On Secrecy Performance of Mixed THz-FSO Relaying System

The use of the Terahertz (THz) frequency band and free-space optical (FSO) communication represents a promising solution to address the spectrum scarcity challenges encountered in emerging applications related to 6G. In this direction, a mixed THz-FSO relaying system is proposed in this paper. The study takes into account the presence of fading effects and pointing errors affecting both THz and FSO links. The THz link is modeled using $\alpha - \mu$ distribution, while the FSO link is characterized using Malaga (M) distribution. The physical layer security (PLS) analysis is conducted, and secure outage probability is derived into closed-form for two scenarios: 1) THz eavesdropping attack and 2) FSO eavesdropping attack. The study further investigates the effect of THz and FSO links parameters on the secrecy performance in detail. The results demonstrate that security attacks on FSO links are more severe than those on THz links. Interestingly, improved THz channel conditions negatively affect the security of the proposed mixed THz-FSO system, whereas better FSO channel conditions provide enhanced secrecy.