30 OQPSK modulation transmission block diagram.
In Fig. 2.31, we show how inserting a bit time delay will force the phase trajectories to avoid the
origin. We will start with the complex symbol at the output of the S/P converter.
As we see from the above example, the QPSK problematic phase trajectory corresponds to a phase
change of or 180 degrees. By inserting a bit time delay, OQPSK was able to split up the 180-degree
phase change into two /2 phase changes and thus avoid the origin.
The OQPSK signal constellation diagram using an SRC with roll-off factor 0.3 is shown in
Fig. 2.32.
The above signal constellation diagram clearly shows the phase trajectories close to the origin
have been avoided. This reduces the PAR of the complex envelope signal.
Ts ??“1
+1
a(t)
Ts ??“1
+1
b(t)
Ts ??“1
+1
a(t)
Ts ??“1
+1
b(t)
Tb
Q
I
Q
I
= a
= b
= c
= d
QPSK OQPSK
Symbols
FIGURE 2.31 Comparison of QPSK and OQPSK Phase trajectories.
MODULATION THEORY 71
OQPSK Constellation Digram
??“2
??“1.5
??“1
??“0.5
0
0.5
1
1.5
2
??“2 ??“1.5 ??“1 ??“0.5 0 0.5 1 1.5 2
I-Channel
Q-Channel
FIGURE 2.32 OQPSK signal constellation diagram.
The signal constellation diagram for the OQPSK modulation signal at the output of a nonlinear
PA is given in Fig.
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