KEYNOTE: Sub-GHz Simultaneous Wireless Information and Power Transfer Using Distributed Antenna System

In 2022 Japan approved radiative wireless power transfer (WPT) in the sub-GHz band under a licensing regime. This technology is already being used in factory automation and building management systems and is helping to make IoT devices completely wireless. Transmit power is limited to 1 W in Japan, so received power drops below the milliwatt level at several meters, limiting the functionality of batteryless receivers. To address this, we are developing techniques to maximize available power using distributed antennas and to collect sensor data across a wide bandwidth using SWIPT (Simultaneous Wireless Information and Power Transfer). This approach increases received power in a manner similar to beamforming by coherently transmitting at precisely matched frequencies from spatially distributed antennas separated by several wavelengths or more. With N antennas, total transmit power scales by N and, under perfect coherent combining, received power can increase by up to N². Such systems require precise frequency synchronization among spatially separated transmitters and tight timing alignment of modulated signals; distributing high-frequency local frequency, or noise-sensitive reference signals such as 10 MHz sine waves, over distance degrades synchronization accuracy. In this work, we propose a novel frequency-locked loop (FLL) system to lock the frequency of a sub-GHz carrier and maintain coherence across distributed transmitters.