## 3 Results
### Experimental Results  
Figure 3 shows the S21 transfer response with only one headlight being lighted, which is measured by a network analyzer when the distance between the transmitter and the receiver is 1 m. The S21 transfer response indicates the insertion loss when the signal transfer from the port 1, which is the transmitter here, to the port 2, which is the receiver. In other words, the S21 transfer response is the frequency response of the whole system, which shows the attenuation of each frequency component. It is obvious that the frequency response of the LED headlights is particularly uneven. There are lots of peaks and valleys in the response along the frequency. The frequency response affects the SNR of each subcarrier. PS bitloading is an algorithm which can approach the Shannon limit, which means it can reach the ceil of the spectrum efficiency limited by the SNR. Therefore, PS bitloading is very worthy of application in the headlight based VLC, which may achieve the best data rate individually in all the subcarriers. The figure also shows the difference of the frequency response between the two headlights. In addition, the equalized bandwidths of the LED headlights are not the same. In this experiment, the overall bandwidth is selected by averaging the \(-10\) dB bandwidth of two headlights at 350 MHz. Figure 4 shows the spectrum of transmitted signal and the received signal. The difference of the frequency response between the two headlights shows in the spectrum as well. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Equipment** & **AWG** & **DC** & **EA** & **LED Headlight** & **PD** & **ATT** \\ \hline \multirow{2}{*}{**Model**} & Agilent & GW Instek & Mini-Circuits & Xiaofu & Hamamatsu & HXT \\  & M8190A & GPD-4303S & ZHL-2-8-S+ & IP32 LED & S10784 & kT2.5-30/1S-2S \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The experimental equipment. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**QAM Order** & **Subcarrier Number** & **Data Number** & **Upsampling Number** & \(\mathbf{R_{c}}\) & **NGMI Threshold** \\ \hline
256 & 250 & 420 & 2 & 0.82 & 0.92 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The modulation parameters. The maximal ratio combining (MRC) technology is diversity receiving technology whose purpose is to improve the received signal at the receiver side. When signals that pass through different paths are collected by multiple receivers, MRC can be used to weight and summarize the signals on each receiving path, and the paths with good signals are assigned the highest weight to achieve signal improvement at the receiving end. With the PD array and the MRC technology, the subcarriers that work well with only one headlight are kept and the other ones that work well with both headlights are enhanced. To illustrate the gain from one single PD to the PD array, Figure 5 compares the AIR performance when the distance is 1 m under different peak-to-peak voltage (Vpp) and LED bias current. With only one PD, the maximum AIR at 1 m is about 1.7 Gb/s. At the same point, with a PD array and the MRC technology, the maximum AIR increases 0.12 Gb/s to 1.82 Gb/s due to the noise reduction, which is a 7% gain. Figure 5 also illustrates how the AIR changes with the different working points of the LEDs. The overall performance of the VLC system depends on the characteristic of the electronic elements, such as the hardware equalizer, the Vpp of the modulated signal, and the bias current of the LED. According to the actual setup of the system, the optimal selection of the electrical parameters is obtained through the experiment [11; 19]. In this system, when the Vpp of the modulated signal is lower then 0.7 V or the LED bias current is lower than 170 mA, the AIR increases almost linearly. This indicates the linear working range of the LED. The SNR determines the performance of VLC, and the SNR increases with the light emitting power.