Capacity of ultra-wideband OFDM

Several researchers have recently shown that the capacity achievable by spread-spectrum DS-CDMA (wideband) signals on multipath fading channels goes to zero as the bandwidth or the number of equal-energy resolvable multipaths (for the case of a channel with a finite number of time-varying paths) increases. In this paper we use a different kind of wideband signaling, based on a hybrid ultra-wideband OFDM technique, proposed by Tewfik and Saberinia. We show that this scheme can achieve capacity on fading channels, as bandwidth goes to infinity provided the signal is scaled in a manner different from the traditional DS-CDMA scheme. In particular, as bandwidth goes to infinity it is better to add more orthogonal frequencies rather than to reduce the signaling pulse duration.