Microwave wireless power transfer (MWPT) is a key enabler to realize battery-maintenance-free IoT device operation. In MWPT, a transmitting antenna (TX) emits high-power microwaves, and a receiving antenna (RX) converts them into DC with a rectifier. Since the rectifier has a threshold to prevent the conduction of weak incident wave, TX beamforming with an array antenna is essential to focus microwave energy on the RX. However, there is a dilemma; beamforming requires channel state information (CSI) feedback from the RX, while RX power consumption is inevitable to send a feedback signal.
To resolve the dilemma, this study established a novel CSI estimation method utilizing the physical barrier of the rectifier. When the TX power is swept, only the strong incident wave passes the rectifier while the weaker one is reflected at the RX. Thus, the RX behaves as if it transmits the ON/OFF keying signal in spite of the absence of the signal source, and the TX side can estimate the CSI by detecting the quasi modulated signal.
Experiments conducted in a shield room verified the proposed method achieved CSI estimation accuracy of over 80% at 29 out of 33 receiving points in the maximum 2 m transmission range.