## 2 State of the Art in Optical Wireless Communications
### The Contribution of Communications Based on Visible Light in the Transmission of Information
\begin{table}
\begin{tabular}{c c c c} \hline
**Article** & **Direction** & **Achievement** & **Limitation/Challenges** \\ \hline \multirow{4}{*}{[51]} & Optical camera based on RGB LEDs & Transfer rates of approximately & \multirow{2}{*}{\begin{tabular}{c} Simulations performed offline using the procedure in MATLAB. \\ Use of a single LED RGB and much smaller capacity. \\ \end{tabular} } \\  & using UPSOOK and WDM & 140–150 bps and communication & & \\  & communication. & distances of over 50–60 m. & & \\ \hline \multirow{4}{*}{[52]} & Using communication through the & \multirow{4}{*}{\
\begin{tabular}{c} Transfer rates of about 10 kbps and a \\ modulating the intensity using four levels. \\ \end{tabular} } & It cannot provide real-time processing. \\  & & & \\ \cline{1-1}  & Using communication through the & \multirow{2}{*}{
\begin{tabular}{c} Transfer rates of about 10 kbps and a \\ communication distance of about 2 m. \\ \end{tabular} } & It cannot provide real-time processing. \\ \cline{1-1}  & smartphone’s optical camera by & Transfer rates of about 10 kbps and a & Using a single LED and limiting \\ \cline{1-1}  & modulating the intensity using four levels. & Communication distances. \\ \hline \multirow{4}{*}{[53]} & Using OCC and for coding, they & Transfer rates of about 8.64 kbps and & Limited communication distances. \\ \cline{1-1}  & performed the modulation by & a communication distance of & Inability to support multiple \\ \cline{1-1}  & changing the color. & about 4 cm. & connections through LED \\ \hline \multirow{4}{*}{[54]} & Improved optical camera & Using a freeform lens, the packet & The distance for viable \\ \cline{1-1}  & communication systems using a & reception rate increased by 35\% and & communication is much too short \\ \cline{1-1}  & freeform lens. & the BER is reduced by 72\% to a & (160 lux at a distance of 1 m). \\ \hline \multirow{4}{*}{[55]} & Indoor VLC communications using & The transfer rates were not noticeable & Mobility does not exist, and there is \\ \cline{1-1}  & the smartphone’s camera. & being of the order of kbps and at & no support in this directive. \\ \cline{1-1}  & distances of a few cm. & Reduced communication distance. \\ \hline \end{tabular}
\end{table} 
Table 1: Studies and implementations regarding OCC communications, limitations, challenges and results obtained. given conditions, the visible light spectrum is divided into seven bands, then the data are coded using various combinations with three colors, all done according to mapping rules. Those three physical layers coexist but cannot be used simultaneously [61]. Even though indoor applications are often the most versatile and more foldable for this communication protocol because the standard stipulates the use of the optical clock frequency with rates between 3.75 MHz and 120 MHz, when it comes to indoor applications externally, the optical clock rates are below 400 MHz [62; 63]. Much lower frequencies must also be taken into account because LED light for street lighting or traffic lights requires much higher currents, and the switching time is much slower. Regarding OOK applications, this standard specifies the use of a 200 kHz optical clock, and in the case of VPPM, an optical clock frequency of 400 kHz are used. These clock rates have been taken into account to prevent interference with other light sources; the latter could generate harmonics that can have frequencies of several tens of kilohertz. VLC outdoor applications also automatically involve extremely long communication distances, which allows the appearance of the distorted signal and the loss of the communication path [64]. The multitude of types of LEDs also brings into question the appearance of the characteristics for each of them depending on the field in which they are applied.