# Analysis and Investigation of Dual-Polarized Color LED Based Visible Light Communication System
## 3 Result and Discussion
Identically, the OOK VLC\({}_{\text{(Blue)}}\) traffics could also reach 560 and 540 Mbps after 3 and 4 m free-space transmissions to satisfy the FEC target under the corresponding illuminances of 5.4 and 2.3 lux, respectively, as illustrated Figure 5b. The insets of (iii) and (iv) of Figure 5b are the measured eye diagrams at the 560 and 540 Mbps VLC\({}_{\text{(Blue)}}\) rates, respectively. As displayed in Figure 5a,b, the modulation rate of the G-LED seems better than that of B-LED due to the characteristics of the component itself. According to the presented dual-polarized G- and B-LED VLC system, the largest VLC traffics of 1200 Gbps (640\({}_{\text{(Green)}}\)) + 560\({}_{\text{(Blue)}}\) Mbps) and 1120 Mbps (580\({}_{\text{(Green)}}\) + 540\({}_{\text{(Blue)}}\) Mbps) were obtainedunder 3 and 4 m free-space link lengths, respectively, when the equalization method was applied on the AFE side under the very low illuminance for decoding data. In the demonstration, we could not only apply two various color LEDs in the presented dual-polarized VLC system but also we could utilize two same color LEDs. We could also utilize a pair of polarization-multiplexing G-LEDs (or B-LEDs) at the VLC-Tx for delivering wireless VLC traffics. Therefore, the maximum dual-polarized G-LED VLC\({}_{\text{(Green)}}\) and B-LED VLC\({}_{\text{(Blue)}}\) rates of 2 \(\times\) 640 and 2 \(\times\) 580 Mbps; and 2 \(\times\) 560 and 2 \(\times\) 540 Mbps were achieved after 3 and 4 m optical wireless connections, respectively. This means that we can arbitrarily use the combination of two LEDs for dual-polarized VLC transmission. Moreover, the observed BER measurements of dual-polarized VLC\({}_{\text{(Green)}}\) and VLC\({}_{\text{(Blue)}}\) under different modulated rates were similar to Figure 5a,b, therefore we do not exhibit the BER measurements here. As mentioned above, this means that we can arbitrarily use the combination of two LEDs for dual-polarized VLC transmission. Figure 2: (**a**) Measured achievable bandwidths of the G\(-\) and B\(-\)LEDs. (**b**) Detected corresponding illuminances of the G\(-\) and B\(-\)LEDs without using POL versus various VLC lengths before launching into the focusing lens. According to the results of Figures 4 and 5, the achieved largest VLC rate depends on the detected illuminance. The obtained VLC power would affect the signal performance. As the VLC transmission length increases, the available signal capacity will be relatively smaller, due to the weaker corresponding detected power. Therefore, a >1 Gbit/s OOK rate through a 4 m wireless connection can be achieved in the proposed dual-polarized LED VLC system. However, while the polarization of two orthogonal POLs is offset at the VLC-Rx, the obtained VLC signal would be influenced and interfered with due to two mixed VLC traffics. To avoid this problem, the two POLs can be controlled for rotation automatically based on the maximum detected illuminance. In the investigation, we demonstrate and discuss the VLC performance of the G- and B-LEDs under the different offset angles. For example, according to the measured results of Figure 5a, we select the 620 and 440 Mbps OOK modulations under the free-space link of 3 and 4 m for VLC(Green) BER demonstration when the various rotation angle of POL is changed gradually. Figure 6a,b present the observed BER performances of G-LED versus the various rotation angles of 0\({}^{\circ}\) to 90\({}^{\circ}\) with the corresponding illuminances of 0 to 2.4 lux and 0 to 0.5 lux, respectively. To meet with the VLC(Green) traffic below the FEC threshold, the offset angles must be small than 50\({}^{\circ}\) and 45\({}^{\circ}\), respectively, after 3 (illuminance = 0.9 lux) and 4 m (illuminance = 0.3 lux) optical wireless transmissions, as illustrated in Figure 6a,b. Figure 3: Different measured VLC rates of (**a**) G– and (**b**) B–LEDs versus the corresponding BER after 3 and 4 m free–space transmissions under WDM VLC connection, respectively.