# Challenges in Physical Layer Security for Visible Light Communication Systems
## 2 Differences in RF and VLC Security Environments  
PLS can be split into the two main categories. The first is transmitting a message with a well-designed coding technique that allows appropriate redundancy so that only the legitimate user can reliably decode the message, while introducing enough randomness so that an eavesdropper experiences large ambiguity when attempting to decode the message. The second is generating and distributing a secret key between a transmitter and an intended user by utilizing the distinct physical characteristics of their reciprocal channel [3]. The two assumptions that the latter PLS category requires--i.e., the reciprocity and the uncorrelatedness of the channel--are challenging to achieve in indoor VLC systems. First, the reciprocity of the channel between a transmitter and a legitimate user's receiver should be achieved so that they can extract a secret key from the shared channel state information (CSI). However, unlike in RF systems where a single antenna can act as a transmitter and a receiver, in VLC systems, the transmitter and receiver are distinct components, that is, they consist of an LED and a photodiode (PD), respectively. Hence, the reciprocity can hold only when the LED and PD are located at the same place or are in very close proximity and the characteristics of LED and PD for the uplink and downlink are identical. In reality, these are impractical restrictions for indoor VLC systems. Furthermore, an uplink transmission with visible light is undesirable, since the light radiating from the user devices would cause a visual disturbance to users; thus, RF or infrared communication is typically considered for the uplink transmissions. For these reasons, sharing a secret key between a transmitter and a legitimate user by using channel reciprocity is not a natural assumption in VLC systems. Second, the fact that fading does not exist in VLC systems, since the PD's detector area is much larger than the wavelength of visible light, precludes the channel for the intended link from being uncorrelated with that of the eavesdropper. Therefore, instead of studying secret key agreement techniques, the majority of existing PLS studies for VLC systems focused on the former key-less techniques, which require that the legitimate user should retain a superior channel relative to that of the eavesdropper to achieve a positive secure transmission rate. In VLC systems, it might be difficult for a legitimate user to achieve a higher signal-to-noise ratio (SNR) than that experienced by an eavesdropper, which imposes challenges on developing PLS techniques. The SNR is typically determined based on the strength of a received desired signal and noise introduced at a receiving device. Thus, in RF systems, to facilitate a higher SNR at a legitimate user, a transmitter with multiple antennas can enhance the strength of the received signal only at a legitimate site by, for example, utilizing beam-steering transmission, antenna selection, or artificial jamming strategies. An eavesdropper in RF systems does not have many options to increase its SNR under these PLS transmission schemes, except to reduce the noise generated in its electronic circuitry and antenna(s), which is technically limited to some extent. Although an RF eavesdropper with multiple antennas is capable of increasing its SNR and somewhat reducing the secrecy rate, it was shown that it is highly challenging to achieve a higher SNR than that of a legitimate user when suitable multiantenna techniques are employed [15]. In VLC systems, considering the channel model of visible light in [16], the eavesdropper can significantly augment its receiver capability by implementing possible device modifications, such as increasing the area of the PD, adopting a high-gain optical lens, and accurately adjusting the receiver's orientation toward the LED transmitter. The legitimate user can also increase its receiver sensitivity using these methods.