91 General 16-PSK waveform generator.
This example modulation scheme is spectrally efficient at 4 bps/Hz. It does, however, have poor
BER performance when compared to QPSK, there is approximately 7-dB Eb/No degradation. Since
there are quite a few modulation levels, a more complicated receiver is required. Similarly, since the
modulation scheme is not a member of the constant envelope family, it suffers from spectral regrowth
[75??“77].
Consider the 16-PSK signal constellation diagram in Fig. 2.92.
I-Channel
Q-Channel
n is the noise
immunity
vector.
n
FIGURE 2.92 Unfiltered 16-PSK constellation block diagram.
The more levels (or modulation order) you consider, the smaller the noise vector required to be
used before it can introduce an error. Hence, as more modulation levels are added, the BER performance
degrades. This is the opposite behavior described for the M-ary FSK modulation scheme.
Figure 2.93 provides the BER performance versus the SNR per bit for M-ary PSK, where M 2,
4, 16, 32, and 64.
MODULATION THEORY 105
Coherent Detection of M-ary PSK
1.0E??“5
1.0E??“4
1.0E??“3
1.0E??“2
1.0E??“1
1.0E+0
??“2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
Eb/No (dB)
Probability of Bit Error (Pb)
Pb(BPSK/QPSK)
Pb(8-PSK)
Pb(16-PSK)
Pb(32-PSK)
Pb(64-PSK)
FIGURE 2.
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