Now in the receiver, the I-channel must be delayed (or appropriately sampled) in
order for the symbol to be time aligned prior to entering the P/S converter. The timing recovery block
inserted into Fig. 2.35 will manage the sampling time instances.
Using coherent detection as a method of detecting OQPSK signals in the receiver produces a very
interesting result; specifically, the following probability of bit error relationship holds true:
(2.19)
Hence, this modulation technique will produce the same BER performance as that of QPSK
modulation; however, the phase trajectories traveling through or near the origin have been
eliminated [24].
Pb(BPSK) Pb(QPSK) Pb(OQPSK)
FIGURE 2.34 OQPSK eye diagram at the output of the SRC filter.
BPF
R(t)
Quadrature
Demodulator
LPF
LPF
Compensate P/S
Channel
Estimation
~
Decision
Device
cos(wct)
Timing
Recovery
(2k + 1)Tb
(2k)Tb
MODULATION THEORY 73
S(t)
cos( )
+
I(t)
Q(t)
a(t)
sin( )
b(t)
S/P
Look
Up
Table
m(t)
Quadrature
Modulator
Ts
Df(k)
LPF
LPF
q(k -Ts)
q(k)
FIGURE 2.36 DQPSK modulator block diagram.
2.2.4 Differential QPSK (DQPSK)
In referencing the modulation scheme migration shown in Fig. 2.19, we now take a path where the
transmitter PA nonlinearity is not a concern, but the receiver complexity is a more critical point.
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