On Coding and Bandwidth Scaling in Ultra-Wideband Communications

The information theoretic capacity of a single-user ultra-wideband (UWB) communications is investigated for various modulations under specific UWB regimes: low power spectral density, large spreading ratio, and a highly dispersive channel. We demonstrate that biphase modulation provides the best performance among several studied formats. The performance loss due to a fixed low coding rate is negligible in the low signal-to-noise ratio (SNR) regime. It is shown that capacity can be obtained by fixing the spreading ratio and allowing the coding rate to vary. However, little is lost by employing a distancedependent spreading ratio in the presence of a fixed coding rate.