Average Harvested Power in THz WPT Systems Employing Resonant-Tunnelling Diodes

In this paper, we study terahertz (THz) wireless power transfer (WPT) systems, where a single-antenna transmitter sends an energy signal to a single-antenna receiver. Since Schottky diodes, which are utilized in GHz energy harvesting (EH) circuits, are not usable for THz EH, we adopt an EH circuit employing a resonant tunnelling diode (RTD) at the receiver. However, the electrical properties of Schottky diodes and RTDs are different, and unlike EH receivers based on a single Schottky diode, an accurate EH model for RTD-based EH receivers is not available. Therefore, we model the dependency between the instantaneous harvested power at the EH device and the instantaneous received power by a piecewise non-linear function, whose parameters are adjusted to fit circuit simulation data. We formulate an optimization problem for the maximization of the average power harvested at the EH receiver subject to constraints imposed on the average and peak transmit powers. Then, for the solution of the formulated problem, we propose an equivalent monotonic EH model and we show that the optimal transmit signal distribution for WPT employs at most two mass points, which can be obtained as the solution of a min-max optimization problem. Our simulation results reveal that RTDs with low leakage currents and high breakdown voltages are preferable for WPT when the received signal power is low and high, respectively. Furthermore, we demonstrate that the proposed optimal input distribution yields significantly higher average harvested powers compared to a baseline scheme employing Gaussian transmit signals.