# High-Speed Visible Light Communications: Enabling Technologies and State of the Art
## 5 Limitations
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Year** & **Transmitter** & **Receiver** & **Modulation** & **Data Rate** & **Distance** & **Comments** & **Ref.** \\ \hline
2016 & ps\(\pm\)LED & PIN & OFDM (bit-loading) & 1 C/s & 0.6 m & \(2\times 2\) MIMO System & [57] \\ \hline
2015 & RGB LDs & PIN & OFDM(WDM) & 14 C/b/s & 2.8 m & / & [67] \\ \hline
2015 & RGB LDs & APD & OFDM(WDM) & 4.4 C/b/s & 0.2 m & / & [68] \\ \hline
2015 & Blue LDs & APD & OFDM & 9 C/b/s & 5 m & / & [69] \\ \hline
2015 & Blue LD + phosphoror & / & OFDM (adaptive bit and power loading) & 6.52 C/b/s & 0.35 m & / & [60] \\ \hline
2015 & Blue LD + phosphoror & APD & OFDM & 5.2 C/b/s & 0.6 m & / & [71] \\ \hline
2015 & Blue LD + phosphoror & APD & OFDM & 4 C/b/s & 0.5 m & / & [72] \\ \hline
2015 & Blue LD + phosphoror & APD & COK & 4 C/b/s & / & / & [73] \\ \hline
2015 & u\(\pm\)LED + polymer & APD & DCO-OFDM & 2.3 C/b/s & / & / & [74] \\ color converter & APD & (WDM) & 2.3 C/b/s & / & / & [74] \\ \hline
2015 & RYGB LED & PIN & CAP(WDM) & 8 C/b/s & 1 m & Pre-and Post-equalization & [41] \\ \hline
2015 & RYGB LED & APD & OFDM(WDM) & 5.6 C/b/s & 4 m & & [75] \\ \hline
2015 & RGB LED & PIN & CAP & 4.5 C/b/s & 2 m & & [47] \\ \hline
2015 & RGB LED & APD & QAM-OFDM & 750 Mb/s & / & Pre-equalization & [47] \\ \hline
2015 & LED & PIN & OFDM(Spatial) & 1.4 C/b/s & 2.5 m & \(2\times 2\) imaging MIMO-VLC system & [76] \\ \hline
2015 & pp\(\pm\)LED & APD & OFDM(adaptive bit and power loading) & 2 C/b/s & 1.5 m & Pre-equalization & [48] \\ \hline
2015 & pp\(\pm\)LED & PIN & OFDM & 1.6 C/b/s & 1 m & Pre-equalization & [54] \\ \hline
2015 & ps\(\pm\)LED & APD & QAM-OFDM & 1.4 C/b/s & 0.8 m & Pre-equalization & [77] \\ \hline
2015 & ps\(\pm\)LED & APD & OFDM(Spatial) & 1.3 C/b/s & 0.4 m & / & [78] \\ \hline
2015 & ps\(\pm\)LED & PIN & PAM & 1.1 C/b/s & / & Post-equalization & [79] \\ \hline
2015 & ps\(\pm\)LED & PIN & QAM-OFDM & 682 Mb/s & 1 m & Pre-equalization & [80] \\ \hline
2014 & μ\(\pm\)LED & PD & OFDM(adaptive bit and power loading) & 3 C/b/s & 5 cm & / & [81] \\ \hline
2014 & μ\(\pm\)LED + polymer & APD & OFDM & 1.68 C/s & 3 cm & / & [82] \\ color converter(white) & APD & QAM (Wavelength) & 4.22 C/b/s & / & / & [83] \\ \hline
2014 & RGB LED & APD & CAP(WDM) & 1.3 C/b/s & 0.3 m & Pre-and Post-equalization & [49] \\ \hline
2014 & RGBLED & APD & 16-QAM & 1 C/b/s & / & Post-equalization & [84] \\ \hline
2014 & Blue-LED & APD & 4-QAM(Spatial) & 500 Mb/s & 0.3 m & \(2\times 2\) non-imaging MIMO System & [85] \\ \hline
2014 & pp\(\pm\)LED & PIN & OOK-NME & 550 Mb/s & 0.6 m & Pre-and Post-equalization & [86] \\ \hline
2014 & ps\(\pm\)LED & PIN & OOK-NME & 340 Mb/s & 0.4 m & \
\begin{tabular}{} \end{tabular} &
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2014 & pp\(\pm\)LED & PIN & OOK-NME & 340 Mb/s & 0.4 m & \
\begin{tabular}{} \end{tabular} &
\begin{tabular}{} \end{tabular} \\ \hline
2014 & pp\(\pm\)LED & PIN & OOK-NME (Spatial) & 50 Mb/s & 2 m &
\begin{tabular}{} \end{tabular} & [87] \\ \hline \end{tabular} Note: / represents not mentioned in the article.  
\end{table} 
Table 1: _Cont._i.e., the reflected LED light by the corner cube reflector acts as the media for uplink communication. [PERSON] et al. [93] described an uplink using RF. While in some RF restricted areas, this approach is not feasible. [PERSON] et al. [94] experimentally demonstrated a bi-directional transmission link using a LED-based VLC for both downlink and uplink paths. And a Time-division-duplex (TDD) was applied to eliminate the reflection interference. Results show that a 2 m bi-directional transmission with BER \(<10^{-5}\) is achieved. (3) Lights Off Mode. Since the VLC aims to offer simultaneous illumination and high-speed data transmission, how to transmit data when the lights are turned off is a significant problem to be solved, especially for the indoor VLC applications. Utilizing the feature of limited perception to light intensity of human eyes, some data transmission may be realized by making the emitted light perceptibly off. [PERSON] et al.