RIS-Aided Mixed RF-FSO Wireless Networks: Secrecy Performance Analysis With Simultaneous Eavesdropping

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Abstract
In order to meet the demands of diverse services within sixth-generation networks across a range of industries, numerous approaches are being employed to address notable signal degradation resulting from channel obstruction, particularly in the realms of millimeter wave and sub-THz frequencies. One of these solutions is the utilization of reconfigurable intelligent surfaces (RISs), which can reflect or refract signals in the desired direction. This integration offers significant potential to improve the coverage area from a transmitter to a receiver. In this paper, we present a comprehensive framework for analyzing the secrecy performance of an RIS-aided mixed radio frequency (RF)-free space optics (FSO) system employing in terms of physical layer security (PLS). It is assumed that a secure message is transmitted from a RF transmitter to a FSO receiver through an intermediate relay. The RF link experiences Rician fading while the FSO link experiences Málaga distributed turbulence with pointing errors. Three different eavesdropping scenarios are examined: 1) RF-link eavesdropping, 2) FSO-link eavesdropping, and 3) simultaneous eavesdropping attack on both RF and FSO links. We evaluate the secrecy performance using analytical expressions to compute secrecy metrics such as the average secrecy capacity, secrecy outage probability, probability of strictly positive secrecy capacity, effective secrecy throughput, and intercept probability. Our results are confirmed via Monte-Carlo simulations and demonstrate that fading parameters, atmospheric turbulence conditions, pointing errors, and detection techniques play a crucial role in enhancing secrecy performance.

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