Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item Energy Cooperation in Energy Harvesting Communication Systems(2016) Gurakan, Berk; Ulukus, Sennur; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In energy harvesting communications, users transmit messages using energy harvested from nature. In such systems, transmission policies of the users need to be carefully designed according to the energy arrival profiles. When the energy management policies are optimized, the resulting performance of the system depends only on the energy arrival profiles. In this dissertation, we introduce and analyze the notion of energy cooperation in energy harvesting communications where users can share a portion of their harvested energy with the other users via wireless energy transfer. This energy cooperation enables us to control and optimize the energy arrivals at users to the extent possible. In the classical setting of cooperation, users help each other in the transmission of their data by exploiting the broadcast nature of wireless communications and the resulting overheard information. In contrast to the usual notion of cooperation, which is at the signal level, energy cooperation we introduce here is at the battery energy level. In a multi-user setting, energy may be abundant in one user in which case the loss incurred by transferring it to another user may be less than the gain it yields for the other user. It is this cooperation that we explore in this dissertation for several multi-user scenarios, where energy can be transferred from one user to another through a separate wireless energy transfer unit. We first consider the offline optimal energy management problem for several basic multi-user network structures with energy harvesting transmitters and one-way wireless energy transfer. In energy harvesting transmitters, energy arrivals in time impose energy causality constraints on the transmission policies of the users. In the presence of wireless energy transfer, energy causality constraints take a new form: energy can flow in time from the past to the future for each user, and from one user to the other at each time. This requires a careful joint management of energy flow in two separate dimensions, and different management policies are required depending on how users share the common wireless medium and interact over it. In this context, we analyze several basic multi-user energy harvesting network structures with wireless energy transfer. To capture the main trade-offs and insights that arise due to wireless energy transfer, we focus our attention on simple two- and three-user communication systems, such as the relay channel, multiple access channel and the two-way channel. Next, we focus on the delay minimization problem for networks. We consider a general network topology of energy harvesting and energy cooperating nodes. Each node harvests energy from nature and all nodes may share a portion of their harvested energies with neighboring nodes through energy cooperation. We consider the joint data routing and capacity assignment problem for this setting under fixed data and energy routing topologies. We determine the joint routing of energy and data in a general multi-user scenario with data and energy transfer. Next, we consider the cooperative energy harvesting diamond channel, where the source and two relays harvest energy from nature and the physical layer is modeled as a concatenation of a broadcast and a multiple access channel. Since the broadcast channel is degraded, one of the relays has the message of the other relay. Therefore, the multiple access channel is an extended multiple access channel with common data. We determine the optimum power and rate allocation policies of the users in order to maximize the end-to-end throughput of this system. Finally, we consider the two-user cooperative multiple access channel with energy harvesting users. The users cooperate at the physical layer (data cooperation) by establishing common messages through overheard signals and then cooperatively sending them. For this channel model, we investigate the effect of intermittent data arrivals to the users. We find the optimal offline transmit power and rate allocation policy that maximize the departure region. When the users can further cooperate at the battery level (energy cooperation), we find the jointly optimal offline transmit power and rate allocation policy together with the energy transfer policy that maximize the departure region.Item Cognitive Multiple Access for Cooperative Communications and Networking(2009) El Sherif, Amr; Liu, K. J. Ray; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In cooperative communications different network nodes share their antennas and resources to form a virtual antenna array and improve their performance through spatial diversity. This thesis contributes to the advancement of cooperative communications by developing and analyzing new multiple access cooperation protocols that leverage the benefits of cooperation to upper network layers. For speech communications networks, we propose a cooperative multiple access protocol that exploits inherent characteristics of speech signals, namely, long periods of silence, to enable cooperation without incurring bandwidth efficiency losses. Using analytical and simulation results we show that the proposed protocol achieves significant increase in network throughput, reduction in delay, and improved perceptual speech quality. In TDMA networks, we investigate the problem of sharing idle time slots between a group of cooperative cognitive relays helping primary users, and a group of cognitive secondary users. Analytical results reveal that, despite the apparent competition between relays and secondary users, and even in case of mutual interference between the two groups, both primary and secondary users will significantly benefit in terms of maximum stable throughput from the presence of relays. For random access networks, we find a solution to the problem of achieving cooperation gains without suffering from increased collision probability due to relay transmissions. A novel cooperation protocol is developed and analyzed for that purpose. Analytical and simulation results reveal significant improvements in terms of throughput and delay performance of the network. Moreover, collision probability is decreased. Finally, in the framework of a cognitive radio network, we study the negative effects of spectrum sensing errors on the performance of both primary and secondary networks. To alleviate those negative effects, we propose a novel joint design of the spectrum sensing and channel access mechanisms. Results show significant performance improvement in the maximum stable throughput region of both networks.Item DIFFERENTIAL MODULATION FOR BROADBAND SPACE-TIME/COOPERATIVE WIRELESS COMMUNICATIONS(2006-10-24) Himsoon, Thanongsak; Liu, K. J. Ray; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Among various diversity techniques to combat fading in wireless channels, spatial diversity through MIMO coding scheme is an effective way to increase link capacity and system reliability without sacrificing bandwidth efficiency. Recently, cooperative diversity has been introduced as an efficient alternative to improve system performance without the requirement of additional antennas. However, most of existing works on MIMO and cooperative communications are based on an assumption that the destination has perfect knowledge of channel state information of all transmission links and hence introduces high complexity to the receiver. To overcome such problems, this thesis proposes differential modulation schemes for space-time coded MIMO and cooperative communications. By exploiting spatial/cooperative diversity without the requirement of channel state information, the proposed schemes provide an excellent tradeoff between receiver complexity and system performance. First, a matrix rotation based signal design for differential space-time modulation is investigated to minimize the union bound on block error probability. Next, a robust differential scheme for MIMO-OFDM systems is proposed by which the signal transmission of each differentially encoded signal is completed within one OFDM block rather than multiple blocks as in existing works. Then, a differential scheme for UWB systems employing MIMO multiband OFDM is proposed to explore all available diversities by jointly encoding across spatial, temporal, and frequency domains. To exploit cooperative diversity, an amplify-and-forward differential cooperative scheme and a threshold-based decode-and-forward differential cooperative scheme are proposed. The proposed differential cooperative schemes are first considered in a two-node cooperation system, and the proposed works are extended to a general multi-node scenario. Finally, a general framework to improve lifetime of battery-operated devices by exploiting cooperative diversity is proposed such that the device lifetime can be greatly improved by efficiently taking advantages of both different locations and energy levels among distributed nodes in wireless networks.