Beam Avoidance for Human Safety in Radiative Wireless Power Transfer

Prior studies show that exposure to radio-frequency electromagnetic field (RF-EMF) waves can increase the chance of developing health-related problems. Being exposed to even a small amount of RF-EMF wave emitted from a wireless power transfer (WPT) system for a long time may raise the risk of long-term health effects. Due to this potential human safety issue that may arise from the WPT system, in the future, people may be hesitant to use the RF-based WPT system to charge their electrical devices. To overcome such an issue, we provide an analytical study of a WPT system comprising a single power beacon and multiple target receivers with human bodies in close proximity. We employ a phased antenna array as the transmit and receive antennas to increase the energy transfer efficiency, and model the WPT system as a quadratically constrained quadratic optimization program (QCQP). In this work, we propose two adaptive beam-steering algorithms that maximize the received power on the target receivers while limiting the energy beam to the area where human bodies are detected. In the first proposed algorithm, we apply the semidefinite relaxation (SDR) method to approximate the QCQP solution by relaxing the rank-one constraint. The second proposed algorithm is an optimized adaptive beam-steering method developed by constructing the corresponding dual form of the QCQP problem and solving it through the hierarchical-iterative approach, in which we combine the eigenvalue decomposition and the projected subgradient method to obtain the optimal antenna array weight. Based on the analytical results, both proposed algorithms successfully generate the optimal antenna array weight that steers the main beam toward the target receivers while maintaining the RF-EMF radiation exposure toward the human body below the safety limit. Extensive simulation results are provided for verifying the validness of the proposed algorithms and comparing the performance of these two algorithms. Through testbed implementation, we have shown that the experimental results exhibit good agreement with the simulation results and confirmed the validity of the the proposed algorithm.