Satellite channel

TheabovecontentisreviewedandannouncedbytheNationalScienceandTechnologyTermReviewCommittee

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.

Referencemodelofthesatellitechannel

Inordertobetterstudythepropagationcharacteristicsofthesatellitechannel,themodelofthesatellitechannelmustbestudiedwell.Althoughthebestwaytostudytheinfluenceofchannelpropagationcharacteristicsonthereliabilityandeffectivenessofsatellitemobilecommunicationsystemsistotestandanalyzethechannelsintheactualcommunicationenvironment,itisoftenverydifficulttotesttheactualsatellitemobilecommunicationchannelsanytimeandanywhere.Itisdifficulttoachieve,soadoptingachannelmodelthatcanwellreflectthepropagationcharacteristicsofthesatellitemobilecommunicationchannelisacommonlyusedsolution.

Theprobabilitydistributionmodelsofthepropagationcharacteristicsofsatellitemobilecommunicationchannelscommonlyusedathomeandabroadare:C.Loomodel,CorazzamodelandLutzmodel.Thesethreetypicalprobabilitydistributionmodelsmodelthepropagationcharacteristicsofthesatellitemobilecommunicationchannelbasedontheshieldingofthesignalonthepropagationpath.Amongthem,theC.Loomodelassumesthatonlythedirectsignalcomponentofthereceivedsignalissubjecttoshadowocclusionandthemultipathsignalcomponentisnotaffectedbytheshadowocclusion,sothismodelisalsocalledapartialshadowchannelmodel.TheCorazzamodelassumesthatboththedirectsignalcomponentandthemultipathsignalcomponentinthereceivedsignalaresimultaneouslyshadowed,sothismodelisalsocalledthefullshadowchannelmodel.TheLutzmodelassumesthatinthereceivedsignal,onlymulti-passsignalsareshadedbyshadowsanddirectsignalsarenotaffectedbyshadows.

Ofcourse,inadditiontotheabovemodels,manymodelshavebeenproposed,suchasRician-Kmodel,FractalBrownianMotion(FBM),LMSmodel,etc.

Satellitechannelcodingtechnology

Convolutionalcodes

ConvolutionalcodeswereproposedbyEliasin1955,andaresimpleandefficientcodingmethodsproposedafterblockcodes.Inthe1970s,therapiddevelopmentofViterbi'ssoft-decisiondecodingalgorithmpromotedconvolutionalcodestobewidelyusedinvarioussatellitecommunicationsystemsanddeepspaceexplorationdevices.TheViterbialgorithmcanobtainacodinggainof7dBcomparedtotheuncodedsituation.Comparedwiththeharddecisiondecoding,theViterbisoftdecodinghasanadditionalgainof2dB.Thedecodingperformanceisalsodependentontheselectedcodingrateandthetruncationlengthoftheencoder.Itisrelatedtofactorssuchastheoverallconstraintlengthv.

Concatenatedcoding

In1966,Forneyproposedaserialconcatenatedcodingtechnologythatusesshortcomponentcodestoconstructlongercodes.Itusesnon-binaryandlongercodesasoutercodes.,Binaryandshortercodesareusedasinnercodes,andtheinnerandoutercodesarecascadedserially.Thereceivingendusessoftdecisiondecodingalgorithmsandalgebraicdecodingmethodstodecodetheinnerandoutercodesrespectively.Higherrorcharacteristicsanddecodingcomplexityarealsoacceptable.

Inaddition,bycascadingmultipleinnercodesandoutercodes,amulti-levelconcatenatedcodecanbeformed.Whendecoding,theconcatenatedcodeisdecomposedanddecodedinmultiplestagestoobtainbetterTheerrorcodeperformancegreatlyreducesthecomplexityofdecodingandprovidesgreatflexibilityfordifferentcommunicationsystemenvironments.

Turbocode

In1993,C.BerrouandR.Pyndianproposeda"parallelcascade"Turbocode.Turbocodeisparticularlysuitableforuseinthecaseofmediumerrorraterequirementsandlonginformationgrouping.Throughanappropriateiterativedecodingalgorithm,acodinggainofseveraldecibelscanbeobtainedwithoutincreasingthecomplexityoftheequipment.

LDPCcode

TheLDPCcodewasfirstproposedbyGallagerinthe1860s.Itisalinearblockcode.Whentheblocklengthislarge,theperformanceoftheLDPCcodeisclosetotheShannonlimit..Inthe1990s,beginners'researchoniterativedecodingalgorithmsmadethepracticalapplicationofLDPCareality,andtheydiscoveredthatLDPCcodeshavebettererrorperformancethanTurbocodes.ThecodingefficiencyoftheLDPCcodeisclosetotheShannonlimit,thecodinganddecodingissimple,andthetimedelayissmall.Itisverysuitableforhigh-speedinformationtransmissionsystemsandisthepreferredchannelcodingschemeforfuturesatellitecommunicationsystems.

Codedmodulationtechnology

Codedmodulation(codedmodulation)combinesmodulationanderrorcontroltechnology,byincreasingthenumberofmodulationsignalspacepointstooffsetthespectrumbroadeningcausedbyerrorcontrolcoding,Thecodinggainalsoprovidesahighfrequencybandutilizationrate,whiletakingintoaccountthereliabilityandeffectivenessofcommunication,whichisverysuitableforbandwidth-limitedchannelssuchassatellitecommunicationsystems.

Fountaincoding

AtypicalexampleoffountaincodingistheLTcodeproposedbytheAmericanscholarLubyin1998.TheLTcodeisauniversalfountaincode.Thebasicideaof​​codingisthroughthefountain.Theencoder’slimitedinputandinfiniteoutputmappingfeaturesgeneratedatapackets.Whenthereceiverreceivesacertainnumberofdatapackets,theinformationcanberestoredtoensurethereliabilityofdatatransmission.Fountaincodesareverysuitableforbroadcastdeletion.channel.

Satellitechannelallocationondemand

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.Thesatellitechannelsarearrangedina"saddleshape".