信息科学课后答案,王朔中

Unit1ExercisesTranslatethefollowingsentencesintoChinese.Aswithseriesresonance,thegreatertheresistanceinthecircuitthelowertheQand,accordingly,theflatterandbroadertheresonancecurveofeitherlinecurrentorcircuitimpedance.对于串联谐振，电路中的电阻愈大Q值就愈低，相应地线路电流或电路阻抗的谐振曲线也就愈平、愈宽。Awirecarryingacurrentlooksexactlythesameandweighsexactlythesameasitdoeswhenitisnotcarryingacurrent.一根带电的导线其外表与重量都与不带电导线完全一样。Clickmouseonthewaveformanddragittochangethepulserepetitionrate,ordirectlyenteranewvalueoftheperiodintheprovideddialoguebox,whilekeepingthepulsewidthunchanged.在波形上点击鼠标并拖动来改变脉冲重复频率，或者在提供的对话框中直接输入新的周期值，而保持脉冲宽度不变。Electronicsisthescienceandthetechnologyofthepassageofchargedparticlesinagas,inavacuum,orinasemiconductor.Pleasenotethatparticlemotionconfinedwithinametalonlyisnotconsideredelectronics.电子学是一门有关带电粒子在气体、真空或半导体中运动的科学技术。注意，在本书中粒子运动仅限于金属的情况不属于电子学。Hardwaretechnologieshaveplayedvitalrolesinourabilitytouseelectronicpropertiestoprocessinformation,butsoftwareanddataprocessingaspectshavenotdevelopedatthesamespeed.硬件技术在我们使用电子特性来处理信息的能力中一直起着重要作用，而软件及数字处理方面却没能与硬件同步发展。However,inaproperlydesignedDCamplifiertheeffectoftransistorparametervariation,otherthanIco,maybepracticallyeliminatediftheoperationpointofeachstageisadjustedsothatitremainsinthelinearoperationrangeofthetransistorastemperaturevaries.然而在设计得当的直流放大器中，若调节每一级的工作点使之在温度变化时保持在晶体管线性区，就能在实际上消除Ico以外的晶体管参数变化所造成的影响。Fillintheblankswiththemostappropriatechoice.Amessagesignalcananyamplitudevaluebetween0andAvolts.A.assumeB.use C.receiveD.acceptEachtimeapositiveclockedgeoccurs,theflip-flopchangesstate,leadingtohalfasmanypulsesattheoutputtotheclockinput.A.as B.comparedC.due D.soasIntheprevioussectionwediscussedsine-waveorCWmodulationsystemsinwhichthemodulatingsignalofadigitalpulsetrain.A.made B.consistedC.containedD.gotInthisbookweshallemphasizethelimitationsimposedontheinformationtransmittedbythesystemthroughwhichitwas andshallattemptsomecomparisonofdifferentsystems.A.put B.traveledC.passedD.viewedThissignaleitherdoesnotchange,oritchanges byanamountequivalenttooneormorequantumsteps.A.interruptedly B.rudely C.abruptly D.absolutelyWeshallonlybewithlogicalrepresentations,notabsolutevaluesA.interested B.caring C.referred D.concernedChoosethephrasethatisclosestinmeaningtotheunderlinedpart.Itisalltooeasywithlinearcircuits,whichappearrelativelysimplewhencomparedwithtoday,scomplexlogicarrangements,toignoredetailedperformanceparameterswhichcandrasticallyreducetheexpectedperformance.whichhasbadinfluenceuponthesystemperformancewhichisimportanttoensurecircuitlinearitywhichispossibletoenhancethesystemperformancewhichiscrucialformaintainingthedesiredperformanceThereforetherewillbeaslighttimedelay,duetothepropagationdelayoftheflip-flopsbetweenthetimeoneflip-flopchangesstateandthetimethenextonechangesstate,i.e.,thechangeofstateripplesthroughthecounter,andthesecountersarethereforecalledripplecounters.thestatechangeispropagatedthroughthecounterlikeripplesthestatechangescauseripplesinthecounterpassingthroughthecounter,thestateripplechangesthestateofthecounterchangesduetoripplesInsteadoftriggeringoneachpulse,aphaselocktechniqueexaminestherelativephasebetweeneachoscillatorandmanyofitssyncpulsesandadjustsoscillatorfrequencysothattheaveragephasediscrepancyissmall.checkseachoscillatorformanysyncpulseschecksthephaserelationshipbetweenindividualoscillatorsandacollectionofsyncpulseslooksattherelationsbetweenthephaseandthesyncpulsesexamineswhetherthephaseisrelativetotheoscillatorandthesyncpulsesBykeepingtheoffstateslightlybelowthreshold,thedelaybetweentheappliedelectricalpulseandtheresultingopticaloutputpulseisminimized;thisdelaymustindeednotbemorethanthebitintervalsothattheopticalpulsecanaccuratelyreproducetheinputsignal.ensuringthatthesystemisoffthethresholdbyasmallmarginmaintainingthesystem'sshut-downstatesomewhatbelowaprescribedlevelmakingsurethesystemisbelowitsthresholdsothatthestateiskeptoffkeepingthesysteminashut-downstatethatisunderagivenlevelwithatinytoleranceIftheoriginalsignaliswellbehaved,thelocaloscillatorwillneedverylittleinformationtobeabletotrack,andthatinformationcanbeobtainedbyaveragingforalongperiodoftime,therebyeliminatingnoisethatcouldbeverylarge.theLOwillnotrequireanyinformationinordertokeeptrackingtheLOwillmakeuseoftrackinginformationthatisscarceinthesystemtheLOwillrequiresomeinformation,butnotmuch,sothatthesystemcantracktheLOwillusenoinformationotherthansomelittleonetomaintaintrackingThethirdsectiondescribesquantizationschemesthattakeaccountofthecharacteristicsofspeech.takemeasureofthespeechcharacteristicstakethespeechcharacteristicsintoconsiderationcountthespeechcharacteristicsmakeuseofthespeechcharacteristicsInaproperlydesignedDCamplifiertheeffectoftransistorparametervariationmaybepracticallyeliminatediftheoperatingpointofeachstageisadjustedsothatitremainsinthelinearoperationrangeofthetransistorastemperaturevaries.theeffectivevariationofthetransistorparameterstheefforttochangetransistorparametersthetransistorparametersthateffectivelychangetheresultscausedbytransistorparameterchangesStabilityandrepeatabilityarefundamentalattributesofdigitalcircuits,however,andthenecessaryaccuracycanbeobtainedinadigitalversionofthemodulatorbyanappropriatechoiceofwordlengthtorepresentthesignals.byexpressingthesignalswithasuitablewordlengthbyusingawordlongenoughtodescribethesignalsbychoosetocharacterizethesignalsintermsofwordlengthappropriatelybyappropriatelyrepresentingthesignalstochoosealengthofwordsUnit2ExercisesTranslatethefollowingsentencesintoChinese.Ifanalogsignalsaretobetransmitteddigitally,theyfirsthavetobesampledataspecifiedrateandfurtherconvertedtodiscreteamplitudesamplesbyquantization.如果要以数字方式传输模拟信号，首先必须以一定的频率对信号进行采样，然后通过量化进一步将它们转换为离散的幅度样本。Linearfiltersamplifyorattenuateselectedspatialfrequencies,canachievesucheffectsassmoothingandsharpening,andusuallyformthebasisofre-samplingandboundarydetectionalgorithms.线性滤波器放大或衰减选择的空间频率，能够实现像平滑和锐化这些效果，通常是重采样和边缘检测算法的基础。Stabilityandrepeatabilityarefundamentalattributesofdigitalcircuits,however,andthenecessaryaccuracycanbeobtainedinadigitalversionofthemodulatorbyanappropriatechoiceofwordlengthtorepresentthesignals.不过稳定性和可重复性是数字电路的基本性质，在数字式调制器中必要的精度可通过适当地选择代表信号的字长而获得。Thefirstobservationismadeonthefundamentalrelationshipbetweenthenatureofsystemandtheperiodicityofitsfrequencyresponse:acontinuoussystemhasanaperiodicfrequencyresponse,whileadiscretesystemhasaperiodicfrequencyresponse.从系统性质和它频率响应的周期性之间的基本关系中首先可以看到：连续系统有非周期性的频率响应，而离散系统有周期性的频率响应。Themainfunctionofthereceiveristoextracttheinputmessagesignalfromthedegradedversionofthetransmittedsignalcomingfromthechannel.接收器的主要功能是从由信道传输来的退化了的信号中提取出输入消息。Choosethewordorphrasethatisclosestinmeaningtotheunderlinedpart.Roughlyspeaking,thetransistorcanbelikenedtoanelectronicallycontrolledvalvewherebyenergyappliedtooneconnectionofthevalveenablesenergytoflowbetweentwootherconnections.relatedlinkedanalogizedviewedWhilethirdpartydesigntoolswereavailable,therewasnotaneffectivelinkfromthethirdpartydesigntoolstothelayoutandactualsemiconductorprocessperformancecharacteristicsofthevariousASICmanufacturers.WhenAlthoughAsSinceThesesteps,implementedwithalevelofskillcommonintheindustry,almostalwaysproduceafinaldevicethatcorrectlyimplementstheoriginaldesign,unlessflawsarelaterintroducedbythephysicalfabricationprocess.practicedusingskillsgenerallyavailableintheindustrycommonlyperformedatlevelsofskilledpersonnelintheindustryrealizedwiththeskillfulcommonlevelintheindustrycarriedoutbyskillscommonlyusedintheindustryAsecondmajorstepwouldbetodevelopdesignspecificationsthatconsiderthefunctionalityofthehumanwiththesamedegreeofcarethathasbeengiventotherestofthesystem.developdesignprocedurestoaccountforthehumanbehaviorandactivitiespromotedesignarrangementsandtakethehumanfactorsintoconsiderationmakeusedofdesignstepsthatrespectthehumanfunctionalitydraftandeffectdesignrequirementsthattakeintoaccountthehumanfunctionalityClickmouseonthewaveformanddragittochangethepulserepetitionrateordirectlyenteranewvalueoftheperiodintheprovideddialoguebox,keepingthepulsewidthunchanged.simplytypeanewperiodvalueinthegivendialogueboxdirectanentryforthenewperiodvaluetothedialogueboxpointastraightforwardnewperiodvaluetothedialogueboxprovidedenteradirectionofanewperiodvaluetoprovideadialogueboxComparedwithmostofthetextbooksattheintroductorylevelofRFanalogdesign,thisbookpresentssomehands-onexperiencesonthedesignissuesthatreallyworkandhasbeenprovedbymanufacturedchips.thisbookprovidessomeexperiencesathandontheissuesencounteredindesignthisbookprovidessomeactuallyworkingexperiencesaboutthedesignproblemsthisbookpresentssomeavailableexperiencesoverthedesignproblemsthisbookpresentssomeissuesconcerningdesignexperiencesthatarehandedoverForadecadetherehasbeenasearchforaprogrammableprocessorwhich,giventhecostandvolumeconstraints,isgeneralenoughtobeprogrammedwithdifferentalgorithms.withcontrolledmeasuresofpriceandmagnitudegiventheconditionofriskandnumbergiventheconstantsofchargeandweightgiventhelimitsofpriceandsizeInaproperlydesignedDCamplifiertheeffectoftransistorparametervariation,otherthanIco,maybepracticallyeliminatediftheoperatingpointofeachstageisadjustedsothatitremainsinthelinearoperationrangeofthetransistorastemperaturevaries.transistoramplifierIcoparameterUnit3ExercisesTranslatethefollowingsentencesintoChinese.Averycommonlyusedmethodofmicrowavemeasurementsisbasedonthestudyofastandingwavepatternformedalongthelinebecauseoftheinterferenceofincidentandreflectedwaves.一个很常用的微波测量方法是基于对驻波图型的研究，这种驻波是由入射波和反射波之间的干涉而沿着传输线形成的。Computationsshowareductionof6dBintheEMfieldintensitywiththedistancedoubled,andanincreaseof3dBintheintensitywiththetransmittedpowerdoubled.Thisresultprovidesverificationtothealgorithmused.计算表明，当距离增大一倍时，电磁场强度减小6分贝；而发射功率增大一倍时，场强则增加3分贝。这一结果验证了所用算法的正确性。Liketransferofheatbyconduction,convectionandradiation,microwaveheatingcanbeconsideredasanothermodeofheattransfer,inwhichheatisproduceddirectlyatthelocationofthedielectriclosses.就像通过传导、对流、辐射来传递热量一样，微波加热可以看作热传递的另一种方式，即热量直接产生于发生介电损耗的地方。Operationoftheradarisbasedonthemeasurementofthetimeittakesforapulsetransmittedfromanantennatogetreflectedbytheobjecttobedetectedandtoreturnattheantennaandthereceiver.雷达的工作是基于对天线发出的脉冲被所要探测的目标反射，（然后）回到天线和接收机所需时间的测量。OthermotivationsforusingCADincircuitdesignincludesthewishtoacquireconfidenceinadesignthatwasaccomplishedbyothermeans,and,notleastinimportance,asenseofcuriosityorperhapsadesiretodiscovertheunexpected.在电路设计中使用CAD的其它动机包括：想要验证用别的方法设计出来的电路；满足好奇心或者可能是想发现预想不到的现象。后一个动机并非是最不重要的。（取得信心，并非最不重要，意料之夕卜的事）Thespacingbetweentherepeatingamplifiersisafunctionofthecableattenuationandthesystembandwidthsothatthegainprovidedbytheseamplifierscompensatesforthelossintroducedbythecable.中继放大器之间的间距是电缆衰减和系统带宽的函数，以便这些放大器提供的增益补偿电缆弓I起的损耗。Whenthecut-offfrequencyisnolessthanthemaximalfrequencyoftheoriginalsignal,norisitgreaterthanthedifferencebetweenthesamplingfrequencyandthemaximalfrequency,theoriginalsignalmaybecompletelyrebuilt.当截止频率不小于原始信号的最高频率，也不大于采样频率和最高频率之差，原始信号可以完整地重建。WhilethepoliticsofthelastonehundredyearscenteredaroundIndustrialAgetechnology,thepoliticsofthefuturewillbebasedonInformationAgeconcernsorientedtowardsthestorage,protectionandexchangeofinformation.尽管近百年来的政治活动是以工业时代的技术为中心的，未来的政治活动将基于信息时代的事物，面向信息的存储、保护和交换。(2)Choosethewordorphrasethatisclosestinmeaningtotheunderlinedpart.Iftheseotherchargesandcurrentsarecomparableinsizetothesourcesproducingtheaboveelectromagneticfield,thenanewnetelectromagneticfieldwillbeproduced.makecomparisonwiththesizeofthesourcesarecomparedwiththesizeofthesourceshavesimilarsizetothesourcestakeacommonsizeofthesourcesThisrangeofwavelengthshasledmanytoauestionthenamingconventionusedformicrowavesasthenamesuggestsamicrometerwavelength.hasraisedmanyquestionsaboutthenaminghabithascausedmanypeopletochallengethenamingruleshasledmanyquestionsofthenamingconventionhasresultedinmanyproblemsinthenamingtraditionNASAworkedinthe1970sandearly1980storesearchthepossibilitiesofusingSolarPowerSatellite(SPS)systemswithlargesolararraysthatwouldbeampowerdowntotheEarth'ssurfaceviamicrowaves.shedsolarenergytotheEarth,ssurfacefromthesatellitesetthesatellitedowntotheEarth,ssurfacewithpowershutdownthepoweron(to)theEarth,ssurfacefromthesatelliteswitchoffpowertotheEarth,ssurfacefromthesatelliteAsDoDurgentlywantedmilitarycommandandcontrolnetworksthatcouldsurviveanuclearwar,ARPAwaschargedwithinventingatechnologythatcouldgetdatatoitsdestinationreliablyevenifarbitrarypartsofthenetworkdisappearedwithoutwarningasaresultofanuclearattack.winanuclearwarbeusedinpreparationofanuclearwaravoidanuclearwarbestilloperationalafteranuclearwarAtthepresent,stateoftheartmicrowaveamplifiersandoscillatortubescanoperateinafrequencyrangeupto40GHzandsolid-statemicrowavedevicesupto100GHz.recentandmostadvancedhightechrelatedsophisticatedartisticallymanufacturedInsuchafiberraystravelingatlargerangleswithrespecttotheaxishavetotraversealongerpathandhencetakealongertimethanthoserayswhichpropagatewithsmalleranglestotheaxis.havetransmittedoveralongerdistanceareboundtotreatalongerwaymusttravelthroughalongerroutemustdealwithalongertraceBykeepingtheoffstateslightlybelowthreshold,thedelaybetweentheappliedelectricalpulseandtheresultingopticaloutputpulseisminimized;thisdelaymustindeednotbemorethanthebitintervalsothattheopticalpulsecanaccuratelyreproducetheinputsignal.thisdelaymustindeedbenomorethanoneinternalbitindeed,thisdelaymustexceedthebitofintervalasamatteroffact,thisdelayintervalmustbealittlebitlessthisdelaymustindeedbekeptlessthanthespacingbetweendatabitsComputationsshowareductionof6dBintheEMfieldintensitywiththedistancedoubled,andanincreaseof3dBintheintensitywiththetransmittedpowerdoubled.Thisresultprovidesverificationtothealgorithmused.that,witha6dBreduction,theEMfieldintensityistwiceasstrongthattheEMfieldintensityisdecreasedby6dBastherangeisdoubledthat,intheEMfield,intensitywithdoubledistanceleadstoa6dBreductiona6dBintensityreductioncausesadoubledistanceintheEMfieldUnit4ExercisesTranslatethefollowingpassagesintoChinese.Communicationmaybebroadlydefinedasthetransferofinformationfromonepointtoanother.Whentheinformationistobeconveyedoveranydistanceacommunicationsystemisusuallyrequired.Withinacommunicationsystemtheinformationtransferisfrequentlyachievedbysuperimposingormodulatingtheinformationontoanelectromagneticwavewhichactsasacarrierfortheinformationsignal.Thismodulatedcarrieristhentransmittedtotherequireddestinationwhereitisreceivedandtheoriginalinformationsignalisobtainedbydemodulation.Sophisticatedtechniqueshavebeendevelopedforthisprocessbyusingelectromagneticcarrierwavesoperatingatradiofrequenciesaswellasmicrowaveandmillimeterwavefrequencies.However,communicationmayalsobeachievedbyusinganelectromagneticcarrierthatisselectedfromtheopticalrangeoffrequencies.通信可以广义地定义为从一处到另一处的信息传递。当要将信息传输到任何距离以外时就需要有一个通信系统。在通信系统部，信息的传输经常通过将信息叠加或调制到作为信号载体的电磁波上来实现。经调制的载波然后被传送到要求的目的地，在那里被接收，原始信号通过解调被恢复出来。为了实现这一过程，已经利用工作在射频、微波以及毫米波频率上的电磁载波开发了复杂的技术。不过通信也可以用光波频段上的电磁载波来实现。Electroniccommunicationsisthetransferandmovementofdatabetweenlocationsthroughtheuseofcomputers.Anelectroniccommunicationsystemincludestheequipmentneededtosupportthemovementofinformation,thecommunicationlinesandmediatocarrytheinformation,thecomputersoftwareandprogramstocontroltheflowofinformation,thepersonneltoplan,implement,andoperatecommunications,andthemanagementofalltheseresources.Electroniccommunicationsestablisheslinksbetweenpeopleaswellascomputers.电子通信就是通过使用计算机在不同的地点之间传输数据。一个电子通信系统包括：支持信息流动所需要的设备，承载信息的通信线路和媒介，控制信息流的计算机软件和程序，计划、实施、操作通信系统的人员，以及对所有这些资源的管理。电子通信建立起人和人、计算机和计算机之间的联系。Firstgenerationcommunicationssystemscanbecharacterizedbytheuseofanalogtransmissiontechniques,andtheuseofsimplemultiplexaccesstechniquessuchasfrequencydivisionmultipleaccess(FDMA).Theysufferedfromalowusercapacity,andsecurityproblemsduetothesimpleradiointerfaceused.Secondgenerationsystemswereintroducedintheearly1990’s,andallusedigitaltechnology.Thisprovidedanincreaseintheusercapacityofaroundthreetimes,achievedbycompressingthevoicewaveformsbeforetransmission.Thirdgenerationsystemsareanextensiononthecomplexityofsecond-generationsystemsandaretobeintroducedaftertheyear2000.Thesystemcapacitywillbeincreasedtoovertentimesoriginalfirstgenerationsystems,achievedbyusingcomplexmultiplexaccesstechniquessuchascodedivisionmultiplexaccess(CDMA),oranextensionofTDMA,andbyimprovingflexibilityofservicesavailable.第一代通信系统以模拟传输技术和简单复用技术如频分复用为特征。这些系统用户容量低，而且因为使用简单的无线电接口而存在安全问题。第二代通信系统于20世纪90年代初开始引入，全部使用数字技术。通过在传输前将话音波形压缩使得用户容量提高了三倍。第三代通信系统是对第二代系统在复杂性方面的扩展，预计于2000年以后启用。通过使用复杂的复用技术如CDMA或TDMA的扩展，并提高服务的灵活性使得系统容量比原来的第一代系统提高10倍。Foreachcombinationofcommunication(modulation/detection)type,channelfadingmodel,anddiversitytype,theaveragebiterrorrate(BER)and/orsymbolerrorrate(SER)ofthesystemisobtainedandrepresentedbyanexpressioninaformthatcanreadilybeevaluated.Allcasesconsideredcorrespondtorealpracticalchannels,andinmanyinstancestheBERandSERexpressionsobtainedcanbeevaluatednumericallyonahand-heldcalculator.对于通信(调制/检测)类型、信道衰落模型、分集类型的每一种组合，得到系统的平均误码率(BER)和符号错误率(SER)，并以容易求值的形式表示。所有考虑的相应于实际信道的情况，以及许多情况下得到的BER和SER表达式均可用手持计算器来计算。Modulationisthesystematicvariationofsomeattributeofacarrierwaveformsuchastheamplitude,phase,orfrequencyinaccordancewithafunctionofthemessagesignal.Itisusedincommunicationsystemsformatchingsignalcharacteristicstochannelcharacteristics,forreducingnoiseandinterference,forsimultaneouslytransmittingseveralsignalsoverasinglechannel,andforovercomingsomeequipmentlimitations.Aconsiderableportionofthisbookisdevotedtothestudyofhowmodulationschemesaredeignedtoachievetheabovetasks.Thesuccessofacommunicationsystemdependstoalargeextentonthemodulation.调制是载波波形的某些属性，例如幅度、相位和频率，根据消息信号的函数有规则的变化。它用于通信系统以使信号特性匹配信道特性，减少噪声和干扰，在单个信道上同时传输多个信号，并克服某些设备的限制。本书相当大部分容是研究如何设计调制方案以实现上述任务。一个通信系统的成功很大程度上依赖于调制。Choosethewordorphrasethatisclosestinmeaningtotheunderlinedpart.Thereceivingdevicewouldhavethewrongtimethathadbeenagreedintheprotocolforsending/receivingdata,sosomebytescouldbecomecorruptedbylosingbits.thathadbeenconsistentwiththeprotocolofdataexchangethathadbeenthesameastheprotocolforsending/receivingdatathathadbeeninagreementwiththeprotocolofdatatransmissionthathadbeendefinedinthedatatransmissionprotocolSynchronoustransmissionusesnostartandstopbitsbutinsteadsynchronizestransmissionspeedsatboththereceivingandsendingendofthetransmissionusingclocksignalsbuiltintoeachcomponent.usingsynchronizationsignalstobuildeachcomponentusingtimingpulsesthatareincorporatedineachcomponentusingtimingsignalsinordertobuildeachcomponentusingclocksthatcanbuildsignalcomponentsInformationtheory,however,doesnotinvolvemessageimportanceormeaning,asthesearemattersofthequalityofdataratherthanthequantityofdata,thelatterofwhichisdeterminedsolelybyprobabilities.whichisthelateroneofthedeterminedprobabilitiesthelateronebeingprobablydeterministicinwhichthelateroneisdeterministicratherthanprobabilisticthedataquantitybeingdeterminedbyprobabilitiesonlyAnysmalldeformationsintheheightorwidthofthepulsesareirrelevantsinceitisonlynecessarytoknowwhetherthepulseispresentorabsentinordertoretrievetheoriginalmessage.toreformtheoriginalinformationtorecovertheoriginalsignaltore-obtaintheoriginalinformationtorepresenttheoriginalsignalConsequently,electronicequipmentisconstantlysubjectedtounwantedsourcesofenergyandisconstantlyproducingenergythatadjacentequipmentisnotdesignedtoaccept.continuallymakingunacceptedinterferencetoadjacentequipmentcontinuouslycausingneighboringequipmentnottoacceptexpectedenergyconstantlyproducingenergythatisacceptableonlytoneighboringequipmentceaselesslygeneratingenergythatisconsideredacceptableinthedesignofadjacentequipmentEarlierweintroducedtheconceptthattheidentityofanamplitudemodulatedmessagesignalmayundercertaincircumstancesbecommunicatedbytransmittingregularsamplesofthemessage,ratherthanthecontinuoussignal.maytransmitsamplesthatareregularincommunicationsundersomeconditionsmaybeinthecircumstancesthatregularmessagesamplesarecommunicatedmaybeconveyedbysendingsignalsampledatregularintervalsundersomeconditions

maycommunicatewithcertaincustomersbysendingregularsamplesofthemessageForeachofthebuildingblocksoftheRFtransceiver,whichhasbeenbrokenintoseparatechapters,alistofessentialreferencepublicationsalsoguaranteesdepthandacademicvalueoftheresearchachievementspresented.eachofthebasiccomponentsoftheRFtransceivereachofthegroupsthatarebuiltintotheRFtransceivertheindividualelementsintheRFtransceivereachofthebuildingsthatblockstheRFtransceiverWearetodevelopacodesuchthatfewerbitsareassignedtocodewordsrepresentinggraylevelshavinghigherprobabilityofoccurrence,andviceversa.inotherwords,graylevelsthatmorefrequentlyoccuraregivenmorebitsandmorebitsareusedforgraylevelsoflowerprobabilityofoccurrenceotherwise,morebitsareneededinthesimilarcaseinotherwords,moreprobablegraylevelsrequireshortercodewordsUnit5

ExercisesTranslatethefollowingpassageintoChinese.OFDMconsistsofalargenumberofsubcarriersequallyspacedinafrequencyband,eachmaybedigitallymodulatedbyasameschemesuchasPSK,QAM,etc.,orbydifferentschemes.Aseriallytransmittedsequenceisdividedintoanumberofsections,eachhavingNsymbols,andtheNsymbolsineachsectionareusedtomodulateNcarriersforsimultaneoustransmission.ThereforeOFDMisessentiallyaparallelmodulationsystem.Whenthenumberofsubcarriersissufficientlylarge,thesystemcanresistmultipathinterference.Thisisbecausethat,inthetimedomain,asymboldurationlongerthanthemultipathdelaycanbechosen,whileinthefrequencydomain,eachsymbolonlyoccupiesasmallportionofthechannel’sfrequencyband.Thus,theeffectofmultipathfadingspreadsovermanysymbols,resultinginslightdistortiontomanysymbolsratherthancompletedestroyofafewsymbols.Inthisway,correctdemodulationisnotaffectedsothatthesignalcanbeaccuratelyrecoveredatthereceiver.工n=0InanOFDMsystem,theprincipleofchoosingthesubcarrierintervalistomakethesubcarriersmutuallyorthogonalwithintheentiresymbolperiod.Thus,evenifspectraloverlapexistsbetweenthesubcarriers,thesymbolscanstillberecoveredwithoutloss.Inordertorealizemaximumspectralefficiency,theintervalbetweensubcarriersisusuallychosentoequalthereciprocalofthesymboldurationT.Thereforethesubcarrierfrequenciesinthebasebandarefn=n/T,(n=0,1,...,N-1).Denotingthen-thmodulatingsymbolasX工n=0(n\(1)X(n)ex3j2兀nt,(1)IT)Samplingthiswaveformatt=T/Nyieldsx(k)=2x(k)=2N-1n=0X(n)exp[j2兀N=2n=0nkX(n)ex3j2兀一，k=1,2,K,N-11N)(2)Itisobservedfromtheaboveexpressionthatx(k)andX(n)formadiscreteFouriertransformpair,thereforethebasebandOFDMwaveformcanbegeneratedfromthediscreteFouriertransformofNmodulatingsymbols.WhenN=2mwheremisaninteger,thefastalgorithmofIDFTiseasytoimplement.OFDM由大量在频率上等间隔的子载波构成。每个子载波可用同样的方案，如PSK,QAM等，或者用不同的方案数字调制。将串行传输的符号序列分成许多段，每一段有N个符号，每段的N个符号被用来分别调制N个载波同时发送。因此，OFDM本质上是一个并行调制系统,当载波数足够大时,这种系统具有抗多径干扰的能力。这是因为在时域上符号周期能选取得比多径延迟时间长,而在频域上每个符号只占整个信道带宽的一小部分,因此多径衰落的影响被分散到许多符号上,其结果是许多符号都发生轻度的畸变,而不是使某些符号被完全破坏。这样,不影响正确的解调,信号能在接收器中准确地恢复。在OFDM系统中，选择子载波间隔的原则是使子载波在整个符号周期相互正交，这样。

即使在子载波间存在频谱部分重叠，符号仍能无损失地恢复。为了实现最大的频谱效率，子载波之间的间隔通常选取为符号周期的倒数。因此基带子载波频率为力=n/T,(n=0,1,...,N-1)，第n个调制符号记为X(n)，在一个符号周期的OFDM波形可表示：记n=记n=0( n\X(n)expj2兀-t,I TJ(1)在t=T/N时刻采样这个波形得到:x(k)=2x(k)=2N-1n=0X(n)exp[j2兀N=2n=0k=1,2,K,N-1(2)从上面的表示中可以看到，x(k)和X(n)构成了一对离散傅里叶变换，因此基带OFDM波形可根据N个调制符号的离散傅里叶变换来生成。当N=2m，其中m为整数，IDFT的快速算法很容易实现。(2)Choosethewordorphrasethatisclosestinmeaningtotheunderlinedpart.Theprocessgainofasystemindicatesthegainorsignaltonoiseimprovementexhibitedbyaspreadspectrumsystembythenatureofthespreadinganddespreadingprocess.byvirtueofthespreading/dispreadingprocessbypro