Retroreflective Transceiver Array Using a Novel Calibration Method Based on Optimum Phase Searching

The retrodirective or retroreflective method has been used as a radio-frequency beamforming method for massive antenna array, for wireless communication systems, radars, and microwave power transfer (MPT) systems. In particular, the retroreflective method is suitable for optimum beamforming for both near- and far-field zones. Each transmitter (Tx) and receiver (Rx) circuit in the Tx array has its own phase offset, which should be calibrated for optimum retroreflective operation. This article presents a simple and effective phase offset calibration method for the retroreflective beamforming arrays using a one-by-one basis optimum phase searching method with a receiver at a reference point. To verify the proposed calibration method for a massive retroreflective beamforming array, a 5.2 GHz short- and mid-range (\leq4 m) MPT system, including a Tx array based on 8 × 8 antennas and an Rx using six antennas (five rectennas for power reception, and 1 additional for pilot transmission) for charging mobile handsets was designed and implemented. Optimal retroreflective beamforming from the Tx array, which was calibrated using the proposed method, was experimentally verified for elevation angles of within \pm 45^\circ for both xz- and yz-planes. The implemented Tx and Rx circuits achieved a received dc power of about 100 mW at a distance of 4 m.