Phase-shift keying represents digital symbols by changing carrier phase.

In binary phase-shift keying, or BPSK, the two symbols can be represented by opposite constellation points, for example

$$+A\qquad\text{and}\qquad -A.$$

The corresponding carrier waveforms differ by a phase shift of $180^\circ$. For two points constrained to the same magnitude on a one-dimensional signal axis, this places them as far apart as possible.

Quadrature phase-shift keying, or QPSK, uses four phases. Its four I/Q constellation points represent

$$\log_2 4=2$$

bits per symbol.

QPSK can be viewed as two independent binary symbol components carried on orthogonal in-phase and quadrature axes. For the same bit rate it therefore requires half the symbol rate of BPSK.

Both schemes require the receiver to estimate the carrier phase well enough to interpret the constellation orientation correctly.