0E ??’ 5
1.0E ??’ 4
1.0E ??’ 3
1.0E ??’ 2
1.0E ??’ 1
1.0E + 0
0 2 4 6 8 10 12 14 16 18 20
Eb/No (dB)
Probability of Bit Error (Pb)
DPSK
DQPSK
8-DPSK
FIGURE 2.96 Differential detection of PSK modulation schemes.
MODULATION THEORY 107
LPF
+
X
X
cos(wct)
??“sin(wct)
I(t)
Q(t)
Quadrature Modulator
LPF
Group
4 Bits
into a
Symbol
Lookup
Table
(LUT)
m(t)
PA
Bits
Symbol
Index
Symbols
FIGURE 2.97 General 16-QAM waveform generator.
2.2.11 M-ary Quadrature Amplitude Modulation (QAM)
Let us consider the example of 16-QAM modulation scheme. The transmit signal is generated where
not only is the phase varied, but also the amplitude. The waveform generator shown in Fig. 2.97 is
similar to what was presented earlier for the M-ary PSK waveforms. The difference is in the contents
of the LUT.
This modulation scheme is spectrally efficient with 4 bps/Hz. The BER performance is better than
the PSK counterpart, because of the larger noise immunity vector with 16-QAM. In an AWGN channel,
16-QAM is approximately 4 dB better than 16-PSK. Since there are more modulation levels, when
compared to QPSK, a more linear and complex receiver is needed to perform demodulation [78, 79].
The signal constellation diagram for 16-QAM is shown in Fig.
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