Method of quadrature-phase amplitude modulation

Abstract

A rectangular array of signal points forming a QAM signal point constellation is modified by relocating signal points from adjacent corners of the rectangular array to positions extrapolated from the rectangular array and at reduced distances from the origin, corresponding to reduced peak amplitude levels. The location 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+1 points, with n=4 or more and points with I and Q amplitudes 1, 3, 5 . . . units in the rectangular array, a plurality of points in each quandrant are relocated to positions, relative to positions which they would have in the rectangular array, rotated through an angle of 180.degree. about a point having I and Q amplitude co-ordinates of (2.sup.n, 2.sup.n). In addition, in each quadrant each of a plurality of the signal points which are relocated is located in a position, relative to the position which it would have in the rectangular array, in which one of its I and Q amplitude co-ordinates is increased by 2.sup.n and the other is reduced by 2.sup.n. Specific examples are described for 512-QAM (n=4).