# Experimental Demonstration of 3 \(\times\) 3 MIMO LED-to-LED Communication Using RGB Colors
## 4 Conclusions  
In this paper, we proposed LED-to-LED communication as another subset of OWC, because LED-to-LED communication is not only an effective solution for UDN and IoT but can also be a commercialization strategy for OWC. Secondly, we presented mathematical models and experimental demonstrations of 3 \(\times\) 3 MIMO LED-to-LED communication systems using RGB colors. We have shown that MIMO LED-to-LED communication with RGB LEDs forms sufficiently low correlated channels due to the special characteristics of the LED as an optical receiver, for which the acceptable wavelength range varies depending on its color. The MIMO LED-to-LED communication experiments were conducted with a QAM DCO-OFDM modulated signal, the SMP MIMO algorithm, and the ZF equalizer. Assuming a communicable threshold of a \(10^{-3}\) BER, the maximum reachable data rate of the MIMO LED-to-LED communication was 30.62 kbps at a 10 cm transmission distance. Additionally, the results showed that the MIMO LED-to-LED system outperforms SISO LED-to-LED systems. We expect that the system can be improved further by applying a ZF-SIC or MMSE-SIC equalizers in the MIMO LED-to-LED system.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{2}{c}{**Data Rate (kbps)**} & \multicolumn{2}{c}{**BER Results at a Transmission Distance of**} \\ \cline{3-4}  & & **10 cm** & **15 cm** \\ \hline \multirow{3}{*}{8.17} & RtoR & \textless{}6.25\times 10^{-5}\) & 0.025 \\  & GtoG & \textless{}6.25\times 10^{-5}\) & 0.019 \\  & BtoB & \(6.25\times 10^{-5}\) & \(5.63\times 10^{-4}\) \\ \hline \multirow{3}{*}{8.17 \(\times\) 3} & RGBtoRGB & \(2.08\times 10^{-5}\) & 0.015 \\ \cline{2-4}  & RtoR & \textless{}6.25\times 10^{-5}\) & 0.055 \\ \cline{2-4}  & GtoG & \(1.25\times 10^{-3}\) & 0.081 \\ \cline{2-4}  & BtoB & \textless{}6.25\times 10^{-5}\) & \(6.25\times 10^{-4}\) \\ \hline \multirow{3}{*}{9.8 \(\times\) 3} & RGBtoRGB & \(4.17\times 10^{-4}\) & 0.046 \\ \cline{2-4}  & RtoR & \(2.93\times 10^{-3}\) & 0.098 \\ \cline{1-1}  & GtoG & \(3.12\times 10^{-3}\) & 0.11 \\ \cline{1-1}  & BtoB & \textless{}6.25\times 10^{-5}\) & \(2.69\times 10^{-3}\) \\ \hline \multirow{3}{*}{12.25 \(\times\) 3} & RGBtoRGB & \(2.02\times 10^{-3}\) & 0.071 \\ \cline{1-1}  & & & \\ \end{tabular}
\end{table}
Table 2: BER comparison of the 3 \(\times\) 3 MIMO LED-to-LED system at a transmission distance of 10 and 15 cm.  
**Author Contributions:** Conceptualization, [PERSON]; methodology, [PERSON]; software, [PERSON]; validation, [PERSON] and S.-M.K.; formal analysis, [PERSON]; investigation, [PERSON]; resources, [PERSON] and S.-M.K.; data curation, [PERSON]; writing--original draft preparation, [PERSON]; writing--review and editing, S.-M.K.; visualization, [PERSON]; supervision, S.-M.K.; project administration, S.-M.K.; funding acquisition, S.-M.K. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (Ministry of Education) (No. NRF-2021R11A3047031). This research was also supported by Kyungsung University Research Grants in 2021.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.