稀疏碼多重存取(Sparse Code Multiple Access, SCMA)為一種非正交多重存取技術(Non-Orthogonal Multiple Access, NOMA)，相對正交多重存取技術能提供更多接取資源。行動通訊系統之上行鏈路(Uplink)採用免授權(Grant-Free)機制，可大量減少排程的延遲。為了滿足5G在高可靠低延遲通訊(uRLLC)及大規模機器型通訊(mMTC)中的需求，免授權上行鏈路的稀疏碼多重存取系統被提出。
本論文即針對基於SCMA的免授權上行鏈路，探討其完整系統設計，尤其一般SCMA接收端採用訊息傳遞演算法(Message Passing Algorithm, MPA)作為多用戶解碼，而用戶的實際存在與否對MPA解碼處理效能影響甚大。因此本論文特別研討活動(active)用戶的偵測技術。除了一般以通道估測作為用戶活動性的判斷外，本論文另採用多區塊組合判斷，以提高用戶活動性偵測之正確率。本論文最終提出一完整系統架構包括用戶活動性偵測、多用戶通道估測、多用戶資訊偵測及通道編碼。

Sparse code multiple access (SCMA) is one of the non-orthogonal multiple access (NOMA) technique, which can supply more access resource comparing to orthogonal multiple access technique. Grant-Free transmission is considered to reduce the scheduling latency in uplink mobile communication systems. To support the needed of the scenario in 5G, ultra-reliable and low latency communications (uRLLC) and massive machine type communications (mMTC), uplink grant-free transmission based on SCMA has been proposed.
This thesis aim to explore the system designed based on uplink grant-free SCMA. In particular, SCMA system adopts message passing algorithm (MPA) as multi-user detection, which considers all UEs to be active. Thus, whether the existence of UE is crucial to MPA decoding. In addition to the channel estimation based algorithm for active UE decision making, diversity of multi-block combination are considered to enhance the accuracy of active UE detection. This thesis will propose a complete system structure including user activity detection, multi-user channel estimation, multi-user detection and channel coding.

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