数据通信与网络编码信息

5.1Digital-to-DigitalEncoding5.2Analog-to-DigitalEncoding5.3Digital-to-AnalogEncoding5.4Analog-to-AnalogEncoding5.5SummaryContentsIntroductionofEncodingTheinformationordatamustbeencodedintosignalsbeforeitcanbetransportedacrossthecommunicationmedia.

Thesignalmustbemanipulatedsothatitcontainsidentifiablechangesthatarerecognizabletothesenderandreceiverasrepresentingtheinformationintended.DifferentencodingschemesEncodingDigital/digitalAnalog/digitalDigital/analogAnalog/analogIntroductionofEncodingHowinformationisencodedependsonitsoriginalformatandontheformatusedbycommunicationhardware.5.1Digital-to-digitalencodingDigital-to-digitalencodingistherepresentationofdigitalinformationbyadigitalsignal.Boththeoriginaldataandtransmitteddataaredigital.Thebinary1sand0saretranslatedintoasequenceofvoltagepulsethatcanbepropagatedoverwire.Typesofdigital-to-digitalencodingDigital/digitalencodingUnipolarPolarBipolar5.1Digital-to-digitalencoding单极性编码极化编码双极性编码5.1.1Unipolar(单极性编码)Digitaltransmissionsystemsworkbysendingvoltagepulsesalongamedialink(wireorcable).Inmosttypesofencoding,onevoltagelevelstandsforbinary0andanotherlevelstandsforbinary1.Thepolarityofpulse(脉冲极性)refertowhetheritispositiveornegative.Unipolarencodingissonamedbecauseitusesonlyonepolarity.5.1.1Unipolar(单极性编码)The1sareencodedaspositivevalue;

The0sareencodedaszero,oridle.振幅时间01000111Theideaofunipolarencoding.Advantage:VerySimpleandStraightforward;inexpensivetoimplementDisadvantages:1、同步(synchronization)

whenasignalisunvarying,thereceivercannotdeterminethebeginningandendingofeachbits.Therefore,asynchronizationprobleminunipolarencodingcanoccurwheneverthedatastreamincludesalonguninterruptedseriesof1sor0s.5.1.1Unipolar(单极性编码)synchronizationTocorrectlyinterpretthesignalsreceivedfromthesender,thereceiver'sbitintervalsmustcorrespondexactlytothesender'sbitintervals.Ifthereceiverclockisfasterorslower,thebitintervalsarenotmatchedandthereceivermightmisinterpretthesignals.Thesolutionofsynchronizationparallellinecarryingsynchronizationclockpulse.5.1.1Unipolar(单极性编码)Example:synchronization

Givenanexpectedbitrateof1000bps,ifthereceiverdetectsapositivevoltagelasting0.005s,itreadsone1per0.001s,orfive1s.(Propagationdelaysdistortitto0.006s)5.1.1Unipolar(单极性编码)2、DCcomponent(直流分量、直流成分)当数字信号中的电平保持一段时间的恒定时，频谱会产生很低的频率(据傅立叶分析)。这些接近于零的频率称为DCcomponent。直流分量会对不允许通过低频的系统或使用电子耦合的系统带来严重的影响。例如：线不能通过低于200Hz的频率；长距离链路一般会用多个变压器来隔离线路的不同部分。5.1.1Unipolar(单极性编码)5.1.2Polar(极化编码)Polarencodingusestwovoltagelevel:onepositiveandonenegative.Advantage:Byusingbothlevels,inmostpolarencodingmethodstheaveragevoltagelevelonthelineisreducedandtheDCcomponentproblemofunipolarencodingisalleviated.TypesofpolarencodingpolarNRZRZBiphaseNRZ-LNRZ-IManchesterDifferentialManchester5.1.2Polar(极化编码)INNRZencoding,thelevelofthesignalisalwayseitherpositiveornegative.※Ifthelineisidleitmeansnotransmissionisoccurringatall.Thetwomostpopularmethods:NRZ-L(非归零电平编码)NRZ-I(非归零反相编码)DisadvantageofNRZBoth

NRZ-LandNRZ-IhaveaDCcomponentproblem.5.1.2.1NRZ(Non-Return-to-Zero)TwomethodsofNRZ:InNRZ_L,thelevelofthesignaldependsonthetypeofbititrepresents.0spositivevoltage1snegativevoltageInNRZ_Ithesignalisinvertedifa1isencountered.

无论当前处于什么状态(高电平或低电平)，只要下一位是1，则跳转到相反的状态。即电平是否反相决定了其位值。5.1.2.1NRZ(Non-Return-to-Zero)5.1.2.1NRZ(Non-Return-to-Zero)跳变，由于下一比特为1振幅时间时间NRZ-L(非归零电平编码)NRZ-I(非归零反向编码)00001111编码规则：在NRZ-L编码方式中，电平的极性具有特定的含义：正代表比特0，负代表比特1；在NRZ-I编码方式中，每段比特间隔的电平值是没有意义的，接收端是以检测每个比特开始处是否有电平的跳变来识别比特1，而在每比特开始处保持当前电平不变的则表示比特0。5.1.2.1NRZ(Non-Return-to-Zero)NRZ-I相对NRZ-L的优点：由于每遇到比特1都发生电平跃迁，因此提供了一种同步(Synchronization)机制。一串7个比特1会导致7次电平跃迁，每次跃迁都使接收方能根据信号的实际到达来对本身时钟进行重同步调整。(据统计，连续的比特1出现的几率比连续的比特0出现的几率大，因此对比特1的连续串进行同步在保持整体消息同步上更有优势。)一串连续的比特0仍会造成麻烦，但由于连续0串出现不频繁，对于解码来说其妨碍就小了许多。

5.1.2.1NRZ(Non-Return-to-Zero)RZencodingusesthreevalues:

positive,zero,negative.0positive(actually,negative-to-zero)1negative(actually,positive-to-zero)

※halfwaythrougheachbitinterval,thesignalreturntozero.InRZ,thesignalchangesnotbetweenbits,butduringeachbit(位于位间隔之间).InRZ,thesignalchangesduringeachbitbeusedforsynchronization.5.1.2.2RZ(Return-to-Zero)encoding5.1.2.2RZ(Return-to-Zero)encoding跳变可以用于同步信号值时间00001111正跳(从负到零电平)表示0；负跳(从正到零电平)表示1。由于每位都在其位间隔中间位置归零，并保持半个位间隔时间，因此每位的开始都是由零开始跳变。若下一位是1(由高电平到零电平的跳变表示，负向跳变)，则首先由零电平跳变到高电平，持续半个位间隔(可理解为准备阶段)，然后在1/2位间隔处由高电平归零(代表1)，并保持半个位间隔时间；若下一位是0(由低电平到零电平的跳变表示，正向跳变)，则首先由零电平跳变到低电平，持续半个位间隔(可理解为准备阶段)，然后在1/2位间隔处由低电平归零(代表0)，并保持半个位间隔时间。5.1.2.2RZ(Return-to-Zero)encodingDisadvantageofRZ:requirestwosignalchangestoencodeonebit,thereforeoccupiesmorebandwidth.Usethreelevel.Advantage:Agoodencodeddigitalsignalmustcontainaprovisionforsynchronization.Thereisasignalchangeforeachbit,soprovidingsynchronization.5.1.2.2RZ(Return-to-Zero)encoding5.1.2.3Biphase(双相位编码)Inbiphaseencoding,thesignalchangesatmiddleofbitinterval,butdoesnotreturntozero.Instead,itcontinuestotheoppositepole.Twotypesofbiphaseencodingmethods:Manchester;DifferentialManchester.Thebiphaseencodingisthebestsolutiontotheproblemofsynchronization.InManchesterencoding,thedurationofthebitisdividedintotwohalves.Thevoltageremainsatonelevelduringthefirsthalfandmovestotheotherinthesecondhalf.Thetransitionatthemiddleofthebitprovidessynchronization.DifferentialManchestercombinestheideasofRZandNRZ-I.Thereisalwaysatransitionatthemiddleofthebit,butthebitvaluesaredeterminedatthebeginning(point)ofthebit.Ifthenextbitis0,thereisatransition;Ifthenextbitis1,thereisnone.5.1.2.3Biphase(双相位编码)在曼彻斯特编码中，由位间隔处的电平变化方向来表示数据，正电平到负电平的跳变为0

，负电平到正电平的跳变为1

。在差分曼彻斯特编码中，由每位开始时是否发生跳变来确定比特值，如果发生(反向)跳变则为0，如果保持当前电平则为1。(注意在每位中间，无论0或1，都需要反向跳变。)

对于位间隔点（起始点），1是通过，0是跳变。5.1.2.3Biphase(双相位编码)5.1.2.3Biphase(双相位编码)同步机制在曼彻斯特编码与差分曼彻斯特编码中，每个1/2位间隔处都有反向跳变，提供了同步机制。数据表示曼彻斯特编码利用电平值跳变方向表示数据；差分曼彻斯特编码利用每位开始时是否发生跳变来确定比特值。5.1.2.3Biphase(双相位编码)5.1.3Bipolar(双极性编码)Inbipolarencoding(sometimescalledmultilevelbinary),

therearethreevoltagelevels:positive,negative,andzero.Thevoltagelevelforonedataelementisatzero,whilethevoltagelevelfortheotherelementalternatesbetweenpositiveandnegative.0~0电平；

1~高低电平交替。(双极性)传号交替反转(AMI)双极性8连0替换(B8ZS)3阶高密度双极性(HDB3)TypesofBipolarEncodingBipolarAMIB8ZSHDB35.1.3Bipolar(双极性编码)双极性传号反转双极性8连0替换3阶高密度双极性5.1.3.1BipolarAlternateMarkInversion(AMI)双极性传号交替反转码(AMI)Acommonbipolarencodingschemeiscalledbipolaralternatemarkinversion(AMI).Inthetermalternatemarkinversion(AMI),thewordmark(传号)comesfromtelegraphyandmeans1.SoAMImeansalternate1inversion.Aneutralzerovoltagerepresentsbinary0;Binary1sarerepresentedbyalternatingpositiveandnegativevoltages.振幅时间00001111正负电平交替变换代表比特15.1.3.1BipolarAlternateMarkInversion(AMI)AdvantageofAMI:1、NoDCcomponentToalongsequenceof1s,thevoltagelevelalternatesbetweenpositiveandnegative,itisnotconstant.Therefore,thereisnoDCcomponent.Foralongsequenceof0s,thevoltageremainsconstant,butitsamplitudeiszero,whichisthesameashavingnoDCcomponent.Inotherwords,asequencethatcreatesaconstantzerovoltagedoesnothaveaDCcomponent.2、1scanbeusedtosynchronization.(0cannot.)5.1.3.1BipolarAlternateMarkInversion(AMI)5.1.3.2Bipolar8-ZeroSubstitution(B8ZS)双极性8连0替换B8ZSistheconventionadoptedinNorthAmericatoprovidesynchronizationoflongstringsof0s.Inmostsituations,B8ZSfunctionsidenticallytobipolarAMI.Thedifferenceoccurswhenevereightormoreconsecutive0sareencounteredinthedatastream.Thesolutionistoforceartificialsignalchanges,calledviolations,withinthe0string.Anytimeeight0soccurinsuccession,B8ZSintroduceschangeinthepatternbasedonthepolarityoftheprevious1.5.1.3.2Bipolar8-ZeroSubstitution(B8ZS)+00000000+000-+0+--00000000-00+-0-+previous1ispositive.previous1isnegative.0D8ZS将8个连续的零置换为000VB0VB。

V(违反)表示与前一个非零脉冲极性相同的极性；

B(双极)表示与前一个非零脉冲极性相反的极性。5.1.3.2Bipolar8-ZeroSubstitution(B8ZS)+00000000+000-+0+--00000000-000+-0-+000VB0VB000VB0VB非零脉冲非零脉冲D8ZS将8个连续的零置换为000VB0VB。

V(违反)表示与前一个非零脉冲极性相同的极性；即该位违反了AMI编码规则。

B(双极)表示与前一个非零脉冲极性相反的极性。即该位与AMI编码规则保持一致。5.1.3.2Bipolar8-ZeroSubstitution(B8ZS)5.1.3.3High-DensityBipolar3(HDB3)3阶高密度双极性编码HDB3iscommonlyusedoutsideofNorthAmerica.Inthis

technique,whichismoreconservativethanB8ZS,fourconsecutivezero-levelvoltagesare

replacedwithasequenceof000VorB00V.Thereasonfortwodifferentsubstitutionsisto

maintaintheevennumberofnonzeropulsesaftereachsubstitution.5.1.3.3High-DensityBipolar3(HDB3)Tworulescanbestatedasfollows:1.Ifthenumberofnonzeropulsesafterthelastsubstitutionisodd,thesubstitutionpatternwillbe000V,whichmakesthetotalnumberofnonzeropulseseven.2.Ifthenumberofnonzeropulsesafterthelastsubstitutioniseven,thesubstitutionpatternwillbeB00V,whichmakesthetotalnumberofnonzeropulseseven.5.1.3.3High-DensityBipolar3(HDB3)3阶高密度双极性编码HDB3编码中，如果遇到连续4个0比特，根据前导1的极性和自上一次替换后传输的1的比特数的奇偶性，以4种方式改变对应的比特模式。+0000-0000+000+-000-最近一次替换后比特流中的1数目为奇5.1.3.3High-DensityBipolar3(HDB3)000V000V+0000-0000+-00--+00+最近一次替换后比特流中的1数目为偶5.1.3.3High-DensityBipolar3(HDB3)B00VB00V5.1.3.3High-DensityBipolar3(HDB3)记忆方式：总体原则：变换后比特1的总数为偶数，即当前比特1的个数为奇数，则替换1位(奇替换)；若当前比特1的个数为偶数，则需要替换两位(偶替换)。奇替换中，利用0000的最后一个0与前导比特1形成违例，即相同极性，违背了AMI的原则；偶替换中，利用0000的第1和第4位形成一个违例，即保持同极性，来违背AMI原则。(注意：第一位与前导比特1不能违例)。+0000-0000+000+-000-奇替换，替换最后一位，并且替换位与前导的比特1极性相同，产生一个违例。5.1.3.3High-DensityBipolar3(HDB3)+0000-0000+-00--+00+偶替换，替换0000中的第一和第四位，并且两个替换位产生一个违例(同极性)。注意：第一位与前导位不违例。5.1.3.3High-DensityBipolar3(HDB3)5.1.4Example振幅时间Example1:

采用B8ZS，对比特流10000000000100进行编码(假设第一个1的极性为正)。100000000001005.1.4ExampleSolution:000VB0VB振幅时间违例违例变换规则：000VB0VB;红牌为违例位重新开始计位100000000000100振幅时间5.1.4ExampleExample2:

采用HDB3，对比特流10000000000100进行编码。(假设第一个1的极性为正且到目前1比特的个数为奇数)。振幅时间5.1.4ExampleSolution:奇违例：变一位(最后一位)，且该位变换后与前导位产生一个违例。偶违例(经过前次变换后，1的个数已经为偶数)：变两位(1，4位)，且这两位变换后产生一个违例。(第一位与前导位不违例)注意：第二组0000的前导位是第一组0000变换后的最后一位。10000000000100违例违例5.1.5Digital-to-DigitalEncodingConclusion5.1.6ExercisesDrawthewaveformfor11000101inRZ、NRZ-L、NRZ-I、Manchester、DifferentialManchester.WhatisthevoltagepatternifthefollowingbitsareencodedusingB8ZSandHD3B。(Assumethatthenumberof1ssofarisoddandthefirst1ispositive.)

100000000000100001105.2Analog-to-digitalencodingAnalog/digitalencodingAnalog-to-digitalencodingistherepresentationofanaloginformationbyadigitalsignal.Inanalog-to-digitalencoding,wearerepresentingtheinformationcontainedinacontinuouswavefromasseriesofdigitalpulses.5.2Analog-to-digitalencodingThemostimportantproblemishowtotranslateinformationfromaninfinitenumberofvaluestodiscretenumberofvaluewithoutsacrificingsenseorquality.5.2.1PulseAmplitudeModulation(PAM)PAM(脉冲振幅调制)PAMtakesanaloginformation,samplesit,andgeneratesaseriesofplusesbasedontheresultsofthesampling.Samplingmeansmeasuringtheamplitudeofthesignalatequalintervals.PAMisthefoundationofpulsecodemodulation(PCM,脉码调制).脉冲振幅调制(PAM)振幅振幅时间5.2.1PulseAmplitudeModulationThelimitationofPAM:AlthoughPAMtranslatestheoriginalwaveformtoaseriesofpulse,thesepulsesarestillofanyamplitude(stillanalogsignal).Note:

PAMhassomeapplications,butitisnotusedbyitselfindatacommunication.However,itisthefirststepinPCM.5.2.1PulseAmplitudeModulation5.2.2PulseCodeModulation(PCM,脉码调制)Themostcommontechniquetochangeananalogsignaltodigitaldata(digitization)

iscalledpulsecodemodulation(PCM).PCMmodifiesthepulsescreatedbyPAMtocreateacompletelydigitalsignal.PCMismadeupoffourseparateprocesses:

①PAM;②quantization;③binaryencoding;④digital-to-digitalencoding.振幅振幅时间5.2.2.1Step1:PulseAmplitudeModulation5.2.2.2Step2:QuantizedPAMsignalQuantizationisamethodofassigningintegralvaluesinaspecificrangetosampledinstances.5.2.2.3Step3:BinaryencodingAftereachsampleisquantizedandthenumberof

bitspersampleisdecided,eachsamplecanbechangedtoannb-bitcodeword.5.2.2.3Step3:BinaryencodingNotethatthenumberofbitsforeach

sampleisdeterminedfromthenumberofquantizationlevels(量化等级).

Ifthenumberofquantization

levels

isL,thenumberofbitsis:

nb

=log2LBitRate:

Bitrate=samplingratexnumberofbitspersample5.2.2.4Step4:digital-to-digitalencoding在第三步中我们已经得到了数字数据(digitaldata),因此可以利用本章第一节所介绍的digital-to-digital技术将其转换为数字信号(digitalsignals).ProcessoffromanalogsignaltoPCMdigitalcode5.2.2.5TheentireprocessofPCM5.2.3SamplingRate(采样速率)Asyoucansee,theaccuracyofanydigitalreproductionofananalogsignaldependsonthenumberofsamplestaken.ThemostimportantisHowmanysamplesaresufficient?AccordingtotheNyquisttheorem(奈奎斯特定理),toensuretheaccuratereproductionofanoriginalanalogsignalusingPAM,thesamplingratemustbeatleasttwicethehighestfrequencyoftheoriginalsignal.5.2.3SamplingRateAccordingtotheNyquisttheorem,asamplingrateoftwiceafrequencyofxHZmeansthatthesignalmustbesampledevery1/2xseconds.5.2.4MoreaboutNyquisttheoremFirst,wecansampleasignalonlyifthesignalisband-limited.Inotherwords,asignalwithaninfinitebandwidthcannotbesampled.Second,thesamplingratemustbeatleast2timesthehighestfrequency,notthebandwidth.Iftheanalogsignalislow-pass,thebandwidthandthehighestfrequencyarethesamevalue.Iftheanalogsignalisband-pass,thebandwidthvalueislowerthanthevalueofthemaximumfrequency.5.2.4MoreaboutNyquisttheoremthevalueofthesamplingratefortwotypesofsignals.5.2.5每个样本多少位规定了采样频率后，另一个重要的问题是确定每一样本要发送的比特数。这取决于所要求的精度，即选择的比特数要使重新复制的原始信号能在振幅上满足预期的精度。5.2.6比特率根据采样频率和样本位数可以得到比特率：比特率=采样频率×每个样本的位数5.3Digital-to-AnalogEncodingDigital-to-analogconversionistheprocessofchangingoneofthecharacteristicsofananalogsignalbasedontheinformationindigitaldata.理论基础一个简单模拟信号(正弦波)可以通过三个特征来表述：振幅、频率、相位。当改变其中任意一个或几个时，就产生了波的另外一种形式。因此，通过改变简单电器信号的某一(几)方面的特性，就可以用其来表示不同的数字数据。5.3Digital-to-AnalogEncoding5.3.1AspectsofDigital-to-AnalogEncodingTypesofdigital-to-analogencodingamplitudeshiftkeying(ASK,幅移键控)

frequencyshiftkeying(FSK,频移键控)

phaseshiftkeying(PSK,相移键控)

quadratureamplitudemodulation(QAM,正交调幅).5.3.1AspectsofDigital-to-AnalogEncodingBitrateandBaudrate(比特率与波特率)Bitrateisthenumberofbitspersecond.Baudrateisthenumberofsignalunits(信号单元)persecond.Baudrateislessthanorequaltothebitrate.比特率等于波特率乘以每个信号单元的的比特数；波特率等于比特率除以每个信号单元的比特数。用类比来区分波特和比特概念：如在运输中，波特类似轿车，比特类似乘客，一辆轿车可载运一个或多个乘客。如果1000辆轿车每辆仅载运一个乘客(司机)从一个地方到另一个地方，则可以运输1000个乘客。如果每辆车载4个乘客，则能够运输4000个乘客。注意，是轿车的辆数而不是乘客人数确定了交通情况。类似地，是波特数而不是比特数确定所要求带宽！5.3.1AspectsofDigital-to-AnalogEncodingExample1Ananalogsignalcarries4bitsineachsignalunit.If1000signalunitsaresentpersecond,findthebaudrateandthebitrate.SolutionBaudrate=1000baudspersecond(baud/s)Bitrate=1000x4=4000bps5.3.1AspectsofDigital-to-AnalogEncodingExample2Thebitrateofasignalis3000.Ifeachsignalunitcarries6bits,whatisthebaudrate?SolutionBaudrate=3000/6=500baud/s5.3.1AspectsofDigital-to-AnalogEncodingCarrierSignal(载波信号)Inanalogtransmission,thesendingdeviceproducesahigh-frequencysignalthatactsasabasisfortheinformationsignal.Thisbasesignaliscalledthecarriersignalorcarrierfrequency.Thereceivingdeviceistunedtothefrequencyofthecarriersignalthatitexpectsfromthesender.Digitalinformationthenchangesthecarriersignalbymodifyingoneormoreofitscharacteristics(amplitude,frequency,orphase).Thiskindofmodificationiscalledmodulation(shiftkeying).5.3.1AspectsofDigital-to-AnalogEncodingInamplitudeshiftkeying,theamplitudeofthecarriersignalisvariedtocreatesignalelements(0or1).Bothfrequencyandphaseremainconstantwhiletheamplitudechanges.5.3.2AmplitudeShiftKeying(ASK)EncodingFeaturesofASK:InASK,theamplitudeofcarriervariedtorepresentbinary1or0;Bothfrequencyandphaseremainconstant;ASKtransmissionishighlysusceptibletonoiseinterference.noise(噪声)：在传输过程中，由于某些其它的因素所产生的热、电磁感应等现象所引发的，线路中非预期的电压值变化。通常，噪声仅对振幅产生影响，因此对ASK威胁最大！5.3.2AmplitudeShiftKeying(ASK)EncodingApopularASKiscalledon-off-keying(OOK,开关键控),1or0isrepresentbynovoltage.Advantage：reductionintheamountofenergyrequiredtotransmitinformation.

5.3.2AmplitudeShiftKeying(ASK)Encoding5.3.2.1BandwidthforASKBandwidthrequirementsforASKencodingarecalculatedusingtheformula:BW:Bandwidth:baudrated:factorrelatedtotheconditionofthelineRelationshipbetweenbaudrateandbandwidthinASK5.3.2.1BandwidthforASK传输所需的最小带宽等于波特率！(d=0)Example3FindtheminimumbandwidthforanASKsignaltransmittingat2000bps.Thetransmissionmodeishalf-duplex.SolutionInASKthebaudrateandbitratearethesame.Thebaudrateistherefore2000.AnASKsignalrequiresaminimumbandwidthequaltoitsbaudrate.Therefore,theminimumbandwidthis2000Hz.5.3.2.2ExampleExample4Givenabandwidthof5000HzforanASKsignal,whatarethebaudrateandbitrate?SolutionInASKthebaudrateisthesameasthebandwidth,whichmeansthebaudrateis5000.ButbecausethebaudrateandthebitratearealsothesameforASK,thebitrateis5000bps.5.3.2.2ExampleExample5Givenabandwidthof10,000Hz(1000to11,000Hz),drawthefull-duplexASKdiagramofthesystem.Findthecarriersandthebandwidthsineachdirection.Assumethereisnogapbetweenthebandsinthetwodirections.SolutionForfull-duplexASK,thebandwidthforeachdirectionis BW=10000/2=5000HzThecarrierfrequenciescanbechosenatthemiddleofeachband fc(forward)=1000+5000/2=3500Hz fc(backward)=11000–5000/2=8500Hz5.3.2.2Example5.3.2.2ExampleSolutionExample5Infrequencyshiftkeying,thefrequencyofthecarriersignalisvariedtorepresentdata.Thefrequencyofthemodulatedsignalisconstantforthedurationofonesignalelement,butchangesforthenextsignalelementifthedataelementchanges.Bothpeakamplitudeandphaseremainconstantforallsignalelements.5.3.3FSK(频移键控)EncodingFSK避免了ASK中的大多数噪声问题，因为它通过频率的变化来识别不同的比特值。FSK技术的限制因素是载波的物理容量。5.3.3FSK(频移键控)EncodingOnewaytothinkaboutbinaryFSKistoconsidertwocarrierfrequencies.(thefirstifthedataelementis0,thesecondifthedataelementis1.Notethatthisisanunrealisticexampleusedonlyfordemonstrationpurposes!Normallythecarrierfrequenciesareveryhigh,andthedifferencebetweenthemisverysmall.5.3.3FSKENCODINGRelationshipbetweenbaudrateandbandwidthinFSK5.3.3FSKENCODINGTheFSKspectrumisthecombinationoftwoASKspectracenteredaroundfc0

andfc1,so

BW=baudrate+(fc1

-fc0)(fc1-fc0)称为频移值(两个载波频率的差值)。Note：

FSK虽然仅有两个载波频率，但是调制过程产生的是一个合成信号，它是许多不同频率的简单信号的合成！5.3.3FSKENCODINGExample6FindtheminimumbandwidthforanFSKsignaltransmittingat2000bps.Transmissionisinhalf-duplexmode,andthecarriersareseparatedby3000Hz.SolutionForFSK

BW=baudrate+fc1

-fc0

AndforFSKbaudratesameasbitrate,soBW=bitrate+fc1-fc0=2000+3000=5000Hz5.3.3FSKENCODINGExample7FindthemaximumbitratesforanFSKsignalifthebandwidthofthemediumis12,000Hzandthedifferencebetweenthetwocarriersis2000Hz.Transmissionisinfull-duplexmode.SolutionBecausethetransmissionisfullduplex,only6000Hzisallocatedforeachdirection.BW=baudrate+fc1-fc0Baudrate=BW-(fc1-fc0)=6000-2000=4000Butbecausethebaudrateisthesameasthebitrate,thebitrateis4000bps. 5.3.3FSKENCODINGInphaseshiftkeying,thephaseofthecarrierisvariedtorepresenttwoormoredifferentsignalelements.Bothpeakamplitudeandfrequencyremainconstantasthephasechanges.Today,PSKismorecommonthanASKorFSK.5.3.4PhaseShiftKeying(PSK,相移键控)5.3.4PhaseShiftKeying(PSK)2-PSK(二相位PSK)，在调制过程中使用了两个不同的相位值。PSKconstellation5.3.4PhaseShiftKeying(PSK)Constellationdiagram(星座图)orphase-statediagram(相位状态图)

可以被用于表示相位值与比特值之间的相互关系。理论基础：PSK调制的质量不易受噪声的影响，但如FSK一样受带宽的限制。接受方可以可靠的检测到更小的信号变化。因此，可以采用更多的信号变化，即更多的相位值(4种、8种)，使每种变化表示更多的比特(2比特、3比特)。

---增加信号单元中的比特数。5.3.4PhaseShiftKeying(PSK,相移键控)5.3.4.14-PSK(Q-PSK)4-PSKcharacteristics(4个相位)8-PSKcharacteristics5.3.4.28-PSK采用8个相位，每个相位代表一个3位组。5.3.4.3BandwidthforPSKPSK调制所需的最小带宽等于ASK调制所需的最小带宽，即等于波特率。PSK调制传输中的最大比特率(bitrate)大于ASK调制传输中的最大比特率。相同的带宽条件下，ASK与PSK的波特率相同，但是PSK的比特率是ASK的2倍(4-PSK)、3倍(8-PSK)，甚至更多！5.3.4.3BandwidthforPSK5.3.5QuadratureAmplitudeModulation(QAM)Quadratureamplitudemodulation(QAM,正交调幅)isacombinationofASKandPSKsothatamaximumcontrast(最大反差)betweeneachsignalunit(bit,dibit,tribit,andsoon)isachieved.Theideaofusingtwocarriers,onein-phaseandtheotherquadrature,withdifferentamplitudelevelsforeachcarrieristheconceptbehindquadratureamplitudemodulation(QAM).ThepossiblevariationsofQAMarenumerous.QAM方案的星座图(Constellationdiagrams)5.3.5QuadratureAmplitudeModulation(QAM)4-QAMand8-QAMconstellations5.3.5QuadratureAmplitudeModulation(QAM)Timedomainforan8-QAMsignal5.3.5QuadratureAmplitudeModulation(QAM)每波特包含3比特。16-QAMconstellations5.3.5QuadratureAmplitudeModulation(QAM)相位数与振幅数的比率越高，抗噪性越好！QAM传输所需的最小带宽与ASK和PSK相同。5.3.4.3BandwidthforQAMConceptofBitandBaud5.3.6Bit/BaudComparison不同数字/模拟调制技术中比特率/波特率的比较ModulationUnitsBits/BaudBaudrateBitRateASK,FSK,2-PSKBit1NN4-PSK,4-QAMDibit2N2N8-PSK,8-QAMTribit3N3N16-QAMQuadbit4N4N32-QAMPentabit5N5N64-QAMHexabit6N6N128-QAMSeptabit7N7N256-QAMOctabit8N8N5.3.6Bit/BaudComparisonExample8Aconstellationdiagramconsistsofeightequallyspacedpointsonacircle.Ifthebitrateis4800bps,whatisthebaudrate?SolutionTheconstellationindicates8-PSKwiththepoints45degreesapart.Since23=8,3bitsaretransmittedwitheachsignalunit.Therefore,the baudrateis4800/3=1600baud5.3.5QuadratureAmplitudeModulation(QAM)5.4Analog-To-AnalogEncodingAnalog-to-analogencodingistherepresentationofanaloginformationbyananalogsignal.Radioisanexampleofananalog-to-analogcommunication.Typesofanalog-to-analogencodingAmplitudemodulation(AM,调幅)Frequencymodulation(FM,调频)Phasemodulation(PM,调相)5.4Analog-To-AnalogEncoding5.4.1AmplitudeModulation(AM)InAMtransmission,thecarriersignalismodulatedsothatitsamplitudevarieswiththechangingamplitudesofthemodulatingsignal(调制信号).Thefrequencyandphaseofthecarrierremainthesame;onlytheamplitudechangestofollowvariationsintheinformation.Themodulatingsignalbecomesanenvelope(包络线)tothecarrier.5.4.1AmplitudeModulation(AM)TheconceptofAM※调制信号成为载波信号的包络线。AMbandwidth5.4.1AmplitudeModulation(AM)Themodulationcreatesabandwidththatistwicethebandwidthofthemodulatingsignalandcoversarangecenteredonthecarrierfrequency.AMbandallocation

AMstationsareallowedcarrierfrequenciesanywherebetween530and1700kHz(1.7MHz).Howeve