Wireless location finding has emerged as an essential public safety feature of future cellular systems. This has been in response to a recent federal order issued by the federal communications commission (FCC), which mandates all wireless service providers to provide public safety answering points with information to locate an emergency 911 caller with certain accuracy. Of course, wireless location also has many other potential applications in areas such as location sensitive billing, fraud protection, mobile yellow pages, fleet management, and person/asset tracking.
Wireless location requires accurate estimates of the time and/or amplitude of arrival of the mobile station (MS) signal when received at various base stations (BSs). Obtaining such estimates is usually difficult due to the low signal to noise ratios, fast channel fading, and multipath propagation conditions encountered in wireless propagation environments.
This dissertation develops estimation algorithms for the time and amplitude of arrival of a CDMA signal transmitted over a fading multipath channel, which are robust to high interference levels, fast fading, and overlapping multipath propagation. The dissertation also proposes an advanced location system for public safety applications and discusses efficient hardware architectures for implementing the proposed algorithms.
Among the contributions of the thesis are the following. A derivation of a (sub-optimal) maximum likelihood estimation algorithm for the time and amplitude of arrival of a signal over a single path fading channel is given in Chapter 3. The algorithm is optimized to enhance performance in fast channel fading and low signal-to-noise ratio conditions. The chapter also presents a noise and fading biases equalization technique for amplitude of arrival estimation that improves the estimation accuracy significantly.
The dissertation then moves to study the resolution of fading overlapping multipath rays. At first, a least-squares multipath resolving algorithm, which is robust to fast channel fading, is derived and optimized in Chapter 4. The solution however is sensitive to data ill-conditioning. A technique for detecting overlapping multipath components is then developed and optimized in Chapter 5. This detection technique is then used as part of a successive projections adaptive algorithm in Chapter 6 to develop more accurate alternative to the least-squares method.
The dissertation also presents a wireless location system that aims to enhance the location accuracy of mobile cellular devices in Chapter 7. The system relies on using base-station-like mobile location tracking units to pinpoint wireless devices. Finally, Chapter~8 presents a combined architecture for location searchers and conventional RAKE receivers.
Acknowledgment This work was supported in part by the National Science Foundation under grants CCR-9732376, CCR-0208573, and ECS-0401188. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.