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AM, FM, and Digital Modulated Systems Chap. 5
solution to the dilemma is to use offset QPSK (OQPSK) or p4 QPSK, each of which has a
lower amount of AM. (OQPSK and p4 QPSK are described after the next section.)
Quadrature Amplitude Modulation
Quadrature carrier signaling, as shown in Fig. 5–31, is called quadrature amplitude modulation
(QAM). In general, QAM signal constellations are not restricted to having permitted
signaling points only on a circle (of radius A c , as was the case for MPSK). The general
QAM signal is
s(t) = x(t) cos v c t - y(t) sin v c t
(5–94)
where
g(t) = x(t) + jy(t) = R(t)e ju(t)
(5–95)
For example, a popular 16-symbol (M = 16 levels) QAM constellation is shown in Fig. 5–32,
where the relationship between (R i , u i ) and (x i , y i ) can readily be evaluated for each of the 16
signal values permitted. This type of signaling is used by 2,400-bit/s V.22 bis computer
modems. Here, x i and y i are each permitted to have four levels per dimension. This 16-symbol
QAM signal may be generated by using two (2 = 2)-bit digital-to-analog converters and
quadrature-balanced modulators as shown in Fig. 5–31b.
Imaginary
axis
(quadrature)
y
s 1
x
Real axis
(in phase)
s 2
Figure 5–32
16-symbol QAM constellation (four levels per dimension).