The ever-increasing demand of mobile Internet and multimedia services poses unique and significant challenges for current and future generation wireless networks. These challenges are mainly related to the support of massive ubiquitous connectivity, low latency, and highly efficient utilization of spectrum resources. Therefore, it is of a paramount importance to address them in the design and deployment of future wireless networks. To this end, this paper provides a comprehensive overview of multiple access schemes in the context of visible light communications (VLC). Specifically, we initially provide a thorough overview of frequency-domain multiple access techniques for single- and multi-carrier systems, which is then followed by an in-depth discussion on the technical considerations of optical code-division multiple access techniques and their adoption in indoor VLC applications. Furthermore, we address space-division multiple access and, finally, we revisit and analyze a new promising technology, namely, non-orthogonal multiple access (NOMA). It is shown that NOMA exhibits significant benefits in VLC systems that can outperform conventional multiple access schemes, rendering it a particularly effective solution. Furthermore, it is demonstrated that it can coexist with the above optical multiple access schemes, which can maximize the performance and efficiency of future VLC systems. However, it is also shown that the potential of NOMA in VLC systems requires efficient addressing of a series of related challenges and constraints, such as fast and effective successive interference cancellation techniques, compensation and mitigation of LED non-linearity, and imperfect and/or outdated channel state information.
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