Hybrid Space Division Multiple Access and Quasi-Orthogonal Multiple Access for Multi-User Underwater Visible Light Communication

Due to severe inter-beam interference and high cost of conventional full space division multiple access (SDMA)-based multi-user underwater visible light communication (MU-UVLC) systems, partial SDMA has been considered to substantially reduce inter-beam interference and costs by enabling closely located users to share the same light beam. Moreover, non-orthogonal multiple access (NOMA) is generally adopted to multiplex users in the same beam. However, for users with similar channel gains, the use of NOMA may suffer from severe error propagation. To this end, we propose and demonstrate a hybrid SDMA and quasi-orthogonal multiple access (QOMA) scheme for performance enhancement of partial SDMA-based MU-UVLC systems. Simulation and experimental results show that, in two-user UVLC system applying hybrid SDMA/QOMA, the bit error rate (BER) performance of the near user can be substantially improved and a much wider range of effective power allocation ratio can be achieved compared with that using conventional hybrid SDMA/NOMA.