## 3 Experimental Setup  
Figure 2 shows the experimental Li-Fi link, which was implemented to experimentally verify the proposed technique for reducing flicker noise. In the implemented Li-Fi link, the maximum light output after 1 m optical wireless transmission was 900 Lux. The frequency response of the Li-Fi link was about 10 MHz. Both NRZ-OOK signal generation and flicker noise reduction in the area corresponding to the dashed box are handled offline using MATLAB. The NRZ-OOK signals are produced using pseudo-random bit sequence (PRBS) with length of \(2^{17}-1\). The number of samples per bit was set to 4. The bandwidth of NRZ-OOK signal was 100 kHz. The NRZ-OOK signal was loaded onto an arbitrary waveform generator (AWG: AWG70002A, Tektronix) sampled at 400 ks/s. The NRZ-OOK signal from the AWG was amplified using the low noise amplifier (LNA: PE15A63012, Pasternack). The light from the white LED (LUW-W5 AM, OSRAM(r), Munich, Germany) was directly modulated by the NRZ-OOK signal biased using a bias-tee. The used white LED was manufactured in surface mount technology (SMT) package type. Its forward voltage was 3.2 V. It was implemented based on ThinGaN. Its light output was 116 lm and the viewing angle of 50% light output was 170\({}^{\circ}\). The switch-mode power supply (SMPS), which was used to apply DC voltage to white LED, employed 220 V AC power as input power and then selectively generates output DC voltage from 3 V to 12 V. The modulated light was transmitted wirelessly over a distance of 1 m and then received at the Avalanche Photodetector (APD: C5331-11, Hamamatsu, Shizuoka, Japan, 100-MHz bandwidth) after a biconvex lens (LB1723-A, Thorlabs, Jessup, MD, USA). Since the light generated from the white LED has a diffuse property, the amount of light received at the photodetector decreases as the transmission distance increases. Therefore, it is necessary to increase the amount of received light. We used a biconvex lens to increase the amount of the received light. The role of the biconvex lens is to collect the diffused light so that the amount of light received at the photodetector is maximized. The specification of the biconvex lens is as  
follows. Its shape was biconvex type, and the focal length was 60 mm. Its diameter was 50.8 mm. The wavelength passband of light was 350 to 700 nm. The used APD operates at 5 V, and the wavelength range of the received optical signal was from 400 nm to 1000 nm. The maximum optical power that the APD could receive without saturation was 2000 Lux. In addition, the flicker modulation of the transmitted light was measured using a flicker meter (BTS256-EF, Gigahertz-Optik, Turkenfeld, Germany). Here, the flicker modulation is an index indicating the intensity of the flicker noise [8]. The flicker modulation is obtained using Equation (7) by measuring the maximum luminance and minimum luminance values during one period of flicker noise in the light emitted from the white LED [8]. \[F_{m}=100\times\frac{L_{max}-L_{min}}{L_{max}+L_{min}} \tag{7}\]  
where \(F_{m}\) is a flicker modulation in units of percentage. \(L_{max}\) is a maximum luminance, and \(L_{min}\) is a minimum luminance within one period. The waveform of the received signal was captured using a real-time oscilloscope (MSO 71604C, Tektronix, Beaverton, OR, USA) sampled at 1.6 MS/s and then stored on a computer. The regularization parameter was 5. We recovered the NRZ-OOK signal from the stored waveform using the proposed technique. Next, the recovered NRZ-OOK signal waveform has been finely adjusted according to the input signal level. For your information, the proposed technique was implemented by offline processing in the dashed box. The waveforms at the bottom of Figure 2 show the NRZ-OOK signal waveforms measured at each point. As shown in the waveform measured at point (A), the input NRZ-OOK signal waveform was generated correctly. In the waveform measured from the real-time oscilloscope (point (B)), we found that the NRZ-OOK signal waveform was loaded on the flicker noise.