Improved error performance of variable PPM for visible light communication

We propose a variable multi-pulse pulse-position-modulation (VMPPM) system with a dimming control that combines the use of pseudo-orthogonal codes and MPPM. In VMPPM, r slots are selected from n MPPM slots and codes for data are generated in each selected r slot according to source data ⌊log₂ n : r⌋ + r [bit/frame]. This enables VMPPM to achieve a higher data-transmission rate than the conventional variable PPM (VPPM). Additionally, VMPPM can represent the dimming level of n - r + 1 stages by using the empty n - r slots. In this paper, the symbol error rate (SER) of VPPM is investigated through theoretical analysis. In addition, the SER performance and spectral efficiency of VMPPM is compared with those of VPPM and variable rate MPPM (VR-MPPM). Specifically, the SER performance is evaluated by taking into account the human eye response. Consequently, VPPM can greatly decrease the maximum ratio of electrical signal power per bit to noise at SER = 10^-7 for conventional dimmable modulation schemes about 20 dB. Moreover, VMPPM is superior to VPPM in the spectral efficiency.