## 3 Results
### Experimental Results
When the voltage or current exceeds the point, the power  
Figure 4: The spectrum of (a) the transmitted signal, (b) the received signal after MRC algorithm, (c) the received signal with only light 1 on, and (d) the received signal with only light 2 on. Figure 3: The measured S21 transfer response from each light to the receiver at 1 m.  
response of the LEDs will not be linear to the power input anymore. Such nonlinear distortion causes a huge loss in the performance of the VLC system. As shown in the subfigures in Figure 5a,b, the width of the PS becomes narrower at the nonlinear points. In the following experiments, the operating point is selected as Vpp at 0.7 V and bias current at 170 mA. The next step is estimating the SNR at each calibration point to generate the entire SNR table. The measured SNR at the discrete calibration points is shown in Figure 6a. The \(y\)-axis is the distance that includes five discrete points. The \(z\)-axis shows the SNR in dB, which can be calculated as \(SNR_{\text{dB}}=10\log_{10}SNR_{\text{ratio}}\). The VLC signal is coupled to a DC voltage by the bias tee which blocks the DC component in the VLC signal, hence the VLC signal coupling is not a simple low-pass system but a band-pass system. Furthermore, VLC is a direct modulation direct detection (DMDD) system. The characteristic of PD brings signal-to-signal beating interference (SSBI) noise, which is distributed in the low frequency spectrum with the shape of an inverted triangle. Therefore, there exists higher noise in the low frequency of VLC than the high frequency, which causes the decrease of SNR. The SNR in the subcarriers indexing from 40 to 160 is large, indicating a good frequency response of the VLC system. The corresponding frequency range is approximately 50 MHz to 220 MHz. The rest subcarriers with the frequency above 250 MHz are limited by the bandwidth of the LED. Figure 6a shows a significant drop of the SNR from 3 m to 4 m due to the low SNR in the high frequency subcarriers. Interpolating the SNR at the calibration points, the SNR table is finished. However, the SNR does not decrease linearly as the distance increases. In the near field, the light intensity is high enough to support the communication, thus the SNR changes linearly with the distance changes. However, in the far field, the light intensity is not enough to support the communication in the highest data rate. As the intensity decreases nonlinearly, the SNR decreases nonlinearly as well. The great decrease of SNR between 3 m to 4 m indicates that the light intensity is not enough for the communication beyond the  
Figure 5: Comparison of the AIR with the PD array and single PD at 1 m under (**a**) different Vpp with 170 mA bias current, and (**b**) LED bias current with 0.7 Vpp. Figure 6: The measured SNR (**a**) at the calibration points and the SNR table interpolated with a (**b**) linear method and a (**c**) partially nonlinear method. distance of 3 m. Therefore, the interpolation method should alter from the linear one to the nonlinear one beyond 3 m. Figure 6b shows the interpolated SNR table with all linear methods, while Figure 6c shows the one with nonlinear interpolation between 3 m to 5 m. The performance of these two SNR tables is evaluated in the testing phase. In the testing phase, the SNR tables are then tested at both the calibration points and the other points. Figure 7a-e shows the receiving GMI and the source entropy at the calibration points. The unevenness in the frequency response of the LED headlights is reflected on both the GMI and the source entropy. The noise level of LEDs in the low frequency part is higher than the higher frequency part. Therefore, the GMI is very low at the first subcarrier, and increases in the following subcarriers.