S. Gezici, Impulse Radio Ultra Wideband Receivers for ...

Faculty of Engineering and Natural Sciences
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Impulse Radio Ultra Wideband Receivers for Communications and Geolocation

Sinan Gezici

After the U.S. Federal Communications Commission approved limited use of ultra wideband (UWB) technology in February 2002, communication systems that employ UWB signals have drawn considerable attention. Commonly, impulse radio (IR) systems, which transmit very short duration pulses with low duty cycle, are employed to implement UWB systems. IR UWB technology holds great promise for a variety of applications such as short-range high-speed data transmission and precise location estimation.

In an IR-UWB system, there are two types of processing gain. The first one is the pulse combining gain, which is the number of pulses transmitted per information symbol, and the other is the pulse spreading gain, which is the ratio between the average pulse transmission period and the pulse width. Trade-off between these two types of processing gain is studied in the presence of timing jitter in frequency-selective multiuser environments and the opposing effects of timing jitter and multiple access interference (MAI) are investigated for different values of processing gain. Also, polarity codes to randomize polarities of pulses for a given information symbol are considered, and their impact on reducing the effects of MAI is quantified. After the effects of polarity codes in MAI suppression are observed, a more detailed analysis of IR-UWB systems with polarity randomization codes is performed for Rake receivers with various combining schemes. Closed-form BEP expressions are derived for large number of pulses per information symbol.

After the performance analysis study, design of optimal and suboptimal IR-UWB receivers is investigated. In order to optimize the system performance, both the selection of multipath components to be employed at the receiver and the combination of those selected components for symbol detection should be performed in an optimal fashion. Therefore, the optimal finger selection problem is considered first to determine the set of multipath components to be used at the receiver. It is formulated as an integer programming problem with a non-convex objective function, which also proves the suboptimality of the conventional techniques. Due to the complexity of the optimal solution, three suboptimal solutions with much lower complexity, two based on Taylor approximation and constraint relaxation, and one based on genetic algorithms (GAs), are proposed, which perform considerably better than the conventional techniques. After the finger selection problem, optimal and suboptimal combining algorithms are considered for IR-UWB receivers. First, the optimal linear combining based on a minimum mean square error (MMSE) criterion is investigated assuming frame-rate sampling at the receiver. Due to the computational complexity of the MMSE-based scheme, two-step combining algorithms are considered and optimal frame combining (OFC), optimal multipath combining (OMC) and two-step MMSE receivers are proposed. Depending on performance and complexity requirements, a suitable combining scheme can be chosen for a given IR-UWB receiver.

In addition to receiver design considerations for UWB communications purposes, time of arrival (ToA) estimation for positioning applications is studied. Due to its high resolution, a UWB signal can provide very accurate ranging information, which can be used in the location estimation of a mobile user. However, this high resolution also makes design of practical TOA estimation algorithms very challenging. Therefore, a two-step ToA estimation algorithm is proposed, which can provide an accurate estimate in a reasonable time interval, using low-rate signal samples.

Sinan Gezici received the BS degree from Bilkent University, Turkey, in 2001 and MA degree from Princeton University in 2003. He is currently working toward the Ph.D. degree at the Department of Electrical Engineering at Princeton University.

His research interests are in the statistical signal processing and wireless communications fields. Currently, he has a particular interest in synchronization, positioning, performance analysis and multi-user aspects of ultrawideband (UWB) communications. He has contributed UWB ranging algorithms to the emerging IEEE 802.15.4a PHY standard. Among his publications are two book chapters on UWB geolocation and multiple-access interference mitigation in UWB systems. He has recently started co-authoring on a book titled Range Estimation in Ultrawideband Wireless Systems (Cambridge University Press, 2007).

April  7, 2006, 13:40, FENS G035