Free-space optics (FSO)-based satellite communication systems have recently received considerable attention due to their enhanced capacity compared to their radio frequency (RF) counterparts. This paper analyzes the performance of physical layer security of space-to-ground intensity modulation/direct detection FSO satellite links under the effect of atmospheric loss, misalignment, cloud attenuation, and atmospheric turbulence-induced fading. Specifically, a wiretap channel consisting of a legitimate transmitter Alice (i.e., the satellite), a legitimate user Bob, and an eavesdropper Eve over turbulence channels modeled by the Fisher-Snedecor $\mathcal{F}$ distribution is considered. The secrecy performance in terms of the average secrecy capacity, secrecy outage probability, and strictly positive secrecy capacity are derived in closed-form. Simulation results reveal significant impacts of satellite altitude, zenith angle, and turbulence strength on the secrecy performance.

翻译：基于自由空间光学的卫星通信系统因其相较于射频系统具有更高的容量而近期受到广泛关注。本文分析了在受大气损耗、指向偏差、云层衰减及大气湍流致衰落效应影响下，采用强度调制/直接检测方式的空间-地面自由空间光学卫星链路的物理层安全性能。具体而言，考虑由合法发射端Alice（即卫星）、合法用户Bob及窃听者Eve构成的窃听信道模型，其中湍流信道服从Fisher-Snedecor $\mathcal{F}$分布。以闭式表达式推导了平均保密容量、保密中断概率及严格正保密容量等保密性能指标。仿真结果表明，卫星高度、天顶角及湍流强度对保密性能具有显著影响。