A Superimposed Codebook Design for Downlink Sparse Code Multiple Access Visible Light Communication System

The capacity performance of visible light communication (VLC) systems can be improved by using sparse code multiple access (SCMA). Since SCMA is a codebook-based multiple access scheme, the design of the codebook is crucial. SCMA codebook performance is severely affected by shot noise. Although several codebook design metrics have been proposed to compensate for the constellation distortion caused by shot noise, the improvement in performance is limited. In order to solve this problem, we use a separable codebook design structure to derive the analytical expression of the symbol error rate (SER) for the SCMA-VLC system under shot noise. According to the SER expression, we formulate an optimization problem and obtain a multi-user superimposed codebook to achieve a minimum SER for the SCMA-VLC system. The simulation results show that the proposed codebook significantly improves the performance of the SCMA-VLC system, especially in the case of high shot noise.