Energy focusing through distributed beamforming in internet of things : mechanisms and applications
Creators: name: Fan, Xiaoran
Publisher: name: No Publisher Supplied
Year: 2020

description: In this thesis, we discuss the feasibility of using distributed antenna systems to facilitate the deployment of IoT devices. Our approaches are inspired by Fresnel zone plates for focusing light. In our design, in a manner analogous to creating a Fresnel zone plate, we discretize the zone plates into multiple independent phase shifters. Each phase shifter is a far-field RF transmitter in our system. Specifically, by coherently combining the phase of each RF transmitter in a 3D distributed antenna system, the system forms an energy ball at the target location where the energy density level is significantly higher than the energy density level at any other locations. Our results demonstrate that this energy ball has great potential to be leveraged to solve many fundamental problems in IoT and enable exciting IoT applications.In the first part of this thesis, we discuss how a distributed antenna system contributes to an IoT system's confidentiality gains. Ensuring confidentiality of communication is fundamental to securing the operation of a wireless IoT system, where eavesdropping is easily facilitated by the broadcast nature of the wireless medium. By applying distributed beamforming among a coalition, we show that a new approach for assuring physical layer secrecy, without requiring any knowledge about the eavesdropper or injecting any additional cover noise, is possible if the transmitters frequently perturb their phases around the proper alignment phase while transmitting messages. This approach is readily applied to amplitude-based modulation schemes, such as PAM or QAM. We present our secrecy mechanisms, prove several important secrecy properties, and develop a practical secret communication system design.In the next part of this thesis, we discuss how a distributed antenna system contributes to an IoT system's energy efficiency gains. In order to meet the ever-growing energy demand from the next billion IoT devices, we present a new wireless power transfer (WPT) approach by aligning the phases of a collection of radio frequency (RF) energy chargers at the target receiver device. Our approach can ship energy over tens of meters and to mobile targets. More importantly, our approach leads to a highly asymmetric energy density distribution in the charging area: the energy density at the target receiver is much higher than the energy density at other locations. It is a departure from existing beamforming based WPT systems that have high energy along the energy beam path. Such a technology can enable a large array of batteryless IoT applications and render them much more robust and long-running. Thanks to its asymmetric energy distribution, our approach potentially can be scaled up to ship a higher level of energy over longer distances.We design, prototype, and evaluate the proposed distributed antenna system. We implement the testbed that consists of 17 N210 and 4 B210 Universal Software Radio Peripheral (USRP) nodes, yielding a 20 x 20 m2 experiment area. Depending on system parameter settings, we measure that the eavesdroppers failed to decode 30%-60% of the bits across multiple locations while the intended receiver has an estimated bit error ratio of 3 x 10-6. Our results also show the system can deliver over 0.6mw RF power that enables batteryless mobile sensors at any point across the area.In the last part of this thesis, we build a distributed beamforming system that can continuously charge tiny IoT devices placed in hard-to-reach locations (e.g. medical implants) with consistent high power, even when the implant moves around inside the human body. To accomplish this, we exploit the unique energy ball pattern of the distributed antenna array and devise a backscatter-assisted beamforming algorithm that can concentrate RF energy on a tiny spot surrounding the medical implant. Meanwhile, the power levels on other body parts stay at a low level, reducing the risk of overheating. We prototype the system on 21 software-defined radios and a printed circuit board (PCB). Extensive experiments demonstrate that the proposed system achieves 0.37 mW average charging power inside a 10 cm-thick pork belly, which is sufficient to wirelessly power a range of commercial medical devices. Comparison with state-of-the-art powering approaches shows that our system achieves 5.4x-18.1x power gain when the implant is stationary, and 5.3x -7.4x power gain when the implant is in motion.