Department of Electrical and Computer Engineering
Ph.D. Thesis Abstract
NONCOHERENT MULTICARRIER COMMUNICATIONS AND MULTIUSER DETECTION
The Internet revolution has created the need for wireless technologies that can deliver data at high speeds in a spectrally efficient manner. However, supporting such high data rates with sufficient robustness to radio channel impairments requires careful selection of modulation techniques. Currently, the most suitable choice appears to be OFDM (Orthogonal Frequency Division Multiplexing). OFDM is currently being used in Europe for digital audio and video broadcasting. In the U.S., OFDM is being used by wireless LANs operating in the unlicensed bands and is also being considered as a serious candidate for fourth generation cellular systems.
In order to serve multiple users simultaneously, an OFDM based system would require a multiple access technology. One such access scheme called MC-MFSK (Multicarrier Multilevel Frequency Shift Keying) is proposed in this thesis. The MC-MFSK scheme is a new spread-spectrum, multiple-access scheme and it shares some of the advantages of both FH (Frequency-Hopping) and DS (Direct-Sequence) spread-spectrum systems, while overcoming some of their disadvantages. The MC-MFSK scheme transmits a symbol in parallel over multiple sub-channels using OFDM and has the desirable properties of frequency diversity, near-far resistance, and multipath immunity. This thesis contends that the above benefits not only make MC-MFSK a simple yet competitive spread-spectrum technology but also make it a strong candidate for future high-speed wireless systems.
Systems operating in the unlicensed bands can experience rapidly changing, uncontrolled interference from other co-located systems. This rapid variation of the channel can make phase estimation, and hence coherent detection, extremely difficult if not impossible. Instead, noncoherent detection schemes may be more feasible in such scenarios. Since the proposed MC-MFSK scheme uses noncoherent detection, it can be an attractive alternative for the unlicensed bands as well.
This thesis also investigates the general area of noncoherent multiuser detection. Multiuser detectors are useful in jointly demodulating a superposition of signals received from different sources to yield improved performance. This thesis investigates noncoherent multiuser detection techniques that achieve near optimum performance while retaining reasonable complexity. A technique called selective filtering is proposed which exploits certain a priori information regarding a user’s transmitted signal. It is shown that selective filtering offers improved performance over the noncoherent counterparts of the existing near-optimum multiuser detectors. Both deterministic as well as blind adaptive implementations of selective filtering are considered. Numerical comparisons are provided to demonstrate the near-optimum performance of the proposed detectors.
Ph.D. Dissertation Director: Professor Roy D. Yates
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