# Ambient LED Light Noise Reduction Using Adaptive Differential Equalization in Li-Fi Wireless Link
## 4 Experimental Results
In addition, it was found that the SNR gain has increased by 7.5 dB with the help of the first general FEC. For reference, in the case of a signal with a modulation index higher than OOK, the required SNR for successful transmission increases than NRZ-OOK. For example, to obtain the 1 st general FEC threshold (3.8 \(\times\) 10\({}^{-5}\) BER), the pulse amplitude modulation (PAM)-4 signal requires about 7.5 dB more signal-to-noise ratio than the NRZ-OOK signal. Therefore, it is important to determine what kind of modulation format to use according to the wireless service to be operated in the system as well as the transmission environment. Figure 3: Signal\(-\)to\(-\)noise ratio (left) and RF spectrum (right) of NRZ\(-\)OOK signal before and after using ADE technique, (**a**) in case of transmitting only white LED light, (**b**) in case of transmitting fluorescent lamp, and dimmable LED as well as white LED light. On the other hand, since the regularization parameter mentioned in Equation (8) determines the shape of the recovered NRZ-OOK signal waveform, it is necessary to find out the impact of the regularization parameter on the transmission performance of the NRZ-OOK signal. Furthermore, as mentioned in Section 2.2, the complexity of DSP depends on the number of iterations calculation into the algorithm of the ADE technique. Hence, it is necessary for us to reduce the number of iterative calculations while maintaining a low BER. By the way, it is worth noting that the number of iterative calculation depends on the regularization parameter because it is executed until the output NRZ-OOK signal shows the waveform most similar to the input NRZ-OOK signal, and the difference between adjacent sampling points is less than the threshold determined in the algorithm. Therefore, we also checked how the number of iteration calculation would change by the change of the regularization parameter. Figure 5 shows the measured BERs (left y-axis) of recovered NRZ-OOK signal, and the number of iterative calculations (right y-axis) after ADE technique as the regularization parameter increases from 1 to 14. The received optical power of APD was fixed at \(-10\) dBm. As shown in Figure 5, if the use of the FEC technique can be accomplished, in the case of the ambient light noise power of \(-16\) dBm, the optical wireless transmission was achieved successfully within the regularization parameter range from 3 to 14 with the help of 1 st generation FEC. In addition, it is possible to transmit the NRZ-OOK signal wirelessly within the regularization parameter range from 5 to 14 when the optical power of the ambient light was \(-5\) dBm. In summary, the BER result in Figure 5 indicates that the proposed scheme is less dependent on the value of a specific regularization parameter. In addition, the regularization parameters with minimum BER were measured as 9 and 11 at the optical power of each ambient light. In terms of the number of iterative calculations, as the regularization parameter value increases to 14, the iterations number gradually decreases from 400 to 150 or 100 at the respective optical power of the ambient light. These results indicate that it is important to optimize the regularization parameter taking into account the number of iterative calculations in order to maximize transmission performance while reducing interference noise and shot noise using the proposed ADE technique in the real Li-Fi network. Figure 4: Bit error rate (BER) changes against the SNR of the input NRZ\(-\)OOK signal. In Equation (10) presented in Section 2, the Poisson distribution function was used as a weighting function to increase the reduction efficiency of shot noise. Therefore, it is necessary to find out whether the choice of the Poisson distribution function can help us to increase the reduction efficiency of shot noise. Figure 6 shows the BER measured according to the ambient light power change for two different weighting functions.