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 A Cross-Layer Study of the Scheduling Problem(2009) Pantelidou, Anna; Ephremides, Anthony; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis is inspired by the need to study and understand the interdependence between the transmission powers and rates in an interference network, and how these two relate to the outcome of scheduled transmissions. A commonly used criterion that relates these two parameters is the Signal to Interference plus Noise Ratio (SINR). Under this criterion a transmission is successful if the SINR exceeds a threshold. The fact that this threshold is an increasing function of the transmission rate gives rise to a fundamental trade-off regarding the amount of time-sharing that must be permitted for optimal performance in accessing the wireless channel. In particular, it is not immediate whether more concurrent activations at lower rates would yield a better performance than less concurrent activations at higher rates. Naturally, the balance depends on the performance objective under consideration. Analyzing this fundamental trade-off under a variety of performance objectives has been the main steering impetus of this thesis. We start by considering single-hop, static networks comprising of a set of always-backlogged sources, each multicasting traffic to its corresponding destinations. We study the problem of joint scheduling and rate control under two performance objectives, namely sum throughput maximization and proportional fairness. Under total throughput maximization, we observe that the optimal policy always activates the multicast source that sustains the highest rate. Under proportional fairness, we explicitly characterize the optimal policy under the assumption that the rate control and scheduling decisions are restricted to activating a single source at any given time or all of them simultaneously. In the sequel, we extend our results in four ways, namely we (i) turn our focus on time-varying wireless networks, (ii) assume policies that have access to only a, perhaps inaccurate, estimate of the current channel state, (iii) consider a broader class of utility functions, and finally (iv) permit all possible rate control and scheduling actions. We introduce an online, gradient-based algorithm under a fading environment that selects the transmission rates at every decision instant by having access to only an estimate of the current channel state so that the total user utility is maximized. In the event that more than one rate allocation is optimal, the introduced algorithm selects the one that minimizes the transmission power sum. We show that this algorithm is optimal among all algorithms that do not have access to a better estimate of the current channel state. Next, we turn our attention to the minimum-length scheduling problem, i.e., instead of a system with saturated sources, we assume that each network source has a finite amount of data traffic to deliver to its corresponding destination in minimum time. We consider both networks with time-invariant as well as time-varying channels under unicast traffic. In the time-invariant (or static) network case we map the problem of finding a schedule of minimum length to finding a shortest path on a Directed Acyclic Graph (DAG). In the time-varying network case, we map the corresponding problem to a stochastic shortest path and we provide an optimal solution through stochastic control methods. Finally, instead of considering a system where sources are always backlogged or have a finite amount of data traffic, we focus on bursty traffic. Our objective is to characterize the stable throughput region of a multi-hop network with a set of commodities of anycast traffic. We introduce a joint scheduling and routing policy, having access to only an estimate of the channel state and further characterize the stable throughput region of the network. We also show that the introduced policy is optimal with respect to maximizing the stable throughput region of the network within a broad class of stationary, non-stationary, and anticipative policies.Item Cross-Layer Design for Multi-Antenna Ultra-Wideband Systems(2005-11-28) Siriwongpairat, Wipawee; Liu, K. J. Ray; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ultra-wideband (UWB) is an emerging technology that offers great promises to satisfy the growing demand for low cost and high-speed digital wireless home networks. The enormous bandwidth available, the potential for high data rates, as well as the potential for small size and low processing power long with low implementation cost, all present a unique opportunity for UWB to become a widely adopted radio solution for future wireless home-networking technology. Nevertheless, in order for UWB devices to coexist with other existing wireless technology, the transmitted power level of UWB is strictly limited by the FCC spectral mask. Such limitation poses significant design challenges to any UWB system. This thesis introduces various means to cope with these design challenges. Advanced technologies including multiple-input multiple-output (MIMO) coding, cooperative communications, and cross-layer design are employed to enhance the performance and coverage range of UWB systems. First a MIMO-coding framework for multi-antenna UWB communication systems is developed. By a technique of band hopping in combination with jointly coding across spatial, temporal, and frequency domains, the proposed scheme is able to exploit all the available spatial and frequency diversity, richly inherent in UWB channels. Then, the UWB performance in realistic UWB channel environments is characterized. The proposed performance analysis successfully captures the unique multipath-rich property and random-clustering phenomenon of UWB channels. Next, a cross-layer channel allocation scheme for UWB multiband OFDM systems is proposed. The proposed scheme optimally allocates subbands, transmitted power, and data rates among users by taking into consideration the performance requirement, the power limitation, as well as the band hopping for users with different data rates. Also, an employment of cooperative communications in UWB systems is proposed to enhance the UWB performance and coverage by exploiting the broadcasting nature of wireless channels and the cooperation among UWB devices. Furthermore, an OFDM cooperative protocol is developed and then applied to enhance the performance of UWB systems. The proposed cooperative protocol not only achieves full diversity but also efficiently utilizes the available bandwidth.Item Physical layer issues and cross-layer design in wireless networks(2004-09-27) Li, Yun; Ephremides, Anthony; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Comparing to wired networks, wireless networks have some special features in the physical layer, medium access control (MAC) layer, and the network layer. This work discusses several research topics in the physical layer, and studies the cross-layer design of wireless networks. First, we consider a Code-division multiple-access (CDMA) system with multiuser detection when the presence of a subset of the users is unknown to the receiver. The performance of the system in terms of Signal-to-Interference and noise-Ratio (SIR) and user capacity is given, by assuming symmetric signals. Then, we study the power control problem with multiple flow types. Each node has multiple flow types requiring different QoS, (for example in a multimedia system,) and has the constraint of using the same power level for all of the flow types. The conditions for solution to exist are given; and the characteristics of the solution are provided. Next, we propose a passive rate adaptation, in which some bits are dropped at the receiver end of a link, for the ad hoc network to use in the temporary channel fluctuation. We study the performance of this passive rate control scheme in terms of both symbol error probability and mean square distortion. Finally, we study the coupling between layers of the network structure, and the cross-layer design. We explore the coupling between the physical layer and the MAC sublayer first, and propose the scheduling algorithm with power control. Then we consider the coupling between MAC sublayer and the network layer, and propose the joint scheduling and routing algorithm. The simulation results demonstrate that the joint algorithm improves the performance significantly.