通信原理(英文版)课件

PrinciplesofCommunications1PrinciplesofCommunications1Chapter1Introduction1.1HistoricalReviewofCommunicationOriginofancientcommunicationTwomodesofcommunicationDevelopmentofmoderncommunication2Chapter1Introduction21.2Message,information&signalMessage：speech,letters,figures,images…Information：effectivecontentofmessage.DifferenttypesofmessagesmaycontainthesameinformationSignal：thecarrierofmessage Whattransmittedinacommunicationsystemissignal. 31.2Message,information&sigMeasurementofinformation： #“quantityofmessage”informationcontent #Ex:“Rainfallwillbe1mmtomorrow”–informationcontentsmall

“Rainfallwillbe1mtomorrow”–informationcontentlarge “Thesunwillriseintheeasttomorrowmorning”–informationcontentequalszero

#Informationcontent

I=I[P(x)]，P(x)–Occurrenceprobability

#Definition：I=loga

[1/P(x)]=-logaP(x)

#Usually,set

a=2,theunitoftheinformationcontentwillbecalledabit.

#Foranequalprobabilitybinarysymbol:

I=log2[1/P(x)]=log2[1/(1/2)]=1bit4Measurementofinformation：4#ForanequalprobabilityM-arysymbol:

I=log2[1/P(x)]=log2[1/(1/M)] =log2

M bit

If

M=2k，then

I=kbit5#ForanequalprobabilityM-a1.3DigitalCommunication 1.3.1BasicconceptTwocategoriesofsignals

Analogsignal：Itsvoltageorcurrentcanbeexpressedbyacontinuousfunctionoftime.Forexample,speechsignal.

Digitalsignal：Itsvoltageorcurrentcanonlytakefinitenumberofdiscretevalues.Forexample,digitalcomputerdatasignal.61.3DigitalCommunication6AnalogSignal&DigitalSignalAnalogsignalsDigitalsignalsts(t)ts(t)s(t)tSymbolts(t)7AnalogSignal&DigitalSignalTwokindsofcommunicationsystems

Analogcommunicationsystem Requirement－Highfidelity Criterion－Signaltonoiseratio Basicissue－parameterestimation Digitalcommunicationsystem Requirement－correctdecision Criterion－Errorprobability Basicissur－statisticaldecisiontheory8Twokindsofcommunicationsys1.3.2AdvantagesofDigitalCommunicationFinitenumberofpossiblevaluesofsignalsCorrectdecisionmaybeachieved

Fig.1.3.2Distortionandrestorationofdigitalsignalwaveforms(a)Waveformsofdistoreddigitalsignal(b)Waveformsofdigitalsignalaftershaping91.3.2AdvantagesofDigitalCoErrorcorrectingtechniquescanbeused.Digitalencryptioncanbeused.Differentkindsofanalog&digitalmessagecanbeintegratedtotransmitDigitalcommunicationequipment:DesignandmanufactureareeasierWeight&volumearesmallerDigitalsignalcanbecompressedbysourcecodingtoreduceredundency.OutputS/Nincreaseswithbandwidthaccordingtoexponentiallaw.10ErrorcorrectingtechniquescaDigitalcommunicationsystemmodelTransmitterReceiverInformationsourceChannelcodingModulationChannelCompressioncodingDemodulationInformationdestinationEncryptiondecodingChanneldecodingCompressiondecodingEncryptioncodingNoiseSynchro-nizationSourcecodingSourcedecoding1.3.3DigitalCommunicationSystemModel11DigitalcommunicationsystemmAnalogcommunicationsystemmodelTransmitterReceiverNoiseModulationChannelDemodulationInformationdestinationInformationsource12Analogcommunicationsystemmo1.3.4SpecificationsofDigitalCommunicationSystemsRelationshipbetweenefficiency&reliability(rate~accuracy)Transmissionrate：Symbolrate:RB－BaudInformationrate:Rb－bit/second ForM-arysystem：Rb=RBlog2

MMessagerate:RM Errorprobability：SymbolerrorprobabilityPe

=numberofreceivedsymbolsinerror/totalnumberoftransmittedsymbols131.3.4SpecificationsofDigitaBiterrorprobabilityPb=numberofreceivedbitsinerror/totalnumberoftransmittedbitsWorderrorprobabilityPw=numberofreceivedwordsinerror/totalnumberoftransmittedwordsRelationshipbetweensymbolerrorprobabilityandbiterrorprobabilityPb=PexM/[2(M-1)]Pe/214BiterrorprobabilityPb=numRelationshipbetweenworderrorprobabilityandbiterrorprobability Forbinarysystem, Ifawordisconsistedofkbits,then Pw＝1–(1–Pe)k

UtilizationfactoroffrequencybandUtilizaitionfactorofenergy15Relationshipbetweenworderro1.4Channel

1.4.1WirelesschannelOriginofwirelesscommunicationRequirementofelectromagneticwaveemissiononwavelengthDivisionoffrequencyband(wavelength)161.4Channel 16Divisionoffrequencyband

Frequency

Name Typicalapplication

band(kHz)3–30VerylowfrequencyLong-distancenavigation, (VLF) Underwatercomm.Sonar30–300Lowfrequency Navigation,underwatercomm. (LF) radiobeaconing300–3000 MediumfrequencyBroadcasting,maritimecomm. (MF) direction-finding,distress calling,coastguard17DivisionoffrequencybandFreDivisionoffrequencyband

Frequency

Name Typicalapplication

band(MHz)3–30 Highfrequency Long-distancebroadcasting,telegraph, (HF) telephone,fax,searchandlifesaving, comm.betweenaircrafts&ships,and betweenship&coast,amateurradio 30–300 VeryhighfrequencyTV,FMbroadcasting,landtraffic,air (VHF) traffic,control,taxi,police,navigation, aircraftcommunication300–3000 Ultrahighfrequency TV,cellularphonenetwork,microwave (UHF) link,,radiosounding,navigation, satellitecommunication,GPS, surveillanceradar,radioaltimeter18DivisionoffrequencybandFreDivisionoffrequencyband

Frequency Name Typicalapplicationband(GHz)3–30SuperhighfrequencySatellitecomm.,radioaltimeter, (SHF) microwavelink,aircraftradar, meteorologicalradar,public landvehiclecommunication30–300 Extremelyhigh Radarlandingsystem,satellite frequency(EHF) comm.,vehiclecomm.,railway traffic300–3000 Submillimeterwave Experiment,notdesignated （0.1–1mm）19DivisionoffrequencybandFreDivisionoffrequencyband

Frequency Name Typicalapplicationband(THz)

43–430 Infrared

Opticalcommunication

(7–0.7m)

430–750Visiblelight

Opticalcommunication

(0.7–0.4m)

750–3000 Ultraviolet Opticalcommunication (0.4–0.1m)Note:kHz=103Hz,MHz=106Hz,GHz=109Hz, THz=1012Hz,mm=10-3m,m=10-6m20DivisionoffrequencybandFreGroundwaveFrequency：below2MHzDiffraction：Propagationdistance：hundredstothousandsofkmGroundsurface21GroundwaveFrequency：below2MHDlayer:60~80kmElayer:100~120kmFlayer:150~400kmF1layer:140~200kmF2layer:250~400kmAtnight:Dlayer:disappearsF1layer:disappears

(Or,F1andF2arecombinedasFlayer)

IonosphereStructureDEFF2F1Groundsurface22Dlayer:60~80kmIonosphereSky-waveIonosphereHeight：60～400kmOpagationdistance：4000kmPropagationdistancebymulti-hops:>10000kmFrequency：2～30MHzFigure1.4.2Sky-wavepropagationPropagationpathofsignalTransmittingantenna

Receivngantenna

IonosphereGround23Sky-waveFigure1.4.2Sky-waveddDhrReceivingantennaTransmittingantennaGroundSignalPropagationFrequency：>30MHzPropagationdistance:d2+r2=(h+r)2, or

h

D2/50(m)whereD－kmLine-of-sightpropagationFigure1.4.3Line-of-sightpropagation24ddDhrReceivingantennaTransmitRadiorelayFigure1.4.4RadiorelayTransmittingantennaReceivingantenna25RadiorelayFigure1.4.4RadioGeostationarysatelliteequator26GeostationarysatelliteequatorStratospherecommunicationHAPS(HighAltitudePlatformStation)27StratospherecommunicationHAPSAttenuation

(dB/km)VaporOxygenFrequency(GHz)(a)Attenuationofoxygen&vapor（concentration7.5g/m3）Attenuation(dB/km)RainfallrateFrequency(GHz)(b)AttenuationofrainfallFigure1.4.5AtmosphereattenuationAtmosphereattenuation28Attenuation(dB/km)VaporOxygenEffectivescatteringregionTransmittingantennaEarthReceivingantennaFigure1.4.6TropospherescatteringcommunicationScattercommunicationIonospherescatteringFrequency:30~60MHzTropospherescatteringFrequency:100~4000MHzMeteor-tailscatteringFrequency:30~100MHzGroundFigure1.4.7Meteor-tailscatteringcommunication29EffectivescatteringregionTra1.4.2WiredchannelOpenwiresSymmetricalcablesCoaxialcablesFig.1.4.8301.4.2WiredchannelFig.1.4.83Table1.4.3GeneralelectricalcharacteristicsofwiredchannelsKindsofchannelCommunicationcapacity(channels)Frequencyrange(kHz)Transmissiondistance(km)Openwire1+303.~27300Openwire1+3+120.3~150120Symmetricalcable2412~10835Symmetricalcable6012~25212~18Smallcoaxialcable30060~13008Smallcoaxialcable96060~41004Mediumcoaxialcable1800300~9,0006Mediumcoaxialcable2700300~12,0004.5Mediumcoaxialcable10800300~60,0001.531Table1.4.3GeneralelectricalOpticalfiberStructureFigure1.4.9SketchofthestructureofmultimodeopticalfibersReflectionindexn2n1(Cladding)(Core)Multimodegraded-indexopticalfiber(Cladding)Multimodestep-indexopticalfiberReflectionindexn2n1(Core)2a32OpticalfiberFigure1.4.9SketTransmissionlossFigure1.4.10Relationshipbetweenlossandwavelength1.55m0.7 0,9 1.1 1.3 1.5 1.7Wavelengthoflightwaves（m）Loss(dB/km)1.31m33TransmissionlossFigure.3ChannelmodelsModulationchannelmodel:

eo(t)=f[ei(t)]+n(t)式中ei(t)－inputsignal

eo(t)－outputsignal

n(t)－additivenoise

f[ei(t)]－functionrelatinginputandoutputsignalsei(t)eo(t)Time-variantlinearnetwork341.4.3Channelmodelsei(t)eo(t)Usually,assumef[ei(t)]canbeexpressedask(t)ei(t) So,

eo(t)=k(t)ei(t)+n(t)wherek(t)iscalledmultiplicativeinterference,andisacomplicatedfunctionwhichreflectsthecharacteristicsofthechannel. Inthesimplestcondition:k(t)=const.,expressingattenuation. Whenk(t)=const.,itisaconstantparameterchannel. Forexample,coaxialcable.

Whenk(t)const.,itiscalledarandomparameterchannel. Forexample,vehicalcellularnetworkcommunicationchannel.35Usually,assumef[ei(t)]canCodingchannelmodel:Binarycodingchannelmodelwhere,P(0/0),P(1/1)－corrrecttransferprobabilities

P(0/1),P(1/0)－errortransferprobabilities

P(0/0)=1-P(1/0)

P(1/1)=1-P(0/1)0110P(0/0)P(0/1)P(1/1)P(1/0)36Codingchannelmodel:0110P(0/0012332104-arycodingchannelmodel

TransmittingendReceivingendFigure1.4.124-arycodingchannelmodel37012332104-arycodingchannelm1.4.4InfluenceofchannelcharacteristicsonsignaltransmissionConstantparameterchannel~time-invariantlinearnetworkLink：asegmentofphysicallinewherenoexchangeexistsAmplitude~frequencycharacteristics:Attenuation(dB)TypicalcharacteristicofatelephonechannelIdealcharacteristicf(Hz)30030000381.4.4InfluenceofchannelchaCompensationoffrequencydistortion0Af(a)Channelcharacteristicwithfrequencydistortion0Af(b)Characteristicoflinearcompensationnetwork0Af(c)Channelcharacteristicaftercompensation39CompensationoffrequencydistPhase~frequencycharacteristics:Idealcharacteristic:phase---()=k; groupdelay---()=d()/d=k Influenceofdistortion:waveformdistortion,inter-symbolinterferenceLineardistortionincludingfrequencydistortion&phasedistortioncanbecorrectedbylinearcompensationnetwork.Nonlineardistortion:nonlinearamplitudecharacteristic,frequencydeviation,phasejittering,…0ω()Idealcharacteristic()0Idealcharacteristic40Phase~frequencycharacteristRandomparameterchannelCommoncharacteristics－attenuation:varyingwithtime transmissiondelay:varyingwithtime multi-pathpropagation:fastfadingCharacteristicsofreceivedsignal:

LettransmittingsignalbeAcos0t，aftertransmissionthroughnpaths,thereceivedsignalR(t)canbeexpressedas:

whereri(t)－amplitudeofreceivedsignalpassingoveri-thpath i(t)－delayofthereceivedsignalpassingoveri-thpath i(t)=-0i(t) Xc(t) Xs(t)41Randomparameterchannel41where V(t)－envelopeofthereceivedsignalR(t)

(t)－phaseofthereceivedsignalR(t)i.e.,Becauseri(t)andi(t)areslowlyvaried,ri(t)andi(t)arealsoslowlyvaried.Hence,Xc(t),Xs(t)andV(t),(t)arealsoslowlyvaried.Hence,R(t)canbeconsideredanarrowbandsignal(randomprocess).4242ItcanbeseenfromthefollowingequationAftertransmission,thetransmittingsignalAcos0t：

*amplitudeAbecomesslowlyvariedamplitudeV(t); *phase0becomesslowlyvariedphase(t)； *spectrumbecomesnarrowbandspectrumfromsinglefrequency.tff043ItcanbeseenfromthefollowFrequencyselectivefadingAssume:thereareonlytwopathswithidenticalattenuationanddifferentdelays,Transmittingsignalisf(t)，receivedsignalsareaf(t-0)andaf(t-0-)；spectrumoftransmittingsignalisF()。then f(t)F()

af(t

-0)aF()e-j0

af(t

-0-)a

F()e-j(0+)

af(t

-0)

+af(t

-0-)aF()e-j0(1+e-j)

H()=aF()e-j0(1+e-j)/F()=ae-j0(1+e-j) |1+e-j|=|1+cos-jsin|=|[(1+cos)2+sin2]1/2| =2|cos(/2)|3categoriesofsignal：

*deterministicsignal *randomphasesignal *fluctuationsignal44Frequencyselectivefading441.5NoiseinChannelClassifiedaccordingtoorigins：Man-madenoise：electricsparks,…Naturalnoise：lightning,atmospherenoies,thermalnoise,...Classifiedaccordingtocharacteristics:

impulsenoisenarrowbandnoisefluctuationnoiseMainnoiseinvolvedinthefollowingdiscussiononcommunicationsystemsis: whitenoise–thermalnoiseisakindoftypicalwhitenoise1.6BriefSummary451.5NoiseinChannelClassifiedPrinciplesofCommunications46PrinciplesofCommunications1Chapter1Introduction1.1HistoricalReviewofCommunicationOriginofancientcommunicationTwomodesofcommunicationDevelopmentofmoderncommunication47Chapter1Introduction21.2Message,information&signalMessage：speech,letters,figures,images…Information：effectivecontentofmessage.DifferenttypesofmessagesmaycontainthesameinformationSignal：thecarrierofmessage Whattransmittedinacommunicationsystemissignal. 481.2Message,information&sigMeasurementofinformation： #“quantityofmessage”informationcontent #Ex:“Rainfallwillbe1mmtomorrow”–informationcontentsmall

“Rainfallwillbe1mtomorrow”–informationcontentlarge “Thesunwillriseintheeasttomorrowmorning”–informationcontentequalszero

#Informationcontent

I=I[P(x)]，P(x)–Occurrenceprobability

#Definition：I=loga

[1/P(x)]=-logaP(x)

#Usually,set

a=2,theunitoftheinformationcontentwillbecalledabit.

#Foranequalprobabilitybinarysymbol:

I=log2[1/P(x)]=log2[1/(1/2)]=1bit49Measurementofinformation：4#ForanequalprobabilityM-arysymbol:

I=log2[1/P(x)]=log2[1/(1/M)] =log2

M bit

If

M=2k，then

I=kbit50#ForanequalprobabilityM-a1.3DigitalCommunication 1.3.1BasicconceptTwocategoriesofsignals

Analogsignal：Itsvoltageorcurrentcanbeexpressedbyacontinuousfunctionoftime.Forexample,speechsignal.

Digitalsignal：Itsvoltageorcurrentcanonlytakefinitenumberofdiscretevalues.Forexample,digitalcomputerdatasignal.511.3DigitalCommunication6AnalogSignal&DigitalSignalAnalogsignalsDigitalsignalsts(t)ts(t)s(t)tSymbolts(t)52AnalogSignal&DigitalSignalTwokindsofcommunicationsystems

Analogcommunicationsystem Requirement－Highfidelity Criterion－Signaltonoiseratio Basicissue－parameterestimation Digitalcommunicationsystem Requirement－correctdecision Criterion－Errorprobability Basicissur－statisticaldecisiontheory53Twokindsofcommunicationsys1.3.2AdvantagesofDigitalCommunicationFinitenumberofpossiblevaluesofsignalsCorrectdecisionmaybeachieved

Fig.1.3.2Distortionandrestorationofdigitalsignalwaveforms(a)Waveformsofdistoreddigitalsignal(b)Waveformsofdigitalsignalaftershaping541.3.2AdvantagesofDigitalCoErrorcorrectingtechniquescanbeused.Digitalencryptioncanbeused.Differentkindsofanalog&digitalmessagecanbeintegratedtotransmitDigitalcommunicationequipment:DesignandmanufactureareeasierWeight&volumearesmallerDigitalsignalcanbecompressedbysourcecodingtoreduceredundency.OutputS/Nincreaseswithbandwidthaccordingtoexponentiallaw.55ErrorcorrectingtechniquescaDigitalcommunicationsystemmodelTransmitterReceiverInformationsourceChannelcodingModulationChannelCompressioncodingDemodulationInformationdestinationEncryptiondecodingChanneldecodingCompressiondecodingEncryptioncodingNoiseSynchro-nizationSourcecodingSourcedecoding1.3.3DigitalCommunicationSystemModel56DigitalcommunicationsystemmAnalogcommunicationsystemmodelTransmitterReceiverNoiseModulationChannelDemodulationInformationdestinationInformationsource57Analogcommunicationsystemmo1.3.4SpecificationsofDigitalCommunicationSystemsRelationshipbetweenefficiency&reliability(rate~accuracy)Transmissionrate：Symbolrate:RB－BaudInformationrate:Rb－bit/second ForM-arysystem：Rb=RBlog2

MMessagerate:RM Errorprobability：SymbolerrorprobabilityPe

=numberofreceivedsymbolsinerror/totalnumberoftransmittedsymbols581.3.4SpecificationsofDigitaBiterrorprobabilityPb=numberofreceivedbitsinerror/totalnumberoftransmittedbitsWorderrorprobabilityPw=numberofreceivedwordsinerror/totalnumberoftransmittedwordsRelationshipbetweensymbolerrorprobabilityandbiterrorprobabilityPb=PexM/[2(M-1)]Pe/259BiterrorprobabilityPb=numRelationshipbetweenworderrorprobabilityandbiterrorprobability Forbinarysystem, Ifawordisconsistedofkbits,then Pw＝1–(1–Pe)k

UtilizationfactoroffrequencybandUtilizaitionfactorofenergy60Relationshipbetweenworderro1.4Channel

1.4.1WirelesschannelOriginofwirelesscommunicationRequirementofelectromagneticwaveemissiononwavelengthDivisionoffrequencyband(wavelength)611.4Channel 16Divisionoffrequencyband

Frequency

Name Typicalapplication

band(kHz)3–30VerylowfrequencyLong-distancenavigation, (VLF) Underwatercomm.Sonar30–300Lowfrequency Navigation,underwatercomm. (LF) radiobeaconing300–3000 MediumfrequencyBroadcasting,maritimecomm. (MF) direction-finding,distress calling,coastguard62DivisionoffrequencybandFreDivisionoffrequencyband

Frequency

Name Typicalapplication

band(MHz)3–30 Highfrequency Long-distancebroadcasting,telegraph, (HF) telephone,fax,searchandlifesaving, comm.betweenaircrafts&ships,and betweenship&coast,amateurradio 30–300 VeryhighfrequencyTV,FMbroadcasting,landtraffic,air (VHF) traffic,control,taxi,police,navigation, aircraftcommunication300–3000 Ultrahighfrequency TV,cellularphonenetwork,microwave (UHF) link,,radiosounding,navigation, satellitecommunication,GPS, surveillanceradar,radioaltimeter63DivisionoffrequencybandFreDivisionoffrequencyband

Frequency Name Typicalapplicationband(GHz)3–30SuperhighfrequencySatellitecomm.,radioaltimeter, (SHF) microwavelink,aircraftradar, meteorologicalradar,public landvehiclecommunication30–300 Extremelyhigh Radarlandingsystem,satellite frequency(EHF) comm.,vehiclecomm.,railway traffic300–3000 Submillimeterwave Experiment,notdesignated （0.1–1mm）64DivisionoffrequencybandFreDivisionoffrequencyband

Frequency Name Typicalapplicationband(THz)

43–430 Infrared

Opticalcommunication

(7–0.7m)

430–750Visiblelight

Opticalcommunication

(0.7–0.4m)

750–3000 Ultraviolet Opticalcommunication (0.4–0.1m)Note:kHz=103Hz,MHz=106Hz,GHz=109Hz, THz=1012Hz,mm=10-3m,m=10-6m65DivisionoffrequencybandFreGroundwaveFrequency：below2MHzDiffraction：Propagationdistance：hundredstothousandsofkmGroundsurface66GroundwaveFrequency：below2MHDlayer:60~80kmElayer:100~120kmFlayer:150~400kmF1layer:140~200kmF2layer:250~400kmAtnight:Dlayer:disappearsF1layer:disappears

(Or,F1andF2arecombinedasFlayer)

IonosphereStructureDEFF2F1Groundsurface67Dlayer:60~80kmIonosphereSky-waveIonosphereHeight：60～400kmOpagationdistance：4000kmPropagationdistancebymulti-hops:>10000kmFrequency：2～30MHzFigure1.4.2Sky-wavepropagationPropagationpathofsignalTransmittingantenna

Receivngantenna

IonosphereGround68Sky-waveFigure1.4.2Sky-waveddDhrReceivingantennaTransmittingantennaGroundSignalPropagationFrequency：>30MHzPropagationdistance:d2+r2=(h+r)2, or

h

D2/50(m)whereD－kmLine-of-sightpropagationFigure1.4.3Line-of-sightpropagation69ddDhrReceivingantennaTransmitRadiorelayFigure1.4.4RadiorelayTransmittingantennaReceivingantenna70RadiorelayFigure1.4.4RadioGeostationarysatelliteequator71GeostationarysatelliteequatorStratospherecommunicationHAPS(HighAltitudePlatformStation)72StratospherecommunicationHAPSAttenuation

(dB/km)VaporOxygenFrequency(GHz)(a)Attenuationofoxygen&vapor（concentration7.5g/m3）Attenuation(dB/km)RainfallrateFrequency(GHz)(b)AttenuationofrainfallFigure1.4.5AtmosphereattenuationAtmosphereattenuation73Attenuation(dB/km)VaporOxygenEffectivescatteringregionTransmittingantennaEarthReceivingantennaFigure1.4.6TropospherescatteringcommunicationScattercommunicationIonospherescatteringFrequency:30~60MHzTropospherescatteringFrequency:100~4000MHzMeteor-tailscatteringFrequency:30~100MHzGroundFigure1.4.7Meteor-tailscatteringcommunication74EffectivescatteringregionTra1.4.2WiredchannelOpenwiresSymmetricalcablesCoaxialcablesFig.1.4.8751.4.2WiredchannelFig.1.4.83Table1.4.3GeneralelectricalcharacteristicsofwiredchannelsKindsofchannelCommunicationcapacity(channels)Frequencyrange(kHz)Transmissiondistance(km)Openwire1+303.~27300Openwire1+3+120.3~150120Symmetricalcable2412~10835Symmetricalcable6012~25212~18Smallcoaxialcable30060~13008Smallcoaxialcable96060~41004Mediumcoaxialcable1800300~9,0006Mediumcoaxialcable2700300~12,0004.5Mediumcoaxialcable10800300~60,0001.576Table1.4.3GeneralelectricalOpticalfiberStructureFigure1.4.9SketchofthestructureofmultimodeopticalfibersReflectionindexn2n1(Cladding)(Core)Multimodegraded-indexopticalfiber(Cladding)Multimodestep-indexopticalfiberReflectionindexn2n1(Core)2a77OpticalfiberFigure1.4.9SketTransmissionlossFigure1.4.10Relationshipbetweenlossandwavelength1.55m0.7 0,9 1.1 1.3 1.5 1.7Wavelengthoflightwaves（m）Loss(dB/km)1.31m78TransmissionlossFigure.3ChannelmodelsModulationchannelmodel:

eo(t)=f[ei(t)]+n(t)式中ei(t)－inputsignal

eo(t)－outputsignal

n(t)－additivenoise

f[ei(t)]－functionrelatinginputandoutputsignalsei(t)eo(t)Time-variantlinearnetwork791.4.3Channelmodelsei(t)eo(t)Usually,assumef[ei(t)]canbeexpressedask(t)ei(t) So,

eo(t)=k(t)ei(t)+n(t)wherek(t)iscalledmultiplicativeinterference,andisacomplicatedfunctionwhichreflectsthecharacteristicsofthechannel. Inthesimplestcondition:k(t)=const.,expressingattenuation. Whenk(t)=const.,itisaconstantparameterchannel. Forexample,coaxialcable.

Whenk(t)const.,itiscalledarandomparameterchannel. Forexample,vehicalcellularnetworkcommunicationchannel.80Usually,assumef[ei(t)]canCodingchannelmodel:Binarycodingchannelmodelwhere,P(0/0),P(1/1)－corrrecttransferprobabilities

P(0/1),P(1/0)－errortransferprobabilities

P(0/0)=1-P(1/0)

P(1/1)=1-P(0/1)0110P(0/0)P(0/1)P(1/1)P(1/0)81Codingchannelmodel:0110P(0/0012332104-arycodingchannelmodel

TransmittingendReceivingendFigure1.4.124-arycodingchannelmodel82012332104-arycodingchannelm1.4.4InfluenceofchannelcharacteristicsonsignaltransmissionConstantparameterchannel~time-invariantlinearnetworkLink：asegmentofphysicallinewh