User pairing and power allocation strategies for downlink NOMA-based VLC systems: An overview

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In current era, non-orthogonal multiple access (NOMA) has been explored as a multiple access scheme for visible light communication (VLC) systems owing to its many advantages over the traditional VLC systems employing orthogonal multiple access (OMA) techniques. The implementation of NOMA may undoubtedly increase the overall system throughput, spectral efficiency and facilitate massive connectivity in wireless communication networks. Also, the energy efficiency of multi-user VLC systems may be maximized by implementing NOMA with an optimal power allocation strategy. There are still many issues which need to be addressed further in order to ensure the effective and efficient use of NOMA in VLC systems. The user pairing and power allocation are two such important issues which play a significant role in improving the overall performance of NOMA-based VLC systems. In literature, a lot of research works exist which are focused on user pairing mechanisms and power allocation methods in this area of research. In this paper we present a comprehensive overview of user pairing and power allocation techniques for NOMA-based VLC systems. In particular, we present the state-of-the-art works on various user pairing schemes, such as conventional near-far pairing, uniform channel gain difference (UCGD) pairing, hybrid pairing, divide-and-next largest gain user pairing (D-NLUPA), vertical user pairing, virtual user pairing, etc., with their pros and cons to enhance the overall system performance. We also provide an overview of various power allocation strategies for NOMA-based VLC systems, viz., static power allocation or fixed power allocation (FPA), fractional transmit power allocation (FTPA) including gain ratio power allocation (GRPA), normalized gain difference power allocation (NGDPA) and normalized logarithmic gain ratio power algorithm (NLGRPA) strategies. An analysis based on various performance parameters such as sum rate, bit error rate (BER), outage probability, etc. has been discussed. We have also highlighted the further research issues and associated challenges related to user pairing and power allocation strategies in this field. In a nutshell, this paper provides an overview of the existing and ongoing research trends in user pairing and power allocation strategies for NOMA-based VLC systems, which may be useful to the research community working in this domain.