复件专业英语课后答案清华

UnitUnit1(1)(1)TranslatethefollowingsentencesintoAswithseriesresonance,thegreatertheinthecircuitthelowertheQand,accordingly,theflatterandbroadertheresonancecurveofeitherlinecurrentorcircuitAwirecarryingacurrentlooksexactlythesameandweighsexactlythesameasitdoeswhenitisnotcarryingacurrent.Clickmouseonthewaveformanddragittochangethepulserepetitionrate,ordirectlyenteranewvalueoftheperiodintheprovideddialoguebox,whilekeethepulsewidth提供的框中直接输入新的周期值而保持脉冲宽度Electronicsisthescienceandthetechnologyofthepassageofchargedparticlesinagas,inavacuum,orinasemiconductor.Pleasenotethatparticlemotionconfinedwithinametalonlyisnotconsideredelectronics.Hardwaretechnologieshaveplayedvitalrolesinourabilitytouseelectronicpropertiestoprocessinformation,butsoftwareanddataprocessingaspectshavenotdevelopedatthesamespeed.However,inaproperlydesignedDCamplifiertheeffectoftransistorparametervariation,otherthanIco,maybepracticallyeliminatediftheoperationpointofeachstageisadjustedsothatitremainsinthelinearoperationrangeofthetransistorastemperaturevaries.Ico以外的晶体管参数变化所造成的影响。(2)(2)FillintheblankswiththemostappropriateAmessagesignalcan anyamplitudevaluebetween0andAvolts.A.assumeB.useC.receiveD.Eachtimeapositiveclockedgeoccurs,theflip-flopchangesstate,leadingtohalfasmanypulsesattheoutputtotheclockinput.A. B. C.dueD.soIntheprevioussectionwediscussedsine-waveorCWmodulationsystemsinwhichthemodulatingsignal ofadigitalpulseA.madeB. C.containedInthisbookweshallemphasizethelimitationsimposedontheinformationtransmittedbythesystemthroughwhichit and differentA. B.traveledC. D.Thissignaleitherdoesnotchange,oritchanges anamountequivalenttooneormorequantumsteps.A.interruptedly B.rudely C.abruptlyD.Weshallonlybe withlogicalrepresentations,absoluteA.interestedB. C.referredD.(3)Choosethephrasethatisclosestinmeaningtounderlined(3)ChoosethephrasethatisclosestinmeaningtounderlinedwhichhasbadinfluenceuponthesystemwhichisimportanttoensurecircuitwhichispossibletoenhancethesystemwhichiscrucialformaintainingthedesiredThereforetherewillbeaslighttimedelay,duetothepropagationdelayoftheflip-flopsbetweenthetimeoneflip-flopchangesstateandthetimethenextonechangesstate,i.e.,thechangeofstateripplesthroughthecounter,andthesecountersarethereforecalledripplecounters.thestatechangeispropagatedthroughthecounterlikethestatechangescauseripplesinthepassingthroughthecounter,thestateripplethestateofthecounterchangesduetoInsteadoftriggeringoneachpulse,aphaselocktechniqueexaminestherelativephasebetweeneachoscillatorandmanyofitssyncpulsesandadjustsoscillatorfrequencysothattheaveragephasediscrepancyissmall.checkseachoscillatorformanysync oscillatorsandacollectionofsyncpulseslooksattherelationsbetweenthephaseandthesyncexamineswhetherthephaseisrelativetotheoscillatorandthesyncpulsesBykeetheoffstateslightlybelowthreshold,thedelaybetweentheappliedelectricalpulseandtheresultingopticaloutputpulseisminimized;thisdelaymustindeednotbemorethanthebitintervalsothattheopticalpulsecanaccurayreproducetheinputsignal.ensuringthatthesystemisoffthethresholdbyasmallmaintainingthesystem’sshut-downstatesomewhatbelowaprescribedlevelmakingsurethesystemisbelowitsthresholdsothatthestateiskeptoffkeethesysteminashut-downstatethatisunderagivenlevelwithatinytoleranceIftheoriginalsignaliswellbehaved,thelocaloscillatorwillneedverylittleinformationtobeabletotrack,andthatinformationcanbeobtainedbyaveragingforalongperiodoftime,therebyeliminatingnoisethatcouldbeverylarge.theLOwillnotrequireanyinformationinordertokeeptheLOwillmakeuseoftrackinginformationthatisscarceinthesystemtheLOwillrequiresomeinformation,butnotmuch,sothatthesystemcantracktheLOwillusenoinformationotherthansomelittleonetomaintaintrackingThethirdsectiondescribeszationschemesthattakeaccountofthecharacteristicsofspeech.takemeasureofthespeechtakethespeechcharacteristicsintocountthespeechmakeuseofthespeechInaproperlydesignedDCamplifiertheeffectoftransistorparametervariationmaybepracticallyeliminatediftheoperatingpointofeachstageisadjustedsothatitremainsinthelinearoperationrangeofthetransistorastemperaturetheeffectivevariationofthetransistortheefforttochangetransistorthetransistorparametersthateffectivelytheresultscausedbytransistorparameterStabilityandrepeatabilityarefundamentalattributesofdigitalcircuits,however,andthenecessaryaccuracycanbeobtainedinadigitalversionofthemodulatorbyanappropriatechoiceofwordlengthtorepresentthebyexpressingthesignalswithasuitablewordbyusingawordlongenoughtodescribethebychoosetocharacterizethesignalsintermsofwordlengthappropriaybyappropriayrepresentingthesignalstochoosealengthofwordsUnitUnit2(1)TranslatethefollowingsentencesintoIfogsignalsaretobetransmitteddigitally,theyfirsthavetobesampledataspecifiedrateandfurtherconvertedtodiscreteamplitudesamplesbyzation.Linearfiltersamplifyorattenuateselectedspatialfrequencies,canachievesucheffectsassmoothingandsharpening,andusuallyformthebasisofre-samplingandboundarydetectionalgorithms.Stabilityandrepeatabilityarefundamentalattributesofdigitalcircuits,however,andthenecessaryaccuracycanbeobtainedinadigitalversionofthemodulatorbyanappropriatechoiceofwordlengthtorepresentthesignals.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Themainfunctionofthereceiveristoextracttheinputmessagesignalfromthedegradedversionofthetransmittedsignalcomingfromthechannel.的主要功能是从由信道传输来的了的信号中(2)Choosethewordorphrasethatisclosestin(2)ChoosethewordorphrasethatisclosestinmeaningtheunderlinedWhilethirdpartydesigntoolswereavailable,therewasnotaneffectivelinkfromthethirdpartydesigntoolstothelayoutandactualsemiconductorprocessperformancecharacteristicsofthevariousASICmanufacturers.Thesesteps,implementedwithalevelofskillcommonintheindustry,almostalwaysproduceafinaldevicethatcorrectlyimplementstheoriginaldesign,unlessflawsarelaterintroducedbythephysicalfabricationprocess.practicedusingskillsgenerallyavailableinthecommonlyperformedatlevelsofskilled nelintherealizedwiththeskillfulcommonlevelinthecarriedoutbyskillscommonlyusedintheAsecondmajorstepwouldbetodevelopdesignspecificationsthatconsiderthefunctionalityofthehumanwiththesamedegreeofcarethathasbeengiventotherestofthesystem.developdesignprocedurestoaccountforthehumanbehaviorandactivitiespromotedesignarrangementsandtakethehumanfactorsintoconsiderationmakeusedofdesignstepsthatrespectthehumandraftandeffectdesignrequirementsthattakeintoaccountthehumanfunctionalityClickmouseonthewaveformanddragittochangethepulserepetitionrateordirectlyenteranewvalueoftheperiodintheprovideddialoguebox,keethepulsewidthunchanged.simplytypeanewperiodvalueinthegivendialoguedirectanentryforthenewperiodvaluetothedialoguepointastraightforwardnewperiodvaluetothedialogueboxprovidedenteradirectionofanewperiodvaluetoprovideadialogueboxComparedwithmostofthetextbooksattheintroductorylevelofRFogdesign,thisbookpresentssomehands-onexperiencesonthedesignissuesthatreallyworkandhasbeenprovedbymanufacturedchips.thisbookprovidessomeexperiencesathandontheissuesencounteredindesignthisbookprovidessomeactuallyworkingexperiencesaboutthedesignproblemsthisbookpresentssomeavailableexperiencesoverthedesignproblems experiencesthatarehandedoverForadecadetherehasbeenasearchforaprogrammableprocessorwhich,giventhecostandvolumeconstraints,isgeneralenoughtobeprogrammedwithdifferentwithcontrolledmeasuresofpriceandgiventheconditionofriskandgiventheconstantsofchargeandgiventhelimitsofpriceandInaproperlydesignedDCamplifiertheeffectoftransistorparametervariation,otherthanIco,maybepracticallyeliminatediftheoperatingpointofeachstageisadjustedsothatitremainsinthelinearoperationrangeofthetransistorastemperaturevaries.Unit (1)(1)TranslatethefollowingsentencesintoAverycommonlyusedmethodofmicrowavemeasurementsisbasedonthestudyofastandingwavepatternformedalongthelinebecauseoftheinterferenceofincidentandreflectedwaves.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而沿着传输Computationsshowareductionof6dBintheEMfieldintensitywiththedistancedoubled,andanincreaseof3dBintheintensitywiththetransmittedpowerdoubled.Thisresultprovidesverificationtothealgorithmused.6分贝；Liketransferofheatbyconduction,convectionandradiation,microwaveheatingcanbeconsideredasanothermodeofheattransfer,inwhichheatisproduceddirectlyatthelocationofthedielectriclosses.Operationoftheradarisbasedonthemeasurementofthetimeittakesforapulsetransmittedfromanantennatogetreflectedbytheobjecttobedetectedandtoreturnattheantennaandthereceiver.的工作是基于对天线发出的脉冲被所要探测的目（OthermotivationsforusingCADincircuitdesignincludes plishedbyothermeans,and,notleastinimportance,asenseofcuriosityorperhapsadesiretodiscovertheCAD的其它动机包括：想要验证用Thespacingbetweentherepeatingamplifiersisafunctionofthecableattenuationandthesystembandwidthsothatthegainprovidedbytheseamplifierscompensatesforthelossintroducedbythecable.Whenthecut-offfrequencyisnolessthanthealfrequencyoftheoriginalsignal,norisitgreaterthanthedifferencebetweenthesamplingfrequencyandthealfrequency,theoriginalsignalmaybecompleyrebuilt.WhilethepoliticsofthelastonehundredyearsaroundIndustrialAgetechnology,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、保护和交换。(2)Choosethewordorphrasethatisclosestinmeaningtotheunderlinedpart.Iftheseotherchargesandcurrentsarecomparableinsizetothesourcesproducingtheaboveelectromagneticfield,thenanewnetelectromagneticfieldwillbeproduced.makecomparisonwiththesizeofthearecomparedwiththesizeofthehavesimilarsizetothetakeacommonsizeoftheThisrangeofwavelengthshasledmanytoquestionthenamingconventionusedformicrowavesasthenamesuggestsamicrometerwavelength.hasraisedmanyquestionsaboutthenaminghascausedmanypeopletochallengethenaminghasledmanyquestionsofthenaminghasresultedinmanyproblemsinthenamingNASAworkedinthe1970sandearly1980storesearchthepossibilitiesofusingSolarPowerSalite(SPS)systemswithlargesolararraysthatwouldbeampowerdowntotheEarth’ssurfaceviamicrowaves.shedsolarenergytotheEarth’ssurfacefromthesasetthesalitedowntotheEarth’ssurfacewithshutdownthepoweron(to)theEarth’ssurfacefromtheswitchoffpowertotheEarth’ssurfacefromthesaAsDoDurgentlywantedmilitarycommandandnetworksthatcouldsurviveanuclearwar,ARPAwaschargedwithinventingatechnologythatcouldgetdatatoitsdestinationreliablyevenifarbitrarypartsofthenetworkdisappearedwithoutwarningasaresultofanuclearattack.winanuclearbeusedinpreparationofanuclearavoidanuclearbestilloperationalafteranuclearAtthepresent,stateoftheartmicrowaveamplifiersandoscillatortubescanoperateinafrequencyrangeupto40GHzandsolid-statemicrowavedevicesupto100GHz.recentandmosthightechartisticallyInsuchafiberraystravelingatlargerangleswithrespecttotheaxishavetotraversealongerpathandhencetakealongertimethanthoserayswhichpropagatewithsmalleranglestotheaxis.havetransmittedoveralongerareboundtotreatalongermusttravelthroughalongermustdealwithalongerBykeetheoffstateslightlybelowthreshold,thedelaybetweentheappliedelectricalpulseandtheresultingopticaloutputpulseisminimized;thisdelaymustindeednotbemorethanthebitintervalsothattheopticalpulsecanaccurayreproducetheinputsignal.thisdelaymustindeedbenomorethanoneinternalindeed,thisdelaymustexceedthebitofasamatteroffact,thisdelayintervalmustbealittlebitthisdelaymustindeedbekeptlessthanthespacingbetweendatabitsComputationsshowareductionof6dBintheEMfieldintensitywiththedistancedoubled,andanincreaseof3dBintheintensitywiththetransmittedpowerdoubled.Thisresultprovidesverificationtothealgorithmused.that,witha6dBreduction,theEMfieldintensityistwiceasstrongthattheEMfieldintensityisdecreasedby6dBastherangeisdoubledthat,intheEMfield,intensitywithdoubledistanceleadstoa6dBreductiona6dBintensityreductioncausesadoubledistanceintheEMfieldUnitUnit (1)TranslatethefollowingpassagesintoCommunicationmaybebroadlydefinedasthetransferofinformationfromonepointtoanother.Whentheinformationistobeconveyedoveranydistanceacommunicationsystemisusuallyrequired.Withinacommunicationsystemtheinformationtransferisfrequentlyachievedbysuperimposingormodulatingtheinformationontoanelectromagneticwavewhichactsasacarrierfortheinformationsignal.Thismodulatedcarrieristhentransmittedtotherequireddestinationwhereitisreceivedandtheoriginalinformationsignalisobtainedbydemodulation.Sophisticatedtechniqueshavebeendevelopedforthisprocessbyusingelectromagneticcarrierwavesoperatingatradiofrequenciesaswellasmicrowaveandmillimeterwavefrequencies.However,communicationmayalsobeachievedbyusinganelectromagneticcarrierthatisselectedfromtheopticalrangeoffrequencies.要将信息传输到任何距离以外时就需要有一个通信系Electroniccommunicationsisthetransferandmovementofdatabetweenlocationsthroughtheuseofcomputers.Anelectroniccommunicationsystemincludestheequipmentneededtosupportthemovementofinformation,thecommunicationlinesandmediatocarrytheinformation,thecomputersoftwareandprogramstocontroltheflowofinformation,theneltoplan,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Firstgenerationcommunicationssystemscanbecharacterizedbytheuseofogtransmissiontechniques,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2000CDMA10倍。Foreachcombinationofcommunication(modulation/detection)type,channelfadingmodel,anddiversitytype,theaveragebiterrorrate(BER)and/orsymbolerrorrate(SER)ofthesystemisobtainedandrepresentedbyanexpressioninaformthatcanreadilybeevaluated.Allcasesconsideredcorrespondtorealpracticalchannels,andinmanyinstancestheBERandSERexpressionsobtainedcanbeevaluatednumericallyonahand-heldcalculator.（SERSER表达式均可用手持计算器来计算。Modulationisthesystematicvariationofsomeattributeofacarrierwaveformsuchastheamplitude,phase,orfrequencyinaccordancewithafunctionofthemessagesignal.Itisusedincommunicationsystemsformatchingsignalcharacteristicstochannelcharacteristics,forreducingnoiseandinterference,forsimultaneouslytransmittingseveralsignalsoverasinglechannel,andfor someequipmentlimitations.Aconsiderableportionofthisbookisdevotedtothestudyofhowmodulationschemesaredeignedtoachievetheabovetasks.Thesuccessofacommunicationsystemdependstoalargeextentonthe相当大部分内容是研究如何设计调制方案以实现上述underlinedThereceivingdevicewouldhavethewrongtimethathadbeenagreedintheprotocolforsending/receivingdata,sosomebytescould ecorruptedbylosingbits.thathadbeenconsistentwiththeprotocolofdata sending/receivingdatathathadbeeninagreementwiththeprotocolofdatathathadbeendefinedinthedatatransmissionSynchronoustransmissionusesnostartandstopbitsbutinsteadsynchronizestransmissionspeedsatboththereceivingandsendingendofthetransmissionusingclocksignalsbuiltintoeachcomponent.usingsynchronizationsignalstobuildeach usingtimingsignalsinordertobuildeachusingclocksthatcanbuildsignalInformationtheory,however,doesnotinvolvemessageimportanceormeaning,asthesearemattersofthequalityofdataratherthanthetyofdata,thelatterofwhichisdeterminedsolelybyprobabilities.whichisthelateroneofthedeterminedthelateronebeingprobablyinwhichthelateroneisdeterministicratherthanthe tybeingdeterminedbyprobabilitiesAnysmalldeformationsintheheightorwidthofthepulsesareirrelevantsinceitisonlynecessarytoknowwhetherthepulseispresentorabsentinordertoretrievetheoriginaltoreformtheoriginaltorecovertheoriginaltore-obtaintheoriginaltorepresenttheoriginalConsequently,electronicequipmentisconstantlysubjectedtounwantedsourcesofenergyandisconstantlyproducingenergythatadjacentequipmentisnotdesignedtoaccept.continuallymakingunacceptedinterferencetoadjacentcontinuouslycausingneighboringequipmentnottoacceptexpectedenergyconstantlyproducingenergythatisacceptableonlytoneighboringequipment acceptableinthedesignofadjacentequipmentEarlierweintroducedtheconceptthattheidentityofanamplitudemodulatedmessagesignalmayundercertaincircumstancesbecommunicatedbytransmittingregularsamplesofthemessage,ratherthanthecontinuoussignal.maytransmitsamplesthatareregularincommunicationsundersomeconditionsmaybeinthecircumstancesthatregularmessagesamplesarecommunicatedmaybeconveyedbysendingsignalsampledatregularintervalsundersomeconditionsmaycommunicatewithcertaincustomersbysendingregularsamplesofthemessageForeachofthebuildingblocksoftheRFtransceiver,whichhasbeenbrokenintoseparatechapters,alistofessentialreferencepublicationsalsoguaranteesdepthandacademicvalueoftheresearchachievementspresented.eachofthebasiccomponentsoftheRFeachofthegroupsthatarebuiltintotheRFtheindividualelementsintheRFeachofthebuildingsthatblockstheRFWearetodevelopacodesuchthatfewerbitsareassignedtocodewordsrepresentinggraylevelshavinghigherprobabilityofoccurrence,andviceversa.inotherwords,graylevelsthatmorefrequentlyoccuraregivenmorebits probabilityofoccurrenceotherwise,morebitsareneededinthesimilarinotherwords,moreprobablegraylevelsrequireshortercodewordsUnit (1)(1)TranslatethefollowingpassageintoOFDMconsistsofalargenumberofsubcarriersequallyspacedinafrequencyband,eachmaybedigitallymodulatedbyasameschemesuchasPSK,QAM,etc.,orbydifferentschemes.Aseriallytransmittedsequenceisdividedintoanumberofsections,eachhavingNsymbols,andtheNsymbolsineachsectionareusedtomodulateNcarriersforsimultaneoustransmission.ThereforeOFDMisessentiallyaparallelmodulationsystem.Whenthenumberofsubcarriersissufficientlylarge,thesystemcanresistmultipathinterference.Thisisbecausethat,inthetime,asymboldurationlongerthanthemultipathdelaycanbechosen,whileinthefrequency,eachsymbolonlyoccupiesasmallportionofthechannel’sfrequencyband.Thus,theeffectofmultipathfadingspreadsovermanysymbols,resultinginslightdistortiontomanysymbolsratherthancompletedestroyofafewsymbols.Inthisway,correctdemodulationisnotaffectedsothatthesignalcanbeaccurayrecoveredatthereceiver.InanOFDMsystem,theprincipleofchoosingtheintervalistomakethesubcarriersmutuallyorthogonalwithintheentiresymbolperiod.Thus,evenifspectraloverlapexistsbetweenthesubcarriers,thesymbolscanstillberecoveredwithoutloss.Inordertorealizeumspectralefficiency,theintervalbetweensubcarriersisusuallychosentoequalthereciprocalofthesymboldurationT.Thereforethesubcarrierfrequenciesinthebasebandarefn=n/T,(n=0,1,...,N1).Denotingthen-thmodulatingsymbolasX(n),theOFDMwaveformwithinasymboldurationcanbeexpressedas:x(t)N1X(n)expj2nt 0t

T T Samplingthiswaveformatt=T/Nx(k)N1X(n)expj2kTN1X(n)expj2nk

k1,2,,N

NNNN

Itisobservedfromtheaboveexpressionthatx(k)andX(n)formadiscreteFouriertransformpair,thereforethebasebandOFDMwaveformcanbegeneratedfromthediscreteFouriertransformofNmodulatingsymbols.WhenN=2mwheremisaninteger,thefastalgorithmofIDFTiseasytoimplement.载波频率为fn=n/T,(n=0,1,...,N1)，第n个调制符号记为x(t)N1X(n)expj2nt 0t

T T 在tT/Nx(k)N1X(n)expj2kTN1X(n)expj2nk

k1,2,,N

NNNN

从上面的表示中可以看到，x(k)和X(n)构成了一对离散傅(2)Choosethewordorphrasethatisclosestinmeaningtotheunderlinedpart.Theprocessgainofasystemindicatesthegainorsignaltonoiseimprovementexhibitedbyaspreadspectrumsystembythenatureofthespreadinganddespreadingprocess.byvirtueofthespreading/dispreadingbyprocessingthespreading/despreadingaccordingtothespreading/despreadingusingthenaturalspreading/dispreadingDuetotheunavoidabletimingerrorsbetweentheusers,thereislittlepointinusingWalshcodesastheywillnolongerbeorthogonal.usingWalshcodeswillhavesomepointduetotheabsenceoforthogonalityusingWalshcodesismeaninglesssinceorthogonalitywillnotexistanymoreWalshcodecannotbeusedsincetheyarenot orthogonalityislostForexample,whenapartofthechannelbandwidthisfaded,frequencyinterleavingensuresthatthebiterrorsthatwouldresultfromthosesubcarriersinthefadedpartofthebandwidtharespreadoutinthebit-streamratherthanbeingthatwouldbeproducedinthosesubcarrierssufferingfromfrequencyselectivefadingthatwouldcausesomesubcarriersbeingfadedinsomepartofthebandwidththatwouldcamefromthefadingsubcarriersfallinginpartofthebandwidththatwouldresultinthosefadedsubcarrierofthepartofthebandwidthThebasicideaofapplyingthebeamformertechniquetothisapproachisverypromisingandmightgiveasignificantimprovementforsourcelocalization.mightobtainformidablegaininlocalprovidepossibilitiestosubstantiallyenhancesourcemightpresentamajoradvanceinlocalizingislikelytoconsiderablyrefinetechniquestolocateEarlyweintroducedtheconceptthattheidentityofanamplitudemodulatedmessagesignalmayundercertaincircumstancesbecommunicatedbytransmittingregularsamplesofthemessage,ratherthanthecontinuoussignal.valuesofthemessagesignaltakenatregularformallyobtainedmessagesamplesofthemessageproducedinaspecificsamplesoftheregularlyobtainedHavingdefinedthesefunctions,wewillproceedtoshowhowtheycanbeusedtoevaluatecertainintegralscharacteristicofsystemerrorperformance.evaluatecharacteristicsofcertainintegralsinsystemerrorperformanceassesscertainintegralsthataretypicalindescribingerrorperformanceofthesystemsdecidethevalueofsomecharacteristicintegralsoferrorperformanceofthesystemscarryoutevaluationsontheintegratedcharacteristicsofsystemerrorperformanceModulationisthesystematicvariationofsomeattributeofacarrierwaveformsuchastheamplitude,phase,orfrequencyinaccordancewithafunctionofthemessageintermsofamessagecarriedbytheaccordingtothebehavioroftheduetotheperformanceoftheinrelationwith tyderivedfromtheOverrecentyearsthispotentialhaslargelybeenrealizedinthecostsoftheopticalfibertransmissionmediumthatforbulkpurchasesisnow ingcompetitivewithcopperthatisspecificallyforpurchases,andis superiortocopperwiresthatisbulkilycomparableinpricewithcopperthatiscompetingwithcopperwiresintermsoftheofboughtboughtinathat ingachallengetocopperwiresUnit6(1)(1)Translatethefollowingsentencesintopointtoanother.Thistransmissionrequirestheabilitytovarysignalswithtimeinamannerwhichisunpredictabletothereceiver.Itistrueofanycommunicationsystemthattheshapeandamplitudeofthetransmittedsignalwillbecontinuouslydegradedbytheintroductionofnoise,andtheattenuationalongthetransmissionpath.传输的信号会不断地被噪声的引入和线的衰减所损Sincetheverybeginning,communicationssystemshaveconsistedofthreemajorparts,namely,theequipment,thethecloselinkof municationswiththecomputertechnologyhasbroughtabouttremendouschangesincommunications,fromtheconcepts,tothecontentsandthe通信从概念到内容和都发生了重大的变化。Theavailabilityofsuchalargecollectionofsystemperformancecurvesinasinglecompilationallowstheresearcherorsystemdesignertoperformtrade-offstudiesamongthevariouscommunicationtype/fadingchannel/diversitycombinationssoastodeterminetheoptimumchoiceinthefaceofhisorheravailable系统设计者能在各种通信类型、信道、分集组合之间进行比较研究，从而在他或她所的限制之下做出Thechannelprovidestheelectricalconnectionbetweentheinformationsourceandtheuser.Thechannelcanhavemanydifferentformssuchasamicrowaveradiolinkoverfreespace,apairofwires,oranopticalfiber.Thedegradationofthetransmittedsignalisaresultofsignaldistortionduetoimperfectresponseofthechannelandduetoundesirableelectricalsignals(noise)andinterference.Noiseandsignaldistortionaretwobasicproblemsofelectricalcommunication.传输信号的是信号失真的结果，而这种失真又是由Choosethephrasethatisclosestinmeaningtotheunderlinedpart.Thisispossiblebecausestationsthatarephysicallyfarenoughfromeachothercanusethesamecarrierfrequency(frequencyreuse)withnegligiblemutualinterference,exceptatsomepointinthemiddlewherethestationsarereceivedwithcomparablesignallevels.wherethesignalsfromdifferentstationsaresimilarinwherethereceivingstationscanbecomparedwiththesignallevelswherethestationscanreceivecomparativelyhighlevelSpacediversity