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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:
*
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