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1、CommunicationSystemAgeneralizedcommunicationsystemhasthefollowingcomponents:(a)InformationSource.Thisproducesamessagewhichmaybewrittenorspokenwords,orsomeformofdata.(b)Transmitter.Thetransmitterconvertsthemessageintoasignal,theformofwhichissuitablefortransmissionoverthecommunicationchanne

2、l.(c)CommunicationChannel.Thecommunicationchannelisthemediumusedtransmitthesignal,fromthetransmittertothereceiver.Thechannelmaybearadiolinkoradirectwireconnection.(d)Receiver.Thereceivercanbethoughtofastheinverseofthetransmitter.Itchangesthereceivedsignalbackintoamessageandpassesthemessag

3、eontoitsdestinationwhichmaybealoudspeaker，teleprinterorcomputerdatabank.Anunfortunatecharacteristicofallcommunicationchannelsisthatnoiseisaddedtothesignal.Thisunwantednoisemaycausedistorionsofsoundinatelephone,orerrorsinatelegraphmessageordata.FrequencyDiversionMultiplexingFrequencyDivers

4、ionMultiplexing(FDM)isaoneofanalogtechnologies.Aspeechsignalis0~3kHz,singlesidebandamplitude(SSB)modulationcanbeusedtotransferspeechsignaltonewfrequencybands,foursimilarsignals,forexample,movedbySSBmodulationtosharethebandfrom5to20kHz.Thegapsbetweenchannelsareknownasguardspacesandtheseall

5、owforerrorsinfrequency,inadequatefiltering,etcintheengineeredsystem.Oncethisnewbasebandsignal,a"group"of4chEmnels,hasbeenfoimeditismovedaroundtheLrunknetworkasasingleunit.Ahierarchycanbesetupwithseveralchannelsfonninga"group".severalgroupsa"supergroup"andseveral"supergraup"eichera"nmsrerg

6、roup"or"hypergroup".Groupsorsupergroupsaremovedaroundassingleunitsbythecommunicationsequipmentanditisnotnecessaryfortheradiostoknowhowmanychannelsareinvolved.Aradiocanhandleasupergroupprovidedsufficientbandwidthisavailable.Thesizeofthegroupsisacompromiseastreatingeachchannelindividuallyin

7、volvesfarmoreequipmentbecauseseparatefilters,modulatorsandoscillatorsarerequiredforeverychannelratherthanforeachgroup.Howeverthefailureofonemodulewillloseallofthechannelsassociatedwithagroup.TimeDiversionMultiplexingItispossible,withpulsemodulationsystems,tousethebe