Indoor visible light communication (VLC) is considered secure against attackers outside the confined area where the light propagates, but it is still susceptible to interception from inside the coverage area. A new technology, intelligent reflecting surfaces (IRS), has been recently introduced, offering a way to enhance physical layer security (PLS). Most research on IRS-assisted VLC assumes the same time of arrival from all reflecting elements and overlooks the effect of time delay and the associated intersymbol interference. This paper tackles, for the first time, the effect of time delay on the secrecy rate in VLC systems. Our results show that, at a fixed light-emitting diode (LED) power of 3W, the secrecy rate can be enhanced by up to 253\% at random positions for the legitimate user when the eavesdropper is located within a 1-meter radius of the LED. Our results also show that careful allocation of the IRS elements can lead to enhanced PLS even when the eavesdropper has a more favourable position and, thus, a better channel gain than the legitimate user.

翻译：室内可见光通信（VLC）在光传播受限区域之外被认为具有安全性，但仍易受到覆盖区域内窃听者的截获。近年来引入的智能反射表面（IRS）新技术为增强物理层安全（PLS）提供了新途径。当前大多数针对IRS辅助VLC的研究假设所有反射单元的信号到达时间相同，忽略了时间延迟及其引发的码间串扰效应。本文首次探讨了时间延迟对VLC系统保密速率的影响。结果表明，在3W固定发光二极管（LED）功率下，当窃听者位于LED周围1米半径内时，合法用户在随机位置上的保密速率最高可提升253%。研究还揭示，即使窃听者占据更有利位置、获得比合法用户更优的信道增益时，通过合理分配IRS单元仍可增强PLS。