PERFORMANCE OF NONUNIFORM CONSTELLATIONS ON THE GAUSSIAN-CHANNEL

Testing of high-speed voiceband modems has revealed a significant incr ease in distortion for points near the perimeter of a QAM signal const ellation. This distortion increases with distance from the center of t he constellation and limits performance at data rates above 19.2 kb / s. The perimeter distortion can be reduced by transforming the signal constellation so that points near the center are closer together, and points near the perimeter are further apart. When the channel SNR is h igh,;such a transformation reduces immunity to Gaussian noise because points near the center of the transformed constellation are closer tog ether than in a uniformly spaced constellation with the same average p ower. This paper demonstrates theoretically that for channel SNR's of practical interest, there is actually a small gain in immunity to Gaus sian noise. In fact, an appropriate coded modulation scheme can produc e gains of about 0.25 dB.