通信原理英文版 1.ppt

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1、PrinciplesofCommunications1Chapter1Introduction1.1HistoricalReviewofCommunicationOriginofancientcommunicationTwomodesofcommunicationDevelopmentofmoderncommunication21.2Message,information&signalMessage：speech,letters,figures,images…Information：effectivecontentofmessage.Differenttypesofmessagesmayco

2、ntainthesameinformationSignal：thecarrierofmessageWhattransmittedinacommunicationsystemissignal.3Measurementofinformation：#“quantityofmessage”informationcontent#Ex:“Rainfallwillbe1mmtomorrow”–informationcontentsmall“Rainfallwillbe1mtomorrow”–informationcontentlarge“Thesunwillriseintheeasttomorrowmo

3、rning”–informationcontentequalszero#InformationcontentI=I[P(x)]，P(x)–Occurrenceprobability#Definition：I=loga[1/P(x)]=-logaP(x)#Usually,seta=2,theunitoftheinformationcontentwillbecalledabit.#Foranequalprobabilitybinarysymbol:I=log2[1/P(x)]=log2[1/(1/2)]=1bit4#ForanequalprobabilityM-arysymbol:I=log2[

4、1/P(x)]=log2[1/(1/M)]=log2MbitIfM=2k，thenI=kbit51.3DigitalCommunication1.3.1BasicconceptTwocategoriesofsignalsAnalogsignal：Itsvoltageorcurrentcanbeexpressedbyacontinuousfunctionoftime.Forexample,speechsignal.Digitalsignal：Itsvoltageorcurrentcanonlytakefinitenumberofdiscretevalues.Forexample,digit

5、alcomputerdatasignal.6AnalogSignal&DigitalSignalAnalogsignalsDigitalsignalsts(t)ts(t)s(t)tSymbolts(t)7TwokindsofcommunicationsystemsAnalogcommunicationsystemRequirement－HighfidelityCriterion－SignaltonoiseratioBasicissue－parameterestimationDigitalcommunicationsystemRequirement－correctdecisionCrite

6、rion－ErrorprobabilityBasicissur－statisticaldecisiontheory81.3.2AdvantagesofDigitalCommunicationFinitenumberofpossiblevaluesofsignalsCorrectdecisionmaybeachievedFig.1.3.2Distortionandrestorationofdigitalsignalwaveforms(a)Waveformsofdistoreddigitalsignal(b)Waveformsofdigitalsignalaftershaping9Errorco

7、rrectingtechniquescanbeused.Digitalencryptioncanbeused.Differentkindsofanalog&digitalmessagecanbeintegratedtotransmitDigitalcommunicationequipment:DesignandmanufactureareeasierWeight&volumearesmallerDigital