# Analysis and Investigation of Dual-Polarized Color LED Based Visible Light Communication System
## 3 Result and Discussion
Figure 4: Measured corresponding illuminances of the G\(-\) and B\(-\)LEDs after passing through two POLs with parallel polarization before launching into the focusing lens. Figure 5: Measured different VLC rates of (**a**) G\(-\) and (**b**) B\(-\)LEDs versus the corresponding BER after 3 and 4 m free-space transmissions. Insets are the corresponding eye diagrams at the maximum VLC rate below the FEC limit of BER = 3.8 \(\times\) 10\({}^{-3}\). Next, to understand the effect of offset angle for the B-LED-based VLC traffic, we applied 520 and 500 Mbps OOK VLC signals for the BER measurements under various offset angles according to the obtained results of Figure 5b. Therefore, Figure 7a,b exhibit the observed BER presentations of B-LED versus the various rotation angles of 0\({}^{\circ}\) to 90\({}^{\circ}\) under the corresponding illuminances of 0 to 5.4 lux and 0 to 2.4 lux, respectively. To fit the VLC(Blue) signal below the FEC threshold, the offset angles must be small than 45\({}^{\circ}\) and 35\({}^{\circ}\) after 3 (illuminance = 1.6 lux) and 4 m (illuminance = 2.9 lux) free-space connections, as demonstrated in Figure 7a,b. The influence of the offset angle on the maximum VLC modulation rate is also a critical issue. As mentioned above, the maximum VLC modulation rates are 580 and 540 Mbps for the G- and B-LED VLC transmissions under a 4 m free-space link, respectively. In the measurement, when the offset angle of POL is increased gradually from 0\({}^{\circ}\) to 50\({}^{\circ}\), the corresponding largest modulation rates of G- and B-LED VLC traffics can be obtained within the FEC target, as shown in Figure 8. In this measurement, the maximum G-LED VLC of 500, 440 and 300 Mbps are achieved under the offset angle of 30\({}^{\circ}\), 40\({}^{\circ}\) and 50\({}^{\circ}\), respectively. The 500, 450 and 400 Mbps traffics of B-LED VLC link are also observed under the offset angle of 30\({}^{\circ}\), 40\({}^{\circ}\) and 50\({}^{\circ}\), respectively. This is because the illuminance of the G-LED itself is smaller than that of the B-LED. Once the angle is over 50\({}^{\circ}\), the VLC(Green) and VLC(Blue) traffics would be not obtained due to the larger insertion loss. Therefore, the results show that the corresponding illuminances of the G- and B-LEDs must be greater than 0.2 and 1 lux to obtain optimal VLC performance through a 4 m free-space connection. As exhibited in Figure 8, we can obtain the same data rate of 500 Mbps for the VLC(Green) and VLC(Blue) connections, when the offset angle is rotated at 30\({}^{\circ}\). In addition, with a  
Figure 6: VLC(Green) BER measurements of 620 and 440 Mbps after the free—space link of (**a**) 3 and (**b**) 4 m under different corresponding illuminance. gradual increase in the offset angle, the measured modulation signal of the B-LED would also be larger than that of the G-LED. In the demonstration, owing to two blended orthogonal VLC signals, the VLC traffic also can provide security operation in the physical layer based on channel interference if the polarization of the POLs are not matching. In our previous study [23], the two mixed VLC signals with similar power would cause very serious data interference. To decode the dual-polarized VLC signals, two orthogonal POLs with precise direction controls are required at the VLC-Rx side, when the client is in a state of arbitrary movement. The control of the POL is dependent on the maximum detected illuminance for proper rotation. The schematic diagram of the control design is illustrated in Figure 9 for proof of concept. Hence, the corresponding VLC signal can be detected through proper rotation. Additionally, if there are many clients with only one Rx detector in the same room for receiving a dual-polarized VLC signal, the polarization of the POL for each client can be adjusted properly to share and adapt the total capacity. Therefore, the proposed dual-polarized LED VLC system can not only double the data rate but can also provide a security function according to the polarization-based filter.