elec 691x/498x – broadcast signal transmission fall 2015msoleyma/elec691x... · 2018-02-14 · in...
TRANSCRIPT
Slide 1
Department of Electrical & Computer Engineering
ELEC 691X/498X – Broadcast Signal TransmissionFall 2015
Instructor:DR.RezaSoleymani,Office:EV‐5.125,Telephone:848‐2424ext.:4103.OfficeHours:Wednesday,Thursday,14:00– 15:00Time:Tuesday,2:45to5:30Room: H411
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Department of Electrical & Computer Engineering
Inthislecturewecoverthefollowingtopics:
• Encoders,MultiplexersandDecoders.• ModulatorsandDemodulators.• UpConverters.• DownConverters.• PowerAmplifiers.• LowNoiseAmplifiers.• TransmitterAntennas.• TVReceivingAntennas.• LinkBudget.
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Department of Electrical & Computer Engineering
Inpreviouslectures,wetalkedaboutwhatisneededinanend‐to‐endtelevisionbroadcastingsystem.So,inasensewhatwewilldiscussaboutinthislectureisnottotallynew.Whatisnewisthatwhileinpreviouslecturesweweretalkingaboutdifferentpartsofthebroadcastinglinkseparately,fromthispointonwewillconsiderthemaspartsofanoverallsystem.Wehavetalkedaboutthetechniquesandalgorithmsusedtodovideoencoding/decoding,modulation/demodulation,etc.Andputtingthatknowledgetogetherwithwhatyouhavelearntaboutdesignofdigitalcircuits,youshouldbeabletoimplementthem.Sowewillnottalkabouthowtheequipmentusedforbroadcastingareimplemented,butaboutthespecificationsoftheseequipmentandhowtheycanbeputtogethertoformatransmissionchain.Wewillstartbyreviewinganendtoendbroadcastingsystem.Wewillstartwithaterrestrialsystem.Thisissimplerthanasatellitebroadcastingsystemand,therefore,simplifiesthematters.Laterwewilldiscussasatellitebroadcastinglink.Somepartssuchasencoder/decoder,modulator/demodulatorwehavealreadycoveredandweknowtheirproperties.Some,likeantennasandHPA’swewillcoverbrieflyonourway.Ourgoalwillbethatattheendbeabletomatchourresourceswithourrequirements.Thismeans
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Department of Electrical & Computer Engineering
thatgiventhebandwith andpoweravailable,weassessthenumberandqualityofTVchannelsthatwecanhaveandifthereissomediscrepancywhetherornotitispossibletoclosethegapbyimprovingthedesign.FollowingisablockdiagramofaTVbroadcastingsystem:
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Department of Electrical & Computer Engineering
Thefirstpieceofequipmentistheencoder.AswesawbeforeitconsistsofaVideoEncoderandanAudioEncoder.MultiplexercancombinetheVideoandAudiooutputswithprogrammingdatatomakeaSPTS SingleProgramTransportStream .However,mostoften,programsaredeliveredinbundle.Thatisseveralprogramswiththesameformat,forexamplesportchannels,dramachannelsorcomedychannelscanbeputtogethertoformasocalledBouquet inEurope andvirtualsub‐channel USandCanada .
Inthiscasethemultiplexer MUX takesintheoutputofseveralencodersandcombinesthemintoanMPTS MultipleProgramTransportStream .ItregeneratesthePSIandmayalsoprovideforPIDRe‐mapping,ServiceFiltering,
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Department of Electrical & Computer Engineering
andPSI/SIEditingortoInsertElectronicProgramGuides,ConditionalAccess,andOtherData.Theseextracapabilitiesallowthestationtoremoveaprogramordodropandaddonpermanentoftemporarybasis.Forexampleachildrenchannelthatisviewedonlyduringcertaintimesduringthedaycanbereplacedbyanotherprogramwithaudiencesoutsidethechildren’sviewinghours.TheTransportFormattermayalsobepartoftheMUX.ItgeneratestheASIAsynchronousSerialInterface discussedinLecture2.ThenextblockistheFECencoderthatoptionallyimplementstheReedSolomonRS codeandconvolutionalcodediscussedinLecture6.Whiletheblockdiagramshowndividesthefunctionsveryfinely,thephysicallocationofeachfunctionmaybeimplementationdependent.ForexamplewhileRSandconvolutionalcodingmaybothbeconsideredchannelcodingfunctionsRScodingmaybedoneattheoutputoftheMUXaspartofformingthetransportstreamsintheASIwhileconvolutionalcodingmaybeintegratedwiththemodulationprocessandperformedinthemodulator.Modulatorperformstheconversionfromabitstreamintoawaveform.Theoutputofthemodulatorwillbeabasebandsignal,i.e.,asignalcenteredaroundzerofrequencyonehalfofthesymbolrate.
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Department of Electrical & Computer Engineering
ThisfigureshowstheblockdiagramofaDVB‐Tmodulator:
Thetwoblocksthatwehavenottalkedaboutaretheinterleaver andtheinverseFastFourierTransform IFFT .Aninterleaver isusedinordertospreadthebitsaccordingtoacertainpatternthroughoutthedatastreamandlaterreturnthem
Slide 8
Department of Electrical & Computer Engineering
backtotheoriginalorderatthereceiverendbydoingde‐interleaving.Theobjectiveistocombatagainstthefadingbylettingtheeffectofalongfadebespread.Thatisaburstoferrorsduetofadingofasignaldoesnotaffectawholecodeword makingtheFECuseless.IFFTispartoftheOrthogonalFrequencyDivisionMultiplexing OFDM .
ThisfigureshowstheblockdiagramofaDVB‐SmodulatorwithoptionalFECfunctionsofDVB‐S2,i.e.,BCHandLDPCencoding.
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Department of Electrical & Computer Engineering
Intheabovesystemanupconverter takestheoutputofthemodulatortoafrequencyrangeusedbytheterrestrialcommunicationsystems cellphones .ThisiscalledLband.FollowingdiagramshowsaBlockUpconverter BUC translatingthesignaltoKu‐band.ABUCincludesupconverter plustheamplifier.
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Department of Electrical & Computer Engineering
TheamplifiersusedarecalledHighPowerAmplifier HPA .Thisistomakethemdistinctfromtheamplifiersindifferentpartsofthetransmissioncircuitry,forexample,inmodulatorordemodulatorusedtomakesignalsstrongenoughforfurtherprocessing.HPA’smaybeimplementedusingTravellingWaveTubes TWT .INsuchacasetheyarecalledTravellingWaveTubeAmplifier TWTA .ATWTAinadditiontothetubehaspowersupplyandcontrolcircuitryrequiredtoadjustthegainandfrequency.
Slide 11
Department of Electrical & Computer Engineering
HighPowerAmplifierscanbealsobuiltusingPowerTransistors.Inthiscase,theamplifieriscalledSolidStatePowerAmplifier SSPA orSolidStatePowerBlockSSPB ifitisintegratedwithanupconverter.
SSPA’sareusualy builtinmultiplestages,co‐phasingamplifierswithlowerpowermadebytransistorsofratinglessthanthefinalproduct.
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Department of Electrical & Computer Engineering
Followingistheimplementationofa5kWSSPAusingeight800Wtransistors:
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Department of Electrical & Computer Engineering
TherestofequipmentonthediagramonSlide4arethecounterpartsoftheequipmentmentionedabove.Forexample,eachencoderhasadecodertoundowhattheencoderhasdoneandanymodulatoratthetransmitsiderequiresademodulatoratthereceiverside.ThechannelisanymediumusedtoconveythesignalinthecaseofthesatelliteandterrestrialTV,itconsistsofatransmitandareceiveantennacoupledthroughtheair.Thecharacteristicsofthechannelandthemathematicalmodelrepresentingitmaydifferdependingonthelocationand,possibly,themovementoftheantennas,objectsblockingtheelectromagneticfieldbetweenthetwoantennas,etc.Inthiscourse,weconsiderstationaryantennasandLineofSight LOS pathbetweenthetwoantennas.Thisisvalidwhenthetwoantennas“see”eachotherandthereisasinglepathbetweenthem.Thismodelisaccuratewhentheantennasareinstalledhighenoughsothatonecanignorethegroundreflectionandalsotherearenotobjectsblocking,scatteringorrefractingtheelectromagneticrays.Finally,inordertobringdownthespectrumtobaseband,weneedadownconverteraftertheantennaandbeforefeedingthesignaltothereceiver.LowNoiseAmplifier LNA maybeintegratedwiththedownconverter.ThecombinedunitiscalledALowNoiseBlock LNB .
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Department of Electrical & Computer Engineering
Inpreviouslectures,wediscussedtherelationshipbetweentheBitErrorRateBER andthereceived . where istheratioofthereceivedsignalpowertothenoisepower.Wenowtrytofind foragivenlinkconsistingoftheelementsshownintheslideshownonslide4.Whatwewilldoisfirsttomultiplythetransmitterpowerbyallthegainssuchasamplification,antennagain,etc.andthendividetheresultbyalldifferentattenuationsinordertofindthepoweratthereceiver .ThenweaddallsourcesofnoiseinordertocomeupwiththenoisepowerN.Finally,wedividebyNtoget .OfcoursewhenthequantitiesareindBthemultiplicationand
divisionarereplacedbyadditionandsubtraction.ThisprocedureiscalledLinkBudgetcalculationandisquitestraightforwardandcanbedoneusinganspreadsheetprogram.Therearealotoffreesoftwaredoingthis.Infactalmostanysatelliteserviceproviderorequipmentmanufacturerforterrestrialandsatellitesystemshasoneonitswebsite.
Slide 15
Department of Electrical & Computer Engineering
Thefirstthingtoconsideristhetransmitpower .ThisisthepowerattheoutputofHPA.Thisisthepowerdeliveredtothetransmitantenna.Theelectromagneticfieldleavingtheantennaspreadsradiallyandatadistancethepowerdensityis
where4 isthesurfaceareaofaspherecenteredatthetransmitterandradiusd.
Thisformulaisonlyvalidiftheelectromagneticwavetravelsatalldirections,i.e.,whentheantennaisisotropic.Usuallyantennasaredirective,thatis,theradiationismoreconcentratedinacertaindirection.Thegainoftheantennaistheratioofthetotalvolumeofthespheretothatofthesolidangle.Thepowerdensityofasystemusingatransmitantennawithgain is .
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Department of Electrical & Computer Engineering
Thereceivedpower isproportionaltotheeffectiveareaofthereceiveantenna.So,
4 ,
Therelationshipbetweenthegainandtheeffectiveareaofandantenna alsocalledantennaaperture is:
4λ
whereλIsthewavelengthofthetransmittersignal.Sotheeffectiveareaofthereceiveantennais,
, .Substitutingthisintheformulaforthereceivedpower,weget,
4λ
Slide 17
Department of Electrical & Computer Engineering
where iscalledspacelossorpathlossandrepresentstheeffectofthedistancebetweenthetransmitandreceiveantennas.Ifthereareotherlossessuchasthoseincablesandconnectors,welumpthemtogetheranddenotethem forotherlosses.So,
ChangingthereceivedpowerintodBW dBWatt ,wehave,.
Where and areindBW ordBm dBmW , and areindBi dBcomparedtoanisotropicantenna and and areindB.
10log .Note:Decibel dB takestheunitofthequantityitisappliedtosoifpowerisinWatts,wehavedBW,ifitisinmW thenitsdecibelvalueisindBm.Take100Wattsofpower.Itis20dBW.Butsinceitis100,000mW,itis50dBm.ThevalueindBmis30dBhigherthanthedBW value.DimensionlessentitiessuchasgainarejustindB. One isfound,wecanfind .
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Department of Electrical & Computer Engineering
Example:Atransmittertransmitswith50Wpoweratafrequencyof10GHz.toasatelliteatasatelliteataGeostationaryorbit atanaltitudeof36000km. .FindthereceivedpoweratthesatelliteiftheEarthstationantennahasadiameterof2m.andefficiencyof 0.65.Assumethatthereceiveantennaatthesatellitehasagainof45dBi.
Solution:Thegainofthetransmitantennais,4λ ,
whereλ 0.03 m.and , 0.65 2.04.
So,.
2.04 28,484or44.55dBi.10log 50 17 47 .
10log.
203.57 .Therefore,
47 44.55 45 203.57 67 or‐97dBW.
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Department of Electrical & Computer Engineering
Wehavetalkedabouthowtofindthepoweratthereceiver and,therefore, .Now,let’sfind .ThenoisegeneratedbyacircuitwithresistanceRatatemperatureTandbandwidthWisgivenby4 where 1.38 10 iscalledtheBoltzmannconstant.Thisiscalledthethermalnoise.IthasaflatspectraldensityoverthefrequencybandandisdistributedaccordingtoGaussiandistribution.ThatiswhyitiscalledAdditiveWhiteGaussianNoise AWGN .Modellingthenoisesourcewithavoltagesource 4 andaresistorT,wefindthemaximumpowerthatitcandelivertoaloadis .ThishappenswhentheloadhasaresistanceequaltoR.DividingNbyW,wegetthenoisedensity
.Tiscalledthenoisetemperature.
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Department of Electrical & Computer Engineering
Communicationlinkssuchabroadcastingsystemsconsistsofdifferentequipmentincascade.Forexample,ahomeTVreceptionsystemhasantennaconnectedtoLNB,downconverter,thentothecoaxialcable,thepoweramplifier,thereceiver,etc.Eachofthesecomponentscanbemodeledwithagain oralossthatcanbeconsideredasagainoflessthanunity .
Itwouldnotberealistictoassumeanycomponenttobenoisefree.Themodel withnoiseincludedis,
Slide 21
Department of Electrical & Computer Engineering
Now,assumethattwocomponentswithGains and andnoisetemperaturesand areconnectedtogethertoanoisesourceT.
Theoverallgainwillbe andthenoisepoweraddedbythesystemtoTwillbe.Nowletsdefineanequivalentnoisetemperatureforthesystem.
Wehave,.
So,Ingeneral,fornstages,wehave,
Slide 22
Department of Electrical & Computer Engineering
NoiseFactorisanotherquantityusedtoquantifyhownoisyacircuit,mostoftenanamplifier,is.ItistheratiooftheSignal‐to‐NoiseRatio SNR attheinputofacircuittotheSNRattheoutputofthecircuit.So,itcannotbelessthanone,
1.Lettheinputpowerbe andthenoisepowerattheinputbe .Then .TakingthegainandthenoisetemperatureofthecircuitasGandT,respectively,wehave .Therefore,
1 .
Or 1 .Tohaveacommonbaseforcomparingthedifferentamplifiers,theinput orambient noisetemperatureisfixedat290degreesKelvin.So,T 290 F‐1 .ThenoisefactorindBiscalledtheNoiseFigure,
10 log 10log 1 290