# Ambient LED Light Noise Reduction Using Adaptive Differential Equalization in Li-Fi Wireless Link
## 4 Experimental Results
One of the weighting functions was a function with Gaussian distribution, and the other was a function showing Poisson distribution. As shown in Figure 6, it was observed that, within the full range of the ambient light power, the BERs when using the Poisson distribution function were less than the BERs in the case of using the Gaussian distribution function. In particular, the difference in BER between the two weighting functions gradually increased as the ambient light power exceeded \(-\)8 dBm. For example, at ambient light power of \(-\)5 dBm, the BER was reduced from 1.4 \(\times\) 10\({}^{-4}\) to 1.6 \(\times\) 10\({}^{-5}\) when the Poisson weight function was used. This result can be explained as follows: As the ambient light power increases, the shot noise having the Poisson characteristics increases. Therefore, the use of a weight function based on a Poisson distribution allows us to reduce shot noise more than when using a Gaussian distribution function. Figure 7 shows the measured BERs of recovered NRZ-OOK signal after the ADE technique as the optical power of ambient lights (fluorescent lamp and dimmable LED) increases from \(-\)16 dBm to \(-\)5 dBm. Eye diagrams present below the measured BER results were measured for three kinds of cases at \(-\)5 dBm ambient light power. The three cases here refer to: The first is when the white LED light and the lights of fluorescent lamp, and dimmable LED are transmitted simultaneously, the second is when only the white LED light and fluorescent lamp are transmitted, and the third is when only the white LED light, and dimmable LED light are transmitted. The received optical power after the optical wireless transmission was \(-\)10 dBm. The filled black squares and open black squares represent BER results of the first case before and after the ADE technique and filled blue triangles, and open blue triangles represent the BER results of the second case with and without the ADE technique. In addition, filled red circles and Open red circles indicate the BER results of the third case before and after the ADE technique. Figure 5: BER changes of recovered NRZ\(-\)OOK signal and iteration number against the regularization parameter. As shown in Figure 7, it can be seen that the impact of the interference and shot noises generated from the dimmable LED on the transmission performance of the optical wireless link is greater than that of the fluorescent lamp within the entire range of the measured power of the ambient light. For example, when the optical power of the ambient light is \(-9\) dBm, the BER in the case of the fluorescent lamp is \(8.5\times 10^{-5}\), and under the impact of the dimmable LED, the BER is \(4.5\times 10^{-4}\). This is because, as shown in the RF spectrum of interference noise generated from the ambient lights (fluorescent lamp and dimmable LED) presented at the bottom of the experimental setup in Figure 2, the number of the interference noise component of the dimmable LED was larger than that of the fluorescent lamp within the frequency range of \(1\) kHz. Therefore, the BER in the case of dimmable LED increases more than in the case of a fluorescent lamp due to the more generated interference noise components. Furthermore, for example, the optical power of the ambient light should be less than about \(-12\) dBm in order to obtain a BER of \(1\times 10^{-5}\) in case of not using the ADE technique. However, when the ADE technique is utilized, the optical power of the ambient light source should not exceed \(-6\) dBm. These results tell us that even if the intensity of the ambient light source is increased by about \(6\) dB, the transmission performance can be accomplished sufficiently with the help of the ADE technique. Figure 8 shows the measured BERs of the received NRZ-OOK signal as the received optical power increases from \(-17\) dBm to \(-8\) dBm. The performance of the matched filter was compared in order to objectively evaluate the transmission performance of the proposed ADE technique.