英语 电子信息工程7

Unit7DigitalModulationUnit7DigitalModulationPassageADigitalModulationPassageBBinaryModulatedBandpassSignalingPassageCMultilevelModulatedBandpassSignalingUnit7DigitalModulationPassageADigitalModulation1.IntroductionModerncommunicationsystemsusedigitalmodulationtechniques.Advancementsinverylargeintegration(VLSI)anddigitalsignalprocessing(DSP)technologyhavemadedigitalmodulationlesscostthananalogtransmissionsystems.Digitalmodulationoffersmanyadvantagesoveranalogmodulation.Someadvantagesincludegreaternoiseimmunityandrobustnesstochannelimpairments,easiermultiplexingofvariousformsofinformation(e.g.,voice,data,andvideo),andgreatersecurity.Furthermore,digitaltransmissionsaccommodatedigitalerror-controlcodesthatdetectand/orcorrecttransmissionerrors,andsupportcomplexsignalconditioningandprocessingtechniquessuchassourcecoding,encryption,andequalizationtoimprovetheperformanceoftheoverallcommunicationlink.Unit7DigitalModulationNewmultipurposeprogrammabledigitalsignalprocessorshavemadeitpossibletoimplementdigitalmodulatorsanddemodulatorscompletelyinsoftware.Insteadofhavingaparticularmoderndesignpermanentlyfrozenashardware,embeddedsoftwareimplementationsnowallowalterationsandimprovementswithouthavingtoredesignorreplacethemodem.Unit7DigitalModulationIndigitalcommunicationsystems,themodulatingsignal(e.g.,message)mayberepresentedasatimesequenceofsymbolsorpulses,whereeachsymbolhasmfinitestates.Eachsymbolrepresentsnbitsofinformation,wheren=log2mbits/symbol.［1］Manydigitalmodulationschemesareusedinmoderncommunicationsystems,andmoreandmorearesuretobeintroduced.Someofthesetechniqueshavesubtledifferencesbetweenoneanother,andeachtechniquebelongstoafamilyofrelatedmodulationmethods.Unit7DigitalModulation2.FactorsThatInfluencetheChoiceofDigitalModulationSeveralfactorsinfluencethechoiceofadigitalmodulationscheme.Adesirablemodulationschemeprovideslowbiterrorratesatlowreceivedsignal-to-noiseratios,performswellinmultipathandfadingconditions,occupiesaminimumofbandwidth,andiseasyandcost-effectivetoimplement.Existingmodulationschemesdonotsimultaneouslysatisfyalloftheserequirements.Somemodulationschemesarebetterintermsofthebiterrorrateperformance,whileothersarebetterintermsofbandwidthefficiency.Dependingonthedemandsoftheparticularapplication,tradeoffsaremadewhenselectionadigitalmodulation.Unit7DigitalModulationTheperformanceofamodulationschemesisoftenmeasuredintermsofitspowerefficiencyandbandwidthefficiency.Powerefficiencydescribestheabilityofamodulationtechniquetopreservethefidelityofthedigitalmessageatlowpowerlevels.Inadigitalcommunicationsystem,inordertoincreasenoiseimmunity,itisnecessarytoincreasethesignalpower.However,theamountbywhichthesignalpowershouldbeincreasedtoobtainacertainleveloffidelity(i.e.,anacceptablebiterrorprobability)dependsontheparticulartypeofmodulationemployed.Thepowerefficiency,ηp(sometimescalledenergyefficiency)ofadigitalmodulationschemeisameasureofhowfavorablythistradeoffbetweenfidelityandsignalpowerismade,andisoftenexpressedastheratioofthesignalenergyperbittonoisepowerspectraldensity(Eb/No)requiredatthereceiverinputforacertainprobabilityoferror(say10-5).［2］Unit7DigitalModulationBandwidthefficiencydescribestheabilityofamodulationschemetoaccommodatedatawithinalimitedbandwidth.Ingeneral,increasingthedatarateimpliesdecreasingthepulse-widthofadigitalsymbol,whichincreasesthebandwidthofthesignal.Thus,thereisanunavoidablerelationshipbetweendatarateandbandwidthoccupancy.However,somemodulationschemesperformbetterthantheothersinmakingthistrade-off.Bandwidthefficiencyreflectshowefficiencytheallocatedbandwidthisutilizedandisdefinedastheratioofthethroughputdatarateperhertzinagivenbandwidth.IfRisthedatarateinbitspersecond,andBisthebandwidthoccupiedbythemodulationRFsignal,thenbandwidthefficiencyηBisexpressedas(7.1)HzBRB/bpsUnit7DigitalModulationThesystemcapacityofadigitalcommunicationsystemisdirectlyrelatedtothebandwidthefficiencyofthemodulationschemes,sinceamodulationwithagreatervalueofηBwilltransmitmoredatainagivenspectrumallocation.Thereisafundamentalupperboundonachievablebandwidthefficiency.Shannon’schannelcodingtheoremstatesthatforanarbitrarilysmallprobabilityoferror,themaximumpossiblebandwidthefficiencyislimitedbythenoiseinthechannel,andisgivenbythechannelcapacityformula.NotethatShannon’sboundappliesforAWGNnon-fadingchannels.(7.2)WhereCisthechannelcapacity(inbps),BistheRFbandwidth,andS/Nisthesignal-to-noiseratio.NSBC1log2Unit7DigitalModulationInthedesignofadigitalcommunicationsystem,veryoftenthereisatradeoffbetweenbandwidthefficiencyandpowerefficiency.Whilepowerandbandwidthefficiencyconsiderationsareveryimportant,otherfactorsalsoaffectthechoiceofadigitalmodulationschemes.Forexample,forallpersonalcommunicationsystemswhichservealargeusercommunity,thecostandcomplexityofthesubscriberreceivermustbeminimized,andamodulationwhichissimpletodetectismostattractive.TheperformanceofthemodulationschemeundervarioustypesofchannelimpairmentssuchasRayleighandRiceanfadingandmulitpathtimedispersion,givenaparticulardemodulatorimplementa