Adaptive Subchip Multipath Resolving for Wireless Location Systems
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Adaptive Subchip Multipath Resolving for Wireless Location Systems Nabil R. Yousef,1, 2 Ali H. Sayed,1 and Nima Khajehnouri1 1 Electrical 2 Newport
Engineering Department, University of California, Los Angeles, CA 90095-1594, USA Media Inc., Lake Forest, CA 92630, USA
Received 31 May 2005; Revised 3 November 2005; Accepted 8 December 2005 Reliable positioning of cellular users in a mobile environment requires accurate resolving of overlapping multipath components. However, this task is difficult due to fast channel fading conditions and data ill-conditioning, which limit the performance of leastsquares-based techniques. This paper develops two overlapping multipath resolving methods (adaptive and nonadaptive), and shows how the adaptive solution can be made robust to the above limitations by extracting and exploiting a priori information about the fading channel. Also the proposed techniques are extended when there are antenna arrays at the base station. Simulation results illustrate the performance of the proposed techniques. Copyright © 2006 Hindawi Publishing Corporation. All rights reserved.
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INTRODUCTION
Wireless propagation suffers from multipath conditions. Under such conditions, the prompt ray may be succeeded by multipath components that arrive at the receiver within short delays. If this delay is smaller than the duration of the pulse shape used in the wireless system (i.e., the chip duration Tc in CDMA systems), then the rays overlap. When this situation occurs, it results in significant errors in the estimation of the time and amplitude of arrival of the prompt ray. Figure 1 illustrates the combined impulse response of a two-ray channel using a conventional pulse shape in a CDMA IS-95 system in two situations. In the second situation, where the pulses overlap, the location of the peak is obviously delayed relative to the location of the prompt ray. Such errors in the time-of-arrival are particularly damaging in wireless location applications (a topic of significant relevance nowadays—see, e.g., [1–18]). In these applications, small errors in the timeof-arrival can translate into many meters in terms of location inaccuracy. There have been earlier studies in the literature on resolving overlapping multipath components (see, e.g., [19, 20]). However, there are two sources of impairments that introduce significant errors into the resolution accuracy and which need special attention; these sources of error are particularly relevant in the context of mobile-positioning systems. The first impairment is due to the possibility of fast channel fading, which prohibits the use of long averaging intervals. This is because the estimation period in wireless
location applications can reach up to a few seconds, which may cause the channel between the transmitter and the receiver to vary significantly during the estimation period, even for relatively slow channel variations. The second impairment is the possibility of noise enhancement, which occurs as a result of the ill-conditioning of the data matrices involved in
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