Microwave power transmission has the potential to supply wireless power to portable/mobile electronic devices over long distances (on the order of meters or even kilometers) efficiently. Nevertheless, several technical challenges remain to be resolved in order to accomplish practical microwave power transmission systems, including (i) minimizing power loss due to microwave propagation, (ii) preventing humans and other electrical systems from exposure to excessive microwave radiation, and (iii) reconfiguring wireless power transmission in reaction to environmental changes (such as physical movements of portable devices) in real time. In this chapter, a microwave power transmission scheme based on retro-reflective beamforming is proposed to address the above challenges. In the retro-reflective beamforming, wireless power transmission is guided by pilot signals. To be specific, one or more than one mobile device(s) broadcast pilot signals to their surroundings, and based on analyzing the pilot signals, a wireless power transmitter delivers focused power beam(s) onto the mobile device(s). Preliminary numerical and experimental results are presented to demonstrate the feasibility of the proposed retro-reflective beamforming scheme.
Part of the book: Wireless Power Transfer