Contents I. Introduction
In RECENT years, ultrawideband (UWB) systems have gained prominence due to many promising applications such as indoor multimedia communications and sensor networks [1]. The Federal Communications Commission (FCC) in the US has allocated the 3.1–10.6 GHz spectrum for UWB emissions [2]. According to the current FCC specification, a UWB signal has a bandwidth of between 0.5–7.5 GHz [2], as opposed to only a few KHz for conventional narrowband systems. The antennas and propagation aspects of UWB systems therefore differ significantly from those of narrowband systems [1], [3]. The design of practical antennas that radiate efficiently over an ultrawide bandwidth continues to be a challenging problem [4]. Apart from wideband matching and radiation efficiency, another problem arising from antenna distortion is signal dispersion. UWB antenna radiation patterns vary significantly with frequency, leading to a direction-specific distortion of UWB waveforms [5]–[7]. An indoor UWB channel is characterized by a large number of incident multipath components (MPCs), with three-dimensional scattering and large angular spreads [1]. As a result, individual MPCs experience nonuniform distortion due to the antenna, determined by their directions-of-departure (DODs) and directions-of-arrival (DOAs). In an impulse radio UWB system with a correlation-based receiver [8], this nonuniform multipath distortion can result in large and unpredictable correlation mismatches at the receiver, consequently degrading the system performance. Similarly, signal distortion is experienced due to multiple-antenna arrays that often suffer from beam pattern variations with frequency. While our discussion in this paper will be limited to a single antenna element, the proposed technique can be easily extended to array distortion and its compensation.
