Alouini and Goldsmith : Capacity of Rayleigh Fading Channels

We study the Shannon capacity of adaptive transmission techniques in conjunction with diversity-combining. This capacity provides an upper bound on spectral eeciency using these techniques. We obtain closed-form solutions for the Rayleigh fading channel capacity under three adaptive policies: optimal power and rate adaptation, constant power with optimal rate adaptation , and channel inversion with xed rate. Optimal power and rate adaptation yields a small increase in capacity over just rate adaptation, and this increase diminishes as the average received carrier-to-noise ratio or the number of diversity branches increases. Channel inversion suuers the largest capacity penalty relative to the optimal technique, however the penalty diminishes with increased diversity. Although diversity yields large capacity gains for all the techniques, the gain is most pronounced with channel inversion. For example, the capacity using channel inversion with two-branch diversity exceeds that of a single-branch system using optimal rate and power adaptation. Since channel inversion is the least complex scheme to implement, there is a tradeoo between complexity and capacity for the various adaptation methods and diversity-combining techniques. Permission to publish this abstract separately is granted.