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1、Comparing with transmitting baseband signal directly,to transmit data by modulating a higher frequencycarrier wave has the effect on:controlling the radiated frequency spectrum.more efficient use of the allocated RF bandwidth.flexibility in accommodating different baseband signal formats.第1页/共69页Amp
2、litude Frequency PhaseAM FM PMAnalog modulation(vary continuously)Digital modulation(change in discrete steps)第2页/共69页more efficient use of the radio spectrum.usually requires less power.CDMAover a fading communications channel.more compatible with the use of error correcting codes.In contrast to an
3、alog modulation,digital modulation has:第3页/共69页9.1 Analog Modulation9.2 Binary Digital Modulation9.3 Error Probabilities for Binary Modulation9.4 Effect of Rayleigh Fading on Bit Error Rats9.5 M-ary Digital Modulation第4页/共69页9.1 Analog ModulationBasic analog modulation:AM(SSB,DSB),FM,PMPPM(Pulse Pos
4、ition Modulation)is used in UWB(ultra wideband)system.1.Signal-Sideband Modulation2.DSB-SC Modulation3.DSB-LC Modulation4.Envelope Detection of DSB-LC Modulation5.Frequency Modulation(FM)第5页/共69页1.Signal-Sideband Modulation m(t):bandlimited modulating waveformfM:the maximum frequency of m(t)n0/2:two
5、-sided power spectral density of Gaussian white noise.Contributed by the transmitter channel and noise generated by the input stages of the receiver.第6页/共69页The demodulator LO is identical in frequency and phasewith the modulator LO-synchronous or coherent demodulator.第7页/共69页Input signal power:Outp
6、ut signal power:Input narrowband noise power:Output noise power:The output SNR:Conclusion:SSB demodulator does not degrade the input SNR.第8页/共69页2.DSB-SC ModulationDouble-sideband suppressed carrier(DSB-SC)第9页/共69页 Conclusion:the DSB-SC demodulator improves the input signal-to-noise ratio by a facto
7、r of two.FSSB=2FDSBButConclusion:The coherent SSB and DSB-SC demodulators have the same SNR performance.第10页/共69页3.DSB-LC ModulationAdvantage:the carrier signal can be used as a reference signal to phase-look the local oscillator to synchronization with the incoming signal.Double-Sideband Large-Carr
8、ier(DSB-LC)Double-Sideband Large-Carrier(DSB-LC)Disadvantage:the carrier power increases the total input power but does not contain any modulation information.第11页/共69页 m:modulation index IF m1,DSB-LC DSB-SC m=1 implies a reduction in SNR of 4.8dB.第12页/共69页4.Envelope Detection of DSB-LC Modulation A
9、dvantage of DSB-LC:can be detected by using an envelope detector,without a LO and mixer,results in a much simple receiver circuit,such as broadcast AM radio.noncoherent demodulation 第13页/共69页Serious distortion for small Si/Ni SNRIf|m|1 over modulation,it will not correctly recover the modulating wav
10、eform.第14页/共69页5.Frequency Modulation(FM)Application:Broadcast radio,television sound,two-way voice radio,AMPS cellular telephone system.第15页/共69页An FM waveform:Where,modulating signal Modulation index:Maximum frequency deviation.第16页/共69页Spectrum:sidebands are spaced at fm on either side of the car
11、rier at IF.amplitudes,given by AJn(),decrease for large n.Carsons rule ,B IF bandwidth第17页/共69页Through differentiator and envelope detection,the output voltage:For the equal transmit powerImproving factor:第18页/共69页Conclusion:FM allows an improvement in SNR at the expense of increased bandwidth,while
12、 AM does not.,S/N,B=4 SNRFM=72SNRDSB-LC(m=1)BFM=5BDSB第19页/共69页9.2 Binary Digital ModulationAmplitude shift keying(ASK)Frequency shift keying(FSK)Phase shift keying(PSK)第20页/共69页1.Binary Signals2.Amplitude Shift Keying3.Frequency Shift Keying(FSK)4.PSK5.Carrier Synchronization第21页/共69页1.Binary Signal
13、sReturn-to-Zero codeNon-Return-to-Zero codePolar NRZ code DC=0第22页/共69页2.Amplitude Shift Keyingwhere m(t)=0 or 1(a)ModulatorIdentical to theDSB-SC modulator第23页/共69页Synchronous demodulation:After low-pass filtering(b)Synchronous demodulationNotice:LO has precisely the same phase and frequency as the
14、 incoming signal,or distortion may be introduced.第24页/共69页Envelope detectionnoncoherent,no LO(c)Envelope detection第25页/共69页 3.Frequency Shift Keying(FSK)-frequency deviation第26页/共69页 IF =1,the up branch output:1/2 The down branch output:0 IF =2,the up branch output:0 The down branch output:1/2 rever
15、se phase It requires two coherent LO operating at1,and 2.第27页/共69页(PLL detector,the control voltage of VCO in PLL)Envelope detector第28页/共69页 4.PSKThe phase of the carrier wave is“0”or“180”.m(t)=1 or-1 Due to the sharp transitions caused by phase reversal,the spectrum of the PSK waveform is relativel
16、y wide in bandwidth,resulting that PSK is impractical for multichannel wireless systems.PSK modulator第29页/共69页第30页/共69页ASK:Non-constant envelope modulationCoherent demodulationNoncoherent demodulation(envelope detection)FSK:Constant envelope modulationCoherent demodulationNoncoherent demodulation(af
17、ter conversion)PSK:Constant envelope modulationCoherent demodulation第31页/共69页5.Carrier Synchronization The effect of a phase error is that the output signal is reduced in amplitude by cos,while an error in frequency introduces a factor of cost.The bit error rates of envelope detection are not as goo
18、d as those obtained with coherent detection.Two ways to realize synchronization:transmit a pilot carrier,used to phase-lock the LO.use a carrier-recovery circuit.Use a phase-locked loop or by frequency multiplier and divider.In fact,employing digital signal processing(DSP)circuits to perform all fun
19、ction of signal conditioning,carrier recovery and synchronization demodulation,and signal formatting.第32页/共69页9.3 Error Probabilities for Binary Modulation The presence of noise in a communication channel introduces the possibility that errors will be made during the detection process.1.PCM Signal a
20、nd Detectors2.Synchronous ASK3.Synchronous PSK4.Synchronous FSK5.Bit rate and Bandwidth Efficient6.Comparison of ASK FSK and PSK Systems第33页/共69页1.PCM Signal and DetectorsPulse coded modulation(PCM)第34页/共69页where第35页/共69页Define:bit energyThe output noise powerThe variance of the gaussian probability
21、 distribution function第36页/共69页2.Synchronous ASKIdeal if s(t)=s2(t)=0,s0(T)=0 if s(t)=s1(t)=V,s0(T)=VTPractical if s0(T)+n0(T)VT/2,m(t)=1 第37页/共69页Let 第38页/共69页3.Synchronous PSK If Threshold level:0ASK:VT/2 For the same probability of error,PSK requires only one-fourth the power of an ASK system.Sin
22、ce an ASK signal is off half the time,in terms of average transmit power,the PSK result is better by a factor of two(3 dB).Due to symmetry of the PSK signal and the demodulator.第39页/共69页4.Synchronous FSKThreshold:0 the signal levels are similar to the PSK case.For the noise voltages:n1 and n2 are un
23、correlated.Conclusion:The total noise power of the FSK demodulator is doubled relative to the synchronous ASK or PSK demodulator.第40页/共69页Conclusion:synchronous FSK requires 3 dB more signal power than equivalent PSK system for the same probability of error.synchronous FSK requires 3 dB less power t
24、han an ASK system on a peak power basis.FSK and ASK have equal error rates when compared in terms of average transmit power.第41页/共69页5.Bit Rate and Bandwidth EfficientDefine Rb:dimension of Eb:WSdimension of n0:W/HzEb/n0:dimensionlessthe bit rate of the binary message signaldimension of Rb:bps(bit p
25、er second)The signal power:It means the error rate will increase with an increase in bit rate,for a fixed noise power spectrum density,and is independent of the receiver bandwidth.第42页/共69页IF bandwidth f RF bandwidthRb:depend on the type of modulation f may range from one to several times the bit ra
26、te Rb.S/N before demodulation=S/N after demodulation.Reference to equation(9.2),(9.3),(9.4)and(9.64,(9.65).第43页/共69页bandwidth efficiency(bps/Hz)of 1 bps/Hz.For a binary modulation method,transmitting one bit each bit period.means thatThis is for the baseband,not for the IF or RF.第44页/共69页6.Compariso
27、n of ASK FSK and PSK Systems第45页/共69页Coherent PSK:The lowest error rate,9.6 dB for 10-5,high price for LO and wide signal bandwidth(2Rb4Rb).Best in fading environments.Used in space and satellite communications.Coherent FSK:Requires 34 dB more power than PSK.12.6 dB for 10-5.第46页/共69页Noncoherent FSK
28、:13.4 dB for 10-5,widespread historical application in a wide variety of systems,such as date modems,teletype,fax.Coherent ASK:Transmitter is simple.15.6dB for 10-5,very poor in a fading environment,low data rates limited to short-range,low-lost,used in telemetry and RFID.Noncoherent ASK:Transmitter
29、 and receiver are simple,16.5 dB for 10-5,very poor in a fading environment,low data rates,short-range,low-cost,used in telemetry and RFID.第47页/共69页EXAMPLE 9.3,page 319.ASKFSKPSK4 Eb/n0 2 Eb/n0Eb/n015.6dB12.6dB9.6dBEb/n0 for Pe=10-5第48页/共69页 9.4 Effect of Rayleigh Fading on Bit Error RatsPDF of a Ra
30、yleigh fading is where is the rms value of the distribution of r(t).第49页/共69页1.Effect of Rayleigh Fading on Coherent PSKWhereis the average received bit energy-to-noise power spectral density ratio of the faded received signal.第50页/共69页2.Effect of Rayleigh Fading on Noncoherent FSKFor envelope detec
31、tion of FSK第51页/共69页3.Comparison of Faded and Nonfaded Error RatesConclusion:Fading has the effect of dramatic increasing the required bit energy-to-noise ratio.Error-correcting codes can be used very effectively to improve the error rate for channel fading occurring in short bursts.Nonfaded case:第5
32、2页/共69页Eye diagram EXAMPLE 9.4,p.323.第53页/共69页9.5 M-ary Digital Modulation Binary modulation methods(ASK,FSK and PSK)transmit one bit per signaling interval,with a bandwidth efficiency of 1bps/Hz.The data can be divided into groups.Each group have n binary codes.One symbol in M=2n is transmitted in
33、each signaling interval.Thus a bandwidth efficiency of n bps/Hz is achieved.第54页/共69页1.Quadrature Phase Shift Keying2.Probability of Error for QPSK3.M-ary Phase Shift Keying4.Quadrature Amplitude Modulation (QAM)5.Channel Capacity第55页/共69页1.Quadrature Phase Shift Keyingn=2,M=4+=The carrier of QPSK:第
34、56页/共69页Note:Each QPSK phase state can be used to represent two bits of data.The bandwidth of the QPSK spectrum is narrower than the spectrum of a BPSK signal,because of the average transition between phase states is 900.The output of the QPSK modulator is a double sideband suppressed carrier signal
35、.The QPSK output is a constant envelope signal.The channel should be constant group delay.Block diagram of a QPSK modulator第57页/共69页Gray CodingAdvantage:When an error occurs,it is most likely that only one of the bits will be in error,rather than both bits.S04501,1S113500,1S2-13500,0S3-4501,0第58页/共6
36、9页2.Probability of Error for QPSKThe overall probability of error for a symbol is:第59页/共69页 A symbol error is most likely to cause only a single bit error for Gray coding.Since each symbol contains two bits,the bit error rate for QPSK will be one-half the symbol error rate:for BPSK第60页/共69页 Led to t
37、he extensive use of QPSK modulation in a wide variety of applications,such as CDMA-PCS telephone systems,The Iridium LEO satellite telephone system,and the DBS.Conclusion:with QPSK it is possible to achieve twice the data rate as for BPSK,with less bandwidth and the same error rate.Bandwidth efficie
38、ncy is 2 bps/Hz.EXAMPLE 9.5,page 329.第61页/共69页3.M-ary Phase Shift Keyingfor i=0,1,2 M-1,M=2n M=2 for BPSK M=4 for QPSK bandwidth efficiency n bps/Hz第62页/共69页4.Quadrature Amplitude Modulation(QAM)QPSK:Generally If 4-QAM is QPSK.It encompasses ASK and M-PSK.第63页/共69页A symbol includes four bit.High ban
39、dwidth efficiency Increasingly being used.第64页/共69页M-QAM,the maximum of M is 256.第65页/共69页5.Channel Capacity To achieve as low an error as is desired once Eb/n0 is above a critical value.C is the maximum data rate capacity of the channel(bps).B is the bandwidth(Hz).S is the signal power(W).n0/2 is t
40、he two-sided power spectral density of the gaussian noise(W/Hz).第66页/共69页For a given channel in the presence of additive gaussian noise and bandwidth,the maximum data rata C can be achieved.In practical,only a fraction of C can be achieved.The use of error correcting codes can provide performance close to the limit.第67页/共69页QPSK:constant envelope modulationnonconstant envelope modulation第68页/共69页感谢您的观看！第69页/共69页