3GPP RedCap Non-Terrestrial Networks: Direct-to-Device Feasibility Study

In the emerging New Space ecosystem, direct-to-device (D2D) communication enables direct transmission between a compact terrestrial user equipment (UE), including a handheld or a machine-type device, and satellites without using the terrestrial network. Unlike conventional satellite communication, modern D2D communication connects the satellites with UEs which are primarily designed for terrestrial use. Industry and academia researchers are investigating massive machine type communications (mMTC) technologies and their potential for D2D to support low data rate Internet of things (IoT) applications. This article extends the concept of D2D by advocating for 5G Reduced Capability (RedCap) and low Earth orbit (LEO) satellite integration. The RedCap D2D concept will support verticals, which require data rates higher than the existing mMTC technologies, but with lower rates and complexity than the technologies supporting enhanced mobile broadband (eMBB) applications. More specifically, we conduct a feasibility study in terms of the link budget and the random access scalability analysis to investigate the limitations of a direct connection from an unmodified terrestrial RedCap UE to an Earth orbiting LEO satellite. Despite the challenging link budget, our findings indicate that RedCap D2D communications is still feasible when using the high antenna gain at satellite. We also study the intra-satellite handover rate for two alternative cell size configurations. Finally, we highlight some open challenges and future research directions.