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Thetransmissioncharacteristicsofthesatellitechannel
ThechannelofsatellitecommunicationisdifferentfromOrdinarymobilechannelsaredifferentfromordinarysatelliteandgroundstationchannels.Satellitecommunicationchannelisafusionofsatellitechannelandmobilecommunicationchannel.Thetransmissioncharacteristicsofthesatellitecommunicationchannelarethattheavailablefrequencybandisverywideandthebandwidthrequirementislow.Atthesametime,theothertransmissioncharacteristicsofthesatellitechannelthatarenearlyshortcomingsarelimitedpower,largeinterference,andlowsignal-to-noiseratio.Therefore,satellitecommunicationshavestrongrequirementsforsignalmodulationmethodsanderrorcorrectioncapabilities,requiringtheuseofmorereliablesignalmodulationmethodsandstrongsignalerrorcorrectioncapabilitiestoachieveperfectsatellitecommunications.Themorereliablemodulationanderrorcorrectionmethodsmainlyincludeforwarderrorcorrection(FEC)andquadraturephaseshiftkeyingusedbyDVB-S.Sincethesignaltransmissiondistanceinsatellitecommunicationissuperfar,usuallytensofthousandsofkilometers,signalattenuationisamoreseriousproblem,whichmakesthereliabilityproblemalife-and-deathproblemthatdeterminesthefateofsatellitecommunication.Fortunately,theemergenceofQPSK(QuadraturePhaseShiftKeying)greatlyguaranteesthereliabilityofsatellitecommunications.Thisdigitalsignalmodulationmethodisusedinmoresatellitechannels,anditscircuitimplementationisrelativelysimple.MostofthereceiversinmycountrythatusemicrowaveantennastodirectlyreceivesatelliteTVusethischannelmodulationmethod.
Satelliittikanavan referenssimalli
Inordertobetterstudythepropagationcharacteristicsofthesatellitechannel,themodelofthesatellitechannelmustbestudiedwell.Althoughthebestwaytostudytheinfluenceofchannelpropagationcharacteristicsonthereliabilityandeffectivenessofsatellitemobilecommunicationsystemsistotestandanalyzethechannelsintheactualcommunicationenvironment,itisoftenverydifficulttotesttheactualsatellitemobilecommunicationchannelsanytimeandanywhere.Itisdifficulttoachieve,soadoptingachannelmodelthatcanwellreflectthepropagationcharacteristicsofthesatellitemobilecommunicationchannelisacommonlyusedsolution.
Theprobabilitydistributionmodelsofthepropagationcharacteristicsofsatellitemobilecommunicationchannelscommonlyusedathomeandabroadare:C.Loomodel,CorazzamodelandLutzmodel.Thesethreetypicalprobabilitydistributionmodelsmodelthepropagationcharacteristicsofthesatellitemobilecommunicationchannelbasedontheshieldingofthesignalonthepropagationpath.Amongthem,theC.Loomodelassumesthatonlythedirectsignalcomponentofthereceivedsignalissubjecttoshadowocclusionandthemultipathsignalcomponentisnotaffectedbytheshadowocclusion,sothismodelisalsocalledapartialshadowchannelmodel.TheCorazzamodelassumesthatboththedirectsignalcomponentandthemultipathsignalcomponentinthereceivedsignalaresimultaneouslyshadowed,sothismodelisalsocalledthefullshadowchannelmodel.TheLutzmodelassumesthatinthereceivedsignal,onlymulti-passsignalsareshadedbyshadowsanddirectsignalsarenotaffectedbyshadows.
Tietysti yllä olevien mallien lisäksi on ehdotettu useita malleja, kuten Rician-Kmodel, FractalBrownianMotion (FBM), LMS-malli jne.
Satelliittikanavakoodaustekniikka
Konvoluutiokoodit
KonvoluutiokooditwereproposedbyEliasin1955,andaresimpleandefficientcodingmethodsproposedafterblockcodes.Inthe1970s,therapiddevelopmentofViterbi'ssoft-decisiondecodingalgorithmpromotedconvolutionalcodestobewidelyusedinvarioussatellitecommunicationsystemsanddeepspaceexplorationdevices.TheViterbialgorithmcanobtainacodinggainof7dBcomparedtotheuncodedsituation.Comparedwiththeharddecisiondecoding,theViterbisoftdecodinghasanadditionalgainof2dB.Thedecodingperformanceisalsodependentontheselectedcodingrateandthetruncationlengthoftheencoder.Itisrelatedtofactorssuchastheoverallconstraintlengthv.
Yhdistetty koodaus
In1966,Forneyproposedaserialconcatenatedcodingtechnologythatusesshortcomponentcodestoconstructlongercodes.Itusesnon-binaryandlongercodesasoutercodes.,Binaryandshortercodesareusedasinnercodes,andtheinnerandoutercodesarecascadedserially.Thereceivingendusessoftdecisiondecodingalgorithmsandalgebraicdecodingmethodstodecodetheinnerandoutercodesrespectively.Higherrorcharacteristicsanddecodingcomplexityarealsoacceptable.
Inaddition,bycascadingmultipleinnercodesandoutercodes,amulti-levelconcatenatedcodecanbeformed.Whendecoding,theconcatenatedcodeisdecomposedanddecodedinmultiplestagestoobtainbetterTheerrorcodeperformancegreatlyreducesthecomplexityofdecodingandprovidesgreatflexibilityfordifferentcommunicationsystemenvironments.
Turbokoodi
In1993,C.BerrouandR.Pyndianproposeda"parallelcascade"Turbokoodi.Turbokoodiisparticularlysuitableforuseinthecaseofmediumerrorraterequirementsandlonginformationgrouping.Throughanappropriateiterativedecodingalgorithm,acodinggainofseveraldecibelscanbeobtainedwithoutincreasingthecomplexityoftheequipment.
LDPC-koodi
TheLDPC-koodiwasfirstproposedbyGallagerinthe1860s.Itisalinearblockcode.Whentheblocklengthislarge,theperformanceoftheLDPC-koodiisclosetotheShannonlimit..Inthe1990s,beginners'researchoniterativedecodingalgorithmsmadethepracticalapplicationofLDPCareality,andtheydiscoveredthatLDPC-koodishavebettererrorperformancethanTurbokoodis.ThecodingefficiencyoftheLDPC-koodiisclosetotheShannonlimit,thecodinganddecodingissimple,andthetimedelayissmall.Itisverysuitableforhigh-speedinformationtransmissionsystemsandisthepreferredchannelcodingschemeforfuturesatellitecommunicationsystems.
Koodattu modulaatiotekniikka
Codedmodulation(codedmodulation)combinesmodulationanderrorcontroltechnology,byincreasingthenumberofmodulationsignalspacepointstooffsetthespectrumbroadeningcausedbyerrorcontrolcoding,Thecodinggainalsoprovidesahighfrequencybandutilizationrate,whiletakingintoaccountthereliabilityandeffectivenessofcommunication,whichisverysuitableforbandwidth-limitedchannelssuchassatellitecommunicationsystems.
Fountain koodaus
AtypicalexampleoffountaincodingistheLTcodeproposedbytheAmericanscholarLubyin1998.TheLTcodeisauniversalfountaincode.Thebasicideaofcodingisthroughthefountain.Theencoder’slimitedinputandinfiniteoutputmappingfeaturesgeneratedatapackets.Whenthereceiverreceivesacertainnumberofdatapackets,theinformationcanberestoredtoensurethereliabilityofdatatransmission.Fountaincodesareverysuitableforbroadcastdeletion.channel.
Satelliittikanavan kohdistaminen pyynnöstä
Inseveralexistingsatellitecommunicationsystems,thesingle-channelsingle-carrier(SCPC)applicationofsatellitechannel(frequency)resourcesisverycommon.Accordingtotheapplicationofthesystemtotheusercommunicationchannel,itcanbedividedintothreeallocationmethods:fixedpre-allocation,semi-on-demandallocationandfull-on-demandallocation.Thesethreeallocationmethodsgraduallyincreasetherequirementsfortheintelligentizationofsystemequipment,buttheuseofsatellitechannelsismoreefficientandtheuser'susefunctionsaremorepowerful.Duetotherapiddevelopmentofcomputertechnology,thesatellitecommunicationsystemadoptingtheall-on-demanddistributionmethodhasbecomerelativelymature.ForasatellitecommunicationsystemwithSCPCandallchannelsallocatedondemand,thenetworkmanagementcenterneedstodealwiththefollowingcontradictoryaspects:1.Reducethewaitingtimeforusersafterdialing.
2.Supporttheuseofdifferentchannelbandwidthsduetodifferentcommunicationservices.
3.Satelliittikanavat on järjestetty "satulamuotoon".