Abstract : The increasing demand for massive device access in the Internet of Things (IoT) necessitates the use of non-orthogonal multiple access (NOMA) technology and grant-free (GF) transmission to enhance spectrum efficiency. Semi-grant-free (SGF) transmission has garnered significant research attention in recent years because it enables grant-free (GF) users to share resource blocks with grant-based (GB) users, thereby enhancing spectrum efficiency in large-scale device access scenarios. This study focuses on distributed GF transmission and integrates it with the Sparse Code Multiple Access (SCMA) scheme to address the low-latency needs of large-scale short-packet transmission devices while simultaneously meeting the transmission rate requirements of GB users. The proposed SGF-SCMA scheme, based on a user hierarchical strategy, significantly enhances spectral efficiency and reduces delay overhead for GF users compared to existing schemes. The successful transmission probability of SGF-SCMA scheme is derived, and the proposed control transmission factor is optimized to improve the throughput in massive user access scenarios.. The solution of the derived successful transmission probability was showed through simulation, and the crosstalk problem of the SCMA system was analyzed. Considering the delay of GF users, the throughput can be improved by 20% to 43% compared with the existing SGF scheme.
Index terms : grant-free, SCMA, Internet of Things, NOMA