Multi-Antenna Constant Envelope Wireless Power Transfer

In this paper, we study wireless power transfer in a multiuser multiple-input single-output system, where a base station equipped with multiple antennas wirelessly transfers power to distributed single-antenna users. To reduce the implementation cost, we propose a constant-envelope analog beamforming scheme to simultaneously transfer power to multiple users. The proposed scheme requires only a single radio frequency (RF) chain at the multi-antenna transmitter. We consider the beamforming design to maximize the sum-power transferred. We show that the proposed constant-envelope beamforming design only incurs about 1-dB power loss under homogeneous and independent Rayleigh fading, as compared with the optimal variable-envelope digital beamforming design (that however requires one RF chain for each transmit antenna). We further consider user fairness and study the beamforming design to maximize the minimum received power of all users. We also address the problem of maximizing the sum-power transferred under the quality-of-service constraint for each user. Moreover, we extend our beamforming design to the case with only partial channel state information at the transmitter. Numerical results show that constant-envelope analog beamforming is a promising cost-effective technique for multiuser multi-antenna wireless power transfer.