《電子信息類專業(yè)英語 》課件-第13章

Unit13MicrowaveandSatelliteCommunicationsPassageAMicrowaveandSatelliteCommunicationBasedRadioSystemsPassageBTheCompositeLinkofSatelliteCommunicationsPassageCTheMobileSatelliteChannel

PassageAMicrowaveandSatelliteCommunication

BasedRadioSystems

Initially,theradiobasedcommunicationsthatareoperatedatthemediumtothehighfrequenciesarejustappliedtothefieldssuchasthebroadcasting,themaritimemobileandtheintercontinentalcommunicationswheretheirsuperioritiesaremuchevident.Itisonlyaftertheadventofthetechnologyofmicrowavecommunicationsthatmakeitpossibleforthisradiotechnologytobeappliedfullyscaledinsuchfieldsthathaveeverbeendominatedbythecabletransmissions.

Thus,themicrowaveradio-relaysystemsthatusetheanaloguemodulationschemearehavingbeengrowntobeaveryimportanttransmissionmediumintheoveralltelecommunicationsnetworksinuniverse.Recently,arapidadvanceinsemiconductorandpulsecircuittechnologieshasgivenmuchimpetusontheresearchanddevelopmentofthedigitalradio-relaysystems.Thereisnowaremarkabletrendthatthetelecommunicationnetworksaremovinggraduallytothedomainofdigitalizedcommunicationsystems.

Thefirstmicrowaveradio-relaysystemwasestablishedin1947byBellTelephoneLaboratories(BTL)betweenNewYorkandBoston(300km).Itcarried480channelsoperatinginthe4GHzband,usingfrequencymodulationandheterodynerepeaterscheme.ThefactthatmostofthepresentmicrowavesystemsusethesamemodulationandrepeatermethodisaclearindicationoftheexcellentforesightofBTL.

In1951,BellSystemcompletedatranscontinental4GHzradio-relaysystembetweenNewYorkandSanFrancisco.Itcarried600telephonechannelsoronetelevisionsignal.Afterthat,manycountriesineveryregionoftheworldundertookthedevelopmentofmicrowaveradio-relaysystems.Thepresenthighestcapacitymicrowavesysteminoperationcarries2,700telephonechannels.Recently,workhasstartedfordevelopingasystemwithacapacityof3,600channels.

Microwavesystemsarenowplayingaveryimportantroleinthetelecommunicationsnetworksofalmostallcountries.Invastcountries,suchastheUnitedStatesandCanada,

microwavesystemscarrythemajorportionofthelong-distancetraffic.

Ontheotherhand,inEurope,whichconsistsofmanysmallormediumcountries,microwavesystemsinthenetworkseemtobeslightlylessimportant.

Inmanydevelopingcountries,microwaveradio-relaysystemsplaythekeyroleinthenetwork.Itisquitelogicalbecausethemicrowavesystemhasthefollowingfeatures:

·theinitialinvestmentrequiredfortheconstructionoftransmissionroutesislow;

·theconstructioncanbecompletedwithinarelativelyshortperiod;

·itissuitablefortelevisiontransmission.

Thedigitalsystemsforwhichpreliminaryresearcheswereconductedinthe1940swereunfortunatelysupersededbytherapidlydevelopedfrequencymodulation(FM)systems,mainlybecauseofthelackofhigh-speedpulsetechniqueformultiplexingandtheineffectivenessinfrequencyspectrumutilization.[2]Thestudyonmicrowavedigitaltransmissionsystemswasresumedintheearly1960s.Accordingtovariousinvestigationsonthemicrowavedigitalsystems;theirfeaturescanbesummedupasfollows:

·therealizationoflow-costhigh-speedPCMterminalequipmentcontributesagreatdealtothereductionofthetotalsystemcost;

·thealmostcompletepulseregenerativerepeatingcanbeachievedprovidedthecarrier-to-noisepowerratioisabovethethresholdlevel,andtheaccumulationofnoiseduetomultiplerepeatingisnegligible.Thereforehigh-qualitylong-haultransmissionisattainable;

·theflexibilityintransmittingvariouskindsofsignals,suchasvoice,data,andvideo,isavailablewithoutmutualinteractionsamongdifferentsignals.

Satellitecommunicationhasbecomeapartofeverydaylifeinthelate1980s.Aninternationaltelephonecallismadeaseasilyaslocalcalltoafriendwholivesdowntheblock.Wealsoseeinternationalevents,suchasanelectioninEnglandandatennismatchinFrance,withthesameregularityaslocalpoliticalandsportingevents.Inthiscase,atelevisionnewsprogrambringsthesignalsandsoundsoftheworldintoourhomeseachnight.

Thiscapabilitytoexchangeinformationonaglobalbasis,beitatelephonecalloranewsstory,ismadepossiblethroughapowerfulcommunicationstool-thesatellite.Forthoseofuswhogrewupatatimewhenthespaceagewasnotaparteverydaylife,satellite-basedcommunicationistheculminationofadreamthatstretchesbacktoanerawhenthetermsatellitewasonlyanideaconceivedbyafewinspiredindividuals.[3]These

pioneersincludedauthorssuchasArthurC.Clarke,whofosteredtheideaofaworldwidesatellitesystemin1945.Thisideahassubsequentlyblossomedintoasophisticatedsatellite

networkthatspanstheglobe.

Thefirstgenerationofsatelliteswasfairlyprimitivewhencomparedwithcontemporaryspacecraft.Theseearlysatellitesembodiedactiveandpassivedesigns.

Apassivesatellite,suchastheEchoⅠspacecraftlaunchedin1960,wasnotequippedwithatwo-waytransmissionsystem.Rather,Echowasahugealuminizedmyriadballoonthatfunctionedasareflector.Afterthesatellitewasplacedinalowearthorbit,signalsrelayedtoEchoreflectedorbouncedoffitssurfaceandreturnedtodifferentlocationsontheearth.

IncontrastwiththeEchoseries,theTelstarⅠactivecommunicationssatellitelaunchedin1962carriedreceivingandtransmittingequipment.Itwasanactiveparticipantinthereception-transmissionprocess.Asthesatellitereceivedasignalfromagroundorearthstation,acommunicationscomplexthattransmittedand/orreceivedsatellitesignals,itrelayeditsownsignaltoearth.Telstaralsopavedthewayfortoday's

communicationsspacecraftsinceitcreatedtheworld’sfirstinternationalsatellitetelevisionlink.

DuringthespanofyearsthatseparatesTelstarⅠfromtoday’satellites,therehavebeenanumberofimprovements.Forexample,spacecraftsuchasTelstarandEchowereplacedinlowearthorbits.Inthistypeoforbitalposition,asatellitetraveledatsuchagreatrateofspeedthatitwasvisible,andhenceusable,toanindividualgroundstationforonlyalimitedperiodoftimeeachday.Thesatelliteappearedfrombelowthehorizon,racedacrossthesky,andthendisappearedbelowtheoppositehorizon.

Sincethegroundstationwascutofffromthenowinvisiblesatellite,astationsituatedbelowthehorizonhadtobeactivatedtomaintainthecommunicationlink.Inadifferentscenario,itwouldhavebeennecessarytolaunchaseriesofsatellitestocreateacontinuoussatellite-basedrelayforanygivenearthstation.Asonesatellitedisappeared,itwouldhavebeenreplacedbythenextsatelliteintheseries.[4]

Thelattertypeofsatellitesystemwouldhaveentailedthedevelopmentofaverycomplexandcumbersomeearthandspace-basednetwork.Fortunatelythough,thisproblemwaseliminatedin1963and1964throughthelaunchingoftheSyncomsatellites.Ratherthancirclingtheearthatarapidrateofspeed,thespacecraftappearedtobestationaryorfixedinthesky.Today’scommunicationssatellites,forthemostpart,havefollowedsuitandarenewplacedinwhatarecalledgeo-stationaryorbitalpositionsor“slots”.

Simplystatedasatelliteinageo-stationaryorbitalpositionappearstobefixedoveroneportionoftheearth.Analtitudeof22,300milesabovetheearth’sequator,asatellite

travelsatthesamespeedatwhichtheearthrotates,anditsmotionissynchronizedwiththeearthrotation.Eventhoughthesatelliteismovingatanenormousrateofspeed,itis

stationaryintheskyinrelationtoanobserverontheearth.

Theprimaryvalueofasatelliteinageo-stationaryorbitisitsabilitytocommunicatewithgroundstationsinitscoveragearea24hoursaday.Thisorbitalslotalsosimplifiestheestablishmentofthecommunicationslinkbetweenastationandthesatellite.Oncethestation’santennaisproperlyaligned,onlyminoradjustmentsmayhavetobemadeintheantenna’spositionoveraperiodoftime.Theantennaisrepositionedtoasignificantdegreeonlywhenthestationestablishescontactwithasatelliteinadifferentslot.Priortothisera,agroundstation’santennahadtophysicallytrackasatelliteasitmovedacrossthesky.

Basedontheseprinciples,threesatellitesplacedinequidistantpositionsaroundtheearthcancreateaworld-widecommunicationssysteminthatalmosteverypointonthe

earthcanbereachedbysatellite.ThisconceptwasthebasisofArthurClarke’soriginalvisionofaglobe-spanningcommunicationsnetwork.

Notes

[1]Itisonlyaftertheadventofthetechnologyofmicrowavecommunicationsthatmakeitpossibleforthisradiotechnologytobeappliedfullyscaledinsuchfieldsthathave

everbeendominatedbythecabletransmissions.

僅僅當(dāng)微波通信技術(shù)出現(xiàn)以后,無線電通信才被完完全全地應(yīng)用于那些原本由有線電纜所主導(dǎo)的領(lǐng)域。

·thetechnologyofmicrowavecommunications意為“微波中繼通信”。

·微波中繼通信,一般來說,由于地球曲面的影響以及空間傳輸?shù)膿p耗,每隔50公里左右,就需要設(shè)置中繼站,將電波放大轉(zhuǎn)發(fā)而延伸。這種通信方式,也稱為微波中繼通信。

[2]Thedigitalsystemsforwhichpreliminaryresearcheswereconductedinthe1940swereunfortunatelysupersededbytherapidlydevelopedfrequencymodulation(FM)systems,mainlybecauseofthelackofhigh-speedpulsetechniqueformultiplexingandtheineffectivenessinfrequencyspectrumutilization.

20世紀(jì)針對數(shù)字系統(tǒng)的初步研究很不幸地被快速發(fā)展的頻率調(diào)制系統(tǒng)所取代,主要原因在于缺乏可用于多路復(fù)接的高速脈沖技術(shù)和低效率的頻帶利用率。

·頻率調(diào)制是一種以載波的瞬時頻率變化來表示信息的調(diào)制方式,通過利用載波的不同頻率來表達(dá)不同的信息。

·supersededby的意思是“由……取代”。

·multiplexing的意思是“多路復(fù)用”,多路復(fù)用技術(shù)是把多個低信道組合成一個高速信道的技術(shù),它可以有效地提高數(shù)據(jù)鏈路的利用率,從而使得一條高速的主干鏈路同時為多條低速的接入鏈路提供服務(wù),也就是使得網(wǎng)絡(luò)干線可以同時運(yùn)載大量的語音和數(shù)據(jù)傳輸。

·frequencyspectrumutilization的意思是“頻率利用率”。

[3]Forthoseofuswhogrewupatatimewhenthespaceagewasnotaparteverydaylife,satellite-basedcommunicationistheculminationofadreamthatstretchesbacktoan

erawhenthetermsatellitewasonlyanideaconceivedbyafewinspiredindividuals.

對于那些沒有生長在太空時代的人們而言,衛(wèi)星通信是人們長期以來的一種夢想的最高點(diǎn),這個夢想可以一直追溯到“衛(wèi)星”這個詞只不過是幾個天才頭腦中靈感的想象的那個時代。

·satellite-basedcommunication的意思是“衛(wèi)星通信”。

[4]Sincethegroundstationwascutofffromthenowinvisiblesatellite,astationsituatedbelowthehorizonhadtobeactivatedtomaintainthecommunicationlink.Ina

differentscenario,itwouldhavebeennecessarytolaunchaseriesofsatellitestocreateacontinuoussatellite-basedrelayforanygivenearthstation.Asonesatellitedisappeared,itwouldhavebeenreplacedbythenextsatelliteintheseries.

當(dāng)?shù)孛嬲九c當(dāng)下不可見的衛(wèi)星失去聯(lián)系后,位于地平線以下的地面站就需要被激活以保持通信鏈路。另一方案就是需要發(fā)送一系列衛(wèi)星,能為任何一個地面站提供一個不間斷

的衛(wèi)星中繼;當(dāng)一個衛(wèi)星消失時,隨后的衛(wèi)星將接替其位置。

·invisiblesatellite的意思是“不可見衛(wèi)星”。

·cutofffrom的意思是“與……失去聯(lián)系”。

Exercises

1.Fillintheblanks.

(1)Initially,theradiobasedcommunicationsthatareoperatedatthemediumtothehighfrequenciesarejust

tothefieldssuchasthebroadcasting,themaritimemobileandtheintercontinentalcommunicationswheretheir

aremuchevident.

(2)Thefactthatmostofthepresentmicrowavesystemsusethesamemodulationandrepeatermethodisaclearindicationoftheexcellent

ofBTL.

(3)Thiscapacitytoexchangeinformationonaglobalbasis,beitatelephonecalloranewsstory,ismadepossiblethroughapowerfulcommunicationstool

.

(4)Forthoseofuswhogrewupatatimewhenthespaceagewasnotapartofeverydaylife,satellite-basedcommunicationisa

ofadreamthatstretchesbackto

anerawhenthetermsatellitewasonlyanideaconceivedbyafew

individuals.

(5)Rather,Echowasahuge

myriadballoonfunctionedasa

.

(6)Inadifferentscenario,itwouldhavebeennecessarytolaunchaseriesof

satellitestocreateacontinuoussatellite-basedrelayforanygiven.

(7)Today’scommunicationsatellites,forthemostpart,havefollowedsuitandarenowplacedinwhatarecalled

orbitalpositionor“

”.

(8)Inordertocreatecommunicationslink,thesatelliteuses

,thesatelliteequipmentthatconductsthetwo-wayrelays.

(9)Inlinewiththiscapability,asatellitemaybeabletosupport

trans-mission.

(10)Fortunatelythough,theFCCimplementedaflexibletimetableto

theimpactoftheirdecisions

2.Filltheblankswiththebestchoice.

(1)Themicrowaveradio-relaysystemsthatusetheanaloguemodulationschemearehavingbeengrowntobeaveryimportanttransmissionmediumin

thetelecommunicationsnetworksinuniverse.

a.overallb.totalc.entireplete

(2)Thereisnowa

trendthatthetelecommunicationnetworksaremovinggraduallytothedomainofdigitalizedcommunicationsystems.

a.unusualb.incompatiblec.eccentricd.remarkable

(3)Thestudyonmicrowavedigitaltransmissionsystemswas

intheearly1960s.

a.assumedb.resumedc.presumedd.summed

(4)Theearlysatellitesembodied

designs.

a.activeb.passiveb.bothaandbd.noneofthem

(5)

communicationssatellitelaunchedin1962carriedreceivingandtransmittingequipment.

a.EchoⅠb.TelstarⅠc.IntelsatⅣd.IntelsatⅤ

(6)Basedontheseprinciples,

satellitesplacedinequidistantpositionsaroundtheearthcancreateaworld-widecommunicationssysteminthatalmosteverypointontheearthcanbereachedbysatellite.

a.oneb.twoc.threed.four

(7)TheIntelsatⅣsatellitescarried

transpondersthatprovidedthesatellitewithatotalaveragetransmissioncapacityof

voicecircuits.

a.12;4000b.27;4000

c.2;240d.48;100,000

(8)A

wouldfallonanarrowlydefinedgeographicalzone.

a.footprintb.spotbeam

c.bothaandbd.noneofthem

(9)A

dishcouldreceivethisspotbeamtransmissionwithintheconfinesofthereceptionarea.

a.smallerb.lessexpensive

c.aorbd.aandb

PassageBTheCompositeLinkofSatelliteCommunications

Herewewillanalyzetheoverallend-to-endperformanceofthecommunicationssatellitetransponder.Theoveralllink,comprisingboththeuplinkandthedownlink,isusuallyreferredtoasthecompositelink.WhereasotherdiscussionsdescribedtheperformanceoftheindividualRFlinks,uplinkordownlink,wethusaregoingtolooksatthecombinedeffectsofbothuplinkanddownlinkoncommunicationssystemperformanceanddesign.

Theimpactoflinkdegradationsintroducedinthesatellitecommunicationstransmissionpaths(uplinkanddownlink)isquantitativelydeterminedbyincludingtheminthetransmissionchannelportionofthesatellitecommunicationssystem.Pathlossesareintroducedintheuplinkandthedownlinksignalpaths,andpathnoiseisaddedtothesignalattheuplinkanddownlink,asshowninFigure13.1.Figure13.1InclusionofRFpathlossandpathnoiseinevaluationofsatellitecommunicationsperformance

Pathlossisthesumofoneormoresignalpowerlossescausedbyeffectssuchasgaseousattenuation,rainorcloudattenuation,scintillationloss,angleofarrivalloss,orantenna

gaindegradation.[1]Pathnoiseisthesumofoneormoreadditivenoiseeffectssuchasnoisecausedbyatmosphericgases,clouds,rain,depolarization,surfaceemissions,or

extra-terrestrialsources.

Thetotalsystemcarrier-to-noiseratio(S/N)isdeterminedbydevelopingthesystemequationsforthetotallink,includingthepathdegradationparameters.Figure13.2definestheparametersusedinthelinkcalculations.AsubscriptbeginningwiththeletterGisusedtodenotegroundstationparameters,andasubscriptbeginningwiththeletterSdefinesasatelliteparameter.Also,parametersgiveninuppercaserefertotheparameterexpressedindecibels(dB),whilelowercasereferstotheparameterexpressedasanumberorratio,intheappropriateunits.Figure13.2ParametersforLinkperformancecalculationFigure13.2ParametersforLinkperformancecalculation

Thecommunicationssatellitetransponderisimplementedinoneoftwogeneraltypes,theconventionalfrequencytranslation(FT)satellite,whichcomprisesthevastmajorityofpastandcurrentsatellitesystems,andtwotheon-boardprocessing(OBP)satellite,whichutilizeson-boarddetectionandremodulationtoprovidetwoessentiallyindependent

cascaded(uplinkanddownlink)communicationslinks.

Thetwotypesexhibitdifferentsystemperformance,duetothedifferentfunctionalrelationshipsbetweenthecontributionofdegradationsfromtheuplinkandthedownlink.Eachtypewillbedescribedandanalyzedinfollowingsections.

Aconventionalfrequencytranslation(FT)satellitereceivestheuplinksignalattheuplinkcarrierfrequency,fU,down-convertstheinformationbearingsignaltoan

intermediatefrequency,fIF

,foramplification,up-convertstothedownlinkfrequency,fD,and,afterfinalamplification,re-transmitsthesignaltotheground.Figure13.3(a)showsafunctionalrepresentationoftheconventionalfrequencytranslationtransponder.Analternateversion,the“direct”frequencytranslationtransponder,isshowninFigure13.3(b).Inthedirecttransponder,theuplinkfrequencyisconverteddirectlytothedownlinkfrequency,andafteroneormorestagesofamplification,re-transmittedtotheground.Figure13.3Frequencytranslation(FT)transponder

Noprocessingisdoneon-boardtheFTsatellite.Signaldegradationsandnoiseintroducedontheuplinkaretranslatedtothedownlink,andthetotalperformanceofthesystemwillbedependentonbothlinks.

ForUplink,ThelinkperformanceequationsfortheFTsatelliteuplink,includingthecontributionsofpathlossandpathnoise,willbedevelopedinthissection.Startingattheuplinktransmitter,thegroundtransmitterminaleirp,usingtheparametersdefinedinFigure13.2,is

Thecarrierpowerreceivedatthesatelliteantennaterminals,point(B)onFigure13.2,is

Where?

U

istheuplinkfreespacepathloss,a?U

istheuplinkpathloss,andgGTandgSRarethetransmitandreceiveantennagains,respectively.

Thenoisepoweratthesatelliteantenna,point(B),isthesumofthreecomponents,i.e.

Thethreecomponentsare

wherekisBoltzmann’sconstant,bU

istheuplinkinformationbandwidth,tSA

isthesatellitereceiverantennatemperature,nfSR

isthesatellitereceivernoisefigure,andtU

is

themeantemperatureoftheuplinkatmosphericpath.Therefore,

Theuplinkcarrier-to-noiseratio,atpoint(B),isthengivenby

Thisresultgivestheuplinkcarrier-to-noiseratioexpressedinaformwheretheuplinkpathlossesandnoisecontributionsareexpresslydisplayed-thiswillbeusefulforourlaterevaluationofcompositelinkperformance.

ForDownlink,thedownlinkcarrier-to-noiseratioforthefrequencytranslationsatelliteisfoundbyfollowingthesameprocedurethatwasusedfortheuplink,usingtheequivalentdownlinkparametersasdefinedinFigure13.2.Thus,atpoint(D)

and

Thisresultgivesthedownlinkcarrier-to-noiseratioexpressedinaformwherethedownlinkpathlossesandnoisecontributionsareexpresslydisplayed.

Notes

[1]Pathlossisthesumofoneormoresignalpowerlossescausedbyeffectssuchasgaseousattenuation,rainorcloudattenuation,scintillationloss,angleofarrivalloss,or

antennagaindegradation.

路徑損耗是一個或多個信號功率損耗的總和,是由氣體衰減、雨或云衰減、閃爍、到達(dá)角損失或天線增益退化等原因引起的。

·sumof的意思是“總和”。

[2]Noprocessingisdoneon-boardtheFTsatellite.Signaldegradationsandnoiseintroducedontheuplinkaretranslatedtothedownlink,andthetotalperformanceofthesystemwillbedependentonbothlinks.

FT衛(wèi)星在星上不進(jìn)行(信號)處理。信號的惡化以及在上行(鏈路)引入的噪聲引入到下行(鏈路),系統(tǒng)總的性能依賴兩條鏈路。

·signaldegradations的意思是“信號變差”。

Exercises

1.Fillintheblanks.

(1)Theoveralllink,comprisingboththe

andthe

,isusuallyreferredtoasthe

.

(2)Theimpactoflink

introducedinthesatellitecommunicationstransmissionpaths(uplinkanddownlink)is

determinedbyincludingtheminthetransmission

channel

ofthesatellitecommunicationssystem.

(3)AsubscriptbeginningwiththeletterGisusedto

groundstationparameters,andasubscriptbeginningwiththeletterS

asatelliteparameter.

(4)Thecommunicationssatellitetransponderisinoneoftwogeneraltypes,theconventionalfrequencytranslation(FT)satellite,whichcomprisesthe

majorityofpastandcurrentsatellitesystems,andtwotheon-boardprocessing(OBP)satellite,whichutilizes

detectionand

toprovidetwoessentiallyindependent

(uplinkanddownlink)communicationslinks.

(5)Aconventionalfrequencytranslation(FT)satellitereceivestheuplinksignalattheuplinkcarrierfrequency,fU

,down-convertstheinformation

signaltoanintermediatefrequency,fIF

,foramplification,up-convertstothedownlinkfrequency,fD

,and,

finalamplification,re-transmitsthesignaltotheground.

(6)Thisresultgivestheuplink

ratioexpressedinaform

theuplinkpathlossesandnoisecontributionsareexpressly

thiswillbeusefulforourlaterevaluationofcompositelinkperformance.

(7)Thisresultgivesthedownlinkcarrier-to-noiseratioexpressedinaformwherethedownlinkpathlossesand

noise

are

displayed.

2.TranslatethefollowingparagraphintoChinese.

Point-to-pointradiosareusedtoprovidededicateddataconnectionsbetweentwofixedpoints.Electricutilitycompaniesusepoint-to-pointradiosfortransmissionoftelemetryinformationforthegeneration,transmission,anddistributionofelectricpowerbetweengeneratingstationsandsubstations.Point-to-pointradiosarealsousedtoconnectcellularbasestationstothepublicswitchedtelephonenetwork,andareattractivebecausetheyaregenerallymuchcheaperthanrunninghigh-bandwidthfiber-opticlinesbelowgroundlevel.

Point-to-pointradiosusuallyoperateinthe18,24,or38GHzbands,anduseavarietyofdigitalmodulationmethodstoprovidedataratesinexcessof50Mbps.High-gainantennas

aretypicallyusedtominimizepowerrequirementsandtoavoidinterferencewithotherusers.

PassageCTheMobileSatelliteChannel

ThesatellitecommunicationschannelsweconsideredinpreviouschaptersconsistedPrimarilyofline-of-site(LOS)linksonboththeuplinkanddownlink.Fixedsatelliteservice(FSS)andbroadcastsatelliteservice(BSS)applicationsarepoint-to-pointandpoint-to-multipointapplications,wherethegroundterminalsarefixedandarenotmovingthroughachangingenvironment.

Themobilesatelliteservice(MSS)channelenvironment,however,ismuchmorecomplex.Transmissionto/fromasatellitetoamobileterminalonthegroundisgenerallynolongerasimpleLOSlink.Theradiowavemayencounteramultiplicityofobstaclesinthepath,includingtrees,buildings,andterraineffects,subjectingthetransmittedwavetoreflections,diffraction,andscattering,resultinginamultiplicityofraysreachingthereceiveantenna.Also,becausethetransmitorreceiveterminalismoving,thepowerreceivedisalsovarying,resultinginsignalfading.Theseconditionswillbepresent

whethertheMSSsatelliteisoperatingfromaGSOoraNGSOlocation.

Thecombinationofobstaclesinthepathandamovingtransmitter/receiverresultsinseveralpossiblesignaldegradationsnotfoundintheLOSlink.Thesignalscouldbe

·dispersedintime;

·changedinphaseandamplitude;

·interspersedwithinterferingsignals.

Alargebodyofengineeringanalysishasbeendevelopedtoevaluatetheperformanceanddesignofmobilecommunicationssystems,mostrecentlyfortheterrestrialcellularmobileenvironment.Muchofthisinformationcanbeappliedtothemobilesatellitechannel,however,caremustbeexercisedbecauseofsomeoftheuniquecharacteristicsfoundinthemobilesatellitetransmissionpath.Thischapterwilldeveloptheproceduresandtechniquesusedtoanalyzethemobilecommunicationschannel,withfocusonthesatellitemobilechannel.Theresultwillbetomodifythebasiclinkpowerbudgetequationtoaccountformobilechanneleffects,andprovideabasisforthedesignandperformanceevaluationofmobilesatellitesystems.

IntermsoftheMobileChannelPropagation,thegeneralmobilecommunicationschannelischaracterizedbylocalconditions,includingnaturalterrain,buildings,andother

obstaclesinthevicinityofthemobile.Fixedsatelliteservice(FSS)andbroadcastsatelliteservice(BSS)linksgenerallyhavehighgaindirectiveantennas,whichminimizetheeffects

oflocalterrainandbuildings.Thisisnotthecaseforlandmobilesatelliteserv