Page 318 - Satellite Communications, Fourth Edition

298 Chapter Ten

limit the sidebands produced, and as part of the filtering needed for
the reduction of ISI, as described in Sec. 10.5. The resulting modu-
lated waveform is sketched in Fig. 10.11.
Differential phase-shift keying (DPSK). This is phase-shift keying in
which the phase of the carrier is changed only if the current bit dif-
fers from the previous one. A reference bit must be sent at the start
of message, but otherwise the method has the advantage of not requir-
ing a reference carrier at the receiver for demodulation.
Quadrature phase-shift keying (QPSK). This is phase-shift keying for
a 4-symbol waveform, adjacent phase shifts being equispaced by 90°.
The concept can be extended to more than four levels, when it is
denoted as MPSK for M-ary phase-shift keying.
Quadrature amplitude modulation (QAM). This is also a multilevel
(meaning higher than binary) modulation method in which the ampli-
tude and the phase of the carrier are modulated.
Although all the methods mentioned find specific applications in prac-
tice, only BPSK and QPSK will be described here, since many of the
general properties can be illustrated through these methods, and they
are widely used.

10.6.1 Binary phase-shift keying
Binary phase-shift keying may be achieved by using the binary polar
NRZ signal to multiply the carrier, as shown in Fig. 10.12a. For a binary
signal p(t), the modulated wave may be written as

e(t) p(t) cos t (10.14)
0

Figure 10.12 (a) BPSK modulator; (b) coherent detection of a BPSK signal.