22 QPSK modulation transmission block diagram.
I(t)
Q(t)
FIGURE 2.23 QPSK signal constellation diagram.
TABLE 2.2 QPSK Phase State Table
a(t)-b(t)
0??“0
0??“1
1??“0
1??“1 p/4
p/4
3p/4
3p/4
f(t)
66 CHAPTER TWO
QPSK Signal Constellation
??“2
??“1.5
??“1.5
??“1
??“0.5
??“0.5
0
0.5
0.5
1
1.5
1.5
2
??“2 ??“1 0 1 2
I-Channel
Q-Channel
FIGURE 2.24 SRC Filtered ( 0.3) QPSK signal constellation diagram.
The SRC-filtered signal constellation clearly shows the four phase states with ISI present, as well
as the phase trajectories that traverse through the origin (see Fig. 2.24).
Let??™s discuss some properties of the QPSK modulation scheme. The complex envelope can be
rewritten in the polar coordinate system as follows:
(2.15)
where the commonly used terms to describe the amplitude and phase are given as
(2.16)
(2.17)
If we were to plot the amplitude of the QPSK modulation scheme as a function of time, using an RC
filter with roll-off factor of 0.2 and 0.9, the plot in Fig. 2.25 would be observed.
Here we notice that the amplitude will vary significantly and come close to zero at certain times.
When the amplitude is close to zero, this corresponds to phase trajectories very close to the origin.
Pages:
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132