% am.m
% Matlab demonstration script for DSB-AM modulation. The message signal
% is +1 for 0 < t < t0/3, -2 for t0/3 < t < 2t0/3 and zero otherwise.
echo on
t0=.15;                              	% signal duration
ts=0.001;                            	% sampling interval
fc=250;                              	% carrier frequency
snr=10;                              	% SNR in dB (logarithmic)
a=0.85;                              	% Modulation index
fs=1/ts;                             	% sampling frequency
t=[0:ts:t0];                         	% time vector
df=0.2;                              	% required frequency resolution
snr_lin=10^(snr/10);                 	% SNR
% message signal
m=[ones(1,t0/(3*ts)),-2*ones(1,t0/(3*ts)),zeros(1,t0/(3*ts)+1)];
c=cos(2*pi*fc.*t);                   	% carrier signal
m_n=m/max(abs(m));                   	% normalized message signal
[M,m,df1]=fftseq(m,ts,df);           	% Fourier transform 
M=M/fs;                              	% scaling
f=[0:df1:df1*(length(m)-1)]-fs/2;    	% frequency vector
u=(1+a*m_n).*c;                      	% modulated signal
[U,u,df1]=fftseq(u,ts,df);           	% Fourier transform 
U=U/fs;                              	% scaling
signal_power=power(u(1:length(t)));  	% power in modulated signal
% power in normalized message
pmn=power(m(1:length(t)))/(max(abs(m)))^2;
eta=(a^2*pmn)/(1+a^2*pmn);           	% modulation efficiency
noise_power=eta*signal_power/snr_lin;	% noise power
noise_std=sqrt(noise_power);         	% noise standard deviation
noise=noise_std*randn(1,length(u));  	% generate noise
r=u+noise;                           	% add noise to the modulated signal
[R,r,df1]=fftseq(r,ts,df);           	% Fourier transform 
R=R/fs;                              	% scaling
pause  % Press a key to show the modulated signal power
signal_power
pause  % Press a key to show the modulation efficiency
eta
pause  % Press any key to see a plot of the message
subplot(2,2,1)
plot(t,m(1:length(t)))
axis([0 0.15 -2.1 2.1])
xlabel('Time')
title('The message signal')
pause
pause  % Press any key to see a plot of the carrier
subplot(2,2,2)
plot(t,c(1:length(t)))
axis([0 0.15 -2.1 2.1])
xlabel('Time')
title('The carrier') 
pause  % Press any key to see a plot of the modulated signal
subplot(2,2,3)
plot(t,u(1:length(t)))
axis([0 0.15 -2.1 2.1])
xlabel('Time')
title('The modulated signal')
pause   % Press any key to see a plots of the magnitude of the message and the
	% modulated signal in the frequency domain.
subplot(2,1,1)
plot(f,abs(fftshift(M))) 
xlabel('Frequency')
title('Spectrum of the message signal')
subplot(2,1,2)
plot(f,abs(fftshift(U))) 
title('Spectrum of the modulated signal')
xlabel('Frequency')
pause  % Press a key to see a noise sample
subplot(2,1,1)
plot(t,noise(1:length(t))) 
title('noise sample') 
xlabel('Time')
pause  % Press a key to see the modulated signal and noise
subplot(2,1,2)
plot(t,r(1:length(t))) 
title('Signal and noise')
xlabel('Time')
pause  % Press a key to see the modulated signal and noise in freq. domain
subplot(2,1,1)
plot(f,abs(fftshift(U))) 
title('Signal spectrum')
xlabel('Frequency')
subplot(2,1,2)
plot(f,abs(fftshift(R))) 
title('Signal and noise spectrum')
xlabel('Frequency')