Energy and Pilot Efficient Beamforming for LoS Wireless Power Transfer

Beamforming is a highly effective technique for enhancing wireless power transfer efficiency by focusing transmitted power on receiver nodes. To eliminate the need for a local oscillator in each sensor for uplink pilot transmission and per-node channel estimation, we examine three power beamsteering techniques: beamsharing, where nearby targets share a single uplink pilot; beamtracking, which uses a dedicated pilot per target; and beamfilling, which covers the entire device plane. This approach reduces the energy consumption of the nodes and minimizes the number of uplink pilots. We assess the trade-off between array gain and pilot cost in a scenario with multiple power-receiving nodes, deriving analytical expressions for the received power spot profile based on physical distance. Our findings indicate that beamsharing with a collocated array is more effective than a distributed transmitter array, as the former provides a larger power spot size, thereby serving more sensors and reducing the number of required pilots, resulting in increased energy efficiency. Our analysis further shows that beamfilling does not extend the sensor region that can be wirelessly powered by the transmitter. Additionally, we derive the optimal pilot transmission periodicity for beamtracking to a mobile target, demonstrating that less frequent and coarser beamforming yields the highest dynamic wireless power transfer efficiency. This implies that more sensors can be powered without the need for all to transmit a pilot. Finally, based on the derived power spot size and beam profile, an opportunistic strategy is proposed for sensors to decide whether to transmit pilots autonomously.