Satellite-Communication---6--Multiple-Access

2011/4/[e-mail]6:MultipleAccess1.Introduction2.FrequencyDivisionMultipleAccess3.TimeDivisionMultipleAccess4.OnboardProcessing5.CodeDivisionMultipleAccess1.IntroductionMultipleaccess:abilityofthesatellitetocarrymanysignalsatthesametimeAllowsthecommunicationcapacityofthesatellitetobesharedamongalargenumberofearthstationsToaccommodatethedifferentmixesofcommunicationtrafficthataretransmittedbytheearthstationsDistinctionbetweenmultiplexingandmultipleaccessSometimesblurredFormerappliestosignalsthataregeneratedatonelocationLatterreferstosignalsfromanumberofdifferentgeographicallocations1.IntroductionApplicationsemploymultiple-accesssystemstoallowtwoormoreEarthstationstosimultaneouslysharetheresourcesofthesametransponderorfrequencychannel.ThreebasicmultipleaccesstechniquesFDMA:frequencydivisionmultipleaccess,withanalogordigitalsignalsTDMA:timedivisionmultipleaccess,withdigitalsignalsCDMA:codedivisionmultipleaccess,withdigitalsignalsMultipleaccesstechniques:FDMA,TDMA,andCDMA.NotethatinthedirectsequenceformofCDMAshownhere,allthechannelsoverlapinbothtimeandfrequency.1.IntroductionTDMAandFDMArequireadegreeofcoordinationamongusers:FDMAuserscannottransmitonthesamefrequencyandTDMAuserscantransmitonthesamefrequencybutnotatthesametime.Capacityineithercasecanbecalculatedbasedonthetotalbandwidthandpoweravailablewithinthetransponderorsliceofatransponder.CDMAisuniqueinthatmultipleuserstransmitonthesamefrequencyatthesametime.Thisisallowedbecausethetransmissionsuseadifferentcodeeitherintermsofhigh-speedspreadingsequenceorfrequencyhoppingsequence.2011/4/1022.FrequencyDivisionMultipleAccessFDMAThefirstMAtechniqueusedinSATCOMsystemsEachradiostationisallocatedafrequencyandabandwidth,andtransmitsitssignalswithinthatpieceofthefrequencyspectrumwithoutoverlapping.GuardbandsareessentialinFDMAsystemstoallowthefiltersinthereceiverstoselectindividualchannelswithoutexcessiveinterferencefromadjacentchannels.Typically,10to25%ofthechannelbandwidthareneededtominimizeadjacentchannelinterferenceAdvantage:FilterscanbeusedtoseparatesignalsIntermodulationIntermodulationproducts:GeneratedwhenevermorethanonesignaliscarriedbyanonlineardeviceSometimesfilteringcanbeusedtoremovetheIMproducts,butiftheyarewithinthebandwidthofthetranspondertheycannotbefilteredout.Third-orderintermodulation:ThesaturationcharacteristicofatransponderOftenhavefrequenciesclosetothesignalsthatgeneratetheintermodulationLikelytobewithinthetransponderbandwidthIntermodulationAssumingtwounmodulatedcarriersatfrequenciesf1andf2attheinputoftheamplifier,theamplifierinputsignalisTheamplifieroutputsignalisAbThecubicterm,V3outisexpandedas1122coscosVtVt3311221122lineartermcubicterm()coscoscoscosoutininVAVbVAVtAVtbVtVt33112233331122222211222211coscoscoscos2coscos2coscosoutVbVtVtbVtVtVtVtVtVtIntermodulationThefirsttwotermscontainfrequenciesf1,f2,3f1and3f2.3f1and3f2canberemovedwithBPF.Thesecondtwotermsgeneratethethird-orderIMfrequencycomponentsBytrigidentitycos2x=1/2[cos2x+1]wehaveIMtermsofinterestThetermsatf1andf2addtothewantedoutputoftheamplifier.Bytrigidentitycosxcosy=cos(x+y)+cos(x-y),theoutputoftheamplifiercontainsIMfrequencycomponentsgivenbyFilteroutthesumterms,whilethedifferencetermsmayfallwithinthetransponderbandwidth–third-orderIMproductsofHPA.212212221121coscos2coscoscos2cosIMVbVVtttbVVttt'21212122212121cos(2)cos(2)cos(2)cos(2)IMVbVVttbVVttIntermodulationThird-orderIMproductsV3IMisMagnitudeofV3IMdependsonb,whichdescribesthenonlinearityofthetransponderandthemagnitudeofthesignalsThewantedsignalsThetotalpowerofthewantedoutputfromtheHPA,PoweroftheIMproductsattheoutputofHPA22312122121cos(2)cos(2)IMVbVVtbVVt1122coscosoutVAVtAVt2222212121/21/2()outPAVAVAPP262623312122(1/21/2)()IMPbVbVbPPIntermodulationIMproductsincreaseinproportiontothecubesofthesignalpowerswithpowerlevelsdependontheratio(b/A)2.Thegreaterthenonlinearityoftheamplifier(largeb/Aratio),thelargertheIMproducts2011/4/103ExerciseConsiderthecaseofa36MHzbandwidthC-bandtransponderwhichhasanoutputspectrumfordownlinksignalsinthefrequencyrange3705-3741MHz.Thetranspondercarrierstwounmodulatedcarriersat3715and3725MHzwithequalmagnitudesattheinputtotheHPA.FindtheIMfrequencieswithinthetransponderbandwidthConsidertwosignalscarrymodulationwhichspreadsthesignalenergyintoabandwidthof8MHzaroundeachcarrier.FindtheIMfrequenciesCalculationofC/NwithIMThetransponderC/Nratiosappearsas(C/N)upinthecalculationof(C/N)0ratiointheearthstationreceiver.IMnoiseinthetransponderisdefinedby(C/N)IM,enterstheoverall(C/N)0ratiothrough0(/)1/1/(/)1/(/)1/(/)updnIMCNCNCNCNChannelCapacityThetelephonestransmitBPSKsignalsinLbandwithanoccupiedbandwidthof12kHzwithaguardband4kHzandanoutputpowerlevelbetween0.05and0.5W,suchthatthepowerlevelattheinputtothetransponderisalways-144dBWforanyuplinksignal.TheresultingC/Nratioinclearairconditionsforanyonesignalinthetransponderis16dB.Th