We consider a cell-free massive MIMO system with multiple antennas on the users and access points. In previous works, the downlink spectral efficiency (SE) has been evaluated using the hardening bound that requires no downlink pilots. This approach works well when having single-antenna users. In this paper, we show that much higher SEs can be achieved if downlink pilots are sent since the effective channel matrix does not harden when having multi-antenna users. We propose a pilot-based downlink estimation scheme and derive a new SE expression that utilizes zero-forcing combining. We show numerically how the number of users and user antennas affects the SE.

翻译：本文考虑一个用户和接入点均配备多天线的无蜂窝大规模MIMO系统。以往工作中，下行频谱效率（SE）采用无需下行导频的硬化界进行评估，该方法在用户为单天线时表现良好。本文证明，当用户配备多天线时，由于有效信道矩阵不再呈现硬化特性，通过发送下行导频可实现更高的SE。我们提出一种基于导频的下行估计方案，并推导出采用迫零合并的新SE表达式。通过数值仿真，我们展示了用户数量及用户天线数对SE的影响。