第二章 无线通信基础

©2009GuangmingSong-SoutheastUniversityS022116:无线传感器网络宋光明仪器科学与工程学院东南大学第二章无线通信基础2009-2-24©2009GuangmingSong-SoutheastUniversity2Outline„无线通信的基本概念„模拟信号与数字信号„带宽与通道容量„信号编码与调制©2009GuangmingSong-SoutheastUniversity3电磁信号„电磁信号是一个时间函数„电磁信号也可以表达为一个频率函数†信号由不同的频率分量组成©2009GuangmingSong-SoutheastUniversity4时域„模拟信号－信号强度随时间平滑变化†信号中无暂停与中断„数字信号－信号强度在某一个时间段内维持一个常量，在下一个时间段又跳转到另一个常量©2009GuangmingSong-SoutheastUniversity5时域©2009GuangmingSong-SoutheastUniversity6时域„周期信号－信号波形随时间周期重复变化†模拟或数字信号中最简单的一种†s(t+T)=s(t)-∞t+∞„T为信号周期„非周期信号－信号波形不随时间周期重复变化©2009GuangmingSong-SoutheastUniversity7时域©2009GuangmingSong-SoutheastUniversity8时域„Peakamplitude(A)-maximumvalueorstrengthofthesignalovertime;typicallymeasuredinvolts„Frequency(f)-Rate,incyclespersecond,orHertz(Hz)atwhichthesignalrepeats„Period(T)-amountoftimeittakesforonerepetitionofthesignal†T=1/f„Phase(φ)-measureoftherelativepositionintimewithinasingleperiodofasignal„Wavelength(λ)-distanceoccupiedbyasinglecycleofthesignal†Or,thedistancebetweentwopointsofcorrespondingphaseoftwoconsecutivecycles©2009GuangmingSong-SoutheastUniversity9时域„Generalsinewave†s(t)=Asin(2πft+φ)„Figure2.3showstheeffectofvaryingeachofthethreeparameters†(a)A=1,f=1Hz,φ=0;thusT=1s†(b)Reducedpeakamplitude;A=0.5†(c)Increasedfrequency;f=2,thusT=½S†(d)Phaseshift;φ=π/4radians(45degrees)„note:2πradians=360°=1period©2009GuangmingSong-SoutheastUniversity10时域©2009GuangmingSong-SoutheastUniversity11FrequencyDomainConcepts„Fundamentalfrequency-whenallfrequencycomponentsofasignalareintegermultiplesofonefrequency,it’sreferredtoasthefundamentalfrequency„Spectrum-rangeoffrequenciesthatasignalcontains„Absolutebandwidth-widthofthespectrumofasignal„Effectivebandwidth(orjustbandwidth)-narrowbandoffrequenciesthatmostofthesignal’senergyiscontainedin©2009GuangmingSong-SoutheastUniversity12FrequencyDomainConcepts©2009GuangmingSong-SoutheastUniversity13FrequencyDomainConcepts„Anyelectromagneticsignalcanbeshowntoconsistofacollectionofperiodicanalogsignals(sinewaves)atdifferentamplitudes,frequencies,andphases„Theperiodofthetotalsignalisequaltotheperiodofthefundamentalfrequency©2009GuangmingSong-SoutheastUniversity14RelationshipbetweenDataRateandBandwidth„Thegreaterthebandwidth,thehighertheinformation-carryingcapacity„Conclusions†Anydigitalwaveformwillhaveinfinitebandwidth†BUTthetransmissionsystemwilllimitthebandwidththatcanbetransmitted†AND,foranygivenmedium,thegreaterthebandwidthtransmitted,thegreaterthecost†HOWEVER,limitingthebandwidthcreatesdistortions©2009GuangmingSong-SoutheastUniversity15RelationshipbetweenDataRateandBandwidth©2009GuangmingSong-SoutheastUniversity16DataCommunicationTerms„Analog-continuous„Digital-discrete„Data-entitiesthatconveymeaning,orinformation„Signals-electricorelectromagneticrepresentationsofdata„Transmission-communicationofdatabythepropagationandprocessingofsignals©2009GuangmingSong-SoutheastUniversity17AnalogandDigitalData„Analog†Video†Audio„Digital†Text†Integers©2009GuangmingSong-SoutheastUniversity18AnalogandDigitalData©2009GuangmingSong-SoutheastUniversity19AnalogSignals„Acontinuouslyvaryingelectromagneticwavethatmaybepropagatedoveravarietyofmedia,dependingonfrequency„Examplesofmedia:†Copperwiremedia(twistedpairandcoaxialcable)†Fiberopticcable†Atmosphereorspacepropagation„Analogsignalscanpropagateanaloganddigitaldata©2009GuangmingSong-SoutheastUniversity20DigitalSignals„Asequenceofvoltagepulsesthatmaybetransmittedoveracopperwiremedium„Generallycheaperthananalogsignaling„Lesssusceptibletonoiseinterference„Suffermorefromattenuation„Digitalsignalscanpropagateanaloganddigitaldata©2009GuangmingSong-SoutheastUniversity21DigitalSignals©2009GuangmingSong-SoutheastUniversity22AnalogSignalingofA/DData©2009GuangmingSong-SoutheastUniversity23DigitalSignalingofA/DDataAnalogData©2009GuangmingSong-SoutheastUniversity24HowtoChooseDataandSignalCombinations„Digitaldata,digitalsignal†Equipmentforencodingislesscomplexandlessexpensivethandigital-to-analogequipment„Analogdata,digitalsignal†Conversionpermitsuseofmoderndigitaltransmissionandswitchingequipment„Digitaldata,analogsignal†Sometransmissionmediawillonlypropagateanalogsignals†Examplesincludeopticalfiberandsatellite„Analogdata,analogsignal†Analogdataeasilyconvertedtoanalogsignal©2009GuangmingSong-SoutheastUniversity25AnalogTransmission„Transmitanalogsignalswithoutregardtocontent„Attenuationlimitslengthoftransmissionlink„Cascadedamplifiersboostsignal’senergyforlongerdistancesbutcausedistortion†Analogdatacantoleratedistortion†Introduceserrorsindigitaldata©2009GuangmingSong-SoutheastUniversity26DigitalTransmission„Concernedwiththecontentofthesignal„Attenuationendangersintegrityofdata„DigitalSignal†Repeatersachievegreaterdistance†Repeatersrecoverthesignalandretransmit„Analogsignalcarryingdigitaldata†Retransmissiondevicerecoversthedigitaldatafromanalogsignal†Generatesnew,cleananalogsignal©2009GuangmingSong-SoutheastUniversity27ChannelCapacity„Impairments,suchasnoise,limitdataratethatcanbeachieved„Fordigitaldata,towhatextentdoimpairmentslimitdatarate?„ChannelCapacity–themaximumrateat