A Comprehensive Evaluation of Joint Range and Angle Estimation in Ultra-Wideband Location Systems for Indoors Camillo Gentile, A. Judson Braga, and Alfred Kik National Institute of Standards and Technology Wireless Communication Technologies Group Gaithersburg, Maryland, USA Abstract— Fine time resolution enables Ultra-Wideband (UWB) ranging systems to reliably extract the first multipath arrival corresponding to the range between a transmitter and receiver, even when attenuated in strength compared to later arrivals. Bearing systems alone lack any notion of time and in general select the arrival coinciding with the strongest path, which is rarely the first one in non line-of-sight conditions. Complementing UWB ranging systems with bearing capabilities allows indexing the arrivals as a function of both time and angle to isolate the first, providing precision range and angle. In order to gauge the limits of the joint UWB system, we carry out close to 20000 measurements up to 45 m in non line-of-sight conditions in four separate buildings with dominant wall material varying from sheet rock to steel. In addition, we report performance for varying bandwidth and center frequency of the system. Index Terms— Uniform circular array, frequency-invariant beamforming, spatial-temporal channel modeling I. I NTRODUCTION Location systems with ranging capabilities alone necessitate at least three base stations with knows locations to extract the two-dimensional position of an unknown device through triangulation [1]. In emergency operations such as fire rescue, no such infrastructure exists to date as part of the building code, nor does time permit installation as a crisis unravels. However if both the range and angle of the device were known, then a single base station alone could extract its location. Moreover if the base station itself were a mobile device attached to a fireman, then the system could be used to find a trapped victim equipped with a beacon tag, yielding his or her location with respect to the fireman as he moves about. Fine time resolution and the presence of lower frequencies in the baseband to penetrate walls enable Ultra-Wideband (UWB) ranging systems to reliably extract the first multipath arrival corresponding to the range between a transmitter and receiver, even when attenuated in strength compared to later arrivals. Bearing systems alone lack any notion of time and in general select the arrival coinciding with the strongest path, which is rarely the first one in non line-of-sight conditions. Complementing UWB ranging systems with bearing capabili- ties allows indexing the arrivals as a function of both time and angle to isolate the first, providing precision range and angle. Irahhauten provides a comprehensive overview of the Ultra- Wideband channel propagation measurements taken in recent years to model the temporal properties of the indoor channel [2], however to our knowledge only Scholtz [3] and Keignart [4] report the statistical properties of the time-of-flight besides us: a comprehensive measurement campaign in our previous work [5] shows that UWB technology can deliver precision from a few centimeters to a tens of centimeters based on the operating conditions. Suprisingly there has been very little effort to model the spatial properties of the UWB channel [6], [7], [8], [9], [10], but even these papers lack statistics on the angle of the first arrival, of particular interest in location sys- tems. Analogous to our comprehensive evaluation of the time- of-flight for UWB ranging, we extend the measurement suite to include angle-of-flight as well, and show its performance according to variation in system parameters. Specifically, the main contribution of this paper is a study of the relationship between angle and range errors and their joint location error and: • bandwidth: precision increases with bandwidth, but car- ries diminishing returns with the additional expense; • center frequency: lower frequencies penetrate materials better, but higher frequencies offer better angular resolu- tion; • construction material: compare performance with typical building construction materials varying as sheet rock (easy), plaster, cinder block, to steel (most difficult) to gauge lower and upper bounds on the technology, rather than with building layout (i.e. office, residential typically have the same wall materials); • long range: the high dynamic range of our system allows us to span 45 m and examine the limits in the technology inherent to the interaction with up to 10 walls. The paper reads as follows: Section II introduces the tempo- ral indoor channel propagation model and describes our Ultra- Wideband system to measure its properties. Incorporating a uniform circular array into the system in Section III enables characterizing the joint spatial-temporal properties of the chan- nel from which the time and angle-of-flight can be extracted, as explained in Section IV. Section V provides the details of our equipment setup and Section VI outlines our suite of measurements, presenting results both through statistical metrics and in graphical format, followed by conclusions in the last section. II. THE TEMPORAL INDOOR PROPAGATION CHANNEL The traditional model for the indoor propagation channel is an impulse response composed from K multipath arrivals indexed through k [11] h(t)= K−1  k=0 α k δ(t - τ k ), (1) This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the ICC 2008 proceedings. US Government Work Not Protected by US Copyright 4219 Authorized licensed use limited to: NIST Researchers. Downloaded on November 25, 2008 at 12:46 from IEEE Xplore. Restrictions apply.