Thesis (M.S. in Computer Science)--Naval Postgraduate School, March 2003. / Thesis advisor(s): Valdis Berzins, Paul Young. Includes bibliographical references (p. 127-130). Also available online.

Thesis (M.S. in Systems Technology)--Naval Postgraduate School, June 2003. / Thesis advisor(s): William G. Kemple, John S. Osmundson. Includes bibliographical references. Also available online.

In a wireless network, owing to the time-varying nature of wireless channels, different mobile users typically experience peaks and troughs in channel quality at different times. This diversity in channel quality is known as multiuser diversity. With the aid of rate adaptation, multiuser diversity can be exploited by allowing the mobile user with the best channel to use the channel resource. However, in order to achieve this in most practical systems, the mobile users in the network must feed back their channel state information (CSI) to the transmitting user. In large networks, this feedback overhead can outweigh the multiuser diversity gain. In this thesis dissertation, a centralised wireless medium access control (MAC) scheme, namely Multiuser Diversity with Capture (MDC), is discussed as a solution to obviate the overhead problem. MDC explicitly employs the capture effect in radio receivers to reduce network overhead by allowing multiple mobile stations (MSs) with channels better than a nominal response threshold to simultaneously compete for the wireless channel. Owing to the capture effect, the base station (BS) can determine which MS has the best channel. In comparison with the Medium Access Diversity (MAD) scheme in the literature, the proposed MDC possesses the strong merit that the feedback overhead is independent of the number of MSs in the network. Several aspects of the MDC scheme are investigated in detail. An application of the MDC scheme based on the physical layer and parts of the MAC layer of the IEEE 802.11a standard is considered. A general analytical framework for the goodput performance of MDC is derived. Using this framework, the exact closed form solution for the expected goodput of MDC with rate adaptation over Rayleigh fading channels is calculated. The fairness performance of MDC in networks where some MSs experience better average channel conditions than others is also addressed. MSs with low average channel states tend to use the channel less often in MDC than MSs with high average channel states. This issue is tackled with Fairer Multiuser Diversity with Capture (FMDC), a variant of the MDC scheme designed to share the channel resource more equitably across all of the MSs in the network. In FMDC, instead of using the network-wide response threshold to decide whether to compete for the channel, each MS only competes for the channel when their channel state is greater than a threshold factor multiplied by their average channel state. Finally, the problem of adaptive optimisation of the response threshold for MDC and the threshold factor for FMDC is also considered. In the proposed solution, the response threshold and the threshold factor are adapted heuristically according to the estimated goodput performance of the system. The adaptive heuristic has importance in practical systems because the BS usually does not know the characteristics of the time varying channels of the MSs in the network.

Thesis (M. S.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Wills, D. Scott; Committee Member: Keezer, David; Committee Member: Yalamanchili, Sudhakar

Zugl.: Karlsruhe, Univ., Diss. 2006.

Thesis (M.S. in Information Technology Management) Naval Postgraduate School, September 1996. / Thesis advisor(s): Gus K. Lott, Cynthia E. Irvine, Rex Buddenberg. "September 1996." Includes bibliographical references (p. 135-140). Also available online.

Thesis (Ph. D.)--University of Hong Kong, 2010. / Includes bibliographical references (leaves 88-96). Also available in print.

Thesis (M.S.)--Brigham Young University. Dept. of Computer Science, 2006. / Includes bibliographical references (p. 109-113).

Different from multiple-input multiple-output (MIMO) systems, a major challenge for cooperative communications is the problem of synchronization because multiple transmissions undertaken by cooperative systems may not be synchronized in time and/or frequency. With synchronization errors, conventional space-time (ST) codes may not be directly applicable any longer. To tackle the problem of timing synchronization, space-frequency (SF) coded orthogonal frequency division multiplexing (OFDM) cooperative systems have recently been proposed to achieve asynchronous diversity due to their insensitivity to timing errors. However, these systems still need to face the problem of multiple carrier frequency offsets (CFOs). Since each node in a cooperative system is equipped with its own oscillator, the received signals from different relay nodes may have multiple CFOs which cannot be compensated simultaneously at the destination node. For SF coded OFDM cooperative systems, this problem becomes more complicated because CFOs can lead to inter-carrier interference (ICI). To address this challenge, in this thesis we consider the signal detection problem in cooperative systems having multiple CFOs. / First, we investigate the effect of multiple CFOs on two classic ST codes. They are delay diversity and the Alamouti code. For delay diversity, we find that both its achieved diversity order and diversity product are not decreased by multiple CFOs arising from maximum-likelihood (ML) detection. For the Alamouti code, the diversity product may be decreased by multiple CFOs. In the worst case situation, full diversity order 2 cannot be achieved. / For deeper insights into the SF coded communication system with multiple CFOs, we then carry out diversity analysis. By treating the CFOs as part of the SF codeword matrix, we show that if all the absolute values of normalized CFOs are less than 0.5, then the full diversity order for the SF codes are not affected by the multiple CFOs in the SF coded OFDM cooperative system. We further prove that this full diversity property can still be preserved if the zero forcing (ZF) method is used to equalize the multiple CFOs. This method, by some reasonable approximations, is actually equivalent to the MMSE-F detection method. To improve the robustness of the SF codes to multiple CFOs, we propose a novel permutation method. With this method, the achieved diversity order of SF codes remains the same even when the absolute values of normalized CFOs are equal to or greater than 0.5. To reduce computational complexity, we further propose two full diversity achievable detection methods, namely the ZF-ML-Zn and ZF-ML-PIC detection methods, which are suitable for the case when the ICI matrix is singular. / In summary, in this study, we demonstrate that with proper design, the SF coded OFDM approach can be made robust to both timing errors and CFOs in a cooperative communication system. / Since OFDM systems are robust to timing errors, we turn to an SF coded cooperative communication system with multiple CFOs, where the SF codes are rotational based and can achieve both full cooperative and full multipath diversity orders. We begin with the traditional way of ICI mitigation. To preserve the performance of the SF code, we suggest increasing the SINR of each subcarrier but not equalizing the SF precoding matrix. By exploiting the structure of the SF codes, we propose three signal detection methods to deal with the multiple CFOs problem in SF coded OFDM systems. They are the minimum mean-squared filtering (MMSE-F) method, the two-stage simple frequency shift Q taps (FS-Q-T) method, and the multiple fast Fourier transform (M-FFT) method, all of which offer different tradeoffs between performance and computational complexity. Our simulation results indicate that the proposed detection methods perform well as long as the CFOs between nodes are small. / Tian, Feng. / Adviser: Ching Pak-Chung. / Source: Dissertation Abstracts International, Volume: 71-01, Section: B, page: 0559. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2009. / Includes bibliographical references (leaves 146-160). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. Ann Arbor, MI : ProQuest Information and Learning Company, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese.

Title from PDF of title page (University of Missouri--Columbia, viewed on March 10, 2010). The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file. Thesis advisor: Dr. Wenjun Zeng. Includes bibliographical references.