格狀編碼調變輔助之非正交多重接取分頻多工系統

Abstract

眾所週知，非正交多重接取 (non-orthogonal multiple access , NOMA) 能夠提供更高的行動傳輸容量，目前在3GPP中NOMA架構已經被列入標準制定協議討論，稱為『多使用者疊加傳輸』(multiuser-superposition-transmission , MUST)。在本文中，我們研究了MUST系統架構並且提出利用『格狀編碼』來輔助NOMA系統。我們所提出的系統包含了『位元層級』的『迴旋編碼』以及『符元層級』的『格狀編碼』。透過設計四維的格狀編碼來提供非均等錯誤保護能力 (unequal-error-protection , UEP) 給不同使用者，位元調變方式採用方形QAM映射的NOMA系統，並且提供近使用者兩種位元傳輸速率。能量權重分配一樣被本文列入考量，因而能夠在實際傳輸系統上提供更多的彈性保護空間。在接收端，本文利用遞迴運算解碼之BCJR演算法，來交換內碼(inner code)與外碼(outer code)的位元資訊，進而達到提供子載波擁有符元層級與位元層級的雙重編碼保護，能夠有效的提高對於頻率選擇性衰減通道的抵抗能力。本文所設計的方法相對於現有LTE規格僅需有最小的變動，在實際傳輸上可行性相當高，模擬結果顯示多維格狀編碼輔助之設計方法，其性能表現優於現有的NOMA系統。

It is known that non-orthogonal multiple access (NOMA) can provide higher capacity than orthogonal multiple access. In 3GPP, some NOMA schemes have been standardized, known as multiuser-superposition-transmission (MUST). In this thesis, we study MUST schemes and propose a trellis-coded-modulation (TCM) assisted NOMA system. The proposed system consists of a convolutional code operated in bit-level and a cascaded TCM code operated in symbol-level. To have an unequal-error-protection property for near and far users, four-dimensional TCM is used to construct square-shape QAM constellations, providing two transmission rates for the near user. Power allocation is reserved in the proposed scheme, effectively increasing the flexibility in real-word applications. At the receiver, an iterative decoding scheme is conducted with the BCJR algorithm. With the proposed scheme, dependence is not only introduced in the bit level but also in the symbol level in the subcarriers. This will enhance the resisting capability for frequency-selective fading. The proposed scheme requires minimum changes in current LTE specifications and its real-world application is highly feasible. Simulation shows that the proposed method performs much better than the existing NOMA scheme.