Downlink Rate Splitting Opportunistic Multiple Access for Satellite–Terrestrial Integrated Networks

With the advantages of wide coverage and strong resource collaboration capabilities, satellite-terrestrial integrated networks has become a potential transmission scheme in the sixth generation (6G) communication system. To ensure flexible and seamless coverage, multi-beam scheme is widely used. Meanwhile, with the limitation of resources, multiple access techniques are introduced into satellite-terrestrial integrated networks for providing services to multiple users simultaneously. Non-orthogonal multiple access (NOMA), a popular multiple access technique that utilizes power domain superposition to significantly improve system performance, is widely employed. In order to achieve flexible interference management and further enhance spectral efficiency (SE) performance, rate-splitting multiple access (RSMA) is proposed, which divides information streams into common parts and private parts. For satellite-terrestrial integrated networks, due to specific frequency band constraints and complexity of feedback link construction, channel estimation and information feedback are particularly difficult. Therefore, we consider introducing opportunistic selection into the existing RSMA scheme and propose a novel rate splitting opportunistic multiple access (RSOMA) system for satellite-terrestrial integrated networks. The proposed system utilizes opportunistic selection to combat imperfect CSI and obtain better SE performance. Statistical characteristics of equivalent channels are attained, and we reveal the closed-forms of SE and BER. To maximize the sum SE, a non-convex optimization problem is constructed and is separated into three sub-issues. Finally, a resource iteration optimization algorithm is applied to derive the optimal parameters. Numerical results reveal that the proposed RSOMA system can obtain excellent SE performance without perfect CSI.