Hybrid PWM/DPAM Dimming Control for Digital Color Shift Keying Using RGB-LED Array

Abstract

As a method of dimming support for visible light communications using a massive LED array, we consider a hybrid pulse width modulation/digitally controlled pulse amplitude modulation (PWM/DPAM) system. Especially, in this paper, we consider a digital color shift keying (DCSK) using an RGB-LED array as an optical intensity modulation scheme, that transmits data through the ratio of the optical intensities (i.e., color) emitted by red, green, and blue LEDs. In DCSK, since only one color is activated in each RGB-LED at a time, the color can be represented by the combination of the digitally (i.e., linearly) controlled “ON-OFF” LEDs. In general, for the dimming control system of DCSK, two schemes have been considered. One is the dimming control by PWM, which changes the duty cycle of optical transmit signals, and the other is the dimming control by DPAM, which changes the number of active LEDs in the RGB-LED array. In this paper, PWM and DPAM are combined to realize a higher spectral efficiency than PWM and a wider dimming range than DPAM. We evaluate the error performances of the proposed system, DCSK with PWM, and DCSK with DPAM from a simulation analysis under several measured light dimming levels. The results show that DPAM should be used only for low bit rate systems because the effect of inter-symbol-interference (ISI), caused by the LED frequency response, increases at a high bit rate. While, PWM is significantly robust against ISI because the PWM signal duration is limited in a symbol duration at the low dimming levels and the empty duration can mitigate the effect of ISI even if the bit rate is high. When focusing on symbol error rate performances corresponding to the dimming levels, the hybrid dimming control and the PWM dimming control can achieve lower signal energy-to-noise-ratio at the dimming level of low and high, respectively.