Cell-Free Massive multiple-input multiple-output (MIMO) and reconfigurable intelligent surface (RIS) are two promising technologies for application to beyond-5G networks. This paper considers Cell-Free Massive MIMO systems with the assistance of an RIS for enhancing the system performance under the presence of spatial correlation among the engineered scattering elements of the RIS. Distributed maximum-ratio processing is considered at the access points (APs). We introduce an aggregated channel estimation approach that provides sufficient information for data processing with the main benefit of reducing the overhead required for channel estimation. The considered system is studied by using asymptotic analysis which lets the number of APs and/or the number of RIS elements grow large. A lower bound for the channel capacity is obtained for a finite number of APs and engineered scattering elements of the RIS, and closed-form expressions for the uplink and downlink ergodic net throughput are formulated in terms of only the channel statistics. Based on the obtained analytical frameworks, we unveil the impact of channel correlation, the number of RIS elements, and the pilot contamination on the net throughput of each user. In addition, a simple control scheme for optimizing the configuration of the engineered scattering elements of the RIS is proposed, which is shown to increase the channel estimation quality, and, hence, the system performance. Numerical results demonstrate the effectiveness of the proposed system design and performance analysis. In particular, the performance benefits of using RISs in Cell-Free Massive MIMO systems are confirmed, especially if the direct links between the APs and the users are of insufficient quality with high probability.
翻译:本文在RIS的协助下,审议了无细胞大规模MIMO系统,在RIS工程分散部分的空间相关性下,提高系统性能;在接入点考虑分配最大干线处理;我们采用综合渠道估算方法,为数据处理提供足够的信息,主要有利于减少频道估计所需的管理费用;通过使用允许大量移动和(或)增加RIS元素的免费连接的系统分析,研究考虑的系统;在RIS工程分散部分的空间相关性下,为增强系统性能提供了较低的约束;在接入点考虑分配最大干线处理;我们采用综合渠道估算方法,为数据处理提供充足信息,主要有利于减少频道估计所需的管理费用;通过使用无干扰分析,我们披露频道相关性的影响、RIS要素的数量以及测试要素的数量大幅增加;对于频道能力进行限制的有限限制,为RIS的分散部分;仅以封闭式表达渠道统计数据的方式制定最高干线和下连接网;根据获得的分析框架,我们披露了频道相关性的影响、RIS要素的概率,以及测试性能的试测用户,因此,通过正常的系统显示每个系统的拟议质量分析结果。