《电子信息类专业英语 》课件-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.

仅仅当微波通信技术出现以后,无线电通信才被完完全全地应用于那些原本由有线电缆所主导的领域。

·thetechnologyofmicrowavecommunications意为“微波中继通信”。

·微波中继通信,一般来说,由于地球曲面的影响以及空间传输的损耗,每隔50公里左右,就需要设置中继站,将电波放大转发而延伸。这种通信方式,也称为微波中继通信。

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

20世纪针对数字系统的初步研究很不幸地被快速发展的频率调制系统所取代,主要原因在于缺乏可用于多路复接的高速脉冲技术和低效率的频带利用率。

·频率调制是一种以载波的瞬时频率变化来表示信息的调制方式,通过利用载波的不同频率来表达不同的信息。

·supersededby的意思是“由……取代”。

·multiplexing的意思是“多路复用”,多路复用技术是把多个低信道组合成一个高速信道的技术,它可以有效地提高数据链路的利用率,从而使得一条高速的主干链路同时为多条低速的接入链路提供服务,也就是使得网络干线可以同时运载大量的语音和数据传输。

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

[3]Forthoseofuswhogrewupatatimewhenthespaceagewasnotaparteverydaylife,satellite-basedcommunicationistheculminationofadreamthatstretchesbacktoan

erawhenthetermsatellitewasonlyanideaconceivedbyafewinspiredindividuals.

对于那些没有生长在太空时代的人们而言,卫星通信是人们长期以来的一种梦想的最高点,这个梦想可以一直追溯到“卫星”这个词只不过是几个天才头脑中灵感的想象的那个时代。

·satellite-basedcommunication的意思是“卫星通信”。

[4]Sincethegroundstationwascutofffromthenowinvisiblesatellite,astationsituatedbelowthehorizonhadtobeactivatedtomaintainthecommunicationlink.Ina

differentscenario,itwouldhavebeennecessarytolaunchaseriesofsatellitestocreateacontinuoussatellite-basedrelayforanygivenearthstation.Asonesatellitedisappeared,itwouldhavebeenreplacedbythenextsatelliteintheseries.

当地面站与当下不可见的卫星失去联系后,位于地平线以下的地面站就需要被激活以保持通信链路。另一方案就是需要发送一系列卫星,能为任何一个地面站提供一个不间断

的卫星中继;当一个卫星消失时,随后的卫星将接替其位置。

·invisiblesatellite的意思是“不可见卫星”。

·cutofffrom的意思是“与……失去联系”。

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

Whereℓ

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.

路径损耗是一个或多个信号功率损耗的总和,是由气体衰减、雨或云衰减、闪烁、到达角损失或天线增益退化等原因引起的。

·sumof的意思是“总和”。

[2]Noprocessingisdoneon-boardtheFTsatellite.Signaldegradationsandnoiseintroducedontheuplinkaretranslatedtothedownlink,andthetotalperformanceofthesystemwillbedependentonbothlinks.

FT卫星在星上不进行(信号)处理。信号的恶化以及在上行(链路)引入的噪声引入到下行(链路),系统总的性能依赖两条链路。

·signaldegradations的意思是“信号变差”。

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