Year: 2020
Type: dissertation
Abstract: Wireless power transfer (WPT) technology is envisioned to revolutionize the powering of wireless devices making them truly wireless. Conventional wired-recharging or battery replacement of wireless devices are not only costly and inconvenient, but can be dangerous in hazardous environments or even impossible for powering biomedical implants. This dissertation focuses on far-field WPT using radio frequency (RF) signals. The dual capability of RF signals to transfer both information and power motivates the integration of WPT and communication systems. The main focus of this dissertation is on the analysis and design of communication systems with WPT. These systems can be classified into two broad categories, namely simultaneous wireless information and power transfer (SWIPT) systems and wireless powered communication (WPC) systems. In particular, in SWIPT systems, the same RF signal is used to transfer both information and power, whereas in WPC systems, an energy harvesting (EH) node harvests energy from an RF signal over a WPT downlink channel to facilitate its communication with a desired receiver over a wireless information transfer uplink channel. In the following, we summarize the main contributions of this dissertation. Multi-User Scheduling in SWIPT: In SWIPT systems, practical circuit limitations prevent a device from fully utilizing the same RF signal for both information decoding and EH. Hence, information decoding and EH need to be separated in time, power, or space. This creates a tradeoff between the rate of information transfer to the device and the amount of energy that can be harvested by the device, the so-called rate-energy tradeoff. In a multi-user SWIPT system, the users share the downlink channel to receive both information and energy and each user is interested in optimizing its rate-energy tradeoff. In this dissertation, we propose a novel framework for the online scheduling of multiple users in a downlink SWIPT system, where the users’ rate-energy tradeoff can be controlled. In particular, we aim to answer the following fundamental question: “How should a SWIPT transmitter schedule multiple heterogeneous users in a common downlink channel to achieve desired information transfer rates and harvested powers at the users while ensuring fairness in the resource allocation among the users?” Energy Buffer-Aided WPC systems: Most works in the WPC literature assume that EH nodes consume the harvested energy for wireless information transfer in a slot by slot manner. However, the system performance can be improved if EH nodes store the harvested energy in energy buffers (e.g. batteries) and optimize the use of the stored energy for future communication tasks. Hence, in this dissertation, we aim to answer the following question: “In WPC systems, how can an EH node with an energy buffer optimize its uplink transmit power such that the communication outages with its desired receiver are minimized?” In particular, we propose two simple online uplink transmission policies that do not require knowledge of the amount of harvested energy in the downlink nor of the uplink channel state information. Accordingly, we analyze the probability of communication outages assuming Nakagami-m fading channels and show that the proposed transmission policies provide a near-optimal outage probability performance. Nonlinear Circuit-based Saturation-Aware EH Model: Most works in the WPT literature assume a linear EH model for the EH receiver. In practice, however, RF EH circuits exhibit a nonlinear input-output characteristic. In particular, the amount of harvested power at the EH receiver depends on both the power and the waveform (shape) of the transmit RF signal. This nonlinearity motivates the design of transmit RF signals that enable efficient WPT. To this end, we develop a novel nonlinear EH model by analyzing a single-diode half-wave rectifier circuit, which is commonly used for WPT applications. The proposed EH model captures the saturation of the output power for high input RF powers. Accordingly, we obtain a closed-form expression for the harvested power whose accuracy is confirmed by circuit simulations. Capacity of SWIPT Systems for Nonlinear EH Model: The proposed nonlinear EH model is applied in a SWIPT system with the objective of optimizing the transmit RF waveform for maximization of the system rate-energy region. In particular, we aim to answer the following question: “What is the optimal distribution of the transmit RF signal that maximizes the rate of information transfer to an information decoding receiver while simultaneously satisfying the EH requirements of multiple nonlinear EH receivers?”. Specifically, we study the conditional capacity of a SWIPT system subject to constraints on the average and peak powers at the transmitter and nonlinear EH constraints at the EH receivers. We prove that the optimal distribution of the transmit signal amplitude that maximizes the rate-energy tradeoff is unique, discrete, and finite. In addition, we propose a suboptimal input distribution whose performance closely approaches that of the optimal one.
Author: Rania Morsi
Language: English
Subfield: Electrical and Electronic Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W3139173298