Wireless power transfer scheme with phase alignment based on time-reversal in MU-MIMO systems

Abstract

This paper explores wireless power transfer (WPT) in multi-user multi-input multi-output (MU-MIMO) systems. Single-tone MIMO (ST-MIMO) scheme is optimal when it comes to power transfer in the respect of periodic tone signal. However, it has high complexity when the number of antennas increases. Additionally, it is not appropriate for multi-user system because this scheme has an inclination to send power to one receiver. In this paper, we propose wireless power transfer scheme that increases the received signal power by aligning the phase of the time-reversal (TR) transmit filter in MU-MIMO system considering user fairness. What’s different from the conventional TR-MIMO scheme is that it uses distinctively aligned TR filter. When TR-MIMO scheme is used for power transfer, the received signal is sum of matched signal and unmatched signal and the unmatched signal is sum of inter symbol signal and inter user signal. Unmatched signal makes received power degradation if the phase of it is much different from the phase of matched signal. The proposed scheme changes the phase and magnitude of each TR filter according to the corresponding unmatched signal. In transmitter, energy symbol, channel and TR filter are known so the overall received signal when using TR filter can be predicted. Therefore, unmatched signal can be predicted in transmitter and then TR filter is aligned with scaling factor to match the phase of matched signal to the phase of unmatched signal. In this process, the scaling factor is used for user fairness. This scheme cannot only eliminate received power degradation caused by unmatched signal but also increase received power using phase matching considering user fairness. Also, in MU-MIMO system that large number of transmit antennas are used, its complexity is much lower than ST-MIMO scheme. Simulation shows that the proposed scheme has lower complexity than ST-MIMO scheme as the number of transmit antennas increases and that performance of user fairness is satisfied while maintaining similar received power performance with ST-MIMO scheme. In USRP experiment run on real indoor channel, we can verify that the received power performance of the proposed scheme outperformed direct transmission and conventional TR-MIMO and followed ST-MIMO scheme well satisfying user fairness.