| Recent experimental and physical modeling studies demonstrate that, as opposed to systems with smaller bandwidth, the Ultra-Wideband (UWB) channel exhibits frequency-dependent distortion of individual multipath components. This per-path distortion is particularly significant in outdoor UWB applications, where line-of-sight (LOS) or non-distorted reflected signals might not be available at the receiver (for example, in a canyon-like street). In these cases, the dominant propagation mechanisms involve shadowing (diffraction) and reflection by small objects (e.g. signs or a lamp-posts). In this paper, a physical model is developed to investigate the position-dependent distortion of the UWB pulse. The results indicate that both the shadowed pulse and the reflected pulse (by small objects with dimensions bounded by the wavelengths present in the signal) are distorted. Design of optimal and suboptimal templates for the correlation receiver are investigated. The UWB pulses that accommodate robust template choice given by the transmit pulse shape for all propagation conditions and satisfy the FCC spectral mask for outdoor channels are identified. Finally, we analyze the frequency-dependent propagation gain of the UWB channels in various outdoor conditions. This knowledge quantifies the potential benefits of adapting the transmitted signal to the dominant propagation mechanism. |