A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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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 An ultra-low power voltage regulator system for wireless sensor networks powered by energy harvesting(2014) Wang, Chao; Peckerar, Martin; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A DC-DC converter is an important power management module as it converts one DC voltage level to another suitable for powering a desired electronic system. It also stabilizes the output voltage when fluctuations appear in the power supplies. For those wireless sensor networks (WSNs) powered by energy harvesting, the DC-DC converter is usually a linear regulator and it resides at the last stage of the whole energy harvesting system just before the empowering sensor node. Due to the low power densities of energy sources, one may have to limit the quiescent current of the linear regulator in the sub-uA regime. This severe restriction on quiescent current could greatly compromise other performance aspects, especially the transient response. This dissertation reports a voltage regulator system topology which utilizes the sensor node state information to achieve ultra-low power consumption. The regulator system is composed of two regulators with different current driving abilities and quiescent current consumptions. The key idea is to switch between the two regulators depending on the sensor state. Since the "right" regulator is used at the "right" time, the average quiescent current of the regulator system is minimized, and the trade-off between low quiescent current and fast transient response has been eliminated. In order to minimize the average quiescent current of the system, nano-ampere reference current design is studied, and the proposed reference current circuit is shown (theoretically and experimentally) to reduce the supply voltage dependence by 5X. The regulator system has been fabricated and tested using an ON Semiconductor 0.5 μm process. It has been verified through experiments that the proposed system reduces the quiescent current by 3X over the state-of-the-art in the literature; and, more importantly, it achieves low quiescent current, low dropout voltage, and fast transient response with small output voltage variation all at the same time. The thesis further presents data on the application of energy harvesting system deriving energies from various RF signals to power a commercial off-shelf wireless sensor node.Item Utilization of Channel State Information in Transmission Control for Wireless Communication Networks(2013) Hany, Mohamed Tawfeek Kashef; Ephremides, Anthony; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation deals with the utilization of channel knowledge in improving the performance of wireless communication systems. The first part is about energy harvesting networks. The transmission policies in energy harvesting wireless systems need to adapt to the harvested energy availability and the channel characteristics. We start by considering the scheduling policy for a single energy harvesting source node that operates over a time varying channel. The goal of the source is to maximize the average number of successfully delivered packets per time slot. The transmission decisions depend on the available channel information and the length of the energy queue. Then, we investigate the case in which the source is helped by a relay through a network-level cooperation protocol. We investigate the case of a single relay node in which we optimize the transmission control based on channel measurements. Then, we assess the benefits of using partial relaying. We provide an exact characterization of the stability region of a network which consists of a source, a relay and a destination with random data arrivals to both the source and the relay. We derive the optimal value of the relaying parameter to maximize the stable throughput of the source for a given data arrival rate to the relay. Finally, we introduce the problem of general relaying cost minimization for cooperative energy harvesting networks with multiple relays. Then, we introduce the energy consumption as a cost criterion for the optimization problem to find an energy-efficient partial relaying protocol. In the second part, we investigate the techniques to optimally exploit channel information in transmission control for interfering sources. We discuss the scheduling problem for different levels of channel knowledge because learning instantaneous channels states may be costly or infeasible. We consider a network that consists of two transmitter-receiver pairs which operate over time varying channels. We derive the optimal scheduling policies which maximize the expected weighted sum-rate of the network per time slot. The decision depends on the information about the channels between nodes. In the third part, we investigate the effect of channel estimation on the performance of a secondary network in a cognitive radio system. We focus on estimating the sensing-channel from the primary source to the secondary source which helps in assessing the reliability of the sensing decision. The channel is estimated opportunistically when the secondary source senses the primary source to be active. We consider the performance criterion to be the energy consumed by the secondary system constrained by a required average data transmission rate for the secondary system and an allowable average failure probability for the primary system.Item Scheduling in Energy Harvesting Systems with Hybrid Energy Storage(2013) Shahzad, Khurram; Ulukus, Sennur; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In wireless networks, efficient energy storage and utilization plays a vital role, resulting in a prolonged lifetime and enhanced throughput. This factor becomes even more important in systems employing energy harvesting as compared to utility or battery powered networks, where a constant supply of energy is available. Therefore, it is crucial to design schemes that make the best use of available energy resources, keeping in view the practical constraints. In this work, we consider data transmission with an energy harvesting transmitter which has hybrid energy storage with a perfect super-capacitor (SC) and an inefficient battery. The SC has finite storage space while the battery has unlimited storage space. The transmitter can choose to store the harvested energy in the SC or in the battery, while draining energy from the SC and the battery simultaneously. Under this energy storage setup, we solve throughput optimal energy allocation problem over a point-to-point channel in an offline setting. The hybrid energy storage model with finite and unlimited storage capacities imposes a generalized set of constraints on the transmission policy. We show that the solution is found by a sequential application of the directional water-filling algorithm. Next, we consider offline throughput maximization in the presence of an additive time-linear processing cost in the transmitter's circuitry. In this case, the transmitter has to additionally decide on the portions of the processing cost to be drained from the SC and the battery. Despite this additional complexity, we show that the solution is obtained by a sequential application of a directional glue-pouring algorithm, parallel to the cost-less processing case. Finally, we provide numerical illustrations for optimal policies and performance comparisons with some heuristic online policies.Item Piezoelectric Vibration Energy Harvesting From Coupled Structural-Acoustic Systems(2013) ALADWANI, ABDULAZIZ EBRAHIM; Baz, Amr; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A comprehensive theoretical and experimental study of the fundamentals and the underlying phenomena governing the operation of piezoelectric vibration energy harvesting from coupled structural-acoustic systems is presented. Analytical and finite element models are developed based on variational formulations to describe the energy harvesting from uncoupled structural elements as well as structural elements coupled with acoustic cavities. The models enable the predictions of the structural displacement, output electric voltage, and fluid pressure for various loading conditions on the energy harvesting system. The developed models also include dynamic magnification means to enhance the energy harvesting capabilities and enable harnessing of the vibration energy over a broader operating frequency range. The predictions of all the models are experimentally validated by using structural elements varying from beams to plates. Close agreements are demonstrated between the theoretical predictions and the obtained experimental results. The theoretical and experimental tools developed, in this dissertation, provide invaluable means for designing a wide variety of efficient energy harvesters for harnessing the vibrational energy inside automobiles, helicopters, aircrafts, and other types of structures that interact internally or externally with a fluid medium. With such harnessed energy, a slew of on-board sensors can be powered to enable the continuous monitoring of the condition and health of these structures without the need for external power sources.Item COOPERATIVE NETWORKING AND RELATED ISSUES: STABILITY, ENERGY HARVESTING, AND NEIGHBOR DISCOVERY(2013) Jeon, Jeongho; Ephremides, Anthony; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation deals with various newly emerging topics in the context of cooperative networking. The first part is about the cognitive radio. To guarantee the performance of high priority users, it is important to know the activity of the high priority communication system but the knowledge is usually imperfect due to randomness in the observed signal. In such a context, the stability property of cognitive radio systems in the presence of sensing errors is studied. General guidelines on controlling the operating point of the sensing device over its receiver operating characteristics are also given. We then consider the hybrid of different modes of operation for cognitive radio systems with time-varying connectivity. The random connectivity gives additional chances that can be utilized by the low priority communication system. The second part of this dissertation is about the random access. We are specifically interested in the scenario when the nodes are harvesting energy from the environment. For such a system, we accurately assess the effect of limited, but renewable, energy availability on the stability region. The effect of finite capacity batteries is also studied. We next consider the exploitation of diversity amongst users under random access framework. That is, each user adapts its transmission probability based on the local channel state information in a decentralized manner. The impact of imperfect channel state information on the stability region is investigated. Furthermore, it is compared to the class of stationary scheduling policies that make centralized decisions based on the channel state feedback. The backpressure policy for cross-layer control of wireless multi-hop networks is known to be throughput-optimal for i.i.d. arrivals. The third part of this dissertation is about the backpressure-based control for networks with time-correlated arrivals that may exhibit long-range dependency. It is shown that the original backpressure policy is still throughput-optimal but with increased average network delay. The case when the arrival rate vector is possibly outside the stability region is also studied by augmenting the backpressure policy with the flow control mechanism. Lastly, the problem of neighbor discovery in a wireless sensor network is dealt. We first introduce the realistic effect of physical layer considerations in the evaluation of the performance of logical discovery algorithms by incorporating physical layer parameters. Secondly, given the lack of knowledge of the number of neighbors along with the lack of knowledge of the individual signal parameters, we adopt the viewpoint of random set theory to the problem of detecting the transmitting neighbors. Random set theory is a generalization of standard probability theory by assigning sets, rather than values, to random outcomes and it has been applied to multi-user detection problem when the set of transmitters are unknown and dynamically changing.