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.

中文翻译：

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用于上行链路大规模机器类型通信的无授权稀疏码多址接入及其实时接收机设计

大规模机器类型通信 (mMTC) 是 5G 标准的主要用例。无授权 (GF) 稀疏编码多址 (SCMA) 传输在零星上行链路流量中特别具有频谱效率，这是 mMTC 网络的特征。本文设计并实现了一种具有用户活动检测（UAD）功能的上行GF-SCMA接收机。特别是，提出了几种新技术来增强 SCMA 解码器的性能；它们包括延迟串行更新、提前停止、消息传递算法方程重构、距离近似和求和电路共享。为了满足实时操作要求，我们实现了关键的内部接收器功能电路，例如载频同步、用户签名检测、信道估计和补偿、软SCMA检测等。在 Xilinx KCU1500 FPGA 芯片上。最后，构建了一个空中（OTA）原型，展示了所提出系统的高效可靠的多用户GF-SCMA上行链路传输。