Impact of Correlation and Pointing Error on Secure Outage Performance Over Arbitrary Correlated Nakagami-$m$ and $\mathcal {M}$-Turbulent Fading Mixed RF-FSO Channel

For a secure communication over Radio Frequency-Free Space Optical (RF-FSO) mixed link, a significant enhancement of secrecy capacity (SC) can be achieved via exploiting spatial diversity at the RF path. But besides atmospheric turbulence and pointing error of FSO link, practical multi-antenna systems experience correlation between the antennas at the RF link that affects the SC, remarkably. Secrecy performance analysis of such a network is analyzed in this paper wherein the system consists of four entities: source, relay, destination, and eavesdropper. The RF hop (source-to-relay) and the FSO hop (relay-to-destination) is modeled utilizing arbitrarily correlated Nakagami-m and Malaga (M) distributions, respectively. The correlated signal branches of the RF hop are combined at the relay exploiting equal gain combining reception technique. We assume that the eavesdropper is capable of wiretapping via RF and FSO links separately. We derive the novel closed-form expressions for SOP and SPSC considering heterodyne detection (HD) and intensity modulation with direct detection (IM/DD) technique in order to examine the impact of atmospheric scintillation, pointing error, fading, and correlation on the system's secrecy performance. It is worthy of note that HD technique exhibits a better performance than IM/DD technique. In addition, similar to the pointing error and turbulent fading, the correlation imposes a detrimental impact on SC. Finally, Monte-Carlo simulation is provided for validation of the derived expressions.