Capacity and mutual information of wideband multipath fading channels

In this paper we will investigate the capacity and mutual information of a broadband fading channel consisting of a finite number of time-varying paths. We will show that the capacity of the channel in the wideband limit is the same as that of a wideband 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 white-like signals are used instead (as is common in spread-spectrum systems), the mutual information is inversely proportional to the number of resolvable paths (L) over tilde with energy spread out, and in fact approaches 0 as the number of paths gets large. This is true even when the paths are assumed to be tracked perfectly at the receiver. A critical parameter (L) over tilde(crit) it is defined in terms of system parameters to delineate the threshold on (L) over tilde over which such overspreading phenomenon occurs.