The effect of asynchronism on the total capacity of Gaussian multiple-access channels

The degradation due to complete asynchronism (at the codeword and symbol levels) in the total capacity, maximum rate-sum, of white Gaussian multiple-access channels is investigated. It is shown that asynchronism reduces the total capacity of a K-user channel by at most a factor of K. Moreover, this bound is achieved, in asymptotically high signal-to-noise ratios, by the TDMA signaling strategy. When the signaling strategies are optimally designed to maximize the asynchronous total capacity under bandwidth constraints, the authors find that in a two-user channel: (1) for a certain set of signal-to-noise ratios there is no degradation due to asynchronism, (2) for any bandwidth and signal-to-noise ratios the asynchronous total capacity is at least 88% of the synchronous total capacity, and (3) asynchronism has a vanishing small effect on total capacity for both low and high signal-to-noise ratios