I. Introduction
With the advent of the Internet of Things (IoTs) era and the rapid development of wireless communication technology, a massive number of sensors will be integrated into human bodies and the surrounding environment for more convenient living. Although the advanced 5G technology remarkably promotes the wireless communication performance of sensors, the power supply for most of sensors is still dependent on batteries or cables, which significantly prevent human beings from truly entering into a completely wireless era. As a promising technology, wireless power transfer (WPT) can completely remove the last wire connected to electronic devices and lead human beings into a true wireless era for the future.WPT can be dated back to Tesla who first invented the WPT system based on the capacitive power transfer (CPT) mechanism in 1894 and verified the feasibility of transferring the electric power to the devices wirelessly [1]. In the same period, the inductive power transfer (IPT) method was experimentally demonstrated [2]. Besides the two nonradiated WPT techniques of CPT and IPT, Brown developed a microwave power transmission (MPT) method in 1960s and successfully demonstrated the use of microwaves to wirelessly power the helicopters [3]. In 2007, based on the magnetic resonance coupling (MRC) mechanism [4], Kurs extended the transmission distance of the coil-based nonradiated WPT up to 2 m, which highly stirred up the extensive study of the nonradiated and radiated WPT techniques in both academic and industrial fields [9]. Up to now, CPT [1], [7], IPT [2], [8]–[11], and MRC [4], [12]–[15] methods have been well-developed. However, due to their limited power transfer distance, the power supply of the IoT sensors located far away from the power transmitter is still an open issue. In recent years, MPT has gained more and more attentions for these IoT sensors by virtue of its long-distance power transmission.