Theory is the first term in the Taylor series expansion of practice. Thomas Cover 5 1.2 Signal distributions Table 1.1 Constellation minimum distancesConstellation d min M -PSK 2 sin π M Square M -QAM 6 M −1 M -ary phase shift keying (M -PSK), where s m = e j2π m M +φ0 m = 0, . . . , M − 1 (1.1) with φ 0 an arbitrary phase. Because of symmetry, the points are always equiprobable, p m = 1/M for m = 0, . . . , M − 1. Special mention must be made of binary phase-shift keying (BPSK), corresponding to M = 2, and quadrature phase-shift keying (QPSK), which corresponds to M = 4. Square M -ary quadrature amplitude modulation (M -QAM), where the in-phase and quadrature components of s independently take values in the set 3 2(Nonsquare M -QAM constellations also exist, and they are employed regularly in wireline systems, but seldom in wireless.) Although making the points in a M -QAM constellation equiprobable is not in general optimum, it is commonplace. Note that, except for perhaps an innocuous rotation, 4-QAM coincides with QPSK.