Magnetic Blind Beamforming for Battery-Free Wireless Sensor Networks

It is challenging to maintain a large number of wireless sensors in inaccessible media, such as underground and intra-body. Tiny battery-free sensors are desirable since they create negligible impacts on the environment and do not require battery replacement. Battery-free sensors can harvest energy from wireless signals to support their normal operation. However, they cannot cooperate with the charger due to the lack of energy in the initial stages. This paper proposes the Magnetic Blind Beamforming (MagBB) using tri-axis coils to power up battery-free wireless sensors. Battery-free sensors with randomly orientated coils may experience significant misalignment losses and they may not receive any energy from the charger. MagBB uses a set of optimized current vectors to generate rotating magnetic fields which can ensure that coils with arbitrary orientations can receive sufficient voltages for charging. It does not require any information about the battery-free sensor’s coil orientation or location. A proof-of-concept prototype is developed. The numerical simulations and experimental results show that MagBB can efficiently improve the reliability of wireless charging. Sensor coils with arbitrary orientation and location within the operation range can harvest sufficient energy. This technology has the potential of powering ultra-dense wireless sensors with low communication overhead.