无线系统的微波与射频设计.ppt

Modulation Techniques,,Microwave and RF Design of Wireless Systems,Chapter 9,Dr. Zhang Yonghong,,Comparing with transmitting baseband signal directly,,,,to transmit data by modulating a higher frequency,,carrier wave has the effect on:,①controlling the radiated frequency spectrum.,,②more efficient use of the allocated RF bandwidth.,,③flexibility in accommodating different,baseband,signal formats.,Amplitude Frequency Phase,AM FM PM,Analog modulation (vary continuously),,Digital modulation (change in discrete steps),①more efficient use of the radio spectrum.,,②usually requires less power. CDMA,,③over a fading communications channel.,,④more compatible with the use of error correcting codes.,In contrast to analog modulation,,,digital modulation has:,§9.1 Analog Modulation,,§9.2 Binary Digital,Modulation,,§9.3 Error Probabilities for Binary,Modulation,,§9.4 Effect of Rayleigh Fading on Bit Error,Rats,,§9.5 M-ary Digital,Modulation,§9.1 Analog Modulation,Basic analog modulation:,,AM (SSB, DSB), FM, PM,PPM (Pulse Position Modulation),,is used in UWB (ultra wideband) system.,1. Signal-Sideband Modulation,,2. DSB-SC,Modulation,,3. DSB-LC,Modulation,,4. Envelope Detection of DSB-LC,Modulation,,5. Frequency Modulation (,FM,),1. Signal-Sideband Modulation,,m(t): bandlimited modulating waveform,,f,M,: the maximum frequency of m(t),n,0,/2: two-sided power spectral density of Gaussian white noise.,,Contributed by the transmitter channel and noise generated by the input stages of the receiver.,The demodulator LO is identical in frequency and phase,,with the modulator LO-synchronous or coherent demodulator.,Input signal power:,,Output signal power:,,Input narrowband noise power:,,Output noise power:,,The output SNR:,,Conclusion:,,SSB demodulator does not degrade the input,SNR,.,2. DSB-SC Modulation,Double-sideband suppressed carrier (DSB-SC),,Conclusion:,,the DSB-SC demodulator improves the input signal-to-noise ratio by a factor of two.,F,SSB,=2F,DSB,But,Conclusion:,,The coherent SSB and DSB-SC demodulators have the same SNR,performance,.,3. DSB-LC Modulation,Advantage:,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-Carrier (DSB-LC),Disadvantage:,the carrier power increases the total input power but does not contain any modulation information.,,m: modulation index,IF m>>1, DSB-LC,,DSB-SC,m=1 implies a reduction in SNR of 4.8dB.,4. Envelope Detection of DSB-LC Modulation,,Advantage 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,Serious distortion for small,S,i,/N,i,SNR,,If |m|>1 over modulation, it will not correctly recover the modulating,,waveform,.,5. Frequency Modulation (FM),Application:,,Broadcast radio, television sound, two-way voice radio,,,AMPS cellular telephone system.,An FM waveform:,,Where, modulating signal,,Modulation index:,,Δω: Maximum frequency deviation.,Spectrum:,,①sidebands are spaced at,f,m,on either side of the carrier at ω,IF.,,②amplitudes, given by AJ,n,(,,), decrease for large n.,Carson’s rule,,,↑, B↑,IF bandwidth,Through differentiator and envelope detection,,,the output voltage:,,For the equal transmit power,Improving factor:,Conclusion:,,FM allows an improvement in SNR at the expense of increased bandwidth, while AM does not.,,,↑, S/N↑, B↑,,=4 SNR,FM,=72SNR,DSB-LC (m=1),,B,FM,=5,B,DSB,§9.2 Binary Digital Modulation,Amplitude shift keying (ASK),,Frequency shift keying (FSK),,Phase shift keying (PSK),1. Binary Signals,,2. Amplitude Shift,Keying,,3. Frequency Shift Keying (,FSK,),,4.,PSK,,5. Carrier Synchroniza,tion,1. Binary Signals,Return-to-Zero code,Non-Return-to-Zero code,Polar NRZ code DC=0,2. Amplitude Shift Keying,where m(t) =0 or 1,(a) Modulator,Identical to the,,DSB-SC modulator,Synchronous demodulation,:,After low-pass filtering,,(b) Synchronous demodulation,Notice,: LO has precisely the same phase and frequency as the incoming signal, or distortion may be introduced.,Envelope detection,,noncoherent, no,LO,(c) Envelope detection,,3. Frequency Shift Keying (FSK),,Δω --frequency deviation,,IF ω=ω,1,, the up branch output: 1/2,,The down branch output: 0,,IF ω=ω,2,, the up branch output: 0,,The down branch output: 1/2 reverse phase,,It requires two coherent LO operating atω,1,, and ω,2,.,(PLL detector, the control voltage of VCO in,PLL,),Envelope detector,,4. PSK,The 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 relatively wide in bandwidth, resulting that PSK is impractical for multichannel wireless systems.,PSK modulator,ASK,:,,Non-constant envelope modulation,,Coherent demodulation,,Noncoherent demodulation,,(envelope detection),FSK:,,,Constant envelope modulation,,Coherent demodulation,,Noncoherent demodulation,,(after conversion),PSK:,,Constant envelope modulation,,Coherent demodula,tion,5. Carrier Synchronization,,The effect of a phase error,,is that the output signal is reduced in amplitude by,cos,,,,while an error,Δω,in frequency introduces a factor of,cos,,t.,,The bit error rates of envelope detection are not as good as those obtained with coherent detection.,Two ways to realize synchronization:,,① transmit a pilot carrier, used to phase-lock the LO.,,② 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 function of signal,conditioning,, carrier recovery and synchronization demodulation, and signal format,ting,.,§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 and Detectors,,2. Synchronous,ASK,,3. Synchronous,PSK,,4. Synchronous,FSK,,5. Bit rate and Bandwidth Effici,ent,,6. Comparison of ASK FSK and PSK,Systems,1. PCM Signal and Detectors,Pulse coded modulation (PCM),where,Define,: bit energy,The output noise power,The variance of the gaussian probability distribution func,tion,2. Synchronous ASK,Ideal if s(t)=s,2,(t) =0, s,0,(T) =0,,if s(t)=s,1,(t)=V, s,0,(T)=VT,,,Practical if s,0,(T)+n,0,(T)VT/2, m(t)=1 ×,Let,,3. Synchronous PSK,,If,,Threshold level: 0,ASK: > VT/2,For the same probability of error, PSK requires only one-fourth the power of an ASK system. Since 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.,4. Synchronous FSK,Threshold: 0 →the signal levels are similar to the PSK case.,For the noise voltages:,,n,1,and n,2,are uncorrelated.,,Conclusion:,,,The total noise power of the FSK demodulator is doubled relative to the synchronous ASK or PSK demodulator.,Conclusion:,,①synchronous FSK requires 3 dB more signal power than equivalent PSK system for the same probability of error.,,②synchronous FSK requires 3 dB less power than an ASK system on a peak power basis.,,③FSK and ASK have equal error rates when compared in terms of average transmit,power,.,5. Bit Rate and Bandwidth Efficient,Define R,b:,,dimension of E,b,: W,,S,dimension of n,0,: W,/Hz,E,b,/n,0,: dimensionless,the bit rate of the binary message signal,,dimension of R,b,: bps (bit per 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.,IF bandwidth Δf,