# Challenges in Physical Layer Security for Visible Light Communication Systems
## 4 Future Directions  
We discussed the challenges in securing VLC systems using PLS, explaining that entirely securing VLC transmissions via PLS without knowing the locations and receiver characteristics of passive eavesdroppers may not be guaranteed. Notably, it appears  
Figure 5: An example of a VLC system operating in presence of both registered and passive eavesdroppers. Figure 6: SOP for different densities of passive eavesdroppers (\(\lambda_{E}\)) plotted as a function of distance between legitimate user and registered eavesdropper when registered and passive eavesdroppers cooperate. \(N_{\mathrm{T}}=10\times 10\) LED transmitters are assumed to be uniformly distributed on a square lattice in a room of dimension \(20\times 20\) m\({}^{2}\). obscure to devise a realistic way to prevent the jamming attack created by the legitimately registered eavesdropper of Section 3.3, except for limiting the number of concurrent users to one. However, this would immensely decrease the spatial efficiency of the VLC systems. Nevertheless, the advantages of PLS that significantly complement the security of higher layers must be exploited in forthcoming VLC systems. Therefore, in what follows, we provide potential (although not perfect) guidelines for improving and designing PLS schemes for VLC systems. First, for indoor VLC systems, the users' behaviors and the room layout can be examined with a view to predict the possible (but not exact) locations of the eavesdroppers. For example, considering typical behaviors of office workers in which they mostly work sitting at desks, the probable and approximate locations of the mobile devices are near to the desks rather than in the aisle or the rest area. This anticipation for the locations of eavesdroppers can be mathematically modeled by using tools from stochastic geometry, e.g., inhomogeneous PPP, which can provide additional hints to design and optimize the secrecy metric of PLS schemes in the presence of unknown passive eavesdroppers. For example, ZF precoding in Section 3--A can be improved by considering the statistical locations of the passive eavesdroppers to reduce the SNR in the area that the passive eavesdroppers are highly likely to be located. Second, to combat the augmented eavesdroppers, jamming is still an advantageous approach since the eavesdroppers, even with powerful receiver architectures, cannot extract the information component from the received signal in the presence of randomized jamming signals. In contrast, jamming signals can be designed such that they lie in the null space of the legitimate user's channel so as not to disturb information reception. Therefore, improving beyond the traditional jamming strategies, transmitting the information and jamming signals at very closely located LEDs, as discussed in Section 3.2, or randomly choosing the information transmitter and jammers, disregarding the location of the legitimate user, can be possible strategies. The first approach can further improve by forcing both information and jamming signals to be emitted from a single LED by splitting its transmit power into two components. However, this would sacrifice the SINR of the legitimate user. Besides, with the latter approach, i.e., transmitter hopping, the information reception at the legitimate user may be weakened due to the increase of the distances from the information LEDs; thus, appropriate beamforming strategies with multiple LEDs should supplement the signal strength at the legitimate user. In addition, it is also possible to utilize intelligent reflecting surfaces (IRS) for VLC [23] to enhance jamming strategies since eavesdroppers cannot know which element of programmable metasurfaces or mirror arrays of IRS emits the information or jamming signals, respectively. Third, VLC is viewed as a complementary technology to other RF wireless communication technologies, such as Wi-Fi, mobile networks, mmWave communications, etc., rather than a RF replacement.