The effects of LED modulation characteristics on the performance of visible light communication system

The visible light communication (VLC) has become a promising solution for robot system design in the radio-frequency (RF)-limited environments due to the safety of the visible light spectrum. The modulation characteristic of GaN-based LEDs, used as a light source in VLC system, was modelled using a parallel equivalent circuit model, in which the negative capacitance is considered. It is demonstrated that the calculated amplitude-frequency response curve is very consistent with experiment result. Under the effect of LED modulation characteristic, the constellation points spread and constellation diagram rotate entirely for QAM and CDMA VLC system with time domain transmission, and constellation point amplitude decrease and constellation point rotate independently for OFDM VLC system with multicarrier transmission. As a result, the VLC system performance decline in different degree corresponding to the signal deterioration at different transmission rate. This research is very meaningful for improving the performance of VLC system by digital signal processing (DSP) or LED design.