Uplink Grant-free Multi-codebook SCMA Based on High-overload Codebook Grouping

Fifth generation (5G) wireless networks are expected to support massive connectivity with a great number of users. Current LTE uplink (UL) system is not able to efficiently support massive connectivity, because of the extra overhead and latency with granting. So UL grant-free transmission is proposed for 5G. Sparse code multiple access(SCMA) is a typical non-orthogonal multiple access technology in 5G. With SCMA, multi-user data is spread to same time-frequency resources, such as OFDMA tones, through codebook mapping. In this paper, the general design of high-overload SCMA codebooks is proposed. In the UL grant-free system, Active Users Detection (AUD) in receiver detects the active pilots to determine the active users and their codebooks, and the number of pilots will affect the AUD performance. Every user may use different number of codebooks to encode data to multiple codewords, because of different service, and that is called Multi-codebook SCMA (McSCMA). In this paper, based on grant-free McSCMA, codebook grouping(CG) is proposed for decreasing the number of combinations of codebook(CoC) in order to reduce the number of pilots to improve the AUD performance. The simulation results demonstrate that the design of high-overload codebook is feasible, and CG can improve the AUD performance based on grant-free McSCMA.