Buffer-Aided Secure Relay Networks With SWIPT

In this paper, we propose the use of data and energy buffering in wireless powered relays, while we also design scheduling schemes, with the aim to increase the secrecy throughput. More specifically, it is assumed that Alice transmits information to Bob by using half-duplex randomize-and-forward relaying with simultaneous wireless information and power transfer, under the existence of a passive eavesdropper. First, considering a conventional system, i.e., without energy and data buffering, an optimal baseline policy is designed. Then, focusing on buffer-aided relaying, we maximize the average secrecy throughput under the stability requirement for data and energy queues by using the framework of Lyapunov optimization, according to which, we transform the initial stochastic optimization problem into a sequence of online subproblems. The proposed online adaptive transmission policy dynamically allocates the available resources, i.e., time and power, with respect to the channel state information, as well as the states of the buffers. In addition, our analysis reveals the existence of an interesting trade-off between the average secrecy throughput and the average queue delay. Simulation results validate the analysis and demonstrate that the proposed buffer-aided scheme significantly improves the average secrecy throughput compared to the baseline one.