航院毕设英文原文.docx

航院毕设英文原文.docx

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航院毕设英文原文

毕业设计（论文）英文翻译原文

院（系）：

桂林航天工业学院

专业：

计算机通信工程

学生姓名：

杨康

学号：

030111301585

指导教师：

孙山林

职称：

教授

年月日

Applicationofadaptivemodulationforrainattenuation

Abstract

Rainattenuationisadominantsourceofsatellitelinkoutagesinhigherfrequenciessatellitecommunicationsystems.Theintensityanddistributionofrainfallgreatlyaffectstransmissionqualityandlimitssystem’savailability.Inordertoovercomesuchdifficulties,inthearticle,ituseadaptivemodulationandmakeSNRasthejudgementthreshold,thismethodfurthestimprovesthethroughputinsatellitecommunication.IthasIridiumsysteminthebackandOPNETastool,veritablysimulatetheapplicationofadaptivemodulationundertheenvironmentofrainfall.Comprehensivetheoreticalanalysisanddedicatedsimulationsshowthatthesimulationresultsandthetheoreticalresultsarematchedperfectly.

Keywords：

adaptivemodulation;rainattenuation;OPNET;linkbudget;coverage

1INTRODUCTION

Satellitecommunicationlinksareaffectedbyrainfall,andsolaroutage,amongothers.Theseaffectsmustbetakenintoaccountsinsatellitelinkdesignandoperation.Thisarticledescribestheeffectsandtechniquestoquantifyandalleviatetheeffect.

Therapiddevelopmentincommunicationsystemshasforcedsystemdesignertoexplorehigherandhigherfrequencies.Rainisadominantsourceofattenuationathigherfrequencies.Theintensityanddistributionofrainfallgreatlyaffectstransmissionqualityandlimitssystem’savailability.Raineffectsonmicrowavesystemsaremorecriticalintropicalandequatorialzones,whererainfallishigherthanintemperatezones.Intemperateregionstheraineffectsbecomessignificantabove10GHz,whileintropicalclimatesingeneralandinequatorialclimateparticularly,sincetheraindropsarelargerthanintemperateclimates,theincidenceofrainfallonradiolinksbecomesimportantforfrequenciesaslowasabout.

Inordertoovercomesuchdifficulties,inthearticle,ituseadaptivemodulationandmakeSNRasthejudgementthreshold,thismethodcanfurthestimprovethethroughputinsatellitecommunication.IthasIridiumsysteminthebackandOPNETastool,veritablysimulatetheapplicationofadaptivemodulationundertheenvironmentofrainfall.

2THEORETICALANALYSISOFADAPTIVEMODULATION

2.1Basictheoryofadaptivemodulation

Inthetraditionalmodulationmode,inordertoensuretheBERperformanceofthesystem,italwaysselectstheappropriatemodulationonthebasisoftheworstsituationofchannel.Thismethodcanensurethereliabilityofchannelduringthecommunication.Astheworsttimeofthetransmissionisveryshort,soitwillcausehugewaste.Inordertofurthestimprovethethroughputoftheinformationtransmission,inthisarticle,wewilluseadaptivemodulation.Thebasictheoryofadaptivemodulationischangethemodulationmodeinlinewiththesituationofthechannel.Inthesystemofadaptivemodulation,theusersalwayschoosethehighermodulationmodeinidealchannel,andchoosethelowermodulationmodeinnotidealchannel,thismethodnotonlycanensurethereliabilityofchannelduringthecommunication,butalsocanurthestimprovethethroughput.

Inthearticleweusethefixedthresholdmethodasthemodulationmodeinthesystemofadaptivemodulation.ItfirstsetsareliableBERthresholdbasedonthetrafficcharacteristicsinthefixedthresholdmethod,andcalculatetheSNRvaluefortheappointedBER,thencompareeverythresholdvalueandchoosethecorrespondingmodulationmode.

2.2Calculationofjudgementthreshold

Thedesignofadaptivemodulationiscloselyrelatedtothecalculationofjudgementthreshold.Inthearticleweusethecriterionofthresholdasfollowing:

maintainacertainBERandthethroughputisvariable.Theselectionofmodulationmodeexertsanessentialeffectonthesystemofsatellitecommunication.Inthearticle,weuseBPSK_1/2FEC,BPSK.,8PSK,64PSK_2/5FEC,64PSKfivemodulationmodes.Usually,SNRisdirectlyrelatedtochannelstate,inordertoanalyzeeasily,itusetheapproximateexpressionbetweenSNRandchannel,thecalculationofBREforM-aryisgiven:

（1）

Fig1ThecurveofBERandSNRundertheconditionof

adaptivemodulation

Thetransmissionefficiencyofthemodulationmodeaboveisgiven:

（2）

BPSK_1/2FEC（M=2）

BPSK（M=2）

8PSK（M=8）

64PSK_2/5FEC（M=64）

64PSK（M=64）

UnderconditionthatBERisintherangeof10-6to10-5,italwayschoosesheadaptivemodulationmodeofhigheffciency.

Accordingtotheformulaabove,wecandeducetheSNRthresholdvalueofadaptivemodulation:

whenSNRislessthan9dB,itonlycanchooseBPSK_1/2FECmodulationmode,asthereis50%redundancy,sothethroughputwillreducebyahalf;WhenSNRisbetween9dBand13dB,themodulationofbothBPSK_1/2FECandBPSKsatisfytheneedofBER,butthereisnoreductioninBPSKmodulation,sothethroughputishigher;WhenSNRisbetween13dBand23dB,asSNRishigher,soitcanchoosethebettermodulationmode,thethroughputof8PSK,64PSK_2/5FECand64PSKishigh,butonly8PSKsatisfiestheneedofBER,soitchooses8PSK,andthethroughputisthreetimeshigherthantheoriginal;WhenSNRisbetween23dBand28dB,thethroughputofboth8PSKand64PSK_2/5FECsatisfytheneed,inordertogetthehigherthroughput,itchooses64PSK_2/5FEC,andthereis40%redundancy;WhenSNRismorethan28dB,allkindsofmodulationmodesatisfytheneed,butthethroughputof64PSKisthehighest,sowechoose64PSK.

Accordingtotheanalysisabove,wecandeducetheselectionschemeofadaptivemodulationmodeasfollowing:

Table1Schemeofadaptivemodulation

RangeofSNR（dB）

Modulationmode

SNR<9dB

BPSK-1/2FEC

9dB

BPSK

13dB

8PSK

23dB

64PSK-2/5FEC

SNR>28dB

64PSK

3.RAINATTENUATIONMODELING

3.1.Rainareadistribution

AccordingtotheITUstandard,thewholeworldcanbedividedintoA,B,C…Qfifteenrainareas,thefiguresinthefollowingaretherainareadistributionmapsofEurope,Asia,andAmerica.Therelationshipbetweenrainfallintensityandtimeisshownintable2

Fig2RainareadistributionmapsofEurope,Asia,andAmerica

Table2TherelationshipbetweenrainfallintensityandtimeaccordingtoITU

（1）For,theslantpathlengthis

（3）

Forthefollowingformulaisused

（4）

（2）CalculatethehorizontalprojectionLG

（5）

（3）Obtainthespecificattenuation

（6）

（4）Calculatethehorizontalreductionfactor,for0.01%ofthetime.

（7）

（5）Calculatetheverticaladjustmentfactor,forthe0.01%ofthetime

For

else

If

elsex=0

（8）

（6）Theeffectivepathlengthis

（9）

（7）Thepredicatedattenuationexceededfor0.01%ofanaverageyearis

（10）

Therainwillnotonlycauseattenuationofsignalpower,butalsowillcauseincreaseinnoisepower.

Intheclearsky,thetemperatureis

（11）

Aclearisattenuationintheclearsky.,Intherainsky,thetemperatureis

（12）

Theraiseinthetemperaturecausedbytherainis

（13）

WheretheTLANisthetemperatureoftheLAN.

3.2.Coverage

Fig3ThecoverageareaoftheLEOsatellite

Thepathlossisgivenby

（14）

WheredisthedistancebetweentheearthstationandIridiumsatellite,andmaximumandminimumvisibilitydistancesdmax,anddminisgivenby

（15）

Combiningthelasttwoequationsyieldsvisibilitydistancedcriterion,wehave

（16）

Wherermaxismaximumvisibilityradiusand

isthemaximumearthcentralangle.

Fig4ThedistancebetweentheearthstationandIridiumsatellite

ThecurvesinFigure4varybetween780kmand2500kminapproximately2hourperiod.Itimpliesthatasatellitecanbevisibleagainfromthesameplaneinabout2hours.Ineachperiod,thereareseveralpeaksandvalleyswhosevaluesaredifferent,correspondingtothedifferentsatellites’coverage.Eachhandoffhappensatdifferentearth-satellitedistancesandthecoveragelastsabout10minutes.Itisbecausethattheorbitplaneisshiftingallthetime,thecoverageofeachsatelliteisnotsimplyrepeatingbutslightdifferent.

4.OPNETSIMULATION

4.1Networklayermodeling

Fig5IridiumNetworkmodel

Inthesimulation,thenetworkismadeupofsixty-sixIridiumsatellites,twoearthstationsandothermobileterminations,theorbitofIridiumsatellitesiscreatedbySTKandthenetworktopologystructureis.star.Intheprocessofsimulation,IridiumsatellitenodesflyaroundtheearthalongtheorbitofIridium,andthetwoearthstationscommunicatewitheachotherthroughsixty-sixIridiumsatellites.AsthecoverageofIridiumislimited,sotheearthstationswillcommunicatewitheachotherthroughseveralsatellites.

4.2.Nodemodeling

Nodeisoftenconsideredasequipmentorresource,ismadeupofhardwareandsoftwarewhichcanprovidecorrespondinghandlingcapacity.InOPNET,thereisthreekindsofnode:

thefirstisfixednode,suchasrouter,switch,workstation,serverandsoon.;thesecondismobilenode,suchasmobileStation,vehicleandsoon;andthethirdissatellitenodewhichrepresentssatellite.Theattributeofeverynodeis

differentfromeachother,forexample,mobilenodecanusethree-dimensionalortwo-dimentionalmobileorbit,andthesatellitecanusesatelliteorbit.

Fig6EarthstationandIridiumsubnetnodemodel

Forthewirelesschannel,themessagetransmittingprocedurebetweencanbedescribedbyaseriesofstages.ThefourteenstagesintheOPNETcanvividlysimulatetheprocessofsendingandreceivingmessage.

5.CONCLUSIONSSIMULATIONSANDANALYSIS

Th