# Analysis and Investigation of Dual-Polarized Color LED Based Visible Light Communication System
## 3 Result and Discussion  
In the experiment, the bias currents of the G- and B-LEDs are both operated at 10 mA. The measured output spectra of the G- and B-LEDs are also shown in Figure 1b. We observed that the central wavelengths of G- and B-LEDs are 457 and 521 nm, respectively. To reach a broader modulation bandwidth of LED, an analog front end (AFE) circuit with pre-equalization was also designed to carry various on-off keying (OOK) modulation signals for delivering VLC traffic at the VLC-Tx [6; 8]. In addition, to enhance the detected VLC illuminance, two focusing lenses with 5 cm diameter were also in front of the two APDs, as exhibited in Figure 1a. Due to the smaller bias on each LED, the demonstrated LED VLC system also can achieve better energy efficiency. In the demonstration, Figure 2a is the effective bandwidth measured by G-LED and B-LED itself by using the 400 MHz bandwidth APD, respectively. Here, the corresponding detected bandwidths of G- and B-LEDs are nearly 415 and 380 MHz within a 10 dB bandwidth range, respectively. This is due to the limitation of the original 400 MHz APD. According to the achieved results of Figure 2a, the modulation rate of the G- and B-LEDs should be able to reach 400 Mbit/s initially. The valuable bandwidth of the G-LED is better than that of the B-LED in the observation. Furthermore, the original illuminances of the G- and B-LEDs are also detected under various VLC transmission lengths before entering the POL or color filter, respectively, as schemed in Figure 2b. The detected illuminances of the G- and B-LEDs are between 7 and 136.1 lux and 39.1 and 255.1 lux, respectively, under the free space connection lengths of 1 to 4 m. First, we executed the bit error rate (BER) performances of WDM G- and B-LEDs through 3 and 4 m VLC connections, respectively, when the POLs are not used and the corresponding color filters are applied in the measurement. Here, the greatest OOK modulation rates of G- and B-LEDs was 670 and 580 Mbit/s and 600 and 560 Mbit/s at the forward error correction (FEC) limit (BER \(\leq\) 3.8 \(\times\) 10\({}^{-3}\)) through 3 and 4 m VLC link lengths, respectively, when the pre- and post-equalizations are applied on the Tx- and Rx-VLC sides, as shown in Figure 3a,b. We only used 400 MHz APD with an equalization circuit in the presented VLC system to decode 670 Mbit/s OOK signal. These have two benefits, the first one can reduce costs, and the other one can increase the decoding bandwidth in the demonstration. Second, we performed the proposed dual-polarized VLC system through different free-space transmission lengths. Hence, the corresponding illuminances of the G- and B-LEDs after passing through two POLs with parallel polarization before launching into the focusing lens were measured first. Figure 4 displays the detected illuminance of the G- and B-LEDs after a free-space link length of 1 to 4 m, respectively. The achievable illuminances of the G- and B-LEDs are obtained between 0.5 and 22.7 lux and 2.3 and 31.9 lux, respectively, as seen in Figure 4. Here, to establish the long indoor VLC transmission length, we selected 3 and 4 m links for the VLC\({}_{\text{(Green)}}\) and VLC\({}_{\text{(Blue)}}\) connections, respectively. Figure 5a,b exhibit the obtained bit error rate (BER) performances of the VLC\({}_{\text{(Green)}}\) and VLC\({}_{\text{(Blue)}}\) traffics versus various OOK modulation rates after an optical wireless transmission length of 3 and 4 m, respectively. In the measurement, the largest VLC\({}_{\text{(Green)}}\) data rates of 640 and 580 Mbps were achieved below the forward error correction (FEC) threshold (BER of 3.8 \(\times\) 10\({}^{-3}\)), respectively, through a wireless link of 3 and 4 m, when the corresponding illuminances were 2.4 and 0.5 lux (before entering the lens), as shown in Figure 5a. The insets of (i) and (ii) of Figure 5a are the corresponding eye diagrams at the 640 and 580 Mbps, respectively.