无线系统的微波与射频设计9.ppt

1、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:,controlling the radiated frequency spectrum. more efficient use of th

2、e allocated RF bandwidth. flexibility in accommodating different baseband signal formats.,Amplitude Frequency Phase,AM FM PM,Analog modulation (vary continuously) Digital modulation (change in discrete steps),more efficient use of the radio spectrum. usually requires less power. CDMA over a fading c

3、ommunications channel. more 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

4、 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.

5、Signal-Sideband Modulation,m(t): bandlimited modulating waveform fM: the maximum frequency of m(t),n0/2: 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 freque

6、ncy 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

7、-SC),Conclusion: the DSB-SC demodulator improves the input signal-to-noise ratio by a factor of two.,FSSB=2FDSB,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 t

8、he 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 m1, DSB-LC DSB-SC,m=1 implies a reduction in SNR of 4.8dB.,4

9、. 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 Si/Ni SNR If |m|1 over modulation, it wil

10、l 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:,: Maximum frequency deviation.,Spectrum: sidebands are spaced

11、at fm on either side of the carrier at IF. amplitudes, given by AJn(), decrease for large n.,Carsons 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

12、increased bandwidth, while AM does not., S/N, B, =4 SNRFM=72SNRDSB-LC (m=1) BFM=5BDSB,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 Synchro

13、nization,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 ph

14、ase and frequency as the incoming signal, or distortion may be introduced.,Envelope detection noncoherent, no LO,(c) Envelope detection,3. Frequency Shift Keying (FSK), -frequency deviation,IF =1, the up branch output: 1/2 The down branch output: 0 IF =2, the up branch output: 0 The down branch outp

15、ut: 1/2 reverse phase It requires two coherent LO operating at1, and 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 r

16、elatively 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 (

17、after conversion),PSK: Constant envelope modulation Coherent demodulation,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 perfor

19、m all function of signal conditioning, carrier recovery and synchronization demodulation, and signal formatting.,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 Sig

20、nal and Detectors 2. Synchronous ASK 3. Synchronous PSK 4. Synchronous FSK 5. Bit rate and Bandwidth Efficient 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 di

21、stribution function,2. Synchronous ASK,Ideal if s(t)=s2(t) =0, s0(T) =0 if s(t)=s1(t)=V, s0 (T)=VT Practical if s0(T)+n0(T)VT/2, m(t)=1 ,Let,3. Synchronous PSK,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 ti

22、me, 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:,n1 and n2 are uncorrelated.,Conclusion: The total

23、noise power of the FSK demodulator is doubled relative to the synchronous ASK or PSK demodulator.,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 than an ASK system on a peak power basi

24、s. FSK and ASK have equal error rates when compared in terms of average transmit power.,5. Bit Rate and Bandwidth Efficient,Define Rb:,dimension of Eb : WS,dimension of n0 : W/Hz,Eb/n0 : dimensionless,the bit rate of the binary message signal dimension of Rb : bps (bit per second),The signal power:,

25、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 f RF bandwidth,Rb: depend on the type of modulation,f may range from one to several times the bit rate Rb.,bandwidth efficiency (bp

26、s/Hz) of 1 bps/Hz.,For a binary modulation method, transmitting one bit each bit period.,means that,This is for the baseband, not for the IF or RF.,6. Comparison of ASK FSK and PSK Systems,Coherent PSK: The lowest error rate, 9.6 dB for 10-5, high price for LO and wide signal bandwidth (2Rb4Rb). Bes

27、t in fading environments. Used in space and satellite communications.,Coherent FSK: Requires 34 dB more power than PSK. 12. 6 dB for 10-5.,Noncoherent FSK: 13.4 dB for 10-5, widespread historical application in a wide variety of systems, such as date modems, teletype, fax.,Coherent ASK: Transmitter

28、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 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 teleme

29、try and RFID.,EXAMPLE 9.3, page 319.,Eb/n0 for Pe=10-5,9.4 Effect of Rayleigh Fading on Bit Error Rats,PDF of a Rayleigh fading is,1. Effect of Rayleigh Fading on Coherent PSK,Where,is the average received bit energy-to-noise,power spectral density ratio of the faded received signal.,2. Effect of Ra

30、yleigh Fading on Noncoherent FSK,For envelope detection of FSK,3. Comparison of Faded and Nonfaded Error Rates,Conclusion: 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 fa

31、ding occurring in short bursts.,Nonfaded case:,Eye diagram,EXAMPLE 9.4, p. 323.,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 cod

32、es. One symbol in M=2n is transmitted in each signaling interval. Thus a bandwidth efficiency of n bps/Hz is achieved.,1. Quadrature Phase Shift Keying 2. Probability of Error for QPSK 3. M-ary Phase Shift Keying 4. Quadrature Amplitude Modulation (QAM) 5. Channel Capacity,1. Quadrature Phase Shift

33、Keying,n=2, M=4,+,=,The carrier of QPSK:,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 doub

34、le sideband suppressed carrier signal. The QPSK output is a constant envelope signal. The channel should be constant group delay.,Block diagram of a QPSK modulator,Gray Coding,Advantage: When an error occurs, it is most likely that only one of the bits will be in error, rather than both bits.,2. Pro

35、bability of Error for QPSK,The overall probability of error for a symbol is:,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,Led to the extensive use of QPSK modulation in a wide variety of applications, such as CDMA-PCS telephon