# Analysis and Investigation of Dual-Polarized Color LED Based Visible Light Communication System
## 2 Proposed VLC Architecture and Experiment  
Generally, to obtain a higher VLC data rate via the LED-based device, wavelength-division-multiplexing (WDM) LEDs were proposed and studied [15; 16]. Hence, the red (R), green (G) and blue (B) LEDs could be used to serve as the optical transmitter (Tx) for WDM VLC transmission at the VLC-Tx side, respectively. Here, the three various modulation signals could be applied to each LED to deliver optical wireless traffic. After a VLC transmission length, the blended WDM RGB signals could be received and decoded by the corresponding optical receiver (Rx), when the RGB filters were in front of each Rx to avoid channel interference at the VLC-Rx side, respectively. However, the optical color filters were needed to choose and match the output spectrum of RGB-LED. Moreover, the achievable VLC rate of R-, G- and B-LEDs depended on their useful modulation bandwidth.  
In our demonstration, to achieve a higher modulation capacity of LED-based VLC traffic systems, the G- and B-LEDs with orthogonal polarization are applied at the VLC-Tx,as illustrated in Figure 1a. Here, the G- and B-LEDs are utilized simultaneously serving as the wireless VLC signal. To deliver two various VLC modulations on the G- and B-LEDs, respectively, two bias-tees (BTs) are used to connect the individual LED and VLC modulation format, when the proper bias currents (I\({}_{\text{bias}}\)) are applied simultaneously. In addition, two optical polarizers (POLs, 400\(-\)700 nm) are placed in front of the G- and B-LEDs to produce orthogonal x- and y-polarization status and avoid the interference of two VLC signals, respectively, as shown in Figure 1a. Through a length of optical wireless transmission, two wireless green and blue VLC signals (VLC\({}_{\text{Green}}\)) and VLC\({}_{\text{Blue}}\)) with orthogonal polarization can be received by the two 400 MHz avalanche photodiodes (APDs) at the VLC-Rx side, respectively. To enhance two detected VLC power and corresponding VLC signals, the two focusing lenses and POLs are also exploited for VLC demodulation, respectively. To improve the detected power sensitivity of the VLC signal, the APD is applied for signal demodulation. Commonly, the indoor illuminance is between 300 and 750 lux, sometimes the illuminance will be lower. So, to detect and decode the VLC signal effectively under a longer wireless length at a lower or extra-low illuminance indoors, the APD-PD can be applied in an LED VLC system as an alternative. Although APD is expensive, it is another option in the VLC system. As a result, the proposed G and B-LED VLC transmission system could double the traffic rate by utilizing dual-polarized VLC channels. However, the two POLs at the VLC-Rx must match the x- and y-polarization directions of the VLC\({}_{\text{Green}}\)) and VLC\({}_{\text{Blue}}\)) signals. Commonly, the offset of x- and y-polarizations of POL\({}_{\text{x}}\) and POL\({}_{\text{y}}\) would result in different insertion losses when the VLC-Rx module moves arbitrarily. In previous studies [17; 18; 19; 20], there was no mention of VLC problems caused by different offset angles. Therefore, in the proposed VLC-Rx module, to detect the two orthogonal VLC signals from the VLC-Tx completely, two rotatable dual-polarized POLs can be controlled automatically based on the maximum received illuminance for decoding VLC data. Moreover, to examine the bit error rate (BER) performance of the proposed LED VLC system, a BER tester is applied for signal measurement.  
Figure 1: (**a**) Proposed dual-polarized G- and B-LED VLC transmission system. (**b**) Observed output spectra of G- and B-LEDs, when the bias currents are both operated at 10 mA.