dumpmatchedfilterprocessing,where

VTissetto0VforthecaseofBPSK.

Frequency-ShiftKeying(FSK)

TheFSKsignalcanbecharacterizedasoneoftwodifferenttypes,dependingonthemethodusedtogenerateit.Onetypeisgeneratedbyswitchingthetransmitteroutputlinebetweentwodifferentoscillators,asshowninFigure7.4(a).Thistypegeneratesanoutputwaveformthatisdiscontinuousattheswitchingtimes.Itiscalleddiscontinuous-phaseFSK,becauseθ(t)isdiscontinuousattheswitchingtimes.Thediscontinuous-phaseFSKsignalisrepresentedby

wheref1iscalledthemark(binary1)frequencyandf2iscalledthespace(binary0)frequency.θ1andθ2arethestart-upphasesofthetwooscillators.Thediscontinuousphasefunctionis

SinceFSKtransmittersarenotbuiltthisway,wewillturntothesecondtype,showninFigure7.4(b).

Figure7.4GenerationofFSK

Thecontinuous-phaseFSKsignalisgeneratedbyfeedingthedatasignalintoa

frequencymodulator,asshowninFigure7.4(b).ThisFSKsignalisrepresentedby

or

(7.12a)where(7.12b)(7.12c)andm(t)

isabasebandsignal.Althoughm(t)isdiscontinuousattheswitchingtime,thephasefunctionθ(t)

iscontinuousbecauseθ(t)isproportionaltotheintegralofm(t)

.Iftheserialdatainputwaveformisbinary,suchasapolarbasebandsignal,theresultingFSKsignaliscalledabinaryFSKsignal.Ofcourse,amultilevelinputsignalwould

produceamultilevelFSKsignal.

Figure7.516-symbolQANConstellations(FourLevelsPerDimension)

whereD=R/rand(xn,yn)denotes

one

of

the

permitted(xi,yi)

valuesduringthesymboltimethatiscenteredont=nTs=n/Ds(ittakesTsstosendeachsymbol).h1(t)isthepulse

shapethatisusedforeachsymbol.IfthebandwidthoftheQAMsignalisnottoberestricted,thepulseshapewillberectangularandofTssduration.Insomeapplications,thetimingbetweenthex(t)andy(t)componentsisoffsetbyTs/2=1/(2D)s.Thatis,x(t)wouldbedescribedbyEquation(7.15),andtheoffsetwouldbedescribedby

Notes

［1］Morsecoderadiotransmissionisanexampleofthistechnique.

莫尔斯码无线传输就是这种调制技术应用的一个实例。

·Morse即莫尔斯·塞缪尔·芬利·布里斯(1791-1872),美国画家和发明家,也是一名肖像画家,他改变了电报并获得专利,发展了以他的名字命名的电报码(MorseCode)。

［2］Thebandwidthofthedigitalsignalneedstobeminimizedtoachievespectralconservation.Thismaybeaccomplishedbyusingapre-modulationraisedcosine-rollofffiltertominimizethebandwidthofthedigitalsignalandyetnotintroduceISI.

数字信号的带宽应最小化以满足频谱要求。为此,可采用预调制升余弦滚降滤波器使数字信号的带宽最小化,这不会带来码间串扰。

·ISI是intersymbolinterference的缩写,意思是码间干扰。

Exercises

1.PleasetranslatethefollowingphrasesintoEnglish

(1)正交调制(2)开关键控

(3)幅移键控(4)频移键控

(5)空号频率(6)预调制

(7)导频载波(8)科斯塔斯环

(9)相干检测(10)差分编码

(11)星座图(12)正交平衡调制器

2.PleasetranslatethefollowingphrasesintoChinese.

(1)minimum-shift-keyed(MSK)

(2)binaryphase-shiftkeying(BPSK)

(3)raisedcosine-rollofffilter

(4)markfrequency

(5)cosine-rollofffilter

(6)synchronousdetection

(7)squaringloop

(8)digitalmodulationindex

(9)digital-to-analogconverter(DAC)

(10)integrate-and-dumpmatchedfilter

(11)quadratureamplitudemodulation(QAM)

3.Answerthequestions.

(1)Listthebinarybandpasssignalingtechniquesmentionedinthispassage.

(2)WhatisBPSK?

(3)HowisBPSKdetected?

(4)WhatisoptimumdetectionofBPSK?

(5)WhatisFSK?ListthetwodifferenttypesofFSK.

(6)Can

you

identify

what

the

difference

are

upon

the

signal

constellation

between

QAMandMPSK?

PassageCMultiplexing

Intelecommunicationsandcomputernetworks,multiplexing(sometimescontractedtomixing)isamethodbywhichmultipleanalogmessagesignalsordigitaldatastreamsarecombinedintoonesignaloverasharedmedium.Theaimistoshareanexpensiveresourcesuchasthespectrum.Adevicethatperformsthemultiplexingiscalledamultiplexer(MUX),andadevicethatperformsthereverseprocessiscalledademultiplexer(DEMUXorDMX).

Inversemultiplexing(IMUX)hastheoppositeaimasmultiplexing,namelytobreakonedatastreamintoseveralstreams,transferthemsimultaneouslyoverseveralcommunicationchannels,andrecreatetheoriginaldatastream.

OrthogonalFrequencyDivisionMultiplexing

(OFDM)isamultiplexingtechniquethatsubdividestheavailablebandwidthintomultipleorthogonalfrequencysub-carriers.Theinput

datastreamisdividedintoseveralparallelsub-streamsofreduceddatarate(thusincreasedsymbolduration)andeachsub-streamistransmittedonaseparateorthogonalsub-carrier.

TheincreasedsymboldurationimprovestherobustnessofOFDMtochanneldelayspread.Furthermore,theintroductionofthecyclicprefix(CP)cancompletelyeliminateISIaslongas

theCPdurationislongerthanthechanneldelayspread.TheCPistypicallyarepetitionofthelastsamplesofdataportionoftheblockthatisappendedtothebeginningofthedatapayload.TheCPpreventsinter-blockinterferenceandmakesthechannelappearcircularandpermitslow-complexityfrequencydomainequalization.

OFDMmodulationcanberealizedwithefficientInverseFastFourierTransform(IFFT),whichenablesalargenumberofsub-carriers(upto2048)withlowcomplexity.OFDMallowsexploitingfrequencydiversityofthemultipathchannelbycodingandinterleavingtheinformationacrossthesub-carriers

priortotransmission.InanOFDMsystem,resourcesareavailableinthetimedomainbymeansofOFDMsymbolsandinthefrequencydomainbymeansofsub-carriers.Thetimeandfrequencyresourcescanbeorganizedintosub-channelsforallocationtoindividualusers.

OFDMAisamultiple-access/multiplexingschemethatprovidesmultiplexingoperationofuserdatastreamsontothedownlinksub-channelsanduplinkmultipleaccessbymeansofuplinksub-channels.Insomeliterature,downlinkmultiplexingiscalledOFDM.Inthispaper,weusethenotion“downlinkOFDMA”fordownlinkmultiplexingand“uplinkOFDMA”foruplinkmultipleaccess.Thesimilarnotionsarealsoappliedto

TDMAandCDMA.

ScalabilityisoneofthemostimportantadvantagesofOFDMA.WiththeOFDMAsubcarrierstructure,itcansupportawiderangeofbandwidth.Table7.1showsthescalableOFDMAparametersof802.16e.ThescalabilityisachievedbyadjustingFFTsizetochannelbandwidthwhilefixingthesub-carrierfrequencyspacingto10.94kHz.Byfixingthesubcarrierspacingandsymbolduration,thebasicunitofphysical(timeandfrequency)

resourceisfixed.Therefore,theimpacttohigherlayersisminimalwhenscalingthebandwidth.