Although the use of coordinated direct and relay transmission (CDRT) in non-orthogonal multiple access (NOMA) can extend the coverage, its duplicated transmission reduces the spectrum efficiency (SE) of NOMA. To improve the SE, we propose a spectrum-efficient scheme for NOMA-based CDRT over Nakagami- $m$ fading channels. In this scheme, the base station (BS) connects with a cell-center user (CCU) directly while communicating with a cell-edge user (CEU) via a relay and the CCU. Then, the relay and the CCU use network coding to process and retransmit the signals sent by the BS first and the CEU later. Finally, the BS and the relay simultaneously broadcast downlink signals. We derive the closed-form expressions for the average SE, the user fairness index and the energy efficiency (EE) as well as the asymptotic average SE using both perfect and imperfect successive interference cancellation (SIC). Simulations verify the correctness of our theoretical analysis and the superiority of the proposed scheme in SE and EE.

中文翻译：

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在 Nakagami-m 衰落信道中使用 NOMA 和网络编码协调直接和中继传输

虽然在非正交多址（NOMA）中使用协调直接中继传输（CDRT）可以扩大覆盖范围，但其重复传输降低了NOMA的频谱效率（SE）。为了改善 SE，我们为 Nakagami-$m$ 衰落信道上的基于 NOMA 的 CDRT 提出了一种频谱效率高的方案。在该方案中，基站(BS)直接与小区中心用户(CCU)连接，同时通过中继和CCU与小区边缘用户(CEU)通信。然后，中继和CCU使用网络编码对先BS发来的信号，后CEU发来的信号进行处理和重传。最后，基站和中继同时广播下行信号。我们推导出平均 SE 的闭式表达式，用户公平指数和能量效率 (EE) 以及使用完美和不完美连续干扰消除 (SIC) 的渐近平均 SE。仿真验证了我们理论分析的正确性以及所提出方案在 SE 和 EE 中的优越性。