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

192 AMPLITUDE MODULATIONS AND DEMODULATIONS
Fi g
ure 4.39
Frequency
domain signals
during QAM
modulation and
coherent
demodulation for
the first message
mz(t) .
200
C:, 150
100
50
Message I spectrum
0 '-----....-"'-",-'--'-' '--'""""'-- -__,
-600 -400 -200 0 200 400 600
f(Hz)
First demodulator spectrum
250.-------------,
200
s 150
100
200
S 150
c,f 100
50
QAM spectrum magnitude
0 ._ ....... "'-''--,._,,____ .,,..,____,__,.,___,
-600 -400 -200 0 200 400 600
f(Hz)
Recovered spectrum 1
250.---------------.
200
S 150
:;;
100
0
f(Hz)
200 400 600
50
0 '--'--""""- '---'---/ """'--
-600 -400 -200 0 200 400 600
f(Hz)
Fi g ure 4.40
Time domain
signals during
QAM
modulation and
coherent
demodulation for
the second
message mz(t).
2
,;::;-
'g' 0
-1
Message signal 2
2
"' "' 0
-1
QAM modulated signal
-2
-0.02
-0.01 0 0.01 0.02
t(sec)
-2
-0.02 -0.01 0 0.01 0.02
t(sec)
4
Second demodulator output
2
Detected signal 2
3
s
.;-
2
0
-1
0
-1
-2
-0.02
-0.01 0 0.01 0.02
t(sec)
-2
-0.02 --0.01 0 0.01 0.02
t(sec)