Multiple-accessing over frequency-selective fading channels

This article considers the transmission of information from many independent sources to a common receiver over a channel impaired by multipath propagation. In cellular radio communications this is the case of the uplink. We start by examining the achievable rate region of the multiuser frequency-selective fading channel without knowledge of the channel on the transmission end. It has been shown that SSMA (spread spectrum multiple access) is theoretically capable of higher data rates than FDMA (frequency division multiple access) or slow frequency-hopping. When the average received power for all the users is equal, which corresponds to a perfect slow power control, we show that the maximum spectral efficiency of SSMA exceeds that of FDMA or slow frequency-hopping by 0.5772 nats/s/Hz for many users and Rayleigh fading. Also, we formulate the optimal multiple-access scheme when all the channels are known to the transmitters. In turns out that only one user should transmit at any given frequency. Moreover, the input power spectra for the transmitters are water-filling formulae both in frequency and time. It is shown that the spectral efficiency for the optimal scheme is significantly higher than both those of SSMA and FDMA