Energy-Ball

Wireless power transfer (WPT) promises to deliver energy to devices that are otherwise hard to charge or replace batteries for. This paper presents a new power transfer 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 Internet of Things 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 higher level of energy over longer distances. In this paper, we design, prototype, and evaluate the proposed energy transfer approach, referred to as Energy-Ball. We implement an Energy-Ball testbed that consists of 17 N210 and 4 B210 Universal Software Radio Peripheral (USRP) nodes, yielding a 20 x 20 m2 energy delivery area. We conduct carefully designed experiments on the testbed. We demo that the energy density of Energy-Ball at the target spot is considerably higher than the energy density elsewhere, with the peak to average power ratio of 8.72. We show that Energy-Ball can transfer energy to any point within the area. When the receiver moves at a speed of 0.5 m/s, Energy-Ball can transfer 80% of optimal power to the mobile receiver. Further, our results also show Energy-Ball can deliver over 0.6mw RF power that enables batteryless sensors at any point across the area.