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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

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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.

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Department of Electrical & Computer Engineering

HighPowerAmplifierscanbealsobuiltusingPowerTransistors.Inthiscase,theamplifieriscalledSolidStatePowerAmplifier SSPA orSolidStatePowerBlockSSPB ifitisintegratedwithanupconverter.

SSPA’sareusualy builtinmultiplestages,co‐phasingamplifierswithlowerpowermadebytransistorsofratinglessthanthefinalproduct.

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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.

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Thefirstthingtoconsideristhetransmitpower .ThisisthepowerattheoutputofHPA.Thisisthepowerdeliveredtothetransmitantenna.Theelectromagneticfieldleavingtheantennaspreadsradiallyandatadistancethepowerdensityis

where4 isthesurfaceareaofaspherecenteredatthetransmitterandradiusd.

Thisformulaisonlyvalidiftheelectromagneticwavetravelsatalldirections,i.e.,whentheantennaisisotropic.Usuallyantennasaredirective,thatis,theradiationismoreconcentratedinacertaindirection.Thegainoftheantennaistheratioofthetotalvolumeofthespheretothatofthesolidangle.Thepowerdensityofasystemusingatransmitantennawithgain is .

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Thereceivedpower isproportionaltotheeffectiveareaofthereceiveantenna.So,

4 ,

Therelationshipbetweenthegainandtheeffectiveareaofandantenna alsocalledantennaaperture is:

4λ

whereλIsthewavelengthofthetransmittersignal.Sotheeffectiveareaofthereceiveantennais,

, .Substitutingthisintheformulaforthereceivedpower,weget,

4λ

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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,

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Department of Electrical & Computer Engineering

Now,assumethattwocomponentswithGains and andnoisetemperaturesand areconnectedtogethertoanoisesourceT.

Theoverallgainwillbe andthenoisepoweraddedbythesystemtoTwillbe.Nowletsdefineanequivalentnoisetemperatureforthesystem.

Wehave,.

So,Ingeneral,fornstages,wehave,

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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