Constellation optimization for Visible Light Communication under Ergodic Optical Channel

Visible light communication (VLC) has gradually gained increasing attention in recent years, but new researches are still needed for conventional technologies in the VLC system, as the visible light signal must be real-valued and non-negative, and it is transmitted over the specific ergodic channel. In this paper, the definition of ergodic optical channel is firstly proposed, and the coded modulation average mutual information (CMAMI) of the VLC system is investigated under both fixed and ergodic optical channels. The gradient descent (GD) algorithm for constellation optimization of amplitude-phase modulation (APM) symbols is adopted, while the improved method for GD based on central symmetry is also proposed to accelerate the convergence for higher order constellations. The numerical simulation proves that the VLC system with optimized constellations can reach larger CM-AMI based on asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) compared with quadrature amplitude modulation (QAM) systems, and the influence of the ergodic optical channel is also evaluated.