Downlink Multi-user RSMA with Coherent Beamforming in Space-Air-Ground-Sea Networks

Space-Air-Ground-Sea Networks (SAGSNs) are potential scenarios in the sixth generation (6G) wireless communications, which enable wide connectivities among different domains and platforms. For high rates and flexible links, many works have been done in SAGSN. Rate splitting multiple access (RSMA) is one of the effective techniques to achieve efficient and flexible connects for SAGSNs, where an orthogonal and non-orthogonal hybrid multiplexing method is introduced. The flexible multiplexing management strategy enables RSMA schemes to enhance sum spectrum efficiencies (SEs) of systems while meeting users' quality of service (QoS) requirements. To avoid interferences among independent streams in RSMA systems, coherent beamforming vectors are designed based on channel state information (CSI). However, traditional RSMA and coherent beamforming ignore diversity gain introduced by multi-user diversity. Therefore, in order to further promote performances of SAGSNs, this paper proposes a downlink multi-user coherent beamforming RSMA system (MC-RSMA), where user space is applied to obtain extra gain. Then, a non-convex optimization problem is constructed by considering rate division, power allocation and user schedule strategy under the restrictions of users' QoSs. To solve the optimization problem, the original issue is divided into three suboptimal issues, and a joint resource allocation algorithm is raised to derive the optimal solution. Numerical results reveal that the proposed system attains a satisfactory SE in SAGSNs, benefitting from elastic interference management and multiuser diversity.