Retrodirective Multi-User Wireless Power Transfer with Massive MIMO

TL;DR

This paper proposes a low-complexity retrodirective beamforming scheme for multi-user wireless power transfer with massive MIMO, addressing fairness issues through an iterative beacon power adjustment algorithm.

Contribution

It introduces a simple retrodirective WPT scheme with massive MIMO and develops a distributed power control algorithm to mitigate the doubly near-far problem.

Findings

The retrodirective scheme achieves massive MIMO beamforming gain.

The distributed algorithm converges quickly to a fair power distribution.

The proposed method improves energy harvesting fairness among users.

Abstract

This letter studies a radio-frequency (RF) multi-user wireless power transfer (WPT) system, where an energy transmitter (ET) with a large number of antennas delivers energy wirelessly to multiple distributed energy receivers (ERs). We investigate a low-complexity WPT scheme based on the retrodirective beamforming technique, where all ERs send a common beacon signal simultaneously to the ET in the uplink and the ET simply conjugates and amplifies its received sum-signal and transmits to all ERs in the downlink for WPT. We show that such a low-complexity scheme achieves the massive multiple-input multiple-output (MIMO) energy beamforming gain. However, a "doubly near-far" issue exists due to the round-trip (uplink beacon and downlink WPT) signal propagation loss where the harvested power of a far ER from the ET can be significantly lower than that of a near ER if the same uplink beacon power is used. To tackle this problem, we propose a distributed uplink beacon power update algorithm, where each ER independently adjusts the beacon power based on its current harvested power in an iterative manner. It is shown that the proposed algorithm converges quickly to a unique fixed-point solution, which helps achieve the desired user fairness with best efforts.