Design of Multi-Spectral Coded VPPM for optical wireless LED communications

In this paper, we propose a new modulation scheme called Multi-Spectral Coded Variable Pulse Position Modulation (MSC-VPPM) for optical wireless LED communication system. Signals are transmitted using Multi-Spectral Keying (MSK) scheme in spectral domain and Multi-Coded VPPM (MC-VPPM) scheme in time domain. By changing the power ratio of multi-spectral LEDs, MSK data are generated. Then, the power ratio of each LED, which means the average duty ratio of pulses, is used to modulate MC-VPPM. In MC-VPPM part, data are decomposed into two groups. Two groups of signals (Pulse Width Modulation (PWM) and Pulse Position Modulation (PPM) groups) are multi-coded by orthogonal codes for transmitting data simultaneously. Then, each multi-level value of the multi-coded signal is converted to pulse width and position, which results in a processing gain in reception as well as an improved data rate. Through the simulation results, w e c onfirm t hat t he proposed MSC-VPPM scheme shows a reasonable BER curve and much greater achievable data rate.