In this paper, we jointly consider the problems of constellation design and automatic modulation recognition (AMR) for color shift keying (CSK) and optical orthogonal frequency division multiplexing (OOFDM) in visible light communication (VLC) systems. Specifically, we conceive a solution based on the proposed enhanced complex-valued CSK (ECCSK) aided OOFDM (ECCSK-OOFDM) framework for multi-target-color communication (MTCC) scenarios. Firstly, we devise a complex-valued CSK constellation design method under given color/illumination constraints. The new method not only eliminates the color balance distortion issue, but also achieves a better normalized minimum Euclidean distance (NMED). Secondly, an expectation maximization (EM) based ECCSK constellation recognition algorithm is proposed, which enables the system to automatically identify the ECCSK constellation with unknown symbol timing offset (STO) and noise variance. Simulation results demonstrate the effectiveness of the new ECCSK constellation design method and the EM-based AMR algorithm. It shows that the proposed system can provide a performance close to the ideal benchmark scheme in MTCC scenarios.

Colour shift keying (CSK) and optical orthogonal frequency division multiplexing (OOFDM) are among the promising high-rate transmission technologies for visible light communication (VLC). Existing methods that combine CSK and OOFDM typically assume the use of conventional CSK constellations such as those defined by the IEEE 802.15.7 standard. However, such a straightforward CSK-OOFDM combination distorts the colour balance and the modulated VLC signal is therefore no longer white. In this paper, we design optimised CSK constellations that work in the complex domain under a given colour constraint. Simulation results demonstrate the effectiveness of the new design method, which not only produces new constellations with an improved normalised minimum Euclidean distance (NMED) compared with the traditional counterparts, hence improves the achievable bit error rate (BER) performance, but also eliminates the colour balance distortion issue of conventional CSK-OOFDM systems.