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Ultra-wide band (UWB) is a relatively new term used to describe a technology that has been known since the 1960's as carrier free, baseband or impulse technology. The basic concept is to develop, transmit and receive an extremely short duration burst of radio frequency energy-typically a few tens of pico seconds to a few nanoseconds in duration. The resultant waveforms are extremely broadband, so much so that it is often difficult to determine an actual RF center frequency- thus the term carrier free. Since UWB waveforms are of such short time duration, they have some rather unique properties. In communications, for example, UWB pulses can be used to provide extremely high data rate performance in multi-user network applications [1]. For radar applications, these same pulses can provide very fine range resolution and precision distance and/or positioning measurement capabilities. These short duration waveforms are relatively immune to multi-path cancellation effects as observed in mobile and in-building environments. As a consequence, UWB systems are particularly well suited for high-speed, mobile wireless applications. As bandwidth is inversely related to pulse duration, the spectral extent of these waveforms can be made quite large. With proper engineering design, the resultant energy densities (i.e., transmitted watts of power per unit hertz of bandwidth) can be quite low. This low energy density translates into a low probability of detection (LPD) RF signature. An LPD signature is of particular interest for military applications (e.g., for covert communications and radar); however, an LPD signature also produces minimal interference to proximity systems and minimal RF health hazards, significant for both military and commercial applications. In this paper we consider the development of a simulation model to calculate ultra-wide band signal propagation characteristics in urban indoor and outdoor environments. The simulation is accomplished using a hybrid model that combines ray tracing and FDTD. The model takes into account the material characteristics of the surrounding walls and buildings, and other obstructions, and accounts for effects due to multiple reflections. The application operates on a 3D terrain database representation of an urban area. The ultimate goal of the simulation is to determine is to maximize coverage in and urban environment given a fixed number of base stations, or, conversely, to optimize the number and location of base stations given a predetermined coverage pattern.
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