  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 multi-antenna 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 paper, 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
i) fairly optimize the WET process if devices' positioning information is
available, and/or ii) to avoid hazards to human health in terms of specific
absorption rate (SAR). We show that RAB performance even approaches quickly (or
surpasses, for scenarios with 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 conditions compared to other CSI-free WET
schemes, and its generalizability to scenarios where the PB uses other than a
ULA topology.
