通信原理（英文版）第1章

PrinciplesofCommunications1Chapter1Introduction1.1HistoricalReviewofCommunicationOriginofancientcommunicationTwomodesofcommunicationDevelopmentofmoderncommunication21.2Message,information&signalMessage：speech,letters,figures,images…Information：effectivecontentofmessage.DifferenttypesofmessagesmaycontainthesameinformationSignal：thecarrierofmessage Whattransmittedinacommunicationsystemissignal. 3Measurementofinformation：

#“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)]=1bit4#ForanequalprobabilityM-arysymbol:

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

M bit

If

M=2k

，then

I=kbit51.3DigitalCommunication 1.3.1BasicconceptTwocategoriesofsignals

Analogsignal：Itsvoltageorcurrentcanbeexpressedbyacontinuousfunctionoftime.Forexample,speechsignal.

Digitalsignal：Itsvoltageorcurrentcanonlytakefinitenumberofdiscretevalues.Forexample,digitalcomputerdatasignal.6AnalogSignal&DigitalSignalAnalogsignalsDigitalsignalsts(t)ts(t)s(t)tSymbolts(t)7Twokindsofcommunicationsystems

Analogcommunicationsystem Requirement－Highfidelity Criterion－Signaltonoiseratio Basicissue－parameterestimation Digitalcommunicationsystem Requirement－correctdecision Criterion－Errorprobability Basicissur

－statisticaldecisiontheory81.3.2AdvantagesofDigitalCommunicationFinitenumberofpossiblevaluesofsignalsCorrectdecisionmaybeachieved

Fig.1.3.2Distortionandrestorationofdigitalsignalwaveforms(a)Waveformsofdistoreddigitalsignal(b)Waveformsofdigitalsignalaftershaping9Errorcorrectingtechniquescanbeused.Digitalencryptioncanbeused.Differentkindsofanalog&digitalmessagecanbeintegratedtotransmitDigitalcommunicationequipment:DesignandmanufactureareeasierWeight&volumearesmallerDigitalsignalcanbecompressedbysourcecodingtoreduceredundency.OutputS/Nincreaseswithbandwidthaccordingtoexponentiallaw.10DigitalcommunicationsystemmodelTransmitterReceiverInformationsourceChannelcodingModulationChannelCompressioncodingDemodulationInformationdestinationEncryptiondecodingChanneldecodingCompressiondecodingEncryptioncodingNoiseSynchro-nizationSourcecodingSourcedecoding1.3.3DigitalCommunicationSystemModel11AnalogcommunicationsystemmodelTransmitterReceiverNoiseModulationChannelDemodulationInformationdestinationInformationsource121.3.4SpecificationsofDigitalCommunicationSystemsRelationshipbetweenefficiency&reliability(rate~accuracy)Transmissionrate：Symbolrate:RB

－BaudInformationrate:Rb

－bit/second ForM-arysystem：Rb=RB

log2

MMessagerate:RM Errorprobability：SymbolerrorprobabilityPe

=numberofreceivedsymbolsinerror/totalnumberoftransmittedsymbols13BiterrorprobabilityPb

=numberofreceivedbitsinerror/totalnumberoftransmittedbitsWorderrorprobabilityPw

=numberofreceivedwordsinerror/totalnumberoftransmittedwordsRelationshipbetweensymbolerrorprobabilityandbiterrorprobabilityPb

=PexM/[2(M-1)]Pe

/214Relationshipbetweenworderrorprobabilityandbiterrorprobability Forbinarysystem, Ifawordisconsistedofkbits,then Pw＝1–(1–Pe)k

UtilizationfactoroffrequencybandUtilizaitionfactorofenergy151.4Channel

1.4.1WirelesschannelOriginofwirelesscommunicationRequirementofelectromagneticwaveemissiononwavelengthDivisionoffrequencyband(wavelength)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,coastguard17Divisionoffrequencyband

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,radioaltimeter18Divisionoffrequencyband

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）19Divisionoffrequencyband

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=10-6m20GroundwaveFrequency：below2MHzDiffraction：Propagationdistance：hundredstothousandsofkmGroundsurface21Dlayer:60~80kmElayer:100~120kmFlayer:150~400kmF1

layer:140~200kmF2

layer:250~400km

Atnight:Dlayer:disappearsF1

layer:disappears

(Or,F1andF2arecombinedasFlayer)

IonosphereStructureDEFF2F1Groundsurface22Sky-waveIonosphereHeight：60～400kmOpagationdistance：4000kmPropagationdistancebymulti-hops:>10000kmFrequency：2～30MHzFigure1.4.2Sky-wavepropagationPropagationpathofsignalTransmittingantenna

Receivngantenna

IonosphereGround23ddDhrReceivingantennaTransmittingantennaGroundSignalPropagationFrequency：>30MHzPropagationdistance:d2+r2=(h+r)2, or

h

D2/50(m)whereD

－kmLine-of-sightpropagationFigure1.4.3Line-of-sightpropagation24RadiorelayFigure1.4.4RadiorelayTransmittingantennaReceivingantenna25Geostationarysatelliteequator26StratospherecommunicationHAPS(HighAltitudePlatformStation)27Attenuation

(dB/km)VaporOxygenFrequency(GHz)(a)Attenuationofoxygen&vapor（concentration7.5g/m3）Attenuation(dB/km)RainfallrateFrequency(GHz)(b)AttenuationofrainfallFigure1.4.5AtmosphereattenuationAtmosphereattenuation28EffectivescatteringregionTransmittingantennaEarthReceivingantennaFigure1.4.6TropospherescatteringcommunicationScattercommunicationIonospherescatteringFrequency:30~60MHzTropospherescatteringFrequency:100~4000MHzMeteor-tailscatteringFrequency:30~100MHzGroundFigure1.4.7Meteor-tailscatteringcommunication291.4.2WiredchannelOpenwiresSymmetricalcablesCoaxialcablesFig.1.4.830Table1.4.3GeneralelectricalcharacteristicsofwiredchannelsKindsofchannelCommunicationcapacity(channels)Frequencyrange(kHz)Transmissiondistance(km)Openwire1+303.~27300Openwire1+3+120.3~150120Symmetricalcable2412~10835Symmetricalcable6012~25212~18Smallcoaxialcable30060~13008Smallcoaxialcable96060~41004Mediumcoaxialcable1800300~9,0006Mediumcoaxialcable2700300~12,0004.5Mediumcoaxialcable10800300~60,0001.531OpticalfiberStructureFigure1.4.9SketchofthestructureofmultimodeopticalfibersReflectionindexn2n1(Cladding)(Core)Multimodegraded-indexopticalfiber(Cladding)Multimodestep-indexopticalfiberReflectionindexn2n1(Core)2a32TransmissionlossFigure1.4.10Relationshipbetweenlossandwavelength1.55m0.7 0,9 1.1 1.3 1.5 1.7Wavelengthoflightwaves（m）Loss(dB/km)1.31m331.4.3ChannelmodelsModulationchannelmodel:

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-variantlinearnetwork34Usually,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.35Codingchannelmodel: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)36012332104-arycodingchannelmodel

TransmittingendReceivingendFigure1.4.124-arycodingchannelmodel371.4.4InfluenceofchannelcharacteristicsonsignaltransmissionConstantparameterchannel~time-invariantlinearnetworkLink：asegmentofphysicallinewherenoexchangeexistsAmplitude~frequencycharacteristics:Attenuation(dB)TypicalcharacteristicofatelephonechannelIdealcharacteristicf(Hz)3003000038Compensationoffrequencydistortion0Af(a)Channelcharacteristicwithfrequencydistortion0Af(b)Characteristicoflinearcompensationnetwork0Af(c)Channelcharacteristicaftercompensation39Phase~frequencycharacteristics:Idealcharacteristic:phase---()=k; groupdelay---()=d()/d=k Influenceofdistortion:waveformdistortion,inter-symbolinterferenceLineardistortionincludingfrequencydistortion&phasedistortioncanbecorrectedbylinearcompensationnetwork.Nonlineardistortion:nonlinearamplitudecharacteristic,frequencydeviation,phasejittering,…0ω()Idealcharacteristic()0Idealcharacteristic40RandomparameterchannelCommoncharacteristics－

attenuation:varyingwithtime

transmissiondelay:varyingwithtime

multi-pathpropagation:fastfadingCharacteristicsofreceivedsignal:

LettransmittingsignalbeAcos

0t，aftertransmissionthroughnpaths,thereceivedsignalR(t)canbeexpressedas:

where

ri(t)－amplitudeofreceivedsignalpassingoveri-thpath i(t)－delayofthereceivedsignalpassingoveri-thpath i(t)=-0i(t) Xc(t) Xs(t)41where 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).42ItcanbeseenfromthefollowingequationAftertransmission,thetransmittingsignal

Acos

0t：

*