Minimizing transmit power for fading multiple-access channels

The problem of minimizing the sum transmit power for a fading multiple-access channel is considered. While for non-fading channels with equal user attenuations wideband multi-access with cooperative (joint) decoding and orthogonal access are both optimal, wideband access is superior in the case of unequal attenuations or fading channels. Losses due to fading are negligible for wideband access as a result of low rate coding, and near-AWGN performance is achievable. On the other hand, orthogonal access suffers from losses of high rate codes under fading conditions. Wideband access with non-cooperative decoding is clearly suboptimal in any case and only useful for low transmission rates