The downlink (DL) of a non-orthogonal-multiple-access (NOMA)-based cell-free massive multiple-input multiple-output (MIMO) system is analyzed, where the channel state information (CSI) is estimated using pilots. It is assumed that the users are grouped into multiple clusters. The same pilot sequences are assigned to the users within the same clusters whereas the pilots allocated to all clusters are mutually orthogonal. First, a user’s bandwidth efficiency (BE) is derived based on his/her channel statistics under the assumption of employing successive interference cancellation (SIC) at the users’ end with no DL training. Next, the classic max-min optimization framework is invoked for maximizing the minimum BE of a user under per-access point (AP) power constraints. The max-min user BE of NOMA-based cell-free massive MIMO is compared to that of its orthogonal multiple-access (OMA) counter part, where all users employ orthogonal pilots. Finally, our numerical results are presented and an operating mode switching scheme is proposed based on the average per-user BE of the system, where the mode set is given by Mode = { OMA, NOMA }. Our numerical results confirm that the switching point between the NOMA and OMA modes depends both on the length of the channel’s coherence time and on the total number of users.

中文翻译：

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基于自适应 NOMA/OMA 模式切换的无小区大规模 MIMO 性能研究

分析了基于非正交多址 (NOMA) 的无小区大规模多输入多输出 (MIMO) 系统的下行链路 (DL)，其中使用导频估计信道状态信息 (CSI)。假设用户被分组到多个集群中。相同的导频序列分配给相同簇内的用户，而分配给所有簇的导频是相互正交的。首先，假设在用户端采用连续干扰消除 (SIC) 而没有 DL 训练，用户的带宽效率 (BE) 是基于他/她的信道统计得出的。接下来，调用经典的最大最小优化框架，以在每个接入点 (AP) 功率约束下最大化用户的最小 BE。将基于 NOMA 的无小区大规模 MIMO 的最大最小用户 BE 与其正交多址 (OMA) 对应部分的最大-最小用户 BE 进行比较，其中所有用户都使用正交导频。最后，我们给出了我们的数值结果，并基于系统的平均每用户 BE 提出了一种操作模式切换方案，其中模式集由 Mode = { OMA, NOMA } 给出。我们的数值结果证实，NOMA 和 OMA 模式之间的切换点取决于信道相干时间的长度和用户总数。