# Ambient LED Light Noise Reduction Using Adaptive Differential Equalization in Li-Fi Wireless Link
## 4 Experimental Results  
Figure 3 shows the signal-to-noise ratio (left) and RF spectrum (right) of the NRZ-OOK signal before and after using the proposed ADE technique. Figure 3a shows the SNR and RF spectrum of NRZ-OOK signal in case of transmitting only the white LED light modulated by the NRZ-OOK signal except for the fluorescent lamp and dimmable LED. Figure 3b shows the SNR and RF spectrum of the NRZ-OOK signal when the fluorescent lamp and dimmable LED light were transmitted. The part marked in red indicates the result where the proposed ADE method is not used, and the part marked in blue indicates the result of using the proposed ADE technique. The optical power of each light source was set to \(-5\) dBm, and the received optical power at the APD was set to \(-10\) dBm using an attenuator and collimator. As shown in Figure 3a, the SNR of the NRZ-OOK signal increased from 22.5 dB to 23.3 dB after using the ADE technique when only the white LED light is transmitted wirelessly to the APD. The improvement of SNR is because shot noise generated in APD due to the injection of white LED light was reduced by the ADE technique. In Figure 3b, the SNR of 13.9 dB was observed when the ADE technique was not employed. We can see that the SNR of the NRZ-OOK signal was reduced by 8.5 dB than that of Figure 3a without using the ADE technique. This is because the interference noise (see the RF spectra at the bottom of Figure 2) and shot noise generated by the two light sources (fluorescent lamp and dimmable LED) were generated at APD. In addition, it was confirmed that the 4.5 dB SNR (from 13.9 dB to 18.4 dB) was improved with the help of the ADE technique. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Value/Dimension** & **Unit** \\ \hline PRBS length & \(2^{23}-1\) & - \\ Sampling points/bit & 10 & - \\ Sampling rate of AWG70002A & 100 & MS/s \\ Sampling rate of MSO 71604C & 250 & MS/s \\ Transmission rate & 10 & Mb/s \\ Wireless transmission length & 1 & Meter \\ Modulation format & NRZ-OOK & \\ Regularization parameter & 9 & - \\ Optical power of three optical source & \(-5\) & dBm \\
3 dB bandwidth of APD & 100 & MHz \\ \multirow{2}{*}{Collimator} & 15 mm Dia., 0.60 NA, BBAR & - \\  & (350–700 nm) & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Experimental parameters and optical/electrical device. Figure 4 shows BERs of the recovered NRZ-OOK signal after optical wireless transmission, which was repeatedly measured while changing the SNR of the input NRZ-OOK signal. The blue line shows the measured BERs when the total optical power of the two ambient light sources (fluorescent lamp and dimmable LED) is \(-5\) dBm, and the red line shows the measured BERs when the optical power of the two ambient light sources is \(-16\) dBm. Filled red squares and filled blue triangles correspond to the BERs in case of not using the ADE technique, while open red squares and open blue triangles indicate the BERs of when the ADE technique is employed. When the optical power of the two light sources was \(-5\) dBm (filled and open blue triangles), the BERs of the recovered NRZ-OOK signal were repeatedly measured while changing the SNR of the input NRZ-OOK signal from 10 dB to 27 dB. In addition, the BERs were measured while changing the SNR of input NRZ-OOK signal from 16 dB to 37 dB in case that the optical power of the two light sources was \(-16\) dBm (filled and open red squares). When the optical power of two light sources was \(-5\) dBm, the SNR of the input NRZ-OOK signal should be at least 25 dB in order to transmit the modulated light of white LED successfully with the help of ADE and the 7% forward error correction (FEC) technique (threshold: \(3.8\times 10^{-3}\) BER) at the same time. In addition, the SNR should be at least 35 dB if the proposed ADE is used with the first general FEC (threshold: \(8.0\times 10^{-5}\) BER). At the two ambient lights (fluorescent lamp and dimmable LED) power of \(-16\) dBm, the SNR gain of 7 dB was improved based on the 7% FEC threshold.