ASK and FSK and PSK eng and bng and pdf fxm

Cummenication system

1. Dr. Suad El-Geder
EC 331
Computer Engineer Department
DIGITAL MODULATION TECHNIQUES
LECTURE 15

2. ▪Modulationisusedwhenthecarrierpropertyused(e.g.amplitude)ischangedprogressively,becausethe
signaltotransmititselfvariesprogressively.Modulationisassociatedmostlywithanalogsignals
modulatingasinusoidalcarrier.
▪Keyingisusedwhenthecarrierpropertyusedischangedabruptly,becausethesignalitselfchanges
abruptly.Keyingisassociatedmostlywithdigitalsignalsmodulatingasinusoidalcarrier.
▪Forkeyingthecarrierpropertyusedischangedbetweendiscretevaluesonly,e.g.for2valuesthiscanbe
on/offforamplitude,or0°and180°forphase.Thechangeitselfisnamedtheshift,itcanbeanamplitude
shift,afrequencyshift,etc.

3. ▪Digital_To_Analog
oEncodingdigitaldataintoanalogsignalscarriersofdifferentamplitude,frequency,andphaseshifts,
movingthemoveranalognetworks,e.g.,PublictelephonenetworkwithlimitedBW300-3400Hzonly.

4. Transmitting digital data using analog signals.
▪Thismethodisusedtosendcomputerinformationovertransmission
channelsthatrequireanalogsignals,likeafiberopticnetworks,computer
modems,cellularphonenetworks,andsatellitesystems.
▪Ineachofthissystems,anelectromagneticcarrierwaveisusedtocarrythe
informationovergreatdistancesandconnectdigitalinformationusersat
remotelocations.
▪Thedigitaldataisusedtomodulateoneormoreoftheparametersofthe
carrierwave,Thisbasicprocessisgiventhename"shift-keying"to
differentiateitfromthepurelyanalogsystemslikeAMandFM.
▪Aswithanalogmodulation,therearethreeparametersofthecarrierwaveto
varyandthereforethreebasictypesofshiftkeying:
•AmplitudeShiftKeying(ASK)
•FrequencyShiftKeying(FSK),and
•PhaseShiftKeying(PSK).

5. ▪InASK,thetwobinaryvaluesarerepresentedbytwodifferent
amplitudesofthecarrierfrequency.Commonly,oneoftheamplitudesis
zero;thatis,onebinarydigitisrepresentedbythepresence,atconstant
amplitude,ofthecarrier,theotherbytheabsenceofthecarrier.The
resultingtransmittedsignalforonebittimeis

6. ▪First,Amplitudeshiftkeyingisaneffectivetechniquetoincreasetheinputamplitudecharacteristicsincommunications.ButtheseASK
modulatedwaveformsareeasilyaffectedbynoise.Andthisleadstoamplitudevariations.Duetothis,therewillbevoltagefluctuations
intheoutputwaveforms.
▪TheseconddrawbackoftheASKmodulationtechniqueis,ithaslowpowerefficiency.BecauseASKrequirestheexcessivebandwidth.
ItleadstopowerlossinthespectrumofASK.
▪Whenevertomodulatetwoinputbinarysignals,amplitudeshiftkeyingmodulationisnotpreferable.Becauseithastotakeonlyone
inputonly.
▪So,toovercomethisQuadratureAmplitudeShiftKeying(ASK)ispreferred.Inthismodulationtechnique,wecanmodulatetwo
binarysignalswithtwodifferentcarriersignals.Here,thesetwocarriersignalsareinoppositephasewith90degreesdifference.Sin
andcosinesignalsareusedascarriersinquadratureamplitudeshiftkeying.Theadvantageofthisis,ituseseffectivelythebandwidth
ofthespectrum.Itoffersmorepowerefficiencythantheamplitudeshiftkeying.

7. ▪FSK(FrequencyShiftKeying)isatypeofdigitalmodulationtechniqueusedintelecommunicationsand
signalprocessing.Itiswidelyusedinapplicationslikeradiocommunication,telemetry,RFID,
▪ThemostcommonformofFSKisbinaryFSK(BFSK),inwhichthetwobinaryvaluesarerepresented
bytwodifferentfrequenciesnearthecarrierfrequency.Theresultingtransmittedsignalforonebittimeis
▪where??????
1and??????
2aretypicallyoffsetfromthecarrierfrequency??????
??????byequalbutoppositeamounts.

8. ▪BFSKislesssusceptibletoerrorthanASK.
Onvoice-gradelines,itistypicallyusedup
to1200bps.
▪Itisalsocommonlyusedforhigh-frequency
(3to30MHz)radiotransmission.Itcanalso
beusedatevenhigherfrequenciesonlocal
areanetworksthatusecoaxialcable.

9. ▪In PSK, the phase of the carrier signal is shifted to represent data.
▪Two-Level PSK →The simplest scheme uses two phases to represent the two binary digits and is known as
binary phase shift keying. The resulting transmitted signal for one bit time is
▪Because a phase shift of 180°is equivalent to flipping the sine wave or multiplying it by the rightmost
expressions in the above Equation can be used.

10. ▪LetPSKsignalbe where arethevaluesofthePCMsignalduringthebit
interval.
▪Thesquaringofthesignalgives
▪TheoutputoftheBPFyields whichbecomes afteritisfrequencydivided.
▪The output of the mixer is
▪WithalowpassfilteronlytheDCtermremainsandhencewegettheoriginalPCMsignalwhichiseither
positiveornegativevaluedependingonthebitvaluesoneorzero.

11. ▪Majordrawback–rapidamplitudechangebetweensymbolsduetophasediscontinuity,whichrequiresinfinite
bandwidth.
▪BinaryPhaseShiftKeying(BPSK)demonstratesbetterperformancethanASKandBFSK.
▪BPSKcanbeexpandedtoaM-aryscheme,employingmultiplephasesandamplitudesasdifferentstates.

12. ▪Amplitudeandphasecanbemodulatedsimultaneouslyandseparatelytoconveymoreinformationthaneithermethod
alone,butisdifficulttodo.
▪Aneasierwayistoseparatetheoriginalsignalintoasetofindependentcomponentsorchannels:I(In-phase)andQ
(Quadrature).
▪TheIandQcomponentsareconsideredorthogonalorinquadraturebecausetheyareseparatedby90degrees.
▪TheIandQcomponentsaresummedinamodulatorcircuit.

13. ▪Asinglecarriergeneratedbyalocaloscillator(L.O.)circuitis
splitintotwopaths
▪Onepathisdelayedbyanamountoftimeequalto¼ofthe
carrier’scycletime,or90degrees.
▪Thetwocarriersareamplitudemodulated—onebytheIsignal,
theotherbytheQsignal.
▪Thetwomodulatedcarriersareaddedtogetherinasumming
circuit.
▪Theoutputisadigitallymodulatedsignalwhoseamplitudeand
phasearedeterminedbytheamplitudesofthetwomodulating
signals

14. 1.QuadratureRelationship:
1.Theterm"quadrature"comesfromthefactthatthetwosignalsareorthogonaltoeachotherinthetimedomain.
2.Orthogonalitymeansthatthetwosignalsdonotinterferewitheachotherwhencombined,allowingthemtocarry
independentinformation.
▪Thesignalshiftsinincrementsof90degreesfrom45°to135°,-45°(315°),or-135°(225°)
▪DataintothemodulatorisseparatedintotwochannelscalledIandQ
▪Twobitsaretransmittedsimultaneously,oneperchannel
▪Eachchannelmodulatesacarrier
oThetwocarrierfrequenciesarethesame,buttheirphaseisoffsetby90degrees(thatis,theyare“inquadrature”)
▪Thetwocarriersarecombinedandtransmitted
▪Fourstatesbecause2
2
=4

15. ▪Definition:QuadraturePhaseShiftKeying(QPSK)isaformofPhaseShiftKeyinginwhichtwobitsare
modulatedatonce,selectingoneoffourpossiblecarrierphaseshifts(0,90,180,or270degrees).
▪QPSKallowsthesignaltocarrytwiceasmuchinformationasordinaryPSKusingthesamebandwidth.
▪QPSKisusedforsatellitetransmissionofMPEG2video,cablemodems,videoconferencing,cellularphone
systems,andotherformsofdigitalcommunicationoveranRFcarrier.
▪QPSKcanbeinterpretedastwoindependentBPSKsystems(oneontheI-channelandoneonQ-channel),and
thusthesameperformancebuttwicethebandwidth(spectrum)efficiency.
▪QPSKisanM-aryencodingscheme.AQPSKmodulatorisabinary(base2)signal,toproducefourdifferent
inputcombinations,:[00,01,10,and11].Therefore,withQPSK,thebinaryinputdataarecombinedintogroups
oftwobits,calleddibits.Inthemodulator,eachdibitcodegeneratesoneofthefourpossibleoutputphases
(+45◦,+135◦,−45◦,and−135◦).

16. QPSK Constellation

18. ▪A block diagram of a QPSK modulator is shown in the Figure. Two bits (a dibit)
are clocked into the bit splitter. The Q channel get the odd bit and the I channel is
the even bit.
▪After both bits have been serially inputted, they are simultaneously parallel
outputted.
▪The I bit modulates a carrier that is in phase with the reference oscillator (hence
the name ”I” for ”in phase” channel), and the Q bit modulate, a carrier that is 90◦
out of phase.
▪For a logic 1 = + 1 V and a logic 0 = -1 V, two phases are possible at the output of
the I balanced modulator (+ cos(??????
??????�)) and (− cos(??????
??????�)), and two phases are
possible at the output of the Q balanced modulator (+ sin(??????
??????�)), and (−sin(??????
??????�)).
▪When the linear summer combines the two quadrature (90◦out of phase) signals,
there are four possible resultant phasors given by these expressions: +cos(??????
??????�) +
sin(??????
??????�),−cos(??????
??????�) + sin(??????
??????�),+cos(??????
??????�) − sin(??????
??????�), and − cos(??????
??????�) − sin(??????
??????�).

20. ▪For demodulation, the incoming QPSK signal may be any one of the four possible output phases.
▪To illustrate the demodulation process, let the incoming QPSK signal be +cos(??????
??????�) − sin(??????
??????�).
▪Looking at the figure above, and for the I branch

21. ▪Bothsignals�
??????(t)and�
??????(t)gothroughalow-passfilterwhichwillfilteroutthe2??????
??????frequencycomponentsandleavetheDC
componentsonly.
▪Forthisexample,theIchannelwillyieldthevalue+
1
2
whichrepresentsthelogic1,andtheQchannelyieldsthevalue−
1
2
which
representsthelogic0.
▪Hence,theoutputis”10”asitshouldbe.Notethatthefinalstageisjustamultiplexerthatconvertsthetwochannels(I&Q)toone
channel.