Earlier it was mentioned there was a 4-dB advantage of 16-QAM over 16-PSK, and this is
expected when we compare the eye diagram in Fig. 2.101, with 4 levels, to the eye diagram for 16-PSK
with 8 levels in Fig. 2.95.
Let us provide an MPSK and MQAM performance comparison, specifically the peak-to-average
power ratios (PAPRs) of /4-DQPSK, 16-PSK, and 16-QAM modulation schemes for two different
types of transmit pulse-shaping filters (see Table 2.7).
MODULATION THEORY 109
16-QAM Eye Diagram
??“6
??“4
??“2
0
2
4
6
0 4 8 12 16
Normalized Symbol (?—Ts /8)
Eye Value
FIGURE 2.101 16-QAM eye diagram using RC with 0.3.
TABLE 2.7 Modulation Scheme Comparison
Raised cosine Root raised cosine
Modulation scheme Ideal PAPR (with 0.35) (with 0.35)
/4-DQPSK 1 ~1.7 ~1.48
16-PSK 1 ~1.76 ~1.59
16-QAM ~1.4 ~2.29 ~2.11
Note that 16-PSK has less envelope variations when compared to 16-QAM; however, the tradeoff
exists between this and BER performance, which has approximately 4-dB degradation in SNR.
The ideal column corresponds to not using any filter and thus having an infinite BW.
There has been much work to reduce the PAPR of QAM systems in areas such as bit-to-constellation
mapping and NLA linearization techniques.
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