Code-Domain Non-Orthogonal Multiple Access for Visible Light Communications

The key limitation of high-speed indoor visible light communication (VLC) system is the narrow bandwidth of the light sources. Accordingly, in this paper, we introduce the code-domain non-orthogonal multiple access (CD-NOMA) to enhance the achievable throughput in VLC downlink networks. Compared to the power-domain NOMA (PD-NOMA) in VLC systems, the CD-NOMA can be more efficiently combined with the orthogonal frequency division multiplexing (OFDM) transmission so that it will be more attractive for practical implementation. In contrast to the SIC multiuser detection in PD-NOMA, message passing algorithm (MPA) in the factor graph acts as the multiuser detection scheme in the CD-NOMA VLC systems. Hence, combining the features MPA detection and the VLC channel gain, we first develop a power allocation criterion for the superposition signals at each OFDM subcarrier so as to maintain the user fairness. Furthermore, note that under the stable and high signal-to-noise ratio VLC channel, the underlying factor graph's girth indeed dominates the performance of MPA detection. Accordingly, we propose a progressive edge-permutation (PEP) algorithm to optimize the mapping patterns from the users' data to the OFDM subcarriers. Consequently, the girth of the whole factor graph is obviously enlarged and the performance of MPA detection in the CD-NOMA VLC systems is well boosted. The effectiveness of the proposed method is verified via the numerical error performance upper bound analysis and the Monte-Carlo simulation.