Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Phase-shift keying is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal. Any digital modulation scheme uses a finite number of distinct signals to represent digital data. PSK uses a finite number of phases, each assigned a unique pattern of binary bits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase.