Modified 256-QAM for logarithmic PCM

Transmission of 8-bit μ-law PCM (pulse-code-modulated) signals using M-level weighted quadrature amplitude modulation (WQAM), M=16, 64 and 256, over Gaussian and Rayleigh fading channels is examined. The weighting process modifies the positions of the QAM constellation points so that the overall average distortion in the recovered information-bearing source signal is reduced. The PCM bits are mapped onto the WQAM points so that the most significant bits have a lower probability of being in error than the least significant bits. The WQAM systems have been optimized both for the same average and peak signal energy per transmitted symbol as for unweighted QAM. Theoretical and simulation results using speech indicate that 16-level and 256-level WQAM have gains of up to 3 and 5 dB, respectively, over unweighted QAM for the Gaussian channel. The gains due to weighting for 16-level QAM operating over an ideal slow Rayleigh fading channel are up to 5 dB