Linear Optimal Signal Designs for Multi-Color MISO-VLC Systems Adapted to CCT Requirement

In visible light communications (VLC), multi-color light emitting diodes (LEDs) have higher modulation bandwidth and better chromatic property compared with traditional phosphor-converted LEDs. For multi-color multi-input single-output (MISO) VLC systems, multiple colors of one LED are not independent with each other since they are limited jointly with chromaticity constraint. In this paper, we develop a novel linear optimal signal constellation (LOSC) for three-color LEDs and four-color LEDs to maximize the minimum Euclidean distance of received signal constellation under lighting constraints. The main signal design problem is formulated as a max-min optimization problem with continuous variables and discrete variables. To solve this problem, we disassemble it into two parts. In the inner part, we develop a linear optimal signal structure under maximum likelihood detection criterion for the fixed average optical power of each color. In the outer part, we transform the non-convex optimization problem into several convex optimization problems with polar coordinate transformations and slack variables. Combining these two parts, our proposed LOSC design scheme shows a lower computation complexity than the exhaustive search scheme. For different given correlated color temperature (CCT) values, the LOSC would always provide an efficient signal design method adaptively. Simulation results demonstrate that our proposed LOSC achieves better bit error rate performances than the conventional schemes.