Multiple Access in Aerial Networks: From Orthogonal and Non-Orthogonal to Rate-Splitting

Recent years have seen an increased interest in the utilization of unmanned aerial vehicles (UAVs) to realize new civilian applications. Specifically, UAVs can be used in cellular networks as aerial base stations (aBS) to communicate with ground users in remote uncovered areas or dense environments requiring high capacity. In order to satisfy the high requirements of the next generation wireless networks, several orthogonal and non-orthogonal multiple access techniques have been investigated. In particular, it has been demonstrated that space-division multiple access (SDMA) and power-domain non-orthogonal multiple access (NOMA) present promising multiplexing gains. Nevertheless, these gains are limited as they depend on the conditions of the environment. Consequently, a generalized multiple access scheme has been recently proposed, called rate-splitting multiple access (RSMA), which is capable of achieving better spectral efficiency gains compared to SDMA and NOMA. In this paper, we present a comprehensive survey of key multiple access technologies adopted for aerial networks, where aBSs are deployed to serve ground users. Since there have been only sporadic results reported on the use of RSMA in aerial systems, we aim to extend the discussion on this topic by modelling and analyzing the weighted sum-rate performance of a two-user network served by an RSMA-based aBS. Finally, related open issues and future research directions are presented.