2.43, using
a phase state coder, as the mechanism to switch between different
phase state tables as the need arises.
For example, the North American Digital Cellular (NADC)
system uses the phase state table in Table 2.5.
For this example, we notice that quadrants I and III have
swapped their phase changes when comparing Table 2.5 to Table
2.4. Hence with the following implementation of the phase state
coder, we can extend the use of the conventional modulator
through the block diagram in Fig. 2.44 [25]. In fact, the phase state
coder for this example consists of invertors. One can easily calculate
the phase state coder needed for the Japan Digital Cellular
(JDC) system, given its phase state table.
TABLE 2.5 p/4-DQPSK Phase
State Table for NADC
a(t)-b(t)
0??“0
0??“1
1??“0
1??“1 3p/4
p/4
3p/4
p/4
f(k)
LPF
X
I(t)
Q(t)
ak
LPF bk
S/P
mk
Quadrature
Modulator
Ts
Xk
Yk
Ik
Qk
Phase State Coder
FIGURE 2.44 /4-DQPSK modulator using the NADC phase state coder design.
Note that as far as the receiver is concerned, the same demodulator can be used with the change in
the symbol quantizer that performs a different symbol demapping operation. Since the phase state table
is time invariant and known a priori, this should pose no problem.
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