We consider a reconfigurable intelligent surface (RIS) assisted cell-free massive multiple-input multiple-output non-orthogonal multiple access (NOMA) system, where each access point (AP) serves all the users with the aid of the RIS. We practically model the system by considering imperfect instantaneous channel state information (CSI) and employing imperfect successive interference cancellation at the users end. We first obtain the channel estimates using linear minimum mean square error approach considering the spatial correlation at the RIS and then derive a closed-form downlink spectral efficiency (SE) expression using the statistical CSI. We next formulate a joint optimization problem to maximize the sum SE of the system. We first introduce a novel successive Quadratic Transform (successive-QT) algorithm to optimize the transmit power coefficients using the concept of block optimization along with quadratic transform and then use the particle swarm optimization technique to design the RIS phase shifts. Note that most of the existing works on RIS-aided cell-free systems are specific instances of the general scenario studied in this work. We numerically show that i) the RIS-assisted link is more advantageous at lower transmit power regions where the direct link between AP and user is weak, ii) NOMA outperforms orthogonal multiple access schemes in terms of SE, and iii) the proposed joint optimization framework significantly improves the sum SE of the system.

翻译：本文研究一种可重构智能表面（RIS）辅助的无蜂窝大规模多输入多输出非正交多址接入（NOMA）系统，其中每个接入点（AP）在RIS的辅助下为所有用户提供服务。我们通过考虑不完美的瞬时信道状态信息（CSI）以及在用户端采用不完美的连续干扰消除技术，对系统进行了实际建模。首先，考虑RIS的空间相关性，采用线性最小均方误差方法获得信道估计；随后，利用统计CSI推导出下行链路频谱效率（SE）的闭式表达式。接着，我们构建了一个联合优化问题以最大化系统的总SE。我们首先引入一种新颖的连续二次变换（successive-QT）算法，结合块优化思想与二次变换来优化发射功率系数；然后采用粒子群优化技术设计RIS相位偏移。需要指出的是，现有关于RIS辅助无蜂窝系统的研究大多是本文所探讨一般场景的特例。数值结果表明：i）在AP与用户间直接链路较弱的低发射功率区域，RIS辅助链路更具优势；ii）在频谱效率方面，NOMA优于正交多址接入方案；iii）所提出的联合优化框架能显著提升系统总SE。