Hybrid terrestrial-satellite (HTS) communication systems have gained a tremendous amount of interest recently due to the high demand for global high data rates. Conventional satellite communications operate in the conventional Ku (12 GHz) and Ka (26.5–40 GHz) radio-frequency bands for assessing the feeder link, between the ground gateway and the satellite. Nevertheless, with the aim to provide hundreds of Mbps of throughput per each user, free-space optical (FSO) feeder links have been proposed to fulfill these high data rates requirements. In this paper, we investigate the physical layer security performance for a hybrid very high throughput satellite communication system with an FSO feeder link. In particular, the satellite receives the incoming optical wave from an appropriate optical ground station, carrying the data symbols of $N$ users through various optical apertures and combines them using the selection combining technique. Henceforth, the decoded and regenerated information signals of the $N$ users are zero-forcing (ZF) precoded in order to cancel the interbeam interference at the end-users. The communication is performed under the presence of malicious eavesdroppers nodes at both hops. Statistical properties of the signal-to-noise ratio of the legitimate and wiretap links at each hop are derived, based on which the intercept probability metric is evaluated. The derived results show that above a certain number of optical apertures, the secrecy level is not improved further. Also, the system’s secrecy is improved using ZF precoding compared to the no-precoding scenario for some specific nodes’ positions. All the derived analytical expressions are validated through Monte Carlo simulations.

中文翻译：

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具有光馈线的混合高吞吐量卫星的PHY层安全性分析

由于对全球高数据速率的高需求，混合地面卫星（HTS）通信系统近来引起了极大的兴趣。常规卫星通信在常规Ku（12 GHz）和Ka（26.5–40 GHz）无线电频带中运行，以评估地面网关与卫星之间的馈线链路。然而，为了给每个用户提供数百Mbps的吞吐量，已经提出了自由空间光（FSO）馈线链路来满足这些高数据速率要求。在本文中，我们研究了具有FSO馈线链路的混合超高吞吐量卫星通信系统的物理层安全性能。特别是，卫星从适当的光学地面站接收传入的光波，并携带以下数据符号： $ N $ 用户通过各种光学孔径并使用选择合并技术将它们合并。从此以后，解码和再生的信息信号 $ N $ 为了消除最终用户的光束间干扰，对用户进行了零强制（ZF）预编码。在两个跃点上都存在恶意窃听者节点的情况下执行通信。得出每个跃点上合法链路和窃听链路的信噪比的统计属性，并据此评估拦截概率度量。得出的结果表明，在一定数量的光学孔径以上，保密水平没有进一步提高。而且，与某些特定节点位置的非预编码方案相比，使用ZF预编码可改善系统的保密性。通过蒙特卡洛模拟对所有推导的解析表达式进行验证。