PhaseShiftKeying

Phase shift keying (PSK) is a digital modulation technique in which the phase of a carrier signal is varied in discrete steps to represent digital data. Each symbol corresponds to one of several phase states, while the amplitude of the carrier remains constant. The constellation is a set of equally spaced points on the unit circle in the complex plane.

Common schemes include binary phase shift keying (BPSK), which uses two phases separated by 180 degrees; quadrature phase shift keying (QPSK), which uses four phases (often 0, π/2, π, and 3π/2). More generally, M-PSK uses M equally spaced phases to carry log2(M) bits per symbol. Higher-order PSK increases data rate but reduces the minimum distance between symbols, making it more sensitive to noise and phase errors.

Demodulation can be coherent, requiring a stable carrier reference, or differential, which encodes data in the phase difference between successive symbols (DPSK) and can be demodulated without explicit carrier phase recovery. Gray coding is often used to map adjacent constellation points to adjacent bit patterns, reducing the bit error rate for small phase disturbances.

PSK has the advantage of a constant envelope, which makes it robust to amplifier nonlinearities and suitable for power-efficient transmitters. Its performance in additive white Gaussian noise degrades as the order M increases for a fixed energy per bit, trading energy efficiency for higher throughput.

Applications include digital wireless links, satellite communications, and many modem and broadband systems; PSK variants are also used in coherent optical communications and in combination with other modulation formats to suit various channel conditions.