Logarithmic PCM Weighted QAM Transmission Over Gaussian and Rayleigh Fading Channels.

Transmission of 8-bit mu-law PCM signals using M-level weighted QAM (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 distortion in the recovered signal is reduced. The PCM bits are mapped to the WQAM points such that the most significant bits have a lower probability of being in error than the least significant bits. Gray coding of the constellation point is also used. The WQAM systems have been optimized both for unchanged average signal energy per symbol and unchanged peak energy per symbol compared to unweighted arrangements. The theoretical and simulation results using speech indicate that 16-level and 256-level WQAM has a gain of 3 and 5 dB over unweighted QAM over a Gaussian channel. The gains due to weighting for 16-level QAM operating over a Rayleigh fading channel are 5 dB.