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1、Agilent Digital Modulation inCommunications SystemsAn IntroductionApplication Note 12982This application note introduces the concepts ofdigital modulation used in many communicationssystems today.Emphasis is placed on explainingthe tradeoffs that are made to optimize efficienciesin system design.Mos
2、t communications systems fall into one of threecategories:bandwidth efficient,power efficient,orcost efficient.Bandwidth efficiency describes theability of a modulation scheme to accommodatedata within a limited bandwidth.Power efficiencydescribes the ability of the system to reliably sendinformatio
3、n at the lowest practical power level.In most systems,there is a high priority on band-width efficiency.The parameter to be optimizeddepends on the demands of the particular system,as can be seen in the following two examples.For designers of digital terrestrial microwaveradios,their highest priorit
4、y is good bandwidthefficiency with low bit-error-rate.They have plentyof power available and are not concerned withpower efficiency.They are not especially con-cerned with receiver cost or complexity becausethey do not have to build large numbers of them.On the other hand,designers of hand-held cell
5、ularphones put a high priority on power efficiencybecause these phones need to run on a battery.Cost is also a high priority because cellular phonesmust be low-cost to encourage more users.Accord-ingly,these systems sacrifice some bandwidth efficiency to get power and cost efficiency.Every time one
6、of these efficiency parameters(bandwidth,power,or cost)is increased,anotherone decreases,becomes more complex,or does notperform well in a poor environment.Cost is a dom-inant system priority.Low-cost radios will alwaysbe in demand.In the past,it was possible to makea radio low-cost by sacrificing p
7、ower and band-width efficiency.This is no longer possible.Theradio spectrum is very valuable and operators whodo not use the spectrum efficiently could lose theirexisting licenses or lose out in the competition fornew ones.These are the tradeoffs that must beconsidered in digital RF communications d
8、esign.This application note covers the reasons for the move to digital modulation;how information is modulated onto in-phase(I)and quadrature(Q)signals;different types of digital modulation;filtering techniques to conserve bandwidth;ways of looking at digitally modulated signals;multiplexing techniq
9、ues used to share the transmission channel;how a digital transmitter and receiver work;measurements on digital RF communicationssystems;an overview table with key specifications for the major digital communications systems;and a glossary of terms used in digital RF communi-cations.These concepts for
10、m the building blocks of anycommunications system.If you understand thebuilding blocks,then you will be able to under-stand how any communications system,present or future,works.Introduction35567778891010111212121314141515161718192021222223232425262728292930311.Why Digital Modulation?1.1 Trading off
11、 simplicity and bandwidth1.2 Industry trends2.Using I/Q Modulation(Amplitude and Phase Control)to Convey Information2.1 Transmitting information2.2 Signal characteristics that can be modified2.3 Polar displaymagnitude and phase representedtogether2.4 Signal changes or modifications in polar form2.5
12、I/Q formats2.6 I and Q in a radio transmitter2.7 I and Q in a radio receiver2.8 Why use I and Q?3.Digital Modulation Types and Relative Efficiencies3.1 Applications3.1.1 Bit rate and symbol rate3.1.2 Spectrum(bandwidth)requirements3.1.3 Symbol clock3.2 Phase Shift Keying(PSK)3.3 Frequency Shift Keyi
13、ng3.4 Minimum Shift Keying(MSK)3.5 Quadrature Amplitude Modulation(QAM)3.6 Theoretical bandwidth efficiency limits3.7 Spectral efficiency examples in practical radios3.8 I/Q offset modulation3.9 Differential modulation3.10 Constant amplitude modulation4.Filtering4.1 Nyquist or raised cosine filter4.
14、2 Transmitter-receiver matched filters4.3 Gaussian filter4.4 Filter bandwidth parameter alpha 4.5 Filter bandwidth effects4.6 Chebyshev equiripple FIR(finite impulse response)filter4.7 Spectral efficiency versus power consumption5.Different Ways of Looking at a Digitally Modulated Signal Time and Fr
15、equency Domain View5.1 Power and frequency view5.2 Constellation diagrams5.3 Eye diagrams5.4 Trellis diagramsTable of Contents432323233333434353536373737383839393940414142434344466.Sharing the Channel6.1 Multiplexingfrequency6.2 Multiplexingtime6.3 Multiplexingcode6.4 Multiplexinggeography6.5 Combin
16、ing multiplexing modes6.6 Penetration versus efficiency7.How Digital Transmitters and Receivers Work7.1 A digital communications transmitter7.2 A digital communications receiver8.Measurements on Digital RF Communications Systems8.1 Power measurements8.1.1 Adjacent Channel Power8.2 Frequency measurem
17、ents8.2.1 Occupied bandwidth8.3 Timing measurements8.4 Modulation accuracy8.5 Understanding Error Vector Magnitude(EVM)8.6 Troubleshooting with error vector measurements8.7 Magnitude versus phase error8.8 I/Q phase error versus time8.9 Error Vector Magnitude versus time8.10 Error spectrum(EVM versus
18、 frequency)9.Summary10.Overview of Communications Systems11.Glossary of TermsTable of Contents(continued)5The move to digital modulation provides moreinformation capacity,compatibility with digitaldata services,higher data security,better qualitycommunications,and quicker system availability.Develop
19、ers of communications systems face theseconstraints:available bandwidth permissible power inherent noise level of the system The RF spectrum must be shared,yet every daythere are more users for that spectrum as demandfor communications services increases.Digitalmodulation schemes have greater capaci
20、ty to con-vey large amounts of information than analog mod-ulation schemes.1.1 Trading off simplicity and bandwidthThere is a fundamental tradeoff in communicationsystems.Simple hardware can be used in transmit-ters and receivers to communicate information.However,this uses a lot of spectrum which l
21、imitsthe number of users.Alternatively,more complextransmitters and receivers can be used to transmitthe same information over less bandwidth.Thetransition to more and more spectrally efficienttransmission techniques requires more and morecomplex hardware.Complex hardware is difficultto design,test,
22、and build.This tradeoff existswhether communication is over air or wire,analogor digital.Figure 1.The Fundamental TradeoffComplexHardware Less SpectrumSimpleHardware SimpleHardware ComplexHardware More Spectrum1.Why Digital Modulation?61.2 Industry trendsOver the past few years a major transition ha
23、soccurred from simple analog Amplitude Mod-ulation(AM)and Frequency/Phase Modulation(FM/PM)to new digital modulation techniques.Examples of digital modulation include QPSK(Quadrature Phase Shift Keying)FSK(Frequency Shift Keying)MSK(Minimum Shift Keying)QAM(Quadrature Amplitude Modulation)Another la
24、yer of complexity in many new systemsis multiplexing.Two principal types of multiplex-ing(or“multiple access”)are TDMA(Time DivisionMultiple Access)and CDMA(Code DivisionMultiple Access).These are two different ways toadd diversity to signals allowing different signalsto be separated from one anothe
25、r.QAM,FSK,QPSKVector Signals AM,FMScalar SignalsTDMA,CDMATime-VariantSignals Required Measurement CapabilitySignal/System ComplexityFigure 2.Trends in the Industry72.1 Transmitting informationTo transmit a signal over the air,there are threemain steps:1.A pure carrier is generated at the transmitter
26、.2.The carrier is modulated with the informationto be transmitted.Any reliably detectablechange in signal characteristics can carry information.3.At the receiver the signal modifications orchanges are detected and demodulated.2.2 Signal characteristics that can be modifiedThere are only three charac
27、teristics of a signal thatcan be changed over time:amplitude,phase,or fre-quency.However,phase and frequency are just dif-ferent ways to view or measure the same signalchange.In AM,the amplitude of a high-frequency carriersignal is varied in proportion to the instantaneousamplitude of the modulating
28、 message signal.Frequency Modulation(FM)is the most popularanalog modulation technique used in mobile com-munications systems.In FM,the amplitude of themodulating carrier is kept constant while its fre-quency is varied by the modulating message signal.Amplitude and phase can be modulated simultane-o
29、usly and separately,but this is difficult to gener-ate,and especially difficult to detect.Instead,inpractical systems the signal is separated intoanother set of independent components:I(In-phase)and Q(Quadrature).These components areorthogonal and do not interfere with each other.Modify aSignalModul
30、ate Detect the Modifications DemodulateAny reliably detectable change insignal characteristics can carry information AmplitudeFrequencyorPhaseBoth Amplitudeand PhaseFigure 3.Transmitting Information(Analog or Digital)Figure 4.Signal Characteristics to Modify2.Using I/Q Modulation to Convey Informati
31、on82.3 Polar displaymagnitude and phase repre-sented togetherA simple way to view amplitude and phase is withthe polar diagram.The carrier becomes a frequencyand phase reference and the signal is interpretedrelative to the carrier.The signal can be expressedin polar form as a magnitude and a phase.T
32、hephase is relative to a reference signal,the carrierin most communication systems.The magnitude iseither an absolute or relative value.Both are usedin digital communication systems.Polar diagramsare the basis of many displays used in digital com-munications,although it is common to describe thesign
33、al vector by its rectangular coordinates of I(In-phase)and Q(Quadrature).2.4 Signal changes or modifications in polar formFigure 6 shows different forms of modulation inpolar form.Magnitude is represented as the dis-tance from the center and phase is represented asthe angle.Amplitude modulation(AM)c
34、hanges only the magnitude of the signal.Phase modulation(PM)changes only the phase of the signal.Amplitudeand phase modulation can be used together.Frequency modulation(FM)looks similar to phasemodulation,though frequency is the controlledparameter,rather than relative phase.PhaseMag0 degFigure 5.Po
35、lar DisplayMagnitude and PhaseRepresented TogetherPhaseMag0 deg Magnitude Change Phase0 deg Phase Change Frequency Change Magnitude&Phase Change 0 deg 0 deg Figure 6.Signal Changes or Modifications9One example of the difficulties in RF design can be illustrated with simple amplitude modulation.Gener
36、ating AM with no associated angular modula-tion should result in a straight line on a polar display.This line should run from the origin tosome peak radius or amplitude value.In practice,however,the line is not straight.The amplitudemodulation itself often can cause a small amountof unwanted phase m
37、odulation.The result is acurved line.It could also be a loop if there is anyhysteresis in the system transfer function.Someamount of this distortion is inevitable in any sys-tem where modulation causes amplitude changes.Therefore,the degree of effective amplitude modu-lation in a system will affect
38、some distortionparameters.2.5 I/Q formatsIn digital communications,modulation is oftenexpressed in terms of I and Q.This is a rectangularrepresentation of the polar diagram.On a polardiagram,the I axis lies on the zero degree phasereference,and the Q axis is rotated by 90 degrees.The signal vectors
39、projection onto the I axis is its“I”component and the projection onto the Q axis is its“Q”component.0 degIQQ-ValueI-ValueProject signalto I and Q axesPolar to Rectangular ConversionFigure 7.“I-Q”Format102.6 I and Q in a radio transmitterI/Q diagrams are particularly useful because theymirror the way
40、 most digital communications sig-nals are created using an I/Q modulator.In thetransmitter,I and Q signals are mixed with thesame local oscillator(LO).A 90 degree phaseshifter is placed in one of the LO paths.Signalsthat are separated by 90 degrees are also known asbeing orthogonal to each other or
41、in quadrature.Signals that are in quadrature do not interferewith each other.They are two independent compo-nents of the signal.When recombined,they aresummed to a composite output signal.There aretwo independent signals in I and Q that can besent and received with simple circuits.This simpli-fies t
42、he design of digital radios.The main advan-tage of I/Q modulation is the symmetric ease ofcombining independent signal components into asingle composite signal and later splitting such acomposite signal into its independent componentparts.2.7 I and Q in a radio receiverThe composite signal with magn
43、itude and phase(or I and Q)information arrives at the receiverinput.The input signal is mixed with the localoscillator signal at the carrier frequency in twoforms.One is at an arbitrary zero phase.The otherhas a 90 degree phase shift.The composite inputsignal(in terms of magnitude and phase)is thusb
44、roken into an in-phase,I,and a quadrature,Q,component.These two components of the signalare independent and orthogonal.One can bechanged without affecting the other.Normally,information cannot be plotted in a polar formatand reinterpreted as rectangular values withoutdoing a polar-to-rectangular con
45、version.This con-version is exactly what is done by the in-phase andquadrature mixing processes in a digital radio.Alocal oscillator,phase shifter,and two mixers canperform the conversion accurately and efficiently.90 deg Phase Shift Local Osc.(Carrier Freq.)Q I CompositeOutputSignal Local Osc.(Carr
46、ier Freq.)Quadrature Component In-Phase Component CompositeInputSignal 90 degPhase Shift Figure 8.I and Q in a Practical Radio TransmitterFigure 9.I and Q in a Radio Receiver112.8 Why use I and Q?Digital modulation is easy to accomplish with I/Qmodulators.Most digital modulation maps the datato a nu
47、mber of discrete points on the I/Q plane.These are known as constellation points.As the sig-nal moves from one point to another,simultaneousamplitude and phase modulation usually results.To accomplish this with an amplitude modulatorand a phase modulator is difficult and complex.Itis also impossible
48、 with a conventional phase modu-lator.The signal may,in principle,circle the originin one direction forever,necessitating infinite phaseshifting capability.Alternatively,simultaneous AMand Phase Modulation is easy with an I/Q modulator.The I and Q control signals are bounded,but infi-nite phase wrap
49、 is possible by properly phasingthe I and Q signals.12This section covers the main digital modulationformats,their main applications,relative spectralefficiencies,and some variations of the main modulation types as used in practical systems.Fortunately,there are a limited number of modula-tion types
50、 which form the building blocks of anysystem.3.1 ApplicationsThe table below covers the applications for differ-ent modulation formats in both wireless communi-cations and video.Although this note focuses on wireless communica-tions,video applications have also been includedin the table for complete