# Challenges in Physical Layer Security for Visible Light Communication Systems
## 3 Vulnerabilities of Physical Layer Security in VLC Systems
### Artificial jamming  
In reality, since an eavesdropper should escape the vigilance of a legitimate user or a network manager, it should strive for concealment while eavesdropping the transmission. However, since the channel gain in VLC systems largely depends on the distance between the transmitter and the receiver, the eavesdropper must be located in close proximity to the LED transmitter that serves the target user, thus risking being detected. Alternatively, the eavesdropper can augment its receiver sensitivity by, for example, increasing an area of its PD, adopting a high-gain optical lens, or directing its orientation toward the LED transmitter. These modifications can allow the eavesdropper to achieve a higher SNR even when located at a significant distance from the target user. For example, in [19], a telescope was used to increase the gain of the receiver in an experimental test, and the VLC transmission was successfully eavesdropped a large distance from the transmitter. On the other hand, to cope with an eavesdropper equipped with a better receiver than the intended user, various artificial jamming strategies for VLC systems were proposed [8; 9; 11; 20]. The jamming strategy was shown to be a powerful and practical approach to secure VLC systems, particularly against eavesdroppers with enhanced receiver architectures, since it would be impossible to distinguish between the information and jamming signals. On the other hand, two intrinsic properties of VLC systems enable eavesdroppers to overcome the jamming strategy. The first is that the multiple LED transmitters must be spatially distributed to illuminate the entire room evenly within the lighting standards, and the second is that the channel gain in VLC systems largely depends on the distance between the transmitter and the receiver. These two properties lead the jamming strategies to perform in a way that the LEDs near to the intended user emit the information, while the distant LEDs emit the jamming signals [11]. In other words, unlike RF systems, the eavesdropper can anticipate which LED performs as either an information transmitter or a jammer, and selectively receives only the information signal, excluding the jamming signals, by narrowing the receiver FoV and accurately aligning the receiver's orientation toward the target LED transmitter. Note that, in RF systems, multiple antennas being  
Figure 2: Secure outage probability (SOP) plotted as a function of distance between legitimate user and active eavesdropper for different densities \((\lambda_{E})\) of passive eavesdroppers. positioned at the same location simultaneously emit the information and jamming signals, which contrasts significantly with the VLC case. Figure 3 illustrates an example that an eavesdropper narrows its FoV and attempts to wiretap an information-bearing signal while minimizing the reception of jamming signals. In the figure, \(N_{\text{T}}\) LEDs are assumed to be uniformly distributed on a square lattice in a room of \(20\times 20\) m\({}^{2}\), and one legitimate user and one eavesdropper are supposed to be positioned in the region. The nearest LED to the legitimate user is chosen to transmit the information-bearing signal, while all the other LEDs act as jammers. Also, it is assumed that the transmitters know the CSI of both the intended and malicious users. According to [8], the jammers obtain the precoding matrix by designing an optimization problem that maximizes the jamming signal reception at the eavesdropper while aligning it to the null space of the legitimate user's channel. Then, the jammers multiply a random noise signal by the precoding matrix and transmit it to hinder information reception at the eavesdropper site. Figure 4 shows the results from numerical experiments that simulate the signal-to-interference-plus-noise ratio (SINR) for the scenario explained in Figure 3. Figure 4a shows the SINR as a function of the distance between the legitimate user and the eavesdropper for different eavesdropper FoVs (\(\Psi_{\text{E}}\)).