Grant-Free Sparse Code Multiple Access for Uplink Massive Machine-Type Communications and Its Real-Time Receiver Design

Massive Machine-type Communications (mMTCs) is a major use case for the 5G standard. The grant-free (GF) sparse-coded multiple access (SCMA) transmission is particularly spectrum efficient in the sporadic uplink traffic, which is characteristic of the mMTC networks. In this paper, an uplink GF-SCMA receiver with a user activity detection (UAD) function was designed and implemented. In particular, several new techniques were proposed to enhance the SCMA decoder performance; they include delayed serial update, early stopping, message passing algorithm equation reformulation, distance approximation, and sum circuit sharing. To meet the real-time operation requirements, we implemented key inner receiver function circuits, such as carrier frequency synchronization, user signature detection, channel estimation and compensation, soft SCMA detection, etc. on a Xilinx KCU1500 FPGA chip. Finally, an over-the-air (OTA) prototype has been constructed, demonstrating the efficient and reliable multi-user GF-SCMA uplink transmission of the proposed system.