Cell-free Massive MIMO is considered as a promising technology for satisfying the increasing number of users and high rate expectations in beyond-5G networks. The key idea is to let many distributed access points (APs) communicate with all users in the network, possibly by using joint coherent signal processing. The aim of this paper is to provide the first comprehensive analysis of this technology under different degrees of cooperation among the APs. Particularly, the uplink spectral efficiencies of four different cell-free implementations are analyzed, with spatially correlated fading and arbitrary linear processing. It turns out that it is possible to outperform conventional Cellular Massive MIMO and small cell networks by a wide margin, but only using global or local minimum mean-square error (MMSE) combining. This is in sharp contrast to the existing literature, which advocates for maximum-ratio combining. Also, we show that a centralized implementation with optimal MMSE processing not only maximizes the SE but largely reduces the fronthaul signaling compared to the standard distributed approach. This makes it the preferred way to operate Cell-free Massive MIMO networks. Non-linear decoding is also investigated and shown to bring negligible improvements.

中文翻译：

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通过 MMSE 处理和集中实施使无蜂窝大规模 MIMO 具有竞争力

Cell-free Massive MIMO 被认为是一种很有前途的技术，可以满足超 5G 网络中不断增长的用户数量和高速率期望。关键思想是让许多分布式接入点 (AP) 与网络中的所有用户进行通信，可能是通过使用联合相干信号处理。本文的目的是在 AP 之间不同程度的合作下对该技术进行首次综合分析。特别地，分析了四种不同的无小区实现的上行链路频谱效率，具有空间相关衰落和任意线性处理。事实证明，它有可能在很大程度上优于传统的蜂窝大规模 MIMO 和小型蜂窝网络，但仅使用全局或局部最小均方误差 (MMSE) 组合。这与提倡最大比率组合的现有文献形成鲜明对比。此外，我们表明，与标准分布式方法相比，具有最佳 MMSE 处理的集中式实现不仅可以最大化 SE，而且在很大程度上减少了前传信令。这使其成为运行无蜂窝大规模 MIMO 网络的首选方式。还研究了非线性解码并显示其带来的改进可以忽略不计。