# Challenges in Physical Layer Security for Visible Light Communication Systems
## 3 Vulnerabilities of Physical Layer Security in VLC Systems
### Beamforming  
Beamforming strategies are popular methods to improve the secrecy performance in RF systems. The transmitters take the CSI of receivers into account in transmitting the information signals such that the waveforms reach only the UE sites with a high SNR, while suppressing the information signals elsewhere, especially at electron device (ED) sites [18]. Various beamforming approaches were also rigorously proposed and studied for VLC systems to increase secrecy by tailoring RF beamforming to VLC environments; however, the VLC beamforming schemes retain weak points that an ED might exploit. This section explains the VLC beamforming's weakness in the simultaneous presence of active and passive EDs by explaining the ZF (zero-forcing) beamforming [6] as an example. ZF precoding is a well-known and practical PLS transmission technique in VLC systems that eliminates information reception at an unintended user by carefully designing the precoding matrix. ZF was shown to effectively decouple a legitimate user from possible malicious users and significantly improve various security measures. More specifically, according to [6], the precoding matrix can be designed to lie in the nullspace of the eavesdropper's channel. In this case, multiple LED transmitters multiply a data signal by the precoding matrix before transmitting, which can force the reception of the unintended user to zero. Therefore, the achievable secure communication rate under ZF can be almost equal to the achievable rate of the intended user's point-to-point channel. However, ZF has limitations with regard to feasibility and performance. First, due to intrinsic condition that the LED transmitters have to retain the CSI of both the legitimate user and malicious users, ZF cannot cope with the threat of unknown passive eavesdroppers (i.e., their CSI is not known to LEDs), which limits the practical use of this approach. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Channel Properties**} & \multicolumn{2}{c}{**Communication Devices**} & \multicolumn{2}{c}{**PLS Techniques**} \\ \hline \multirow{2}{*}{**Reciprocity**} & \multirow{2}{*}{**Uncorrelatedness**} & \multirow{2}{*}{**Fading**} & \multirow{2}{*}{**Transmitter**} & \multirow{2}{*}{**Receiver**} & \multicolumn{2}{c}{**Key-Less**} & **Secret Key** \\  & & & & & & **Transmission** & **Sharing** \\ \hline RF & \(\bigcirc\) & \(\bigcirc\) & \(\bigcirc\) & Antenna & Antenna & Feasible & Feasible \\ \hline VLC & \(\times\) & \(\times\) & \(\times\) & LED & Photodiode & Feasible & Infeasible \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Differences in security environments between radio frequency (RF) and visible light communication (VLC) systems. Second, ZF can make the VLC system more vulnerable when passive eavesdroppers are present together with an active eavesdropper whose CSI is known, as illustrated in the example in Figure 1. The figure shows the SNRs according to the receiver location when ZF is adopted. A \(10\times 10\) transmitter array is assumed to consist of uniformly distributed LEDs on a square lattice in a room of dimension \(20\times 20\) m\({}^{2}\). One legitimate user and one active eavesdropper are assumed to be present. These receivers are marked with blue and green dots in the figure. Also, multiple passive eavesdroppers are assumed to be distributed according to a Poisson point process (PPP) with the density \(\lambda_{E}\). In Figure 1, even when the active eavesdropper is located close to the legitimate user, ZF effectively minimizes the information reception at the active eavesdropper site by utilizing precoding. From this result, ZF appears to secure the transmission against the eavesdropper. However, from the perspective of the passive randomly located eavesdroppers, as shown in Figure 1, it is possible to achieve a higher SNR than the legitimate user. Note that if any passive eavesdropper is located in the dashed area in Figure 1, it can achieve a higher SNR than the legitimate user.