Capacity and mutual information of broadband multipath fading channels

We investigate the capacity and mutual information of a broadband fading channel consisting of a finite number of time-varying paths. We show that the capacity of the channel in the wide band limit is the same as that of a wide band Gaussian channel with the same average received power. However, the input signals needed to achieve the capacity must be "peaky" in time or frequency. In particular, we show that if broadband white-like signals are used (as is common in spread spectrum systems), the mutual information is inversely proportional to the number of distinguishable paths.