# Challenges in Physical Layer Security for Visible Light Communication Systems
## 3 Vulnerabilities of Physical Layer Security in VLC Systems
### Artificial jamming
We note that the eavesdropper with a narrow FoV can effectively exclude the jamming signals and achieve a higher SINR than the legitimate user when the eavesdropper is located not too far from the legitimate user. According to the VLC channel gain model [16], a receiver equipped with an optical concentration lens with a narrower FoV would yield a better receiver sensitivity. Therefore, the eavesdropper can achieve a higher SINR than the legitimate user to exclude the jamming signal even at a further distance. Also, Figure 4a shows that as the FoV becomes narrower, the eavesdropper can exclude the jamming signal at a further distance. For example, the eavesdropper with \(\Psi_{\text{E}}=15^{\circ}\) should be within 1.4 m of the legitimate user to exclude the effects of the jamming signal. But, an eavesdropper with a narrower FoV, e.g., \(\Psi_{\text{E}}=5^{\circ}\), can wiretap the transmission as far as 5.4 m from the legitimate user. In contrast, when the eavesdropper fails to exclude the jamming signals due to the extended sight of its PD, the eavesdropper's SINR suddenly drops. Figure 3: An example of VLC systems with a jamming strategy, where electron device (ED) is equipped with a narrow field-of-view (FoV) photodiode (PD), oriented directly toward information LED transmitter. On the other hand, Figure 4b gives an insight into how one might cope with an eavesdropper with a narrow FoV. The figure shows the SINR for the different numbers of transmitters as a function of the distance between the legitimate user and the eavesdropper. Note that, to maintain a constant level of illumination (even with different numbers of LEDs), we fix the total optical power emitted from the LED array by reducing or increasing the optical power of each LED for the different sets of results. As the number of LEDs increases within a fixed geometry, the distances among the LEDs decrease, which allows the information transmitter and the jammer to be located closer to each other. Therefore, it would be more difficult for the eavesdropper to exclude the jamming signal by using its narrow FoV receiver. More specifically, as shown in the figure, when \(N_{\text{T}}=7\times 7\) and the nearest distance between the information transmitter and the jammers is 2.86 m, the eavesdropper with \(\Psi_{\text{E}}=10^{\circ}\) can wiretap the information as long as it is  
Figure 4: SINR according to distance between legitimate user and eavesdropper, who are located at center of room and \((d_{\text{E}},d_{\text{E}})\), respectively. \(N_{\text{T}}\) LED transmitters are assumed to be uniformly distributed on a square lattice in room of \(20\times 20\) m\({}^{2}\). FoV of eavesdropper is denoted by \(\Psi_{\text{E}}\). \(N_{\text{T}}=10\times 10\) is used for (**a**), and \(\Psi_{\text{E}}=10^{\circ}\) is used for (**b**). positioned within 4.8 m of the legitimate user; yet, when the LED array size is increased to \(N_{\text{T}}=15\times 15\), the eavesdropper must be positioned within 3.4 m of the legitimate user to intercept the signal (i.e., to achieve a higher SINR). The nonmonotonic results for the long distances between the legitimate user and the eavesdropper are due to the design of the jamming precoding strategy, which is optimized such that the eavesdropper that has the same receiver properties as the legitimate user experiences a high degree of jamming. However, for an eavesdropper with different receiver properties--i.e., narrow FoV aligned with the information LED--jamming is not optimized and instead depends upon geometric factors related to the eavesdropper position relative to the neighboring LEDs.