Secrecy Energy Efficiency in SWIPT Networks with Two-Layer Power-Splitting Receiver

This paper investigates the energy efficiency (EE) for a MISO simultaneous wireless information and power transfer (SWIPT) system where one transmitter intends to transmit information and energy to its legitimate power-splitting (PS) receivers while restricting the information leakage to an eavesdropper (Eve). To guarantee secure transmission, artificial noise (AN) is embedded into the transmit signals. The non-linear energy harvest (EH) model is adopted and a novel two-layer PS receiver architecture is proposed. Our goal is to maximize the system global secrecy EE (GSEE) by optimizing the transmit beamforming vectors, the AN covariance matrix and two-layer PS ratios under the EH requirement and the total available power constraints. To solve the non-convex optimization problem, the second-layer PS ratios are firstly optimized and then, an iterative solution framework is presented to jointly optimize the transmit beamforming vectors, the AN covariance matrix and the first-layer PS ratios. Simulation results demonstrate the validity and efficiency of our proposed solution approach. It is shown that the system GSEE has a saturation point and after passing the saturation point, it cannot be improved by increasing the transmit power anymore. It is also observed that compared with traditional PS receiver architecture, the proposed two-layer PS receiver architecture is able to achieve higher EH efficiency.