Ultrawideband vector antennas for sensing and positioning

Over the last decade, there has been a growing recognition that ultra-wideband (UWB) radios offer several unique capabilities that can enable new sensing, positioning and communication applications. Most UWB systems currently use single- or dual-polarized antennas, which can measure at most two components of the received electromagnetic (EM) signal. Since the received signal consists of six field components, however, potentially useful information is neglected. In principle, a "vector antenna" that can independently detect or excite three or more EM field components enables the UWB system to access additional signaling dimensions, which can be used to enhance performance in the same way as antenna arrays. In this paper, we consider the potential advantages of UWB vector antenna systems for range and direction-of-arrival (DOA) estimation. We first introduce a general model for an UWB vector antenna system that incorporates the possibility of arbitrary space- and frequency-selective antenna coupling. We then use this model to derive a frequency-domain Cramer-Rao lower bound on the position error covariance for arbitrary polarized UWB signals. For two particular 3-element UWB vector antennas, we then use the bound as a design criterion to derive optimal UWB polarized signals that minimize a lower bound on mean-square angular estimation error.