Dissertation/Thesis Abstract

Coding and optimization for ultra-wide-band wireless communications
by Riaz, Usman, Ph.D., University of Southern California, 2008, 119; 3341826
Abstract (Summary)

Ultra-Wide Band Impulse Radio (UWB-IR) is a mechanism of transmission to exploit a large amount of unlicensed bandwidth allocated by the Federal Communications Commission (FCC) for high data rate communications [Fed]. To satisfy FCC's power spectral density constraint, two popular modes of transmission are employed in UWB; namely, impulse radio (IR) [WS98b] and multi-band othorgonal frequency division multiplexing (OFDM) [AE02]. The transmission of very narrow low duty cycle pulses in UWB-IR [WS98a] improves the resolution of multiple paths and, hence, the channel energy captured at the receiver. In contrast with the IR technology, the available spectrum is divided into small chunks of bandwidth in multiband OFDM. This approach is flexible in terms of reducing the interference from UWB systems to other systems operating in a particular subband and reducing the interference from other systems to UWB systems.

Low-complexity low-rate super-orthogonal turbo codes (SOTC) are proposed in this work to replace the implicit repetition code (RC) in ultra-wide band impulse radio (UWB-IR) systems to improve the transmission range and system throughput. Various receivers, including the matched-filter and the RAKE receivers, are examined for data detection in the additive white Gaussian noise (AWGN) channel and the indoor IEEE 802.15.3a channels. The performance of SOTC-coded UWB with perfect and imperfect timing and channel information is analyzed and corroborated by computer simulation. It is demonstrated that the SOTC-based UWB-IR system can achieve significant performance improvement over the conventional direct-sequence UWB (DS-UWB) system encoded by RC over both ISI and ISI-free channels.

A coded ultra-wide band impulse radio (UWB-IR) system that employs the pre-rake as the inner code and the super-othorgonal turbo codes (SOTC) as the outer code is investigated in this work. The pre-rake inner code enhances the signal power at the receiver while the SOTC outer code reduces the bit-error-rate (BER) with a low decoding complexity. The pre-rake coding requires channel information available to the transmitter. The performance of the coded UWB-IR system depends on the number of fed back channel taps. The trade-off between the fed back channel information and several system performance metrics such as the frame number and BER is studied. It is shown that there exists a minimum feedback quantity needed to achieve a target system performance in different communication scenarios. The optimal quantity of fed back channel knowledge, the effect of channel estimation error and inter-symbol interference (ISI) are investigated and verified by computer simulation.

In addition, we examine the tradeoff between diversity and throughput for unitary precoders, including the rotated Walsh-Hadamard (RWH) and the carrier interferometric (CI) precoding schemes, in a multi-band orthogonal frequency division multiplexing ultra-wide band (MBOFDM-UWB) communication system in this work. The MBOFDM-UWB system offers rich frequency diversity due to the highly frequency selective nature of the channel. Although the repetition code (RC) is proposed as a method to achieve diversity in the IEEE 802.15.3a standard, unitary precoders of rate 1 with linear receivers can be used to leverage the diversity offered by the channel [PN07]. In practice, orthogonality of unitary precoders can be lost due to fading, which results in diversity loss. Diversity can be restored using unitary precoders of rate less than 1 by compromising throughput. The diversity characterization of various receivers such as the maximal ratio combining (MRC), the minimum mean squared errors (MMSE), the least squares (LS), the zero forcing (ZF) and the maximum likelihood (ML) receivers with various UWB channels (e.g., CM1 and CM4) is conducted. We derive analytical bounds and perform computer simulation to understand the tradeoff between diversity, orthogonality and rate for unitary precoders of rate less than 1 with linear and ML receivers using perfect and imperfect channel information at the receiver.

Indexing (document details)
Advisor: Kuo, C.-C. (Jay)
Commitee: Golomb, Solomon W., Mikulevicius, Remigijus
School: University of Southern California
Department: Electrical Engineering
School Location: United States -- California
Source: DAI-B 69/12, Dissertation Abstracts International
Subjects: Electrical engineering
Keywords: Coding, Ultrawideband, Wireless communications
Publication Number: 3341826
ISBN: 9780549976639
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