18)
Another issue besides increased receiver complexity is that of phase trajectories traveling through
or near the undesirable origin. This property increases the PAR, causes spectral regrowth, and forces
the PA to operate in the less-efficient operating region. Let us address the second issue by forcing the
phase trajectories to avoid the origin altogether. This is accomplished with the OQPSK modulation
scheme.
2.2.3 Offset QPSK (OQPSK)
For this modulation scheme, we assume the receiver implementation complexity is not an issue.
OQPSK provides a solution to reducing the spectral regrowth created when the signal enters a nonlinearity.
This is accomplished by forcing the phase trajectories to avoid the origin. A block diagram
of the OQPSK transmitter is shown in Fig. 2.30.
In comparing this block diagram to that presented earlier for QPSK, we see a bit time (Tb) delay
was inserted into the complex symbol generation.
Pb(BPSK) Pb(QPSK)
MODULATION THEORY 69
BPF
R(t)
Quadrature
Demodulator
LPF
LPF
Compensate P/S
Channel
Estimation
~
Decision Device
cos(wct)
FIGURE 2.29 A simplified block diagram of QPSK demodulation with special emphasis on coherent detection
operation.
70 CHAPTER TWO
LPF
+
X
X
cos(wc t)
??“sin(wct)
I(t)
Q(t) b(t ??’ Tb)
S(t)
a(t)
Quadrature Modulator
LPF
b(t)
S/P
Tb
m(t)
FIGURE 2.
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