Next we show the PA input and output signal
constellation diagrams for a particular nonlinearity to emphasize the distortion (see Figs. 2.27 and 2.28).
Based on the constellation plots shown in Fig. 2.28, the distortion is clearly visible in two areas.
The first area is for the larger-amplitude phase trajectories; these are mapped to a circle of constant
radius. The second area is for the smaller-amplitude phase trajectories (i.e., those traversing close to
the origin). Although they have a linear amplitude response, they exhibit nonlinear phase distortion,
which also has a contribution to distortion.
MODULATION THEORY 67
QPSK RC Filter with ?± = 0.2 and 0.9
0
0.5
1
1.5
2
2.5
1 81 161
Oversampled Time Instance (?—8)
Constellation Magnitude
RC ?± = 0.9
RC ?± = 0.2
Input
Amplitude, AIn
Output
Amplitude, AOut
AM-AM
Linear
Region
Nonlinear
Region
A
B
FIGURE 2.25 Amplitude variation of RC-filtered QPSK with roll-off factor 0.2 and 0.9.
FIGURE 2.26 Transmitter nonlinearity degradation example.
68 CHAPTER TWO
QPSK Eye Diagram
??“0.2
??“0.15
??“0.1
??“0.05
0
0.05
0.1
0.15
0.2
0 8 12 16
Normalized Symbol Time (Ts/8)
Eye Value
4
FIGURE 2.27 PA input signal constellation diagram for SRC-filtered QPSK with roll-off factor 0.
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