# Ambient LED Light Noise Reduction Using Adaptive Differential Equalization in Li-Fi Wireless Link
## 4 Experimental Results
Filled squares and open squares show the BERs change, respectively before and after using both the ADE technique and the matched RF filter, and the open circles show the BERs of when only the ADE technique is used, and the open triangles represent the BERs of when only the matched RF filter is employed. Figure 6: BER changes of recovered NRZ\(-\)OOK signal against two different weight functions. As shown in Figure 8, the power penalty of less than 0.5 dB was observed due to the use of the ADE technique (open red circles) compared to the matched RF filter (open blue triangles). This result means that the transmission performance of the proposed ADE technique is hardly different from that of matched RF filter. In addition, it can be seen that when the simultaneous use of the ADE technique and the matched RF filter allows us to reduce the power penalty of 1 dB compared to the case of using only the ADE or only matched RF filter based on the 7% FEC threshold. The reduction of the power penalty of 1 dB was founded likewise in the case of the 1 st general FEC threshold. Figure 7: (**a**) BER changes of recovered NRZ\(-\)OOK signal as the optical power of ambient light increases from \(-\)16 dBm to \(-\)5 dBm, (**b**) eye diagrams at the ambient light power of \(-\)5 dBm, which were measured for three kinds of cases. Insets on the far left: enlarged eye patterns of each case before using the proposed ADE. A typical method of filtering the baseband component is to use the matching filter presented in the result of Figure 8. Hence, far, it is known that the performance of improving the signal-to-noise ratio using a matching filter is the best. As shown in Figure 8, the proposed technique shows a power penalty of less than 0.5 dB compared to the method using a matching filter. However, unlike the matching filter, since the proposed technique is based on DSP, there is no insertion loss caused by using the RF matching filter. In addition, since it is not necessary to know the header information of the input signal in advance to reduce interference noise, it can be used in various network systems using various protocols. For reference, for data with a long PRBS length, the time to be affected by various noises generated during the transmission is longer than for data with a short pattern length. Therefore, the probability of generating errors in the detection process increases. This means that the BER increases as the pattern length increases. Accordingly, it is necessary to optimize between the PRBS length and the data rate. In addition, the BER floor clearly occurred when the received optical power exceeded \(-10\) dBm. This is due to the following reasons. As shown in Figure 4, the BERs of output NRZ-OOK signal were measured when the output power of the white-LED was set to \(-5\) dBm and the received optical power was \(-10\) dBm. In the case of using the proposed technique, the BER of \(0.9\times 10^{-5}\) was measured at the input SNR of 37 dB. As reviewers know, the SNR of the input signal is constant regardless of the change in the received optical power because the output power of the white LED was fixed to \(-5\) dBm. Therefore, as shown in Figure 8, even if the received optical power is increased to \(-10\) dBm or more, the signal-to-noise ratio of the input NRZ-OOK signal does not change. Eventually, the BER does not decrease any more. In addition, to further reduce the BER, the following improvement methods can be considered. First, the optical SNR can be improved d by increasing the output optical power of the white LED over \(-5\) dBm. Second, the BER can be reduced by improving the frequency response of the wireless optical transmission system by using a blue filter before receiving the transmitted optical signal or by additionally using an equalizer after optical detection. Figure 8: BER changes of recovered NRZ\(-\)OOK signal against the received optical power.