# Visible Light Communication System Technology Review: Devices, Architectures, and Applications
## 2 VLC Transmitter Light Source  
### Theoretical Background of LED for VLC Application  
At present, most of the light sources used in VLC systems are white light LEDs, which are mainly divided into two categories, RGB-LED and phosphor-converted (pc)-LED. RGB-LED is realized by the utilization of three primary color phosphors which are excited \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Light Source** & **Size (mm2)** & **Limiting Factor** & **Bandwidth** & **Power** & **Eye Safe** \\ \hline LED & 0.1–1 & \(\uptau_{\text{RC}}\) (\(\sim\)1 ns) & \(\sim\)10 MHz & \(>\)1 W & Yes \\ \(\upmu\)-LED & \(<\)0.01 & \(\uptau_{\text{carrier}}\) (\(\sim\)1 ns) & \(<\)1.5 GHz & \(\sim\)\(\upmu\)W & Yes \\ Laser diode & \(<\)0.2 & \(\uptau_{\text{Photon}}\) (\(\sim\)1 ps) & 10–20 GHz & \(>\)1 W & No \\ Laser light & \(\sim\)0.01 & \(\uptau_{\text{Photon}}\) (\(\sim\)1 ps) & 10–20 GHz & \(>\)1 W & Yes \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of light source used in the VLC system. by violet or ultraviolet LEDs; another approach is to package red, green, and blue chips together to emit white light (RGB-LED), as shown in Figure 2a. On the other hand, a pc-LED converts the light source into white light by exciting the yellow phosphor with a blue LED chip, as shown in Figure 2b [28; 29]. Studying the modulation characteristics of the LED device is critical to improving the performance of the new Li-Fi system. The modulation bandwidth of the LED determines the channel capacity and data rate of the communication system. The definition of LED modulation bandwidth is the frequency when the AC optical power output by the LED drops to 50% of a certain reference frequency value (\(-3\) dB). As the photoelectric response of a pc-LED is relatively behindhand [30], the modulation bandwidth of the Li-Fi light source is limited to a few MHz, therefore limiting the data rate of the entire system; even the use of a blue filter failed to improve the defects of the light source. Use of a higher modulation bandwidth by using wavelength division multiplexing (WDM) with three different wavelengths of light increases the channel capacity of RGB-LEDs. The terminal receives three wavelengths of light through filters of each wavelength, which significantly improves the transmission efficiency. However, different colors of RGB-LED have different operating temperatures, depending on the output light flux. To achieve independent color operating at different temperatures, the feedback loop and each single-color LED's driving current need to be controlled individually, which leads to higher cost and a more complex modulation circuit. The response rate and modulation bandwidth of the LED are influenced by the carrier lifetime. In addition, increasing the radiation recombination rate of electrons and holes and reducing the spontaneous radiation lifetime of carriers is a conventional approach for designing a modulation circuit and minimizing the RC delay. The spontaneous emission lifetime of carriers has been decreased by increasing the injected carrier concentration, which can be achieved by increasing the injection current and delta doping. Under high injection current, as the injected carrier concentration increases, the exciton recombination probability increases, the radiation recombination carrier lifetime decreases, and the electro  
Figure 2: Structures for generating white light from fluorescent materials, based on (**a**) UV chip and (**b**) blue chip. Reprinted with permission from [29]. Copyright 2011, American Chemical Society. optical conversion responds quickly. Delta doping technology also results in high carrier injection, thereby reducing carrier lifetime and improving modulation bandwidth at the same current density. Due to the built-in piezoelectric polarization of the LED, the traditional polar LED grown on the c-plane has limited bandwidth, especially in the green spectral region [31].