Method of quadrature-phase amplitude modulation

Abstract

A square array of signal points forming a QAM signal point constellation is modified by relocating signal points from corners of the square to positions extrapolated from the square array and at reduced distances from the origin, corresponding to reduced peak amplitude levels. The relocation is effected in a manner to maintain, at least for a majority of the relocated points, a Hamming distance of one. To this end for a constellation of 2.sup.2n points, with n=4 or more and points with I and Q amplitudes 1, 3, 5, . . . 2.sup.n -1 units in the square array, a plurality of points in each quadrant are relocated to positions, relative to positions which they would have in the square array, rotated through an angle of 180.degree. about predetermined reference points having I and Q amplitude co-ordinates of (2.sup.n,2.sup.n-1) and (2.sup.n-1,2.sup.n). Specific examples are described for 256 QAM (n=4) and 1024 QAM (n=5).