Leveraging Decoupling Multi-Connectivity for Direct Satellite-to-Device Communications in 6G: Architecture, Technologies, and Opportunities

Direct Satellite-to-Device (DS2D) communication is a viable solution in future Sixth Generation (6G) Non-Terrestrial Networks (NTNs) to provide global connectivity, especially in remote and underserved regions, which has attracted growing interests from industry and academia. With the 3rd Generation Partnership Project (3GPP) technical specifications now incorporating NTN elements into the Fifth Generation (5G) architecture, industrial efforts are devoted towards the creation and implementation of DS2D communication. In this article, we summarize the progression of standards for NTN outlined by 3GPP, and propose a novel 6G decoupling Multi-Connectivity (MC) Radio Access Network (RAN) architecture. Specifically, we trace the development from simple transparent forwarding satellite network to complex regenerative satellite network, and analyze the superiority of the proposed 6G decoupling MC RAN architecture. Then, from a service scenario perspective, we start with the spectrum coexistence between the NTN and TN, then advance to NTN-TN service continuity, and culminates in the fusion of MC. Furthermore, we explore key technologies that enhance the compatibility of decoupling MC with DS2D communication. Finally, a case study on multi-satellite cooperation DS2D communication in a 3GPP NTN scenario is presented to demonstrate its superiority.