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Intelligent reflecting surface (IRS) is a promising technology to boost theefficiency of wireless energy transfer (WET) systems. However, for a multiuserWET system, simultaneous multi-beam energy transmission is generally requiredto achieve the maximum performance, which may not be implemented by using theIRS having only a single set of coefficients. As a result, it remains unknownshow to exploit the IRS to approach such a performance upper bound. To answerthis question, we aim to maximize the total harvested energy of a multiuser WETsystem subject to the user fairness constraints and the non-linear energyharvesting model. We first consider the static IRS beamforming scheme, whichshows that the optimal IRS reflection matrix obtained by applying semidefiniterelaxation is indeed of high rank in general as the number of energy receivers(ERs) increases, due to which the resulting rank-one solution by applyingGaussian Randomization may lead to significant loss. To achieve the multi-beamgain, we then propose a general time-division based novel framework byexploiting the IRS's dynamic passive beamforming. Moreover, it is able toachieve a good balance between the system performance and complexity bycontrolling the number of IRS shift patterns. Finally, we also propose atime-division multiple access (TDMA) based passive beamforming design forperformance comparison. Simulation results demonstrate the necessity ofmulti-beam transmission and the superiority of the proposed dynamic IRSbeamforming scheme over existing schemes.

Achieving Multi-beam Gain in Intelligent Reflecting Surface Assisted Wireless Energy Transfer