Optimization of three-dimensional superposed constellation with probabilistic shaping for MIMO visible light communication systems

Superposed constellation technology has received increasing attention due to its robustness to the correlation of multiple-input multiple-output (MIMO) channels in visible-light communication (VLC) systems. In this study, a novel superposed three-dimensional 32-quadrature amplitude modulation (3D-32QAM) constellation scheme combined with probabilistic shaping is proposed for MIMO VLC systems. Tetrahedron-shaped 3D-4QAM and cube-shaped 3D-8QAM signals are transmitted from two separate LEDs, which are superposed at the receiver to obtain a cross-shaped 32QAM signal. In addition to improving the minimum Euclidean distance (MED) by using 3D constellations, the proposed scheme focuses on exploring the advantages of superposed 3D constellations in probabilistic shaping. Both global and local probabilistic shaping can be achieved in terms of superposition of 3D constellations, which increase the MED and reduce the risk of nonlinear distortion of light-emitting diodes, respectively. Experimental results demonstrated that the proposed scheme achieved a better normalized generalized mutual information performance and significantly improved the dynamic range of the driving peak-to-peak voltage when compared with the traditional superposed two-dimensional 32QAM constellation schemes.