On the Physical Layer Security of a Decode and Forward Based Mixed FSO/RF Co-Operative System

In this letter, the secrecy performance of a decode-and-forward (DF) principle based mixed free space optics (FSO) and radio frequency (RF) system is analyzed. The analysis is done from physical layer (PHY) security perspective. Herein, two different eavesdroppers are considered while signal is transmitting from source (S) to destination (D) through a relay (R) point. The FSO based and RF based eavesdroppers are trying to intercept the confidential signal from S-R link and R-D links respectively. The faded FSO links are modeled by Málaga (M) distribution and RF links are characterized by Nakagami-m distribution. Exact closed form expression for secrecy performance metric such as secrecy outage probability is derived and analyzed for the aforementioned system in terms of Meijer's G-function and bivariate Fox's H-function. Furthermore, the asymptotic expressions for these performance metrics are derived by employing high signal to noise ratio (SNR) approximation. Afterwards the results are verified by Monte-Carlo simulations. Finally, the secrecy threshold for the system is derived to get insight of the system reliability and secrecy.