Coordinated direct and relay transmission (CDRT) is a promising strategy to extend cell coverage. However, higher spectral efficiency (SE) cannot be guaranteed in CDRT due to the cooperative process. Thus, non-orthogonal multiple access (NOMA) based CDRT is investigated, where a relaying user assists a base station (BS) in communicating with a cell-edge user (CEU) while the BS directly communicates with a cell-center user (CCU). Specifically, a novel joint downlink-uplink adaptive scheme is proposed for NOMA-based CDRT to improve the SE further. The proposed scheme jointly designs uplink and downlink via NOMA and adaptive transmission to implement bidirectional communication. First, we study the performance of the proposed scheme under both perfect and imperfect successive interference cancellation (SIC), and closed-form expressions are derived for the outage probability, diversity order, outage throughput (OT), ergodic sum rate (ESR), and energy efficiency (EE). Then, an optimal power allocation algorithm using the segment optimization and a modified particle swarm optimization algorithm are proposed to maximize the ESR under perfect and imperfect SIC, respectively. Numerical results verify the superiority of the proposed scheme in terms of the ESR, OT, EE, and the outage performance of CEU. The effectiveness of the proposed power allocation algorithms is also validated.

中文翻译：

---

NOMA 网络的自适应协调直接和中继传输：联合下行链路-上行链路方案

协调直接中继传输 (CDRT) 是扩展小区覆盖范围的一种很有前景的策略。但是，由于协作过程，在 CDRT 中无法保证更高的频谱效率 (SE)。因此，研究了基于非正交多址 (NOMA) 的 CDRT，其中中继用户协助基站 (BS) 与小区边缘用户 (CEU) 通信，而 BS 直接与小区中心用户 (CCU) 通信）。具体而言，针对基于 NOMA 的 CDRT 提出了一种新的联合下行链路-上行链路自适应方案，以进一步改善 SE。所提出的方案通过NOMA和自适应传输联合设计上下行链路以实现双向通信。首先，我们研究了所提出方案在完美和不完美连续干扰消除（SIC）下的性能，并导出了中断概率、分集顺序、中断吞吐量 (OT)、遍历总和率 (ESR) 和能源效率 (EE) 的闭式表达式。然后，分别提出了使用分段优化的最优功率分配算法和改进的粒子群优化算法，以分别在完美和不完美 SIC 下最大化 ESR。数值结果验证了该方案在 ESR、OT、EE 和 CEU 中断性能方面的优越性。还验证了所提出的功率分配算法的有效性。提出了使用分段优化的最优功率分配算法和改进的粒子群优化算法，以分别在完美和不完美 SIC 下最大化 ESR。数值结果验证了该方案在 ESR、OT、EE 和 CEU 中断性能方面的优越性。还验证了所提出的功率分配算法的有效性。提出了使用分段优化的最优功率分配算法和改进的粒子群优化算法，以分别在完美和不完美 SIC 下最大化 ESR。数值结果验证了该方案在 ESR、OT、EE 和 CEU 中断性能方面的优越性。所提出的功率分配算法的有效性也得到了验证。