B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

Figure 4.41
Frequency
domain signals
during QAM
modulation and
coherent
demodulation for
the second
message m2(1) .
Message 2 spectrum
250 250
200
(\
200
150
'-,
'.::) 150
100 .,.,a 100
50 50
O -600
J \
4. 9 MATLAB Exercises
QAM spectrum magnitude
-400 -200 0 200 400 600 O -600 -400 -200 0 200
f(Hz)
f(Hz)
Second demodulator spectrum Recovered spectrun 2
250
250
200
200
A
C:, 150
S 150
Cl
100
i 100
50
50
0 0 -600 -400 -200 0 200 400 600 -600 -400
I \
-200 0 200 400
f(Hz)
f(Hz)
400
600
600
193
% (ExampleQAMdemfilt.m)
% This program uses triangl .m and triplesinc .m
% to illustrate QAM modulation % and demodulation
% of two message signals
clear;c lf;
ts=l .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 dif ferent shapes and spectra
m_sigl=triangl ((t+0.0l)/0.01)-triangl ((t- 0.01)/0.01) ;
m_s ig2=triplesinc (t,Ta) ;
Lm_s ig=length (m_sigl) ;
Lfft=length (t) ; Lfft=2 ceil (log2 (Lfft) );
Ml_fre=fftshift ( fft (m_s igl ,Lfft) );
M2_fre=fftshift ( fft (m_s ig2 ,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_sigl .*cos (2*pi*fc*t) +m_sig2 .*sin(2*pi*fc*t);
Lfft=length (t) ; Lfft=2 A ceil ( log2 (Lfft) +l) ;