# High-Speed Visible Light Communications: Enabling Technologies and State of the Art
## 2 Potentials of VLC System
### Security/privacy and interference  
Radio waves can penetrate walls and are susceptible to eavesdropping on in-room or in-building communications. Furthermore, radio waves are susceptible to mutual interference with electronic devices, which greatly limits the scope of their application. Since lights can be strictly confined to an intended area surrounded by opaque boundaries, the security of VLC is greatly enhanced. However, recent research has shown that eavesdropping on VLC is possible, generally by capturing the light signals that leak through the gap between the floor and door, keyhole, and even partially covered windows [4]. Nonetheless, VLC is still a much more secure option than traditional RF communication. In addition, VLC does not cause any interference with RF signals and has no health concerns [5]. Thus this technology is perfectly suitable for a scenario where sensitive electronic devices are used, such as hospitals, or in industrial and aerospace applications [6]. (4) Low cost and energy efficient  
The cost of VLC devices are comparatively lower than the current RF modules. Study shows that a Bluetooth module, which provides a data ratio up to 1 Mb/s, costs about $5 while a VLC link can offer a data rate of about 4 Mb/s with a cost of only around $1 per module [7]. Furthermore, LED-based VLC has emerged as a green communication technology because of the excellent energy efficiency feature of LEDs. When compared with traditional lighting technologies, LEDs use less energy [8] and generate less heat [9]. The energy consumption of LED is only 1/10 and 1/4 of the incandescent and fluorescent lighting, respectively. According to a report from the U.S. Department of Energy, SSL technology is promising to conserve up to 217 terawatt-hours (TWh) save energy by the year 2025 [10]. Owing to the inherent benefits outlined above, VLC system has broad application prospects, and different application scenarios are summarized as follows. (1) Indoor high-speed network access: Li-Fi  
The term Li-Fi was coined by Prof [PERSON] from Edinburgh University in 2011. It is a high-speed bi-directional fully connected VLC system analogous to Wi-Fi as shown in Figure 1. A speed of up to 10 Gb/s can be obtained using Li-Fi, which is 250 times more than the speed of'super-fast' broadband [11]. Due to the problem of interference with other RF signals, Wi-Fi is forbidden or limited in many electromagnetic wave sensitive areas, such as aircrafts, nuclear power plants, and hospitals. Therefore, Li-Fi can be a better solution in such environments as it does not cause any electromagnetic interference. However, as one cannot use light bulbs to send data to fast-moving objects, or objects behind obstacles, Li-Fi is not a competitor to Wi-Fi, but a complementary technology that will help to free up spectrum space. (2) Underwater communication  
Due to the salty, high conductivity, and high attenuation environment of sea water, RF signals cannot travel well in sea water. Although sound waves travel far in the ocean, they have a low transmission rate, limited bandwidth and high-power consumption. Furthermore, they may interfere with marine animals such as dolphins. VLC uses the visible light of the optical transmission window (420-510 nm) in seawater for data communication, which can effectively overcome some shortcomings of microwave and sound waves in underwater communication. Thus, VLC can be used to send data to submarines or divers with their head lights and other underwater devices. In 2015, [PERSON] et al. from Massachusetts Institute of Technology (MIT) has applied VLC to their underwater robots' communication and localization in a non-destructive nuclear reactor inspection [13]. The underwater use of VLC now still has some limitations, such as the data rate and distance are obviously affected by hydrological conditions, especially turbidity. For example, [PERSON] et al. proposed an underwater  
Figure 1: Example of a Li-Fi system. Reproduced with permission from [12], Elsevier, 2017.