Though visible light communication (VLC) systems are contained to a given room, improving their security is an important criterion in any practical deployment. In this paper, the design of artificial noise (AN) to enhance physical layer security in VLC systems is studied in the context of input signals with no explicit amplitude constraint (e.g., multicarrier systems). In such systems, clipping is needed to constrain the input signals within the limited linear ranges of the LEDs. However, this clipping process gives rise to non-linear clipping distortion, which must be incorporated into the AN design. To facilitate the solution of this problem, a sub-optimal design approach is presented using the Charnes-Cooper transformation and the convex-concave procedure (CCP). Then, a novel AN transmission scheme is proposed to reduce the impact of clipping distortion, thus improving the secrecy performance. The proposed scheme exploits the common structure of LED luminaries that they are often composed of several light-emitting chips. Capitalizing on this property, LED chips in each luminaire are divided into two groups driven by separate driver circuits. One group is used to transmit the information-bearing signal, while the other group transmits the AN. Numerical results show that the clipping distortion significantly reduces the secrecy level, and using AN is advantageous over the no-AN scheme in improving the secrecy performance. Moreover, the proposed AN transmission scheme is shown to achieve considerable secrecy improvements compared with the traditional transmission approaches (e.g., about 1 bit/s/Hz improvement in the achievable secrecy rate when the standard deviation of the LEDs' modulating current is 0.25 A and the signal-to-interference-plus-noise ratio of the eavesdropper's received signal is limited to $0$ dB).

翻译：尽管可见光通信（VLC）系统被限制在特定房间内，但在实际部署中提升其安全性仍是一项重要指标。本文研究了在输入信号无显式幅度约束（如多载波系统）的背景下，增强VLC系统物理层安全的人工噪声（AN）设计问题。在此类系统中，需通过削波将输入信号约束在发光二极管（LED）有限的线性范围内。然而，削波过程会产生非线性削波失真，该失真必须纳入AN设计。为求解该问题，本文提出一种基于Charnes-Cooper变换和凸凹过程（CCP）的次优设计方法。进而提出一种新型AN传输方案以降低削波失真影响，从而提升保密性能。该方案利用LED灯具由多个发光芯片组成的常见结构特性，将每个灯具的LED芯片分为两组并由独立驱动电路控制：一组传输信息承载信号，另一组传输AN。数值结果表明，削波失真会显著降低保密水平，而采用AN方案在提升保密性能方面优于无AN方案。此外，与传统传输方法相比，所提AN传输方案可实现显著的保密增益（例如，当LED调制电流标准差为0.25 A且窃听者接收信号的信干噪比限制为$0$ dB时，可达保密速率提升约1 bit/s/Hz）。