Nonlinear encoding by surface projection

High speed quadrature amplitude modulated modems use dense signal constellations to transmit multiple bits of information in each symbol interval. Modem tests reveal a significant increase in distortion near the perimeter of the signal constellation caused by signal-dependent impairments. A nonlinear encoding technique reduces perimeter distortion. The encoder projects the signal constellation points on a nonlinear surface prior to modulation. This surface may, for example, increase the margin or distance between perimeter signal points. The receiver projects the demodulated signal points on a reciprocal surface prior to decoding, restoring the signal points to their original uniform spacing, and compressing distortion on the perimeter points. The author, along with illustrations of the surface projected signal constellations, presents the nonlinear encoding technique. A data rate of 28800 bit/s is achieved using this technique