# Visible Light Communication System Technology Review: Devices, Architectures, and Applications
## 1 Introduction  
Electromagnetic waves have an extensive range of applications, including cell phone communications, wireless radio broadcast, Wi-Fi, etc. Depending on the wavelength, electromagnetic waves can be roughly divided into radio waves, infrared rays, visible light, ultraviolet rays, etc. Though radio frequency (RF) is not susceptible to low interference, it is capable of wide area coverage [1]. Nevertheless, RF has several disadvantages, such as interference, bandwidth limitations, safety issues, transmission power limitations, and a crowded radio spectrum [2; 3; 4; 5]. Therefore, visible light communication (380 to 780 nm) has been developed to solve these challenges as an alternative solution to the problem mentioned above. Wireless optical communication technology is characterized by its emphasis on the realization of high-frequency bandwidth communication capacity, which is significantly greater than the current radio wireless communication frequency. In the past ten years, light fidelity (Li-Fi) technology, which combines illumination and communication, has risen to the forefront of study. Optical Wireless communication (OWC) has two main advantages: one is that it won't be interfered with by electromagnetic waves. Despite the fact that current flight regulations allow passengers to use electronic devices on planes and vital medical equipment in hospitals, and copper nets are embedded in the walls to prevent mobile phone interference, there are still some chances for interference. On the other hand, wireless optical communication in these sensitive places can avoid interference. The other advantage is power saving; it is more power efficient to turn on and off a LED/LD than to use RF wireless radio frequency to transmit signals. Hand-held devices have limited battery power, so it is more advantageous to use optical wireless communication to transmit signals. Li-Fi technology design is directly combined with the LED bulb of the skylight board to enable hotspot for indoor wireless internet access, and most mobile phones have LED flashes and CMOS camera lenses, which means that they can potentially receive and send information. The beam steering and VLC position combination is a promising solution to directionality between the light source and the receiver [6]. Although the active components of the VLC system will inevitably increase the power consumption, alignment difficulties, cost, and volume of the mobile device, these can be mitigated by utilizing a modulated retroreflector (MRR) in the mobile device; this has been successfully demonstrated recently [7]. In the future, mobile phones may be able to easily exchange information; a schematic of a VLC System is shown in Figure 1.  
Visible light communication is mainly composed of two components: an optical transmitter (Tx) and optical receiver (Rx), as shown in Figure 1. After preprocessing and encoding, a binary bit stream drives the light source (LED/LD), and the electrical signals are converted into optical signals through modulation. Signal distortion generated by other channels is compensated by using pre-processing. The equalization technology can increase the response bandwidth of the LED and increase the transmission data rate. The receiver's post-equalization can compensate for other channel losses, such as phase noise. High-order coding modulation techniques were used to increase the transmission rate and spectrum efficiency. The light sources that are predominantly used in VLC systems are LEDs and laser diodes. White light is preferred for indoor illumination; hence, red, green, and blue LEDs (RGB-LEDs) are mixed to generate white light sources, or yellow phosphors convert blue LED output into white light sources. When compared to RGB three-color hybrid LEDs, employing yellow phosphor Gallium Nitride (GaN) blue LEDs can minimize light source complexity and system cost.