We consider a cell-free massive multiple-input multiple-output (MIMO) system with multiple antennas on the users and access points (APs). In previous works, the downlink spectral efficiency (SE) has been evaluated using the hardening bound that requires no downlink pilots. This approach works well for single-antenna users. In this paper, we show that much higher SEs can be achieved if downlink pilots are sent when having multi-antenna users. The reason is that the effective channel matrix does not harden. We propose a pilot-based downlink estimation scheme, derive a new SE expression, and show numerically that it yields substantially higher performance when having correlated Rayleigh fading channels. In cases with multi-antenna users, the APs can either transmit the same or different data streams. The latter reduces the fronthaul signaling but comes with a SE loss. We propose precoding and combining schemes for these cases and consider whether channel knowledge is shared between the APs. Finally, we show numerically how the number of users, APs, and the number of antennas on users and APs affect the SE.

翻译：本文研究一种在用户端和接入点均配备多天线的无蜂窝大规模多输入多输出系统。在已有研究中，下行频谱效率通常采用无需下行导频的硬化界进行评估。该方法对单天线用户表现良好。本文证明，当用户配备多天线时，若采用下行导频传输可获得显著更高的频谱效率，其根本原因在于有效信道矩阵不具备硬化特性。我们提出一种基于导频的下行信道估计方案，推导出新的频谱效率表达式，并通过数值仿真表明该方案在相关瑞利衰落信道环境下能实现性能的大幅提升。对于多天线用户场景，接入点可选择传输相同或不同的数据流：后者虽能减少前传信令开销，但会导致频谱效率损失。我们针对这两种情况分别提出预编码与合并方案，并探讨接入点间信道知识共享的影响。最后通过数值仿真，系统分析了用户数量、接入点数量以及用户与接入点天线数量对频谱效率的影响规律。