A brief inspection of the above equations reveals that the real part of the conventional demodulator
output consists of the sum of the autocorrelations of the input signal. Also, the imaginary part of
the conventional demodulator output consists of the difference of the cross-correlations of the input
signal. We will make use of this observation in the later chapters.
cak
bk d c Ik1 Qk1
Qk1 Ik1 d # c Ik
Qk d
m I1 # i
cak
bk d c Ik Qk
Qk Ik d # c Ik1
Qk1 d
bk Qk # Ik1 Ik # Qk1
ak Ik # Ik1 Qk # Qk1
MODULATION THEORY 77
S(t)
LPF
X
I(t)
Q(t)
ak
LPF bk
S/P
Phase
State
Coder
mk
Quadrature
Modulator
Ts
Xk
Yk
Ik
Qk
Conventional DQPSK Modulator
FIGURE 2.43 Block diagram showing use of a phase state coder, as the mechanism to switch between different phase state tables.
Phase State Coder. We can use the easily implementable, conventional modulator for any system that
has exactly the same phase state table in Table 2.4. Now if this is not the case, instead of redesigning the
modulator, we would like to reuse it and basically change the input signals. This is accomplished through
what we will call the phase state coder. A block diagram supporting this idea is shown in Fig.
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