移动通信调制技术

AgilentDigitalModulationinCommunicationsSystems—AnIntroductionApplicationNote12982Thisapplicationnoteintroducestheconceptsofdigitalmodulationusedinmanycommunicationssystemstoday.Emphasisisplacedonexplainingthetradeoffsthataremadetooptimizeefficienciesinsystemdesign.Mostcommunicationssystemsfallintooneofthreecategories:bandwidthefficient,powerefficient,orcostefficient.Bandwidthefficiencydescribestheabilityofamodulationschemetoaccommodatedatawithinalimitedbandwidth.Powerefficiencydescribestheabilityofthesystemtoreliablysendinformationatthelowestpracticalpowerlevel.Inmostsystems,thereisahighpriorityonband-widthefficiency.Theparametertobeoptimizeddependsonthedemandsoftheparticularsystem,ascanbeseeninthefollowingtwoexamples.Fordesignersofdigitalterrestrialmicrowaveradios,theirhighestpriorityisgoodbandwidthefficiencywithlowbit-error-rate.Theyhaveplentyofpoweravailableandarenotconcernedwithpowerefficiency.Theyarenotespeciallycon-cernedwithreceivercostorcomplexitybecausetheydonothavetobuildlargenumbersofthem.Ontheotherhand,designersofhand-heldcellularphonesputahighpriorityonpowerefficiencybecausethesephonesneedtorunonabattery.Costisalsoahighprioritybecausecellularphonesmustbelow-costtoencouragemoreusers.Accord-ingly,thesesystemssacrificesomebandwidthefficiencytogetpowerandcostefficiency.Everytimeoneoftheseefficiencyparameters(bandwidth,power,orcost)isincreased,anotheronedecreases,becomesmorecomplex,ordoesnotperformwellinapoorenvironment.Costisadom-inantsystempriority.Low-costradioswillalwaysbeindemand.Inthepast,itwaspossibletomakearadiolow-costbysacrificingpowerandband-widthefficiency.Thisisnolongerpossible.Theradiospectrumisveryvaluableandoperatorswhodonotusethespectrumefficientlycouldlosetheirexistinglicensesorloseoutinthecompetitionfornewones.ThesearethetradeoffsthatmustbeconsideredindigitalRFcommunicationsdesign.Thisapplicationnotecovers•thereasonsforthemovetodigitalmodulation;•howinformationismodulatedontoin-phase(I)andquadrature(Q)signals;•differenttypesofdigitalmodulation;•filteringtechniquestoconservebandwidth;•waysoflookingatdigitallymodulatedsignals;•multiplexingtechniquesusedtosharethetransmissionchannel;•howadigitaltransmitterandreceiverwork;•measurementsondigitalRFcommunicationssystems;•anoverviewtablewithkeyspecificationsforthemajordigitalcommunicationssystems;and•aglossaryoftermsusedindigitalRFcommuni-cations.Theseconceptsformthebuildingblocksofanycommunicationssystem.Ifyouunderstandthebuildingblocks,thenyouwillbeabletounder-standhowanycommunicationssystem,presentorfuture,works.Introduction35567778891010111212121314141515161718192021222223232425262728292930311.WhyDigitalModulation?1.1Tradingoffsimplicityandbandwidth1.2Industrytrends2.UsingI/QModulation(AmplitudeandPhaseControl)toConveyInformation2.1Transmittinginformation2.2Signalcharacteristicsthatcanbemodified2.3Polardisplay—magnitudeandphaserepresentedtogether2.4Signalchangesormodificationsinpolarform2.5I/Qformats2.6IandQinaradiotransmitter2.7IandQinaradioreceiver2.8WhyuseIandQ?3.DigitalModulationTypesandRelativeEfficiencies3.1Applications3.1.1Bitrateandsymbolrate3.1.2Spectrum(bandwidth)requirements3.1.3Symbolclock3.2PhaseShiftKeying(PSK)3.3FrequencyShiftKeying3.4MinimumShiftKeying(MSK)3.5QuadratureAmplitudeModulation(QAM)3.6Theoreticalbandwidthefficiencylimits3.7Spectralefficiencyexamplesinpracticalradios3.8I/Qoffsetmodulation3.9Differentialmodulation3.10Constantamplitudemodulation4.Filtering4.1Nyquistorraisedcosinefilter4.2Transmitter-receivermatchedfilters4.3Gaussianfilter4.4Filterbandwidthparameteralpha4.5Filterbandwidtheffects4.6ChebyshevequirippleFIR(finiteimpulseresponse)filter4.7Spectralefficiencyversuspowerconsumption5.DifferentWaysofLookingataDigitallyModulatedSignalTimeandFrequencyDomainView5.1Powerandfrequencyview5.2Constellationdiagrams5.3Eyediagrams5.4TrellisdiagramsTableofContents432323233333434353536373737383839393940414142434344466.SharingtheChannel6.1Multiplexing—frequency6.2Multiplexing—time6.3Multiplexing—code6.4Multiplexing—geography6.5Combiningmultiplexingmodes6.6Penetrationversusefficiency7.HowDigitalTransmittersandReceiversWork7.1Adigitalcommunicationstransmitter7.2Adigitalcommunicationsreceiver8.MeasurementsonDigitalRFCommunicationsSystems8.1Powermeasurements8.1.1AdjacentChannelPower8.2Frequencymeasurements8.2.1Occupiedbandwidth8.3Timingmeasurements8.4Modulationaccuracy8.5UnderstandingErrorVectorMagnitude(EVM)8.6Troubleshootingwitherrorvectormeasurements8.7Magnitudeversusphaseerror8.8I/Qphaseerrorversustime8.9ErrorVectorMagnitudeversustime8.10Errorspectrum(EVMversusfrequency)9.Summary10.OverviewofCommunicationsSystems11.GlossaryofTermsTableofContents(continued)5Themovetodigitalmodulationprovidesmoreinformationcapacity,compatibilitywithdigitaldataservices,higherdatasecurity,betterqualitycommunications,andquickersystemavailability.Developersofcommunicationssystemsfacetheseconstraints:•availablebandwidth•permissiblepower•inherentnoiselevelofthesystemTheRFspectrummustbeshared,yeteverydaytherearemoreusersforthatspectrumasdemand