In cell-free (CF) massive MIMO, a large number of access points (APs) distributed over the coverage area jointly serve a set of users over the same time/frequency resources. In this letter, we study the impact of channel non-reciprocity (NRC) and imperfect channel state information in CF massive MIMO systems. We derive analytical expressions of capacity lower bounds, including a physically inspired non-reciprocal channel model where the NRC variables vary slowly in time. The conclusion is that under conjugate beamforming, the achievable downlink rate is only sensitive to AP side phase non-reciprocity, hence the calibration requirements can be less restrictive since only the phase reciprocity errors have to be corrected. These findings are new compared to the existing literature where the NRC variables are commonly assumed to be of fast-fading nature.

中文翻译：

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无小区大规模 MIMO 中信道非互易性的影响

在无小区 (CF) 大规模 MIMO 中，分布在覆盖区域内的大量接入点 (AP) 在相同的时间/频率资源上共同为一组用户提供服务。在这封信中，我们研究了信道非互易性 (NRC) 和不完善的信道状态信息在 CF 大规模 MIMO 系统中的影响。我们推导出容量下限的分析表达式，包括一个受物理启发的非互易信道模型，其中 NRC 变量随时间变化缓慢。结论是在共轭波束成形下，可实现的下行链路速率仅对 AP 侧相位非互易性敏感，因此校准要求的限制较少，因为只需校正相位互易性误差。