Large-Scale Multiantenna Multisine Wireless Power Transfer

Wireless Power Transfer (WPT) is expected to be a technology reshaping the landscape of low-power applications such as the Internet of Things. Multi-antenna multi-sine waveforms adaptive to the channel state information have been shown to be a promising building block of WPT. However, the current design is computationally too complex to be applied to large-scale WPT, where signals are sent across a large number (tens) of antennas and frequencies. This paper proposes efficient single/multi-user algorithms based on a generalizable optimization framework, in order to design transmit waveforms that maximize the weighted-sum/minimum rectenna output DC voltage. The study highlights the significant effect of the nonlinearity of the rectenna on the design of waveforms in single/multi-user systems. Interestingly, in the single-user case, the optimal spatial beamforming, obtained before optimizing power allocation across frequencies, turns out to be maximum ratio transmission. Nevertheless, in the general multi-user weighted-sum criterion maximization problem, optimizations in the frequency and the spatial domains are coupled. Assuming channel hardening, we also propose low-complexity algorithms based on asymptotic analysis, where the structure of the asymptotically optimal spatial beamforming can be found before optimizations. Simulation results confirm the inefficiency of the linear model-based design in single/multi-user systems. Moreover, simulation results show that the proposed algorithms (based on the nonlinear model) can benefit from an increasing number of sinewaves at a computational cost much lower than the existing method. Simulation results also indicate that the large-scale WPT architecture can boost the end-to-end power transfer efficiency and the transmission range.