# Challenges in Physical Layer Security for Visible Light Communication Systems
## 1 Introduction  
Over the past few decades, the acceleration of the development of mobile devices, such as smart-phones, tablet computers, wearable devices, and Internet of Things devices, provoked a higher demand for data traffic via wireless communication. However, traditional radio frequency (RF) communication systems, such as WiFi and cellular networks, face difficulties in satisfying the requirements of future networks due to the scarcity of RF spectrum. Thus, there is a need for a new air interface and/or more spectrum to offload the high volume of wireless traffic. As a possible solution, visible light communication (VLC) gained considerable popularity in academia and industry. VLC utilizes visible light, the spectrum of which ranges from 400 THz to 700 THz and is license free; hence, VLC can be exploited for high-speed indoor wireless communication [1]. VLC offers additional advantages. On the one hand, since visible light cannot penetrate an opaque wall, a VLC system can offer high security at the physical layer. However, in large open spaces, such as libraries, open-plan offices, and conference halls, it is still possible for a malicious user to eavesdrop a VLC transmission. Although an authentication process similar to Wired Equivalent Privacy (WEP) and WiFi Protected Access (WPA) would be possible even for a VLC system, such a method imposes large signaling and computational overhead and was shown to be breakable [2]; thus, it would not be secure. As one of many network security approaches, physical layer security (PLS) is a set of techniques that enables a transmitter and a legitimate receiver to securely communicate by utilizing the randomness of the channel between the transmitter and the receiver [3, 4]. PLS can be deemed to be the most secure method of communicating, since security in thiscontext is provable (in the information theoretic sense). For example, in a typical wiretap model, PLS theory intimates that secure communication is possible when the capacity of the intended communication channel is higher than that of the eavesdropping channel. In RF systems, various PLS transmission techniques that allow better signal reception at an intended receiver by utilizing multiple antennas were proposed, and their information-theoretic security performances were analyzed [5]. Also, motivated by the PLS schemes of the RF systems, numerous variants of PLS techniques securing indoor VLC systems were also proposed, such as zero-forcing, robust beamforming, artificial jamming, and light emitting diode (LED) selection, generalized space shift keying (GSSK) modulation, etc. [6; 7; 8; 9; 10; 11; 12; 13; 14]. Yet many intrinsic characteristics of VLC systems are different from RF systems (e.g., the channel, physical properties of the transmitting and receiving devices, and the signal constraints). Hence, it is necessary to take these differences into account when tailoring PLS techniques to VLC systems. A number of investigations considered these unique properties. For example, [7] studied the achievable secrecy rates under constraints on the input signal amplitude for single-input single-output (SISO) and multiple-input single-output (MISO) scenarios and [9] considered various input signal distributions to increase the secrecy rate. Fundamental investigations of PLS typically treat a model whereby a single active jamming device or passive eavesdropper is present in the system, and the location and/or channel properties associated with the third-party device are assumed to be known. In reality, it may be the case that the locations and number of eavesdropping devices are not known. Recent studies in [11; 12] began to address this scenario, where unknown eavesdropper locations are modeled by using tools from stochastic geometry. Furthermore, VLC eavesdroppers use different receiver architectures than legitimate users in an effort to intercept the signal.