Asynchronous Focusing Time Reversal Wireless Power Transfer for Multi-Users With Equal Received Power Assignment

Time reversal wireless power transfer (TR-WPT) provides an efficient radiated method for wirelessly transferring power to multiple users at a middle distance in complicated multipath environments. However, TR-WPT based on the direct time reversal scheme cannot ensure each user to have the equal received power although the maximum transfer efficiency can be achieved. In order to realize the same received power assignment for each user as well as high transfer efficiency, an asynchronous focusing time reversal (AFTR) scheme is proposed in this paper. Different from the TR-WPT based on the direct time reversal technique, the radiated power does not focus on multi-users at the same time. Instead, the AFTR-WPT system operates in an asynchronous focusing mode. For the continuous-wave wireless power transfer, asynchronous focusing means that the phases of the focused power signals received by multi-users are different. By optimizing the phase of the focused signal received by each user, the AFTR-WPT system can not only transfer the equal power to each user but also achieve the optimal transfer efficiency. To demonstrate the proposed scheme, a porotype of multi-user AFTR-WPT system of 16 transmit antennas and 9 receive antennas is developed with the continuous-wave power transfer in an indoor laboratory environment. Utilizing the proposed asynchronous focusing scheme, we successfully demonstrate the equal power assignment for multiple users and achieve the optimal transfer efficiency, approaching the maximum efficiency of the TR-WPT based on direct time reversal scheme for all studied cases.