Secure Wireless Information and Power Transfer in Heterogeneous Networks

In this paper, we investigate secure simultaneous wireless information and power transfer (SWIPT) in a two-tier downlink heterogeneous network (HetNet), wherein the ambient interference signals are exploited for both secure communications and wireless energy harvesting. We assume one macrocell base station (MBS) and several femtocell base stations (FBSs) simultaneously send information to their macrocell users (MUs) and femtocell users, respectively. Meanwhile, the FBSs also transfer energy to some energy receivers, who act as the potential eavesdroppers and are able to wiretap the confidential messages to one MU via the cross-tier interference links. Exploiting interference in the considered HetNet, we jointly optimize the beamforming vectors and artificial noise of the MBS and the FBSs to maximize the secrecy rate of the eavesdropped MU under the quality-of-service, energy harvesting, and transmit power constraints at relevant receivers/transmitters. In particular, we first investigate the ideal case with perfect eavesdropper's channel state information (ECSI) and the optimization problem turns out to be nonconvex. By using the tools of semidefinite relaxation (SDR) and one-dimensional line search, we successfully transfer the original problem into a more tractable two-stage problem to obtain the optimal solution. Furthermore, we extend our study to the imperfect ECSI case, where the worst-case based solution is obtained with the aid of SDR and successive convex approximation. Simulation results demonstrate the effectiveness of the proposed algorithms and also bring useful insights into the design of secure SWIPT in the presence of interference.