Wireless Power Transfer (WPT) is expected to be a technology reshaping thelandscape of low-power applications such as the Internet of Things, RadioFrequency identification (RFID) networks, etc. Although there has been someprogress towards multi-antenna multi-sine WPT design, the large-scale design ofWPT, reminiscent of massive MIMO in communications, remains an open challenge.In this paper, we derive efficient multiuser algorithms based on ageneralizable optimization framework, in order to design transmit sinewavesthat maximize the weighted-sum/minimum rectenna output DC voltage. The studyhighlights the significant effect of the nonlinearity introduced by therectification process on the design of waveforms in multiuser systems.Interestingly, in the single-user case, the optimal spatial domain beamforming,obtained prior to the frequency domain power allocation optimization, turns outto be Maximum Ratio Transmission (MRT). In contrast, in the general weightedsum criterion maximization problem, the spatial domain beamforming optimizationand the frequency domain power allocation optimization are coupled. Assumingchannel hardening, low-complexity algorithms are proposed based on asymptoticanalysis, to maximize the two criteria. The structure of the asymptoticallyoptimal spatial domain precoder can be found prior to the optimization. Theperformance of the proposed algorithms is evaluated. Numerical results confirmthe inefficiency of the linear model-based design for the single and multi-userscenarios. It is also shown that as nonlinear model-based designs, the proposedalgorithms can benefit from an increasing number of sinewaves.
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