# CSI-Free Rotary Antenna Beamforming for Massive RF Wireless Energy Transfer

Radio-frequency (RF) wireless energy transfer (WET) is a key technology that may allow seamlessly powering future massive low-energy Internet of Things (IoT) networks. To enable efficient massive WET channel state information (CSI)-limited/free multiantenna transmit schemes have been recently proposed in the literature. The idea is to reduce/null the energy costs to be paid by energy harvesting (EH) IoT nodes from participating in large-scale time/power-consuming CSI training but still enable some transmit spatial gains. In this article we take another step forward by proposing a novel CSI-free rotary antenna beamforming (RAB) WET scheme that outperforms all state-of-the-art CSI-free schemes in a scenario where a power beacon (PB) equipped with a uniform linear array (ULA) powers a large set of surrounding EH IoT devices. RAB uses a properly designed CSI-free beamformer combined with a continuous or periodic rotation of the ULA at the PB to provide average EH gains that scale as $$0.85\sqrt{M}$$ where $$M$$ is the number of PB’s antenna elements. Moreover a rotation-specific power control mechanism was proposed to: 1) fairly optimize the WET process if devices’ positioning information is available and/or 2) avoid hazards to human health in terms of specific absorption rate (SAR) which is an RF exposure metric that quantifies the absorbed power in a unit mass of human tissue. We show that RAB performance even approaches quickly (or surpasses for scenarios with a sufficiently large number of EH devices or when using the proposed power control) the performance of a traditional full-CSI-based transmit scheme and it is also less sensitive to SAR constraints. Finally we discuss important practicalities related to RAB such as its robustness against non line-of-sight (LOS) conditions compared to other CSI-free WET schemes and its generalizability to scenarios where the PB uses other than a ULA topology.

López Onel L. A., Alves Hirley, Montejo-Sánchez Samuel, Souza Richard Demo, Latva-aho Matti

A1 Journal article – refereed

O. L. A. López, H. Alves, S. Montejo-Sánchez, R. D. Souza and M. Latva-aho, "CSI-Free Rotary Antenna Beamforming for Massive RF Wireless Energy Transfer," in IEEE Internet of Things Journal, vol. 9, no. 10, pp. 7375-7387, 15 May15, 2022, doi: 10.1109/JIOT.2021.3107222

http://dx.doi.org/10.1109/jiot.2021.3107222 http://urn.fi/urn:nbn:fi-fe2022082656395