A Comparison of Secrecy Enhancing Techniques for Secure SWIPT System

Owing to the tendency of RF-powered wireless devices to be located closer to the RF sources in SWIPT systems, the secrecy performance of RF transmissions needs to be carefully studied. In this paper, we study the secrecy performance of a wireless network that is simultaneously transmitting information and power to information receivers (IRs) and energy receivers (ERs), respectively. Considering the ERs as potential eavesdroppers, we provide a generalized model that captures three popular secrecy enhancing techniques: (i) adding artificial noise (AN) to the transmitted signal, (ii) surrounding the information transmitters (ITs) with secrecy guard zones, and (iii) using directional beamforming towards the intended IRs. Assuming an interference-limited system, we use the proposed generalized model to derive the successful connection and the secrecy outage probabilities of the IR (P-con and P-out, respectively) and the energy coverage probability (P-energy) of the ER for each of the three techniques. We then study the influence of the guard zone radius, the AN power splitting ratio, and the beamwidth on these performance metrics. We focus on two parameter selection criteria: (i) minimizing Pout for given constraints on P-con and P-energy, and (ii) maximizing P-con for given constraints on P-out and P-energy. At lower densities of ERs, the results demonstrate the optimality of the GZ technique for the first selection criterion and the BF technique for the second selection criterion. At higher densities of ERs, however, the AN technique is shown to be optimal for both the selection criteria.