基于matlab的正交振幅调制与解调,正交幅度调制和解调（QAM）

% (ExampleQAMdemfilt.m)

% This program uses triangl.m to illustrate QAM modulation

% and demodulation of two message signals.

clear; clf;

ts=1.e-4;

t=-0.04:ts:0.04;

Ta=0.01; fc=300;

% Use triangl.m and triplesinc.m to generate

% two message signals of different shapes and spectra

m_sig1=triangl((t+0.01)/0.01)-triangl((t-0.01)/0.01);

m_sig2=triplesinc(t,Ta);

Lm_sig=length(m_sig1);

Lfft=length(t); Lfft=2^ceil(log2(Lfft));

M1_fre=fftshift(fft(m_sig1,Lfft));

M2_fre=fftshift(fft(m_sig2,Lfft));

freqm=(-Lfft/2:Lfft/2-1)/(Lfft*ts);

B_m=150; %

Bandwidth of the signal is B_m Hz.

% Design a simple lowpass filter with bandwidth B_m Hz.

h=fir1(40, [B_m*ts]);

% QAM signal generated by adding a carrier to DSB-SC

s_qam=m_sig1.*cos(2*pi*fc*t)+m_sig2.*sin(2*pi*fc*t);

Lfft=length(t); Lfft=2^ceil(log2(Lfft)+1);

S_qam=fftshift(fft(s_qam,Lfft));

freqs=(-Lfft/2:Lfft/2-1)/(Lfft*ts);

% Demodulation begins by using a

rectifier

s_dem1=s_qam.*cos(2*pi*fc*t)*2;

S_dem1=fftshift(fft(s_dem1,Lfft));

% Demodulation the 2nd signal

s_dem2=s_qam.*sin(2*pi*fc*t)*2;

S_dem2=fftshift(fft(s_dem2,Lfft));

%

% Using an ideal LPF with bandwidth 150 Hz.

s_rec1=filter(h,1,s_dem1);

S_rec1=fftshift(fft(s_rec1,Lfft));

s_rec2=filter(h,1,s_dem2);

S_rec2=fftshift(fft(s_rec2,Lfft));

Trange=[-0.025 0.025 -2 2];

Trange2=[-0.025 0.025 -2 4];

figure(1)

subplot(221);td1=plot(t,m_sig1);

axis(Trange); set(td1,'Linewidth',1.5);

xlabel('{\it t} (sec)'); ylabel('{\it m}({\it t})');

title('message signal 1 ');

subplot(222);td2=plot(t,s_qam);

axis(Trange); set(td2,'Linewidth',1.5);

xlabel('{\it t} (sec)'); ylabel('{\it s}_{\rm qam}({t})');

title('QAM modulated signal');

subplot(223);td3=plot(t,s_dem1);

axis(Trange2); set(td3,'Linewidth',1.5);

xlabel('{\it t} (sec)'); ylabel('{\it e}({\it t})');

title('first demodulator output');

subplot(224);td4=plot(t,s_rec1);

axis(Trange); set(td4,'Linewidth',1.5);

xlabel('{\it t} (sec)'); ylabel('{\it m}_{d1}({\it t})');

title('detected signal 1 ');

figure(2)

subplot(221);td5=plot(t,m_sig2);

axis(Trange); set(td5,'Linewidth',1.5);

xlabel('{\it t} (sec)'); ylabel('{\it m}({\it t})');

title('message signal 2 ');

subplot(222);td6=plot(t,s_qam);

axis(Trange); set(td6,'Linewidth',1.5);

xlabel('{\it t} (sec)'); ylabel('{\it s}_{\rm qam}({t})');

title('QAM modulated signal');

subplot(223);td7=plot(t,s_dem2);

axis(Trange2); set(td7,'Linewidth',1.5);

xlabel('{\it t} (sec)'); ylabel('{\it e}({\it t})');

title('second demodulator output');

subplot(224);td8=plot(t,s_rec2);

axis(Trange); set(td8,'Linewidth',1.5);

xlabel('{\it t} (sec)'); ylabel('{\it m}_{d2}({\it t})');

title('detected signal 2 ');

Frange=[-700 700 0 250];

figure(3)

subplot(221);fd1=plot(freqm,abs(M1_fre));

axis(Frange); set(fd1,'Linewidth',1.5);

xlabel('{\it f} (Hz)'); ylabel('{\it M}({\it f})');

title('message 1 spectrum');

subplot(222);fd2=plot(freqs,abs(S_qam));

axis(Frange); set(fd2,'Linewidth',1.5);

xlabel('{\it f} (Hz)'); ylabel('{\it s}_{\rm QAM}({f})');

title('QAM spectrum magnitude');

subplot(223);fd3=plot(freqs,abs(S_dem1));

axis(Frange); set(fd3,'Linewidth',1.5);

xlabel('{\it f} (Hz)'); ylabel('{\it E}({\it f})');

title('first demodulator spectrum ');

subplot(224);fd4=plot(freqs,abs(S_rec1));

axis(Frange); set(fd4,'Linewidth',1.5);

xlabel('{\it f} (Hz)'); ylabel('{\it M}_{d1}({\it f})');

title('recovered spectrum 1 ');

figure(4)

subplot(221);fd5=plot(freqm,abs(M2_fre));

axis(Frange); set(fd5,'Linewidth',1.5);

xlabel('{\it f} (Hz)'); ylabel('{\it M}({\it f})');

title('message 2 spectrum');

subplot(222);fd6=plot(freqs,abs(S_qam));

axis(Frange); set(fd6,'Linewidth',1.5);

xlabel('{\it f} (Hz)'); ylabel('{\it s}_{\rm QAM}({f})');

title('QAM spectrum magnitude');

subplot(223);fd7=plot(freqs,abs(S_dem2));

axis(Frange); set(fd7,'Linewidth',1.5);

xlabel('{\it f} (Hz)'); ylabel('{\it E}({\it f})');

title('second demodulator spectrum ');

subplot(224);fd8=plot(freqs,abs(S_rec2));

axis(Frange); set(fd8,'Linewidth',1.5);

xlabel('{\it f} (Hz)'); ylabel('{\it M}_{d2}({\it f})');

title('recovered spectrum 2 ');