# Experimental Demonstration of 3 \(\times\) 3 MIMO LED-to-LED Communication Using RGB Colors
## 3 MIMO LED-to-LED Communication Using RGB Colors
Afterward, IFFT processing was performed to load data symbols onto orthogonally located subcarriers. The resulting signals appear as a real value. After adding ZPs and DC bias, the OFDM signals were directly modulated with arbitrary function generators (AFG) to drive each red, green, and blue LED. After a 10 cm transmission, emitted visible light signals were detected by a red, green, and blue receiver LED and recorded by an oscilloscope (OSC). The received signals were recovered in the sequence of ZF equalization, FFT, and QAM de-mapping. Digital signal processing was carried out via MATLAB. Detailed experimental parameters are listed in Table 1.  
Figure 6: Output current and channel DC gain with respect to the input current when transmitter color is (**a**) red, (**b**) green, and (**c**) blue. The reason why RtoG, RtoB, and GtoB cases are not displayed is that the receiver LEDs cannot receive longer wavelength signals than the wavelength they were designed to as a transmitter. Figure 7: Frequency response of the channel when the transmitter is (**a**) red, (**b**) green, and (**c**) blue. We measured the BER performance of the 3 \(\times\) 3 MIMO LED-to-LED system by varying the bit-rate, and compared the results with the SISO LED-to-LED systems. Note that the bit-rate of each LED channel in the MIMO LED-to-LED system was set to be equal. The BER performances of the 3 \(\times\) 3 MIMO LED-to-LED communication system and the SISO LED-to-LED communication systems are shown in Figure 9. Note that the SISO LED-to-LED system uses the same geographical and power allocation as the MIMO LED-to-LED system. Assuming that a BER of \(10^{-3}\) is a communication-capable threshold, the maximum data rates of the SISO red-to-red, SISO green-to-green, SISO blue-to-blue, and MIMO RGB-to-RGB were measured at 27.2, 15.31, 15.31, and 30.62 kbps, respectively. Figure 10 shows the constellation diagram of (a) RtoR, (b) GtoG, and (c) BtoB SISO LED-to-LED system when the transfer data rate of each channel is 27.2, 15.31, and 15.31 kbps, respectively. Figure 11 shows the constellation diagram of each (a) RtoR, (b) GtoG, and (c) BtoB channel of the MIMO LED-to-LED system when the transfer data rate of each channel is equally set to be 10.21 kbps. Table 2 shows the BER performance of the 3 \(\times\) 3 MIMO LED-to-LED communication system at a transmission distance of 10 and 15 cm. Note that all the experimental settings except the transmit distance were the same. Our results show that the communications are almost impossible at 15 cm. The results show that the BER performance of the system varies significantly in terms of the transmission distance. In the future work, we need to find a way to increase the transmission distance. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline LED diameter & 10 mm & QAM level & 32 \\ \hline LED angle of emission & 30 deg. & OFDM subcarrier number & 66 \\ \hline Transmission distance & 10 cm & Zero padding length & 34 \\ \hline \multirow{3}{*}{Peak wavelength} & R & 629 nm & R & 0.27–36.3 mA \\  & G & 521 nm & Operating current & G & 9.4–25 mA \\  & B & 466 nm & & B & 0.94–2.75 mA \\ \hline \multirow{3}{*}{Average E/O gain} & R & 0.0909 & \multirow{3}{*}{Average E/E gain} & RtoR & \(4.62\times 10^{-6}\) \\  & G & 0.56 & & GtoR & \(1.02\times 10^{-5}\) \\ \cline{1-1}  & B & 0.3715 & & GtoG & \(9.12\times 10^{-6}\) \\ \hline \multirow{3}{*}{System bandwidth} & R & 6000 Hz & \multirow{3}{*}{} & RtoR & \(5.49\times 10^{-6}\) \\  & G & 3000 Hz & & BtoG & \(8.52\times 10^{-5}\) \\ \cline{1-1}  & B & 3000 Hz & & BtoB & \(3.2\times 10^{-5}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Experimental conditions in 3 \(\times\) 3 MIMO LED-to-LED communication system. Figure 8: Experimental setup of the 3 \(\times\) 3 MIMO LED-to-LED communication system using RGB LEDs. Figure 11: Constellation diagram of MIMO LED-to-LED system. (**a**) RtoR, (**b**) GtoG, and (**c**) BtoB when transfer data rate of each channel is 10.21 kbps.