On the Secrecy Capacity of Hybrid FSO-mmWave Wiretap Channels

Hybrid Free-Space Optical (FSO) and millimeter Wave (mmWave) systems have emerged as a promising candidate for high data rate wireless transmissions due to the unique complementary properties against the different channel and environment conditions. Consequently, in this study, we investigate the hybrid FSO-mmWave systems from a physical-layer security point of view in the presence of different types of eavesdroppers, where the communication between two legitimate peers takes place over both FSO and RF links simultaneously. In particular, exponential atmospheric turbulence and Weibull fading channels are considered for FSO and mmWave links, respectively. We examine practical scenarios to eavesdrop the legitimate communication, and discuss the effects of random radio power of mmWave link and optical irradiance of FSO link on the probability of achieving a secure transmission. The impact of the fundamental physical layer parameters on the secrecy performance of the hybrid system is analyzed by obtaining analytical derivations of the probability of strictly positive secrecy capacity (SPSC) for different types of eavesdroppers, namely RF-, FSO- and Hybrid-Eve. In the light of results, we show that the analytical expressions are in exact agreement with the Monte-Carlo simulations.