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）被认为对局限于光传播空间外的攻击者具有安全性，但仍易受到覆盖区域内窃听的威胁。近年来引入的智能反射表面（IRS）技术为增强物理层安全（PLS）提供了新途径。现有关于IRS辅助VLC的研究大多假设所有反射单元的到达时间相同，忽略了时延效应及其引发的符号间干扰。本文首次探讨了时延对VLC系统保密速率的影响。结果表明：在发光二极管（LED）功率固定为3W时，当窃听者位于LED半径1米范围内，合法用户在随机位置处的保密速率最高可提升253%。此外，即使窃听者处于更有利位置（即比合法用户具有更优信道增益），通过合理分配IRS单元仍能增强PLS。