# Visible Light Communication System Technology Review: Devices, Architectures, and Applications
## 3 VLC Receiver Technology  
The following are the basic requirements of photodetectors in a visible light communication system:  
1. The wavelength has a sufficiently high responsivity. For a certain incident optical power, the output photocurrent can be as large as possible. 2. Fast enough response speeds to be implemented to a high-speed broadband system. 3. A sufficiently fast response speed can be applied to high-speed broadband systems. The noise level must be as low as possible to reduce the impact on the device's signal. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Light Source** & \begin{tabular}{c} **Data Rate** \\ **(Gbps)** \\ \end{tabular} & \
\begin{tabular}{c} **Modulation** \\ **Method** \\ \end{tabular} & **Distance (m)** &
\begin{tabular}{c} **Suitable of** \\ **Lighting** \\ \end{tabular} & **Ref.** \\ \hline white phosphor & 1.1 & MIMO OFDM 1 & 1 & Yes & [80] \\ RGB LED & 3.4 & WDM OFDM & 0.3 & Maybe & [81] \\ RGB LED & 6.36 & MIMO OFDM & 1 & Maybe & [82] \\ RGB LED & 3.375 & PAM8 & 1 & Maybe & [83] \\ RGBYC LED 2 & 10.72 & MIMO OFDM & 1 & No & [84] \\ GaN \(\mu\)-LED & 5 & OFDM & 0.05 & No & [85] \\ GaN violet \(\mu\)-LED & 11.95 & OFDM & N/A & No & [86] \\ DUV \(\mu\)-LED 3 & 1 & OFDM & 0.3 & No & [87] \\ InGaN/GaN \(\mu\)-LED & 1.5 & OOK & N/A & No & [61] \\ GaN Blue LD & 2.5 & OOK & 0.5 & No & [20] \\ Red VSECL & 12.5 & OFDM & 5 & No & [88] \\ Blue LD & 9 & OFDM & 5 & No & [22] \\ GaN Blue LD & 4 & OOK & 0.15 & No & [89] \\ Blue LD & 18 & OFDM & 16 & No & [90] \\ Violet LD & 26.4 & DMT & 0.5 & No & [91] \\ Red VSECL & 11.1 & OFDM & 1.2 & No & [92] \\ Red VSECL & 10.6 & OFDM + OOK & 3 & No & [93,94] \\ Blue LD + phosphor & 4 & OFDM & 0.5 & Yes & [24] \\ Blue LD + phosphor & 2 & OOK & 1 & Yes & [23] \\ Blue LD + phosphor & 1.25 & OOK & 1 & Yes & [95] \\ NUV LD + phosphor & 4 & 1.25 & OOK & 0.15 & Yes & [96] \\ Blue LD + phosphor & 2.705 & OFDM & 1.5 & Yes & [97] \\ Blue LD + phosphor & 5.2 & OFDM & 0.6 & Yes & [25] \\ Blue LD + phosphor & 6.915 & OFDM & 1.5 & Yes & [98] \\ RGB LD & 8 & OFDM & 0.5 & Maybe & [27] \\ RGB LD & 20.231 & OFDM & 1 & Maybe & [99] \\ RGBV LD 5 & 26.228 & OFDM & 2 & Maybe & [100] \\ \hline \hline \end{tabular} \({}^{\dagger}\) MIMO: Multiple-input multiple-output. 2  RGBYC LED: Red-green-blue-yellow-cyan LED. 3  DUV \(\mu\)-LED: Deep-ultraviolet \(\mu\)-LED. 4  NUV LD: Near-ultraviolet LD. 5  RGBV LD: Red-green-blue-violet LD.  
\end{table} 
Table 2: Recently proposed high-speed LED and LD-based VLC systems. Figure 7: A schematic of a VSCEL. Reprinted from [79], Copyright 2020, with permission from Elsevier. According to a good linear relationship, the signal conversion process has been guaranteed to spread without distortion. * Small size and has a long service lifetime. There are three critical parameters for photodetector: responsivity, bandwidth, and dark current [101]. Responsivity is a wavelength-dependent metric that measures the gain of an output electrical signal per unit of incident optical power incident on the detector. The responsivity, \(R\), shown in Equation (3) where incident optical power is \(P\), output electrical signal current is \(I\), is usually given in amperes per watt (A/W):  
\[R=\frac{I}{P} \tag{3}\]  
The quantum efficiency \(\eta\), which is relative to the maximum possible current, is also comparable to responsivity. When the photodetector picks up every electron-hole pair generated by the incident photons, the quantum efficiency will be denoted as 100%. Hence, Equation (3) can be rewritten into Equation (4), where \(q\) is the electron charge, \(\lambda\) is the wavelength of the incident light, \(h\) is the Plank constant, and \(c\) is the speed of light. \[R=\eta\frac{q\lambda}{hc} \tag{4}\]  
The bandwidth (BW) corresponds to the rise time (\(t_{r}\)) of the photodetector, which is shown in Equation (5):  
\[BW=\frac{0.35}{t_{r}} \tag{5}\]  
Two crucial parameters influence the bandwidth of a detector: the transit time and the RC parasitic response.