## 1 Introduction  
As the mobile communication network evolves from 5G to 6G, it is expected that the shortage of radio frequency (RF) resources will intensify [1, 2]. Since various wireless communication services may be limited due to the lack of RF bandwidth, visible light communication (VLC) or light-fidelity (Li-Fi) technology can play a complementary role in the next-generation wireless transmission system in both the industrial and telecommunication field [3, 4]. Furthermore, it has been recognized as one of the main alternatives [5, 6]. In the Li-Fi system, after the white light emitting diode (LED) light is modulated by a digital or analog signal and then transmitted wirelessly, the modulated light is received by a photodetector. Here, the flickering phenomenon of the LED may occur in the electric circuit driving the white LED and in the process of the white LED light being modulated by the signal [7, 8]. Flicker is a phenomenon in which the brightness of light changes rapidly and repeatedly over time. The effects of flicker on human health range from reduced visual performance to nonspecific malaise to the development of some forms of epilepsy. The flicker can be divided into two main categories. It can be divided into visible flicker, which can cause epileptic seizures caused by direct light exposure for a few seconds, and imperceptible flicker, which can cause malaise, headache, and visual loss due to long-term indirect light exposure. Visible flicker generally corresponds to the flicker of visible light in the frequency range of from 3 Hz to 70 Hz, while imperceptible flicker refers to the invisible modulation of light at frequencies that the human eye cannot perceive [9; 10]. This occurs in the form of a sinusoidal wave near 100 Hz or 120 Hz, which is twice the frequency of AC voltage [8; 9; 10]. It is easy to recognize the danger of visible flicker since it is immediately observed by the human eye. However, the risk of the imperceptible flicker may be higher because it is not well detected by the human eye. Since the transmission rate of the VLC or Li-Fi system that has been reported so far shows at least 10 Mb/s, the imperceptible flicker is more likely to occur than the visible flicker. Existing techniques for reducing a flicker noise can be broadly classified into two categories [11; 12; 13; 14; 15; 16; 17]. One of them is to use various modulation formats that avoid the 120 Hz frequency where the flicker noise is located [11; 12; 13; 14; 15]. The other is a technique to design a circuit that drives an LED while suppressing the flicker noise [16; 17]. Both techniques are implemented at the transmitter of the Li-Fi system. Techniques using the modulation format can reduce the transmission rate or complicate the hardware structure of the transmitter. The technique of implementing a driving circuit that reduces flicker noise can increase the cost of implementing a Li-Fi system, so it can slow down the commercialization of Li-Fi systems that need to implement lighting and communication functions at the same time. However, our proposed technique is located at the receiver of the Li-Fi system. It can reduce flicker noise regardless of various digital modulation formats. In addition, there is no need to change the structure of the electrical circuit driving the LED to reduce the flicker noise. To our knowledge, a technique to reduce flicker noise in the receiver is proposed for the first time. Furthermore, we know that a technique, which can reduce the flicker noise regardless of the modulation format of the received signal, is also proposed for the first time. Since the proposed technique is implemented based on digital signal processing (DSP) technology, a DSP-based circuit should be designed in the receiver when the Li-Fi link is realized in an actual network. Additionally, the waveform of the input signal should be a digital waveform such as on-off-keying (OOK) and pulse amplitude waveform (PAM) because the proposed technique only works well when the signal and noise have different waveforms.