## 3 Experiment Result
### Communication Test  
Firstly, bit error rate (BER) performance is tested by varying the illuminance (lux) on the license plate from the two WLED-based headlamps. The illuminance is closely linked with the actual link distance on the road. For instance, the levels used in this work are 5 lux, 10 lux, 30 lux, and 100 lux, and they represent the distances of about 80 m, 60 m, 30 m, and 15 m, respectively [6; 15]. The commercial WLED headlamps are modulated with a DCO-OFDM using an adaptive bit and power-loading scheme. The two WLED modules are synchronized and send the same data stream. A detailed description of the modulation technique used here is found in Ref. [16]. The FFT size and cyclic prefix length were set to 512 and 10, respectively. To ensure a reliable experiment, the uniform flood light covering the entire license plate is applied. The measured data rate versus BER plots are shown in Figure 7a.  
Figure 6: (**a**) Comparison of received signal strength with and without beam steering; (**b**) Beamsteering and tracking setup.  
Figure 7: (**a**) BER performance according to different lux levels of LED headlamp; and (**b**) constellations, measured SNR and allocated bits with adaptive bit and power loading.  
Even at 5 lux illuminance, the data rate of 4.5 Mbps under a forward error correction (FEC) with BER threshold of 3.8 \(\times\) 10\({}^{-3}\) is achieved. At 100 lux, the data rate is much improved to 54 Mbps. This is mainly due to the high SNR available from the high illuminance. The received 8 QAM and 16 QAM constellations for the data rate of 52 Mbps at 100 lux case are presented in Figure 7b. The SNR is measured by analyzing the received signal statistically in comparison with the transmitted signal [16]. Additionally, shown are the measured SNR and the allocated bits, accordingly.  
The experiment is also performed for the blue beam-steering channel. The intensity on the license plate is varied by adjusting the driving conditions and monitored by optical power meter (Thorlabs, PM400). The tested intensities on the license plate are 0.01, 0.08, and 0.16 W/m\({}^{2}\). As shown in Figure 8a, the data rates of 194 Mbps, 462 Mbps, and 532 Mbps are achieved, respectively. Additionally, the constellations, the measured SNR, and the allocated bits for the case of 532 Mbps are shown in Figure 8b.  
Next, experiments to check the impact of ambient light on communication performance are conducted. A high-power WLED is used to consider a strong ambient light from adjacent light sources. The ambient light is applied to both the headlamp channel and the blue-beam channel. Levels of 400 lux and 2000 lux are investigated, and the case without the ambient light is also presented as a reference. As shown in Figure 9a, in the case of headlamp channel, the data rate without the ambient light is 54 Mbps with 100 lux. However, the rate is decreased to 38 Mbps and 29.3 Mbps under the ambient light with 400 lux and 2000 lux, respectively, as shown in Figure 9b. Additionally, the data rate of 462 Mbps achieved with 0.08 W/m\({}^{2}\) from the blue LD channel is decreased to 404 Mbps and 367 Mbps under the ambient light with 400 lux and 2000 lux, respectively. The effect of ambient light on the SNR in each case is shown in Figure 9b,d. Although the SNR deteriorated due to the ambient light much more strongly than the communication signal, the proposed system still has a reasonably good performance. This is due to the fact that the APD placed downward in the proposed system does not see the ambient light directly incident on the license plate. Additionally, the FC in front of it only absorbs a certain portion of the incoming ambient light by its wavelength-dependent absorption property [11].  
Figure 8: (a) BER performance according to output power of LD; and (b) constellations, measured SNR and allocated bits with adaptive bit and power loading.