Secrecy performance for a multi-relaying SIMO-satellite/MISO-FSO network

In this paper, physical-layer security performance for a multi-relaying based satellite-free-space optical network, which consists of a single-antenna satellite source (S), M (M >= 1 ) relays (R) with each equipping with L (L >= 1 ) receiving antennas and a photo-aperture transmitter with N (N >= 1 ) lasers, a single-aperture destination (D) and a single-aperture eavesdropper (E), is evaluated. Specifically, a relay, which first adopts a maximum ratio combining protocol to process the received signals from S and then transforms the decoded signal into an optical one before transmitting it to D with a transmit aperture selection scheme, is selected based on a general K-th best relay selection criteria to assist the information forwarding. Additionally, the probability density functions and cumulative distribution functions of the instantaneous signal-to-noise ratio at R and D are formatted, serving as the foundation for deriving both analytical and asymptotic expressions of the lower bound of secrecy outage probability. Finally, the accuracy of those derived expressions is validated through Monte-Carlo simulations.