## 4 Experimental Results  
Figure 3 shows the flicker modulation, which was measured by changing the SNR of the input NRZ-OOK pulse at each biased voltage. The flicker modulation was measured repeatedly using the flicker meter. The inset in Figure 3 shows the measured optical modulation index (OMI) of white LED while changing the bias voltage. The OMI is defined as the ratio of half the peak-to-peak optical signal power and the average optical power. We calculated the OMI by measuring the intensity of the sinusoidal signal just before its output signal waveform is clipped in the process of modulating the light of the white LED by the sinusoidal signal. As shown in the inset of Figure 3, the maximum optical modulation index was observed when the biased voltage was around 4 V. According to IEEE Standard PAR 1789, there is no risk to health if the flicker modulation does not exceed 4% or less at 120 Hz flicker frequency, and the flicker modulation from 4% to 10% indicates low risk [7]. Importantly, if the Flicker Modulation Index exceeds 10%, it means that a person may be exposed to high health risks [7]. Therefore, IEEE Standard 1789 recommends that the light of the white LED be modulated under the operating condition showing a flicker modulation index of 10% or less. As shown in Figure 3, it can be seen that the input SNR can be set up to about 20 dB when the biased voltage is 4 V based on the 10% flicker modulation. In addition, the result of Figure 3 tells us that it is possible to minimize the flicker effect on health when the input SNR is set up to 15.5 dB and 16.5 dB, respectively, at the bias voltage of 2 V and 6 V. This means it is possible to transmit an NRZ-OOK signal with about 4.5 dB greater near the 4 V biased voltage than when the biased voltage is 2 V. The reason why the light from the white LED can be modulated directly by the NRZ-OOK signal with the highest SNR at the biased voltage (4 V) corresponding to the maximum OMI is as follows. If the NRZ-OOK signal is optically modulated at a biased voltage with a low optical modulation index, the output signal waveform can be clipped and distorted as the SNR of the input signal increases. Due to the optical distortion, not only does the intensity of 120 Hz flicker noise increase but also harmonic components of the flicker noise are also generated. Therefore, it is found that the flicker modulation is inversely proportional to the OMI. Therefore, it is important to minimize the intensity of flicker noise by modulating the NRZ-OOK signal in the section where the optical modulation index is maximized. Figure 4 shows the bit error rates (BERs) before and after employing the proposed technique, which were measured repeatedly while increasing the SNR of the input NRZ-OOK signal from 15 dB to 24 dB. In the legend of Figure 4, the filled shapes represent the BERs in the case of not using the proposed technique, and the open shapes correspond to the BERs when the proposed technique is applied. The filled squares and the empty  
Figure 3: Flicker modulation against the SNR change in input NRZ-OOK signal. squares are the measured BERs when the bias voltage is 2 V. The filled triangles and the empty ones are the BERs when the bias voltage is 4 V while the filled and empty circles represent the measured BER when the bias voltage is 6 V. As shown in Figure 4, the BER decreased as the SNR of the input signal increased, and then the BER floor was observed as the SNR exceeded 21 dB. The reason for the BER floor is that even if the SNR increases, the SNR of the output NRZ-OOK signal does not increase continuously and is saturated due to the OMI determined according to the biased voltage. Furthermore, when the bias voltage is 4 V, the BER floor was observed even if the SNR of the input NRZ-OOK signal went up because BERs smaller than the minimum BER (7.63 \(\times\) 10\({}^{-6}\)) corresponding to the PRBS length could not be obtained.