Numerous portable electronic devices (such as laptops, cell phones, digital cameras, and electric shavers) rely on rechargeable batteries and must be routinely charged by the line power. A wireless charging technique capable, of delivering electromagnetic energy to these portable devices would make them tether free and "truly portable." Wireless charging is especially valuable for devices with which wired connections are intractable, e.g., unattended radio frequency identification tags and implanted sensors. In recent years, enormous research efforts have been devoted to wireless charging. In 1990s, a case study is reported in [1] to construct a point-to-point wireless electricity transmission to a small isolated village called Grand-Bassin in France, In 2007, an inductive resonance coupling scheme, which makes use of near-field coupling between two magnetic resonators, was demonstrated able to power a 60-Watt light bulb over two meters by a team of Massachusetts Institute of Technology [2]. In addition, several companies (PowerCast, WildCharge, WiPower, .etc:) have developed products targeting specific applications. Nevertheless, several technical challenges remain to be resolved in order to accomplish practical wireless charging. Specifically, (i) to achieve efficient charging over long distance, severe power loss due to electromagnetic wave propagation must be remedied; (ii) humans' exposure to electromagnetic radiation 'should always be kept below safety level while sufficient power is delivered to devices; and (iii) some existing systems are unsuitable for ubiquitous deployment due to high cost, large size, and/or heavy weight. In this paper, an innovative wireless charging system based on ultra-wideband retro-reflective beamforming is proposed to address the above challenges. The proposed charger consists of multiple antenna elements distributed in space. According to pilot signals (which are short impulses) they receive from the target device, the antenna elements jointly construct a focused electromagnetic beam onto the device (i.e., beamforming). Beamforming enables spatially focused/dedicated power delivery to devices while keeping power level in all the other locations minimal. As a result, the proposed system attains high charging efficiency and leads to little hazard/interference to other objects. Performance of the proposed wireless charging system is demonstrated by some simulation results obtained by a full-wave Maxwell's equations solver.
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