# Challenges in Physical Layer Security for Visible Light Communication Systems
## 3 Vulnerabilities of Physical Layer Security in VLC Systems
### Cooperative Eavesdroppers in Multiuser VLC Systems  
VLC offers the option of dense spatial reuse due to the fact that visible light cannot penetrate an opaque wall and decays quickly with distance. These features enable VLC systems to serve many users simultaneously with high data rates and short time delays. Thus, various PLS transmission techniques for multiuser VLC systems were proposed [21, 22]. These works have shown that the PLS can effectively secure VLC transmissions in the presence of either active or passive eavesdroppers under certain conditions. However, in multiuser VLC systems in large open spaces, it would be challenging to distinguish between legitimate and malicious users and would also be impractical to impose on the number and type of eavesdroppers. Moreover, in a public area, it may be too lenient to issue access authority to network users. These properties of multiuser VLC systems lead us to expect an eavesdropping scenario that a malicious user can legitimately access the network and cooperate with other passive (not registered) eavesdroppers to wiretap the information-bearing signal more efficiently. Here, we will look into such a scenario whereby registered and passive eavesdroppers cooperate. Figure 5 shows an example of the VLC system in question, in which registered and passive eavesdroppers are present. Since the nearest LED is typically assumed to serve the mobile VLC user to maximize the rate of the communication link [10], it is assumed here that LED cooperation does not take place. Thus, registered users, including the legitimate user and the registered eavesdropper, would be served by their nearest LEDs, i.e., \(\text{LED}_{\text{U}}\) and \(\text{LED}_{\text{E}}\), respectively. Also, a few passive eavesdroppers are assumed to be distributed according to a PPP with density \(\lambda_{E}\) around \(\text{LED}_{\text{U}}\) to wiretap the transmission from \(\text{LED}_{\text{U}}\) to the legitimate user. The registered and passive eavesdroppers can cooperate in the following way. Before and while \(\text{LED}_{\text{U}}\) communicates to the legitimate user employing a wiretap coding scheme, the registered eavesdropper induces its serving transmitter \(\text{LED}_{\text{E}}\) to emit a _promised_ signal. Note that this is possible since the registered eavesdropper has the legitimate right to access the network. The promised signal, which is a waveform modulated from any data bits, e.g., a string of random binary numbers, is already disclosed to other passive eavesdroppers and saved in a remote server/node to be requested by the registered eavesdropper. At the same time, the passive eavesdroppers attempt to wiretap the signal being transmitted from \(\text{LED}_{\text{U}}\) to the legitimate user. Here, since the promised signal, which is supposed to interfere with the signal reception of the desired information, is already known to the passive eavesdroppers, they can cancel the interference to extract only the information component. Note that since the legitimate user does not know the promised signal, the signal coming from \(\text{LED}_{\text{E}}\) can interfere with the communication between \(\text{LED}_{\text{U}}\) and the legitimate user, i.e., it acts like a jamming signal. Therefore, the promised signal can reduce the SINR of the legitimate user while not affecting the passive eavesdroppers. Figure 6 shows the SOP for different passive eavesdropper densities \((\lambda_{E})\) as a function of the distance between the legitimate user and the registered eavesdropper. Here, the legitimate user is assumed to be located below its serving LED, i.e., \(\text{LED}_{\text{U}}\). When there is no cooperation among the eavesdroppers, the legitimate user can achieve the highest SINR compared to the eavesdroppers. Thus, as shown in the figure, the SOP without eavesdropper cooperation is zero regardless of \(\lambda_{E}\).