Newnes Radio and RF Engineering Pocket Book, 3rd Edition

129

change of logic state saves bandwidth which, for DPSK, is equal to

the bit rate.

An important advantage of both FFSK and PSK over FSK is that

because the moment of change is predefined it is possible to recover

data more accurately. However, the transition between signalling states

is not smooth requiring large and rapid phase shifts. Multilevel systems

with less phase shift between elements are preferable.

Quaternary, or quadrature, phase shift keying (QPSK) is a four-

level Gray-coded signalling method with 90

◦

phase shift between

adjacent signalling elements (Figure 8.11(b)). If the signal is con-

sidered as a vector the points at π/4 (45

◦

), 3π/4 (135

◦

), 5π/4 (225

◦

)

and 7π /4 (315

◦

) represent the transition points between states and the

binary data. ...