非正交多重接取技術最近引起廣泛的研究，因其有潛力提高細胞邊緣
用戶的吞吐量，並且有效的增加無線頻譜使用效益，其概念是結合重疊
編碼與連續干擾消除從功率層面分離用戶，這點有別於現行的正交分頻
多重接取與分時多工接取。基於合作式通訊為一種有效的空間分集技術
這點，目前有研究將合作式通訊與非正交多重接取技術做結合，在研究
中指出，理想條件下，此種結合確實可以為細胞用戶帶來最大的分集增
益效果。為了使此想法運行於實際的情況，我們將通道的假設改為瑞雷
衰弱結合路徑損耗的模型，並用此通道條件推導出固定用戶與基地台距
離條件下的雙相鍵移與四相鍵移調變符元錯誤率，我們還提出了低複雜
度的用戶分組與配對方法，最後則針對合作時機提出合適的機制設計。
模擬結果指出，在採用合作時機判斷機制的情況下，合作式非正交多重
接取技術可以有效實現分集增益的效果。

Non-orthogonal multiple access (NOMA) has recently attracted much attention for its potential to improve network spectral efficiency and cell-edge user throughput. It allows a base station (BS) to serve multiple users using same resource by combining superposition coding with successive interference cancellation (SIC) in conjunction with power-domain multiplexing. This is different from orthogonal multiple access schemes such as orthogonal frequency division multiple access (OFDMA) and time division multiple access (TDMA). Some recent work combines NOMA with different techniques for further throughput improvement. Particularly, cooperative NOMA (C-NOMA) has been proposed by employing cell-center user as the relay to forward signal to cell-edge user. However, early research is conducted assuming perfect SIC and ignoring path loss
that lead to somewhat optimistic results. In this work, a low-complexity user grouping and pairing scheme is presented rstly. Secondly, we design a cooperative scheme that decides whether direct transmission or cooperative relaying is bene cial to the cell-edge user. Finally, we derive the symbol error probability of quadrature phase shift keying (QPSK) modulations for a given user pair considering channel impairments including Rayleigh fading and path loss. Simulation results verify the accuracy of our analysis and demonstrate the performance of C-NOMA under practical settings in comparison with NOMA and TDMA.

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