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UT EE 381K - Ultrawideband Radar Processing using Channel Information from Communication Hardware

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Bryan Westcott Final Report Abstract— Channel information provided by impulse-radio ultrawideband communications hardware is used in radar applications requiring information about the surrounding physical environment. The methods presented are similar to sonar-based echolocation methods, but have the advantage that complex reflections can be analyzed. The localization and classification of multipath allows sonar-based map-building algorithms to be applied. Novel solutions are presented for reflection classification and map building for multi-user, multi-antenna, and single-antenna hardware. Ultrawideband Radar Processing using Channel Information from Communication Hardware2 MERGING ultrawideband (UWB) technology potentially offers not only hundred-megabit-per-second data rates for wireless communications, but also sub-centimeter resolution radar applications [2]. The short pulse duration of impulse-radio implementations allows most of the dominant multipath reflections in the environment to be individually resolved [1] and this multipath energy may be collected with a Rake receiver, which is common with spread-spectrum communications. Channel estimation is crucial to the performance of wideband receivers, but also provides information about the local environment. This paper explores the options for mapping the local physical environment from channel information already obtained by communications hardware. Location-based services, such as emergency-911, are already proving popular in the cellular industry [3], but do not provide high-precision localization. GPS is extremely popular for navigation, but is also limited in accuracy and does not work well indoors [4]. The algorithms presented could complement existing technologies, particularly useful for indoor and urban navigation. The algorithms would be useful for navigating known environments, such as guiding a person through an unfamiliar building. The algorithms could also be useful for the exploration of unknown environments by robots, dangerous environments by emergency response workers, and hostile environments such as military teams in urban conflicts. This would only require communication hardware, which may already be in use to connect to local hotspots or other users in a team. In addition, location-based security is a great potential application to prevent wireless networks from being accessed outside of a building [5]. Also, future “augmented reality” applications would require extremely precise location measurements. The methods presented in this report are similar to sonar-based ‘echolocation’ algorithms. In these algorithms the reflection time, or time-of-flight (TOF) of wideband ultrasound pulses provide information about the local environment. Both ultrasound and ultrawideband have similar specular propagation [1] where large surfaces have mirror-like reflection, right-angle corners reflect back to all directions, and edges diffract to all directions. If these features can be identified and localized in UWB implementations, the sonar-based map-building algorithms may be used. Sonar implementations typically use directional arrays that measure not only distance but direction to all reflections. This may seem to place omnidirectional communication antennas at a disadvantage; however, directional arrays can only measure reflections from normal surfaces (where the normal projection of the antenna location onto a finite-area surface exists). If done correctly and efficiently, electromagnetic echolocation can provide more complex mapping because signals reflecting off of multiple surfaces and arriving from all directions can be analyzed, and ‘hidden’ non-normal surfaces can be located. One relevant sonar-based method is a relocation algorithm by Lim and Leonard [6]. It is one of the few methods based on range information alone, but requires a known map of the local environment, which this paper is trying to produce. The goal of E3 this method is to find the location on the map that best fits the range data obtained, and this method could easily be extended to any of the hardware configurations presented in this paper. A method designed specifically for map-building is presented by Kuc and Siegel [7]. After locating and then classifying the reflections as corners, edges or walls, a map is constructed with known shadowed regions. Finally, a method by Guivant and Nebot [8] uses an extended Kalman filter to estimate and update the current estimate of position, while simultaneously building and maintaining a map of the local environment. This method is particularly useful as some of the methods in this paper may present multiple solutions. Also, while the methods presented may be complex, they would only have to be performed once if a Kalman filter is used. Any of these methods may be used, but it first requires that reflection location and classification be performed with a single antenna, and with communications hardware. The specular propagation of both methods allows a channel to be estimated with simple ray tracing techniques. A three dimensional room with moderate complexity is represented in two dimensions in simulation. The local and distributed users may be randomly placed, and the peaks in the expected channel response will be determined by this ray tracing. Reflections, diffractions and double reflections are combined into one response, as in a real channel. Error can be simulated, but the receiver operation requires that it be less than the pulse width, which is three orders of magnitude less than typical room dimensions, making it insignificant. The general solutions were obtained using the symbolic math toolbox in Matlab. Three novel algorithms using only the TOF of multipath reflections are presented; an extensive literature survey found no similar methods. The first assumes an environment with distributed sources of communication activity, with no cooperation necessary, where techniques such as successive interference cancellation are used to measure and then subtract the interference of other users to increase communication performance. This environment essentially provides distributed radar sources,


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UT EE 381K - Ultrawideband Radar Processing using Channel Information from Communication Hardware

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