## 5 Conclusions  
We proposed a differential equalization technique with a pseudo-flicker weighting function to reduce the imperceptible flicker noise generated in the Li-Fi transmission link. The imperceptible flicker noise was generated in the white LED driving circuit and mainly has a frequency of 120 Hz (in the case of 220-V AC power). It was modulated simultaneously with the NRZ-OOK signal in the process of directly modulating the light of the white LED. As a result, the 120 Hz imperceptible flicker noise plays the role of reducing the SNR of the NRZ-OOK signal. Using the proposed technique, the following main experimental results were obtained. The communication and illumination environment of the implemented optical wireless link is as follows. At a bias voltage of 4 V with maximum OMI, the light of the white LED was directly modulated by the NRZ-OOK signal with an SNR of 20 dB (corresponding to 10% flicker modulation). After that, the modulated light was transmitted wirelessly by 50 cm and then the received light intensity was 500 Lux. A 2 dB SNR gain was obtained using the proposed technique based on the BER threshold (3.5 \(\times\) 10\({}^{-5}\)) corresponding to 10% flicker modulation, which is known to have no significant impact on human health. This means that even if the input SNR is set to 18 dB from 20 dB, the BER corresponding to 10% flicker modulation can be obtained using the proposed technique as shown in Figure 4.  
Furthermore, it means that it is possible to implement an illumination and communication environment with flicker modulation reduced from 10% to 7%. Therefore, it can be said that it is possible to realize an illuminating Li-Fi space with attenuated flicker noise using the proposed technique. In addition, the proposed technique is expected to be applicable to other fields. For example, speckle noises are known to degrade performance in laser projection systems, and many techniques have been proposed to reduce them [25; 26]. In terms of reducing analog signals mixed with digital signals, the proposed technique can be applied to reduce speckle noises generated in laser projection systems. As in the example mentioned above, it is necessary to analyze whether the proposed technique in other fields can be employed in terms of scalability. Considering the proposed technique in terms of human life, it is possible to build a Li-Fi network in which the flicker effect does not affect human health because the proposed technique can reduce the flicker noise generated in the Li-Fi system.  
Conceptualization, Y.-Y.W. and J.K.; methodology, Y.-Y.W.; software, Y.-Y.W.; validation, J.K. and Y.-Y.W.; formal analysis, Y.-Y.W.; investigation, Y.-Y.W.; resources, J.K.; data curation, Y.-Y.W.; writing--original draft preparation, Y.-Y.W.; writing--review and editing, J.K.; visualization, Y.-Y.W.; supervision, J.K.; project administration, J.K.; funding acquisition, J.K. All authors have read and agreed to the published version of the manuscript.  
This work is supported by the Korea Agency for Infrastructure Technology Advancement (KALA) grant funded by the Ministry of Land, Infrastructure and Transport (22 PQWO-C153354-04).  
Not applicable.  
We would like to thank researcher [PERSON], Department of Electrical and Electronic Engineering, Yonsei University, Korea for his help in establishing the optical wireless experiment link.  
The authors declare no conflict of interest.