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Secure Energy Efficiency in SWIPT Multicarrier Relay Transmission with Direct Links

In this paper, we investigate the energy efficiency maximization for secure simultaneous wireless information and power transfer (SWIPT) relay networks in the presence of direct links between the source and the destination, where an eavesdropper can overhear the information from the source and the relay. Two cases for the decode-and-forward relay using time switching (TS) and power splitting (PS) scheme for information transmission and energy harvesting are studied, separately. A practical non-linear energy harvesting (EH) model is adopted. The energy efficiency maximization problem based on security rate is quite difficult to solve. We first analyze the property of the problem, then transform it to a difference of convex (D.C.) optimization problem, and further propose an iterative-based algorithm to solve the D.C. problem efficiently. Simulation results verify the convergence of the proposed algorithm and compare the performance of the TS and PS schemes in terms of secure energy efficiency under different conditions.