Energy-Preserving Indirect-Feedback for Wireless Power Transfer: Algorithm and Hardware Architecture

Recognising the limitations of existing channel-estimation schemes for energy beamforming, this paper proposes an energy-preserving, indirect-feedback based approach to determine the optimal energy beamforming vector, specially designed for battery-free RF energy harvesting devices. Upon elaborating on the key ideas behind the proposed approach—dynamics of the harvest-then-transmit protocol, non-linearity of harvesting efficiency and the latency associated with the charging process—we present an algorithm and its hardware architecture to concretise the proposed approach. The developed algorithm is supplemented by mathematical analysis and numerical simulations. Furthermore, the designed hardware architecture has been post-route implemented on an FPGA platform, and ASIC post-layout simulated in 90 nm-CMOS technology node. For the configuration in the numerical results section, a low-power sensor node operating at the milliwatt level can save approximately 1 mJ of energy by adopting the proposed scheme, enabling the transmission of 5 additional packets. Moreover, the saved energy—and consequently, the number of additional packets—scales linearly with the number of transmit antennas.