This is very similar to the DQPSK modulation scheme presented earlier,
except now the four allowable phase changes are multiples of /4.
For example, let us consider the phase state table in Table 2.4.
Using this modulation scheme, we will be able to show the
unfiltered signal constellation diagram in Fig. 2.40. Please note this
constellation was generated using the same DQPSK modulation
block diagram presented in the previous section, with the only difference
being the LUT contents. The constellation has eight phase
states where, at any symbol time instance, only four are allowable.
The phase trajectories alternate between ???circles??? and ???boxes.???
MODULATION THEORY 75
TABLE 2.4 p/4-DQPSK Phase
State Table Definitions
a(t)-b(t)
0??“0
0??“1
1??“0
1??“1 p/4
p/4
3p/4
3p/4
f(k)
I-Channel
Q-Channel
FIGURE 2.40 /4-DQPSK signal constellation diagram.
Earlier we presented the DQPSK modulator block diagram, using the polar coordinate system; we
would now like to this opportunity to present the Cartesian coordinate equivalent approach (see Fig. 2.41).
The above approach, when considering the phase variations, is equivalent to the polar coordinate
approach. This is a very simple, robust, and easily implementable architecture.
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