Now let us show the filtered version of the signal constellation diagram using an SRC filter (see
Fig. 2.45). The constellation diagram clearly shows the presence of ISI on all 8 phase states. It also
shows the origin is avoided by the phase trajectories; however, they do come very close.
Also the one-dimensional eye diagram is shown in Fig. 2.46.
Note that the 5-level eye diagram comes from the fact that as we project the two-dimensional
constellation onto one dimension (either the I- or Q-axis), there are five states that become
pronounced.
In fact, if we were to rotate the constellation by /8 in either direction, then the resulting eye
diagrams of the I- and Q-channels will consist of 4 levels instead of the previous 5 levels.
78 CHAPTER TWO
??/4-QPSK Signal Constellation Diagram
??“2
??“1.5
??“1
??“0.5
0
0.5
1
1.5
2
??“2 ??“1 0 1 2
I-Channel
Q-Channel
FIGURE 2.45 /4-DQPSK signal constellation using SRC with roll-off factor 0.3.
??/4-QPSK Eye Diagram
??“0.2
??“0.15
??“0.1
??“0.05
0
0.05
0.1
0.15
0.2
0 4 8 12 16
Normalized Symbol Time (Ts /8)
Eye Value
FIGURE 2.46 /4-DQPSK eye diagram using SRC with roll-off factor 0.3.
We would like to take this moment to briefly pause and review what has been discussed so far.
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