电力系统通信基础DigitalandAnalogCommunicationSystemmessage:words,pictures,voices,valueofvoltage,current…Electricsignal:representingmessage.Signal’sparameterschangeaccordingtomessage.SineWave(正弦波):amplitude(幅值),frequency,phaseoffset(相移).Pulsewave(脉冲波):amplitude,period,markspaceratio(占空比)=marktime/spacetimedutycycle=(hightime/period)*100%.continuousmessage:voices…discretemessage:words,statusofcircuitbreakers…Analogsignaldigitalsignal.Transmitter:dealingwiththesourcem(t)formoreefficienttransmission.Forexample:signalprocessing:analoglowpassfilter(低通滤波器)inanalogcommunicationsystemtorestrictthebandwidth(带宽)ofm(t).analog-to-digitalconverter(模数转换器),usedtotransmitananaloginformationthroughadigitalcommunicationsystem---sourcecoding(信源编码)errorcorrectcoding(差错控制编码)tothedigitalwordtoachieveerrordetectionandcorrectioncausedbynoise---channelcoding(信道编码)carrier(载波)circuits:convertsthebasebandsignal(基带信号)intoafrequencybandsignal(频带信号)thatisappropriateforthetransmissionmedium(传输媒质).--modulation(调制)anddemodulation(解调).Transmissionmedium(channel):twisted-pairtelephonelines:双绞线coaxialcables:同轴电缆fiberopticcables:光纤wireless无线Noise:naturalelectricaldisturbances(e.g.lightning)man-madenoise(e.g.electromagneticinterference)GeneraldigitalcommunicationsystemComparisonbetweenDigitalandAnalogCommunicationsystemErrorscanbedetectedandcorrectedbytheuseofcoding(编码).Datafromvoice,video,anddatasourcesmaybemergedandtransmittedoveracommondigitaltransmissionsystem.Privacyispreservedbyusingdataencryption(加密).Relativelyinexpensivedigitalcircuitsmaybeused.Disadvantages:Generally,morebandwidth(带宽)isrequiredthanthatforanalogsystems.Synchronization(同步)isrequired.CommunicationModeschannelInformationMeasureHowmanyinformationdoesamessagehave?Definition:TheSelf-Informationofthemessagejisgivenbywhere,Pjistheprobabilityoftransmittingthejthmessage.IjiscalledSelf-InformationofeventjTheunitof“I”:ifthebaseofthelogarithmis2,theunitsare“bits”;比特ifthebaseise,theunitsare“nats”;ifthebaseis10,theunitsare“h