《电子信息专业英语》-Chapter 4 Communications

Text1CommunicationSystemToday，communicationentersourdailylivesinsomanydifferentwaysthatitisveryeasytooverlookthemultitudeofitsfacets.Thetelephonesatourhands，theradiosandtelevisionsinourlivingroom，thecomputerterminalswithaccesstotheInternetinourofficesandhomes，andournewspapersareallcapableofprovidingrapidcommunicationsfromeverycorneroftheglobe.Communicationprovidesthesensesforshipsonthehighseas，aircraftinflight，androcketsandsatellitesinspace.Communicationthroughawirelesstelephonekeepsacardriverintouchwiththeofficeorhomemilesaway.下一页返回Text1CommunicationSystemCommunicationkeepsaweatherforecasterinformedofconditionsmeasuredbyamultitudeofsensors.Indeed，thelistofapplicationsinvolvingtheuseofcommunicationinonewayoranotherisalmostendless.Inthemostfundamentalsense，communicationinvolvesimplicitlythetransmissionofinformationfromonepointtoanotherthroughasuccessionofprocesses，asdescribedhere.1.Thegenerationofamessagesignal:voice，music，picture，orcomputerdata.下一页返回上一页Text1CommunicationSystem2.Thedescriptionofthatmessagesignalwithacertainmeasureofprecision，byasetofsymbols:electrical，aural，orvisual.3.Theencodingofthesesymbolsinaformthatissuitablefortransmissionoveraphysicalmediumofinterest.4.Thetransmissionoftheencodedsymbolstothedesireddestination.5.Thedecodingandreproductionoftheoriginalsymbols.6.There-creationoftheoriginalmessagesignal，withadefinabledegradationinquality;thedegradationiscausedbyimperfectionsinthesystem.下一页返回上一页Text1CommunicationSystemAgeneralizedcommunicationsystemhasthefollowingcomponents(asshowninFig.4-1):InformationSourceThisproducesamessagewhichmaybewrittenorspokenwords，orsomeformofdata.TransmitterThetransmitterconvertsthemessageintoasignal，theformofwhichissuitablefortransmissionoverthecommunicationchannel.CommunicationChannelThecommunicationchannelisthemediumusedtotransmitthesignal，fromthetransmittertothereceiver.下一页返回上一页Text1CommunicationSystemThechannelmaybearadiolinkoradirectwireconnection.ReceiverThereceivercanbethoughtofastheinverseofthetransmitter.Ttconvertsthereceivedsignalbackintoamessageandpassesthemessageontoitsdestinationwhichmaybealoudspeaker，teleprinterorcomputerdatabank.Anunfortunatecharacteristicofallcommunicationchannelsisthatnoiseisaddedtothesignal.Thisunwantednoisemaycausedistortionsofsoundinatelephoneorerrorsinatelegraphmessageordata.下一页返回上一页ReadingChannelWhatIsChannel?Thechannelisacarrierwhichtransmitsmessages—apassagewhichsignalpassesthrough.Theinformationisabstract，thechannelthenisconcrete.Forinstance:Iftwopeopleconverse，theairisthechannel;ifthetwocalleachother,thetelephonelineisthechannel;ifwewatchthetelevision，listentotheradio，thespacetoreceiveandsendisthechannel.下一页返回上一页ReadingTheFunctionofChannelThechannelismainlyusedintransmittingandstoringdataintheinformationsystem，butincommunicationssystemit'smainlyusedintransmitting.TheGoaltoResearchChannelStudyingthechannelinthecommunicationssystemismainlytodescribe，measure，analyzedifferenttypesofchannel，calculatetheircapacity，namely，thelimittransmissioncapacity，andanalyzetheircharacteristics.下一页返回上一页ReadingClassificationProjectphysics-transmissionmediumtype;Mathematicsdescriptionway-descriptionwayofsignalanddisturbance;Parametertypeofthechannelitself-changeableandpermanentparameter;Usertype-singleuserandmulti-user.下一页返回上一页Reading下一页返回上一页Reading下一页返回上一页ReadingWhatIsInformationSource?Informationsourceistheoriginofinformation.下一页返回上一页ReadingInpracticalcommunication,thecommonsourcesare:voice，word,picture，data.，IntheInformationTheory,informationsourceistheoriginofmessage(signal)，message(signal)sequenceandcontinuousmessage.Inmathematics，thesourceistheorigintoproducerandomvariableU,randomsequenceUandrandomprocessU(t，ω).MainCharacteristicsofInformationSourceThebasiccharacteristicofsourceisstatisticallyuncertain.Itcanbedescribedbytheprobabilitystatisticalcharacteristics.返回上一页Text2FDMTDMPCMFrequencyDivisionMultiplexing(FDM)Frequencydivisionmultiplexing(FDM)isoneofanaloguetechniques.Aspeechsignalis0~3kHz，andsinglesidebandamplitude(SSB)modulationcanbeusedtotransferspeechsignaltonewfrequencybands，foursimilarsignals，forexample，movedbySSBmodulaiontosharethebandfrom5to20kHz.Thegapsbetweenchannelsareknownasguardspacesandtheseallowforerrorsinfrequency，inadequatefiltering，etc.intheengineeredsystem.下一页返回Text2FDMTDMPCMOncethisnewbasebandsignal，a“group”of4channels，hasbeenformed，itismovedaroundthetrunknetworkasasingleunit.Ahierarchycanbesetupwithseveralchannelsforminga“group，”severalgroupsa“supergroup”andseveral“supergroup”either“mastergroup”or“hypergroup.”Groupsorsupergroupsaremovedassingleunitsbythecommunicationsequipmentanditisnotnecessaryfortheradiostoknowhowmanychannelsareinvolved.Aradiocanhandleasupergroupprovidedsufficientbandwidthisavailable.下一页返回上一页Text2FDMTDMPCMThesizeofthegroupisacompromiseastreatingeachchannelindividuallyinvolvesfarmoreequipmentbecauseseparatefilters，modulatorsandoscillatorsarerequiredforeverychannelratherthaneachgroup.Howeverthefailureofonemodulewillloseallofthechannelsassociatedwithagroup.TimeDivisionMultiplexing(TDM)Itispossible，withpulsemodulationsystems，tousethetimebetweensamplestotransmitsignalsfromothercircuits.下一页返回上一页Text2FDMTDMPCMThetechniqueisknownastimedivisionmultiplexing(TDM).Todothisitisnecessarytoemploysynchronisedswitchesateachendofthecommunicationlinktoenablesamplestobetransmittedinturn，fromeachofseveralcircuits.Thusseveralsubscribersappeartousethelinksimultaneously.Althougheachuseronlyhasperiodicshorttimeslots，theoriginalanaloguesignalsbetweensamplescanbereconstitutedatthereceiver.TypicalTDMsystemisshowninFig.4-2.下一页返回上一页Text2FDMTDMPCMPulse-CodeModulation(PCM)Pulse-codemodulation(PCM)isthemostbasicformofdigitalpulsemodulation.Inpulse-codemodulation，amessagesignalisrepresentedbyasequenceofcodedpulses，whichisaccomplishedbyrepresentingthesignalindiscreteforminbothtimeandamplitude.Thebasicoperationsperformedinthetransmitter(Fig.4-3)ofPCMaresampling，quantizing，andencoding.Thequantizingandencodingoperationsareusuallyperformedinthesamecircuit，regenerationofimpairedsignals，decoding，andreconstructionofthetrainofquantizedsamples.whichiscalledanalogue-to-digitalconverter.下一页返回上一页Text2FDMTDMPCMThebasicoperationsinthereceiver(Fig.4-4)areregenerationofimpairedsignals,decoding,andreconstructionofthetrainofquantizedsamples.Inwhatfollows，wedescribethevariousoperationsthatconstituteabasicPCMsystem.SnmpliugTheincomingmessagesignalissampledwithatrainofnarrowrectangularpulses.下一页返回上一页Text2FDMTDMPCMQuantizatiouThesampledversionofthemessagesignalisthenquantized，therebyprovidinganewrepresentationofthesignalthatisdiscreteinbothtimeandamplitude.EucocliugAnyplanforrepresentingeachofthisdiscretesetofsamplevaluesasagroupofparticularbinarynumbersiscalledencoding.下一页返回上一页Text2FDMTDMPCMRegenerationThemostimportantfeatureofPCMsystemsliesintheabilitytocontroltheeffectsofdistortionandnoiseproducedbytransmittingaPCMsignalthroughachannel.ThiscapabilityisaccomplishedbyreconstructingthePCMsignalbymeansofachainofregenerativerepeaterslocatedsufficientlyclosespacingalongthetransmissionroute.下一页返回上一页Text2FDMTDMPCMDecodingThedecodingprocessinvolvesgeneratingapulse，theamplitudeofwhichisthelinearsumofallthepulsesinthecodeword，witheachpulsebeingweightedbyitsplacevalue(20，21，22，…，2R-1)inthecode，whereRisthenumberofbitspersample.FilteringThefinaloperationinthereceiveristorecoverthemessagesignalbypassingthedecoderoutputthroughalow-passfilterwhosecut-offfrequencyisequaltothemessagebandwidthW.下一页返回上一页ReadingAsynchronousTransferMode(ATM)DemandforrichmediaservicessuchasInternetaccess，videoondemand，digitaltelevisionandvoiceoverIPgrowsmoreclamorouseveryday.So，too，doestheneedforhigh-performancedistributiontechnology.Tomeetthisdemand，serviceprovidersareturningtoATMtechnology-aflexible，scalablewayofmovinghigh-speedvoice，videoanddataacrossnetworks.ATM'ssophisticatedbandwidthutilizationcapabilitiesenableproviderstoefficientlytransportlarge，complexvideopacketswithouttaxinganetwork.下一页返回上一页ReadingThemajorityoftrafficportedovertheATMinfrastructureisvoiceanddata.Videowillsoonbeasprominentandwilldrivetheneedformorehigh-capacityATMnetworks.ThebasisofATMtechnologyishigh-efficiency，low-latencyswitchingandmultiplexingmechanismideallysuitedtoanenvironmentinwhichtherearespecificbandwidthlimitations.ATMallocatesbandwidthondemandbyconstructingvirtualchannelsandvirtualpathsbetweensourceanddestinationpointsontheATMnetworkboundaries.Thesechannelsarenotdedicatedphysicalconnections，butarepermanentvirtualconnectionsorswitchedvirtualconnectionsthataredeconstructedwhennolongerneeded.下一页返回上一页ReadingATMisacell-switchingandmultiplexingtechnologythatcombinesthebenefitsofcircuitswitching(guaranteedcapacityandconstanttransmissiondelay)withthoseofpacketswitching(flexibilityandefficiencyforintermittenttraffic).Itprovidesscalablebandwidthfromafewmegabitspersecond(Mbps)tomanygigabitspersecond(Gbps).Becauseofitsasynchronousnature，ATMismoreefficientthansynchronoustechnologies，suchastime-divisionmultiplexing(TDM).WithTDM，eachuserisassignedtoatimeslot，andnootherusercansendinthattimeslot.下一页返回上一页ReadingIfastationhasmuchdatatosend，itcansendonlywhenitstimeslotcomesup，evenifallothertimeslotsareempty.However,ifastationhasnothingtotransmitwhenitstimeslotcomesup，thetimeslotissentemptyandiswasted.BecaseATMisasynchronous，timeslotsareavailableondemandwithinformationidentifyingthesourceofthetransmissioncontainedintheheaderofeachATMcell.ATMsupportstwotypesofconnections:point-to-pointandpoint-to-multipoint.下一页返回上一页ReadingPoint-to-pointconnectstwoATMendsystemsandcanbeundirectional(one-waycommunication)orbi-directional(two-waycommunication).Multipointconnectsasingle-sourceendsystems(knownastherootnode)tomultipledestinationendsystems(knownasleaves).Suchconnectionsareundirectionalonly.Rootnodescantransmittoleaves，butleavescannottransmittotherootortoeachotheronthesameconnection.下一页返回上一页ReadingItwouldbedesirableinATMnetworkstohavebi-directionalmultipoint-to-multipointconnections.Suchconnectionsareanalogoustothebroadcastingormulticastingcapabilitiesofshared-mediaLANs，suchasEthernetandTokenRing.Abroadcastingcapabilityiseasytoimplementinshared-mediaLANs，whereallnodesonasingleLANsegmentmustprocessallpacketssentonthatsegment.返回上一页Text3OpticalFiberCommunicationsEmergenceofOpticalFiberCommunicationsOneofthemostimportanttechnologicaldevelopmentsduringthe1980shasbeentheemergenceofopticalfibercommunicationsasamajorinternationalindustry.Theadvantagesofopticalfiberasatransmissionmediumareduetothefactthatthefiberismadeofadielectric(glassorplastic)andthesignalthatiscarriedislight.Theresearcherswhoproducedthefirstcladglassopticalfibersinearly1950swerenotthinkingofusingthemforcommunications.Theywantedtomakeimagingbundlesforendoscopy.下一页返回Text3OpticalFiberCommunicationsThefirstlow-loss(20db/km)silicafiberwasdescribedinapublicationwhichappearedinOctoberof1970.Thedateofthispublicationissometimescitedasthebeginningoftheeraoffibercommunications.OpticalCommunicationsSystemTheopticalcommunicationssystemisessentiallybasedontheuseofopticalfibers.FundamentalopticalcommunicationssystemconstructionisshownasFig.4-5.Opticalfibercablefunctionsasatransmissionmediumintheopticalcommunicationssystem.下一页返回上一页Text3OpticalFiberCommunicationsAlightsourceconvertingelectricalsignalstoopticalsignals(E/Oconverting)issetinthetransmitterside.Anopticaldetectorreconvertingtransmittedopticalsignalstoelectricalsignals(O/Econverting)issetinthereceiverside.Thus，thelightsourceanddetectorelement，aswellasopticalfiberarefunctionallyimportantdevicesintheopticalcommunicationssystem.Asemiconductorlightsourceemitsopticalpowerwhenpassinganelectricalcurrentthroughajunction.Thisjunctionisformedinsidethesemiconductorelement.Therefore，opticaloutputcanbeeasilymodulatedbyadjustingtheelectriccurrentaccordingtosignalstobetransmitted.下一页返回上一页Text3OpticalFiberCommunicationsThesemiconductordetectorabsorbsopticalpower,excitingelectronsinthedetector.Thisprocessresultsinopticalsignalconversiontoelectricalsignalcurrent.Theuseofopticalfiberspresentsmanyadvantages，suchaslessloss，lightweight，wideband，flexibilityandetc.OpticalfiberlossislowerthanPEF(polyethylenefoam)orcoaxialcable，andmillimeterwaveguide.Lowlossvaluesoflessthan1dB/kmareattainedinthe1.0to1.7p,mwavelengthregion，excludingthe1.4p,mwavelength.下一页返回上一页Text3OpticalFiberCommunicationsOpticalcablesarelighterthanothercables，increasinginstallationsimplicityanddecreasingcosts.Opticalfibertransmissioncapacityislargerthananyothercable.Sincetransmissioncapacityvariesfromasmalltoalargenumberofchannels，opticalfibersystemdesignisflexible.Itisapplicabletovirtuallyallfieldscurrentlyusingconventionalcommunicationssystems.TheFutureofFiberOpticsThepotentialoffiberopticsinotherareasisonlybeginningtoberealized.下一页返回上一页Text3OpticalFiberCommunicationsFiberopticnetworksforcomputersystemsandofficesbecomemoreprominent.Inthetelephonesystem,theuseoffiberopticsforinterconnectingcentralofficeswithinametropolitanareaandforlowerlevelsintheswitchinghierarchyisstillincreasingrapidly.Manyobserversbelievethatnationaltelephonesystemswilleventuallybeupgradedtohandlevideobandwidthsbyusingfiberoptics.Thesewidebandsubscriberloopsystemswouldprovideaccesstoservicessuchaspicturephone,videoentertainment.Widespreadinstallationofthesebroadbandserviceswillbecomeeconomicallyfeasible.下一页返回上一页ReadingSatelliteCommunicationsSystemsSatellitecommunicationshasbecomeapartofeverydaylifeinthelate1980s.Aninternationaltelephonecallismadeaseasilyasalocalcalltoafriendwholivesdowntheblock.Wealsoseeinternationalevents，suchasanelectioninEnglandandatennismatchinFrance，withthesameregularityaslocalpoliticalandsportingevents.Inthiscase，atelevisionnewsprogrambringsthesightsandsoundsoftheworldintoourhomeseachnight.下一页返回上一页ReadingThiscapabilitytoexchangeinformationonaglobalbasis，beitatelephonecalloranewsstory，ismadepossiblethroughapowerfulcommunicationstool-thesatellite.Simplystated，asatelliteinageostionaryorbitalpositionappearstobefixedoveroneportionoftheearth.Atanaltitudeof22,300milesabovetheequator,asatellitetravelsatthesamespeedatwhichtheearthrotates，anditsmotionissynchronizedwiththeearth'srotation.Eventhoughthesatelliteismovingatanenormousrateofspeed，itisstationaryintheskyinrelationtoanobserverontheearth.下一页返回上一页ReadingTheprimaryvalueofasatelliteinageostationaryorbitisitsabilitytocommunicatewithgroundstationsinitscoveragearea24hoursaday.Thisorbitalslotalsosimplifiestheestablishmentofthecommunicationslinkbetweenastationandthesatellite.Oncethestation'santennaisproperlyaligned，onlyminoradjustmentsmayhavetobemadeintheantenna'spositionoveraperiodoftime.Theantennaisrepositionedtoasignificantdegreeonlywhenthestationestablishescontactwithasatelliteinadifferentslot.Priortothisera，agroundstation'santennahadtophysicallytrackasatelliteasitmovedacrossthesky.下一页返回上一页ReadingBasedontheseprinciples，threesatellitesplacedinequidistantpositionsaroundtheearthcancreateaworld-widecommunicationssysteminthatalmosteverypointontheearthcanbereachedbysatellite.Asatellitesystemconsistsbasicallyofasatelliteinspacewhichlinksmanyearthstationsontheground，asshownschematicallyinFig.4-6.Theusergeneratesthebasebandsignalwhichisroutedtotheearthstationthroughtheterrestrialnetwork.Theterrestrialnetworkcanbeatelephoneswitchoradedicatedlinktotheearthstation.Attheearthstationthebasebandsignalisprocessedandtransmittedbyamodulatedradiofrequency(RF)carriertothesatellite.下一页返回上一页ReadingThesatellitecanbethoughtofasalargerepeaterinspace.ItreceivesthemodulatedRFcarriersinitsuplink(earth-to-space)frequencyspectrumfromalltheearthstationinthenetwork,amplifiesthesecarriers，andretransmitsthembacktoearthinthedownlink(space-to-earth)frequencyspectruminordertoavoidinterference.ThereceivingearthstationprocessesthemodulatedRFcarrierdowntothebasebandsignalwhichissentthroughtheterrestrialnetworktotheuser.下一页返回上一页ReadingMostcommercialcommunicationssatellitestodayutilizea500MHzbandwidthonuplinkanda500MHzbandwidthonthedownlink.Themostwidelyusedfrequencyspectrumisthe6/4GHzband，withauplinkof5.725to7.075GHzandadownlinkof3.4to4.8GHz.The6/4GHzbandforgeostationarysatellitesisbecomingovercrowdedbecauseitisalsousedbycommoncarriersforterrestrialmicrowavelinks.Satellitesarenowbeingoperatedinthe14/12GHzbandusinganuplinkof12.75to14.8GHzandadownlinkofeither10.7to12.3GHzor12.5to12.7GHz.下一页返回上一页ReadingThe14/12GHzbandwillbeusedextensivelyinthefutureandisnotyetcongested.Butoneproblemexists-rain，whichattenuates14/12GHzsignalsmuchmorethanitdoesthoseat6/4GHz.Thefrequencyspectruminthe30/20GHzbandhasalsobeensetasideforcommercialsatellitecommunications，withadownlinkof18.1to21.1GHzandanuplinkof27.5to31GHz.Equipmentforthe30/20GHzbandisstillintheexperimentalstageandisexpensive.返回上一页Text4MobileCommunicationDevelopmentofMobileSystemMobilesystemsoriginallyoperatedinanaloguemodeinthe450MHzband，movinglaterto900MHzwithdigitalGSMandthento1800MHzwithpersonalcommunicationsystems.Thehistoryofmobilitycansplitintogenerations.Thefirstgenerationsystemsweretheadvancedmobilephonesystems(AMPS)intheUS，totalaccesscommunicationsystem(TACS)inmostofEuropeandNordicmobiletelephonesystem(NMT)，whichwereallanaloguesystems.下一页返回Text4MobileCommunicationThesecondgeneration(2G)isverymuchdominatedbythestandardfirstsetoutinEuropebythegroupspecialmobile(GSM)committee，whichwasdesignedasaglobalmobilecommunicationsystem.Foursystemsareinusenow:D-AMPS，GSM，CDMA，andPDC.Thethirdgenerationmobilecommunicationsystem(3G)currentlybeingdevelopedinEuropeisintendedtointegrateallthedifferentservicesofthesecondgenerationsystemsandcoveramuchwiderrangeofbroadbandservices(voice，data，videoandmultimedia)consistentandcompatiblewithtechnologydevelopmentstakingplacewithinthefixedtelecommunicationnetworks.下一页返回上一页Text4MobileCommunicationTheidealmobiletelephonesystemwouldoperatewithinalimitedassignedfrequencybandandwouldserveanalmostunlimitednumberofusersinunlimitedareas.Threemajorapproachestoachievetheidealare:Single-sideband(SSB)，whichdividestheallocatedfrequencybandintomaximumnumbersofchannels.Cellular,whichreusestheallocatedfrequencybandindifferentgeographiclocations.Spreadspectrum，frequency-hopped，whichgeneratesmanycodesoverawidefrequencyband.下一页返回上一页Text4MobileCommunicationIntroductiontoSomePopularMobileTelephoneSystemsHereweintroducesomepopularmobiletelephoneSystems.CellularMobileTelephoneSystemAbasiccellularsystemconsistsofmobilestations，basestationsandamobileswitchingcenter(MSC).Themobilestationcontainsatransceiver,anantenna，andcontrolcircuitry，andmaybemountedinavehicleorusedasaprotablehand-heldunit.Thebasestationsconsistofseveraltransmittersandreceiverswhichsimultaneouslyhandlefullduplexcommunicationsandgenerallyhavetowerswhichsupportseveraltransmittingandreceivingantennas.下一页返回上一页Text4MobileCommunicationThebasestationservesasabridgebetweenallmobileuserinthecellandconnectsthesimultaneousmobilecallsviatelephonelinesormicrowavelinkstotheMSC.TheMSCcoordinatestheactivitiesofallofthebasestationsandconnectstheentirecellularsystemtothePSTN.AtypicalMSChandles100000cellularsubscribersand5000simultaneousconversationsatatime，andaccommodatesallbillingandsystemmaintenancefunctions，aswell.Inlargecities，severalMSCsareusedbyasinglecarrier.下一页返回上一页Text4MobileCommunicationGlobal

System

forMobileCommuuicntious(GSM)Thesuccessofmobilesystemsacrosstheworldisasignthatcommunicationismovingtowardsamorepersonalized，convenientsystem.TheGSMsystemisbasedonacellularcommunicationsprinciple，buttheuseofdigitalradiotransimissionandtheadvancedhandoveralgorithmsbetweenradiocellsinGSMnetworkallowsforsignificantlybetterfrequencyusagethaninanaloguecellularsystems，thusincreasigthenumberofsubscribersthatcanbeserved.下一页返回上一页Text4MobileCommunicationSinceGSMprovidescommonstandard，cellularsubscriberswillalsobeabletousetheirtelephonesovertheentireGSMservicearea.RoamingisfullyautomaticbetweenandwithinallcountriescoveredbyGSMsystem.Inadditiontointernationalroaming，GSMprovidesnewservices，suchashigh-speeddatacommunication，facsimileandshortmessageservices.TheGSMtechnicalspecificationsaredesignedtoworkinconcertwithotherstandards，e.g.ISDN(theintegratedservicesdigitalnetwork).Interworkingbetweenthestandardsisinthiswayassured.下一页返回上一页Text4MobileCommunicationInthelongtermperspectivecellularsystems，usingadigitaltechnologywillbecometheuniversalmethodoftelecommunication.TheGSMsystemoperatesinabursttransmissionmodewith124radiochannelsinthe900MHzband，andtheseburstscancarrydifferenttypesofinformation.Thefirsttypeofinformationisspeech，whichiscodedat6.5kb/sor13kb/s.Thesecondtypeisdata，whichcanbesentat3.6kb/s，6kb/sor12.6kb/s.Thesetwoformsoftransmissionaretheusefulpartsofthetransmission，buthavetobesupportedbyoverheadinformationwhichissentincontrolchannels.下一页返回上一页Text4MobileCommunicationCodeDivisionMultipleAccess(CDMA)CDMAiscompletelydifferentfromAMPS，D-AMPS，andGSM.Insteadofdividingtheallowedfrequencyrangeintoafewhundrednarrowchannels，CDMAallowseachstationtotransmitovertheentirefrequencyspectrumalltime.Multiplesimultaneoustransmissionsareseparatedusingcodingtheory.CDMAalsorelaxestheassumptionthatcollidingframesaretotallygarbled.Instead，itassumesthatmultiplesignalsaddlinearly.ThekeytoCDMAistobeabletoextractthedesiredsignalwhilerejectingeverythingelseasrandomnoise.下一页返回上一页Text4MobileCommunicationCDMAalsohasmanyothercomplicatedfactorsthathavebeenglossedoverhere.Nevertheless，CDMAisacleverschemethatisbeingrapidlyintroducedforwirelessmobilecommunication.Itnormallyoperatesinabandof1.25MHz(versus30kHzforD-AMPSand200kHzforGSM)，butitsupportsmanymoreusersinthatbandthaneitheroftheothersystems.Inpractice，thebandwidthofCDMAavailabletoeachuserisatleastasgoodasthatofGSMandoftenmuchbetter.下一页返回上一页ReadingOnMobileOfficeMobileofficeisthemutualproductofeconomic，scientific，andsocialprogress.Mobileofficehasbecomeasolutionthatprovidesuserswithconvenient，prompt，safe，reliable，andreasonablypricedcommunicationsandofficefacultyanywhereanytimeviathesupportofmobileinterconnectionplatform(MIP)anditsapplicationssystems.UsingmobileofficeandWAPtechnology，peoplecandotheirworkanywhereanytime，cansendandreceivedataviaterminalssuchasmobilephone，palmcomputer,andPDA，andcansu