**5.3 Phase shift keying**

The principle of phase shift keying is that the phase of the carrier wave is modulated in accordance with the digital message signal, i.e., PSK signal represents the binary data in the form of variations in the phase of the carrier signal. Here the carrier signal changes its phase depending on the nature of the transmitted bit, i.e., 0 or 1. In two levels of PSK, the difference of 180-phase shift is used between binary 1 and binary 0 (**Figure 22**).
