We consider a two-way half-duplex decode-and-forward (DF) relaying system with multiple pairs of single-antenna users assisted by a cell-free (CF) massive multiple-input multiple-output (mMIMO) architecture with multiple-antenna access points (APs). Under the practical constraint of imperfect channel state information (CSI), we derive the achievable sum spectral efficiency (SE) for a finite number of APs with maximum ratio (MR) linear processing for both reception and transmission in closed-form. Notably, the proposed CF mMIMO relaying architecture, exploiting the spatial diversity, and providing better coverage, outperforms the conventional collocated mMIMO deployment. Moreover, we shed light on the power-scaling laws maintaining a specific SE as the number of APs grows. A thorough examination of the interplay between the transmit powers per pilot symbol and user/APs takes place, and useful conclusions are extracted. Finally, differently to the common approach for power control in CF mMIMO systems, we design a power allocation scheme maximizing the sum SE.

中文翻译：

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无单元大规模MIMO的多对双向DF中继

我们考虑一种双向多路双工解码和转发（DF）中继系统，该系统具有多对单天线用户，并辅以无单元（CF）大规模多输入多输出（mMIMO）体系结构和多个天线接入点（AP）。在不完善的信道状态信息（CSI）的实际约束下，我们得出了有限数量的AP的可实现的总频谱效率（SE），并以最大比率（MR）线性处理以封闭形式进行接收和传输。值得注意的是，提出的CF mMIMO中继体系结构利用空间分集并提供更好的覆盖范围，优于传统的并置mMIMO部署。此外，随着AP数量的增加，我们阐明了维持特定SE的功率调整法则。对每个导频符号和用户/ AP的发射功率之间的相互作用进行了彻底的检查，并得出了有用的结论。最后，与CF mMIMO系统中用于功率控制的常用方法不同，我们设计了一种使总和SE最大化的功率分配方案。