5
SDJ, a = 0.5, JR = 0.3
SDJ, a = 0.5, JR = 0.1
SDJ, a = 0.5, JR = 0.01
FIGURE 2.82 Square root double-jump frequency response comparisons.
The impulse response of a typical DJ filter is plotted with that of a Nyquist pulse-shaping filter,
and both show satisfying the zero-ISI criteria (see Fig 2.83). Notice that the amplitudes of the tails in
the DJ filter impulse response are smaller than those of the RC filter [59??“61].
MODULATION THEORY 99
Double-Jump Filter Impulse Response
??“0.4
??“0.2
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4
Normalized Time
h(t)
RC
DJ
FIGURE 2.83 Double-jump impulse response.
I(t)
Q(t)
a(t)
b(t)
S/P ??/4-QPSK
Modulator
m(t)
Quadrature
Modulator
LPF
LPF
Cross-
Correlator
V(t)
U(t)
Feher /4-QPSK. In the previous sections, we have presented the successful technique of crosscorrelating
the complex envelope in order to achieve high spectral efficiency and high PA efficiency.
Next we turn our attention to applying this technique to /4-QPSK [62]. As previously shown,
/4-QPSK technique has phase trajectories that avoid the origin; however, they come dangerously
close, using practical values of roll-off factors in defining the SRC filter. The cross-correlator is
applied to /4-QPSK as shown in Fig.
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