For visible light communication (VLC), the light signals are transmitted without optical fibers or any sort of wave-guiding. Due to the inherent broadcast nature, physical-layer security emerges as a promising method to protect information delivery from eavesdropping. As for the secrecy capacity of VLC channel, there exist two features. In one way, the limited optical power makes the common capacity expressions in radio-frequency (RF) communication unapplicable for VLC. In another way, several correlated geometrical parameters directly alters the Lambertian model of indoor VLC channel, which gives the secrecy capacity more meanings. However, the issue considering both aspects has not been studied recently. In this paper, from the practical scenarios, we extract a typical geometrical model to reveal the mobility principles of the legitimate receiver and the eavesdroppers. Then, we character two typical distributions of the geometrical parameter. Correspondingly, we derive the upper and lower bounds on the average secrecy capacity, which have the closed forms. Finally, simulation results show that our upper and lower bounds are tight at high optical signal-to-noise rates (OSNRs). Moreover, the geometrical features of VLC systems and distribution parameters of the receiver mobility are effectively reveal by the bounds.

中文翻译：

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室内可见光通信系统的平均保密容量

对于可见光通信（VLC），光信号的传输无需光纤或任何类型的波导。由于固有的广播性质，物理层安全性作为一种有希望的方法来保护信息传递免遭窃听。关于VLC信道的保密容量，存在两个特征。在一种方式中，有限的光功率使得射频（RF）通信中的通用容量表达式不适用于VLC。换句话说，几个相关的几何参数直接改变了室内VLC通道的Lambertian模型，这为保密能力提供了更多的含义。然而，最近尚未研究考虑到这两个方面的问题。本文从实际情况出发，我们提取了一个典型的几何模型，以揭示合法接收者和窃听者的移动性原理。然后，我们描述了几何参数的两个典型分布。相应地，我们推导了具有保密形式的平均保密容量的上限和下限。最后，仿真结果表明，在高光学信噪比（OSNR）时，我们的上限和下限是紧密的。此外，边界有效地揭示了VLC系统的几何特征和接收机移动性的分布参数。仿真结果表明，在高光学信噪比（OSNR）时，我们的上限和下限是紧密的。此外，边界有效地揭示了VLC系统的几何特征和接收机移动性的分布参数。仿真结果表明，在高光学信噪比（OSNR）时，我们的上限和下限是紧密的。此外，边界有效地揭示了VLC系统的几何特征和接收机移动性的分布参数。