Reciprocity-based downlink beamforming is imperative for a scalable time-division duplex massive multiple-input multiple-output~(MIMO) deployment. Specifically, for a dual-antenna repeater-assisted massive MIMO system, a mismatch between forward and reverse path gains at the repeater can exacerbate the overall calibration error between the user equipments (UEs) and the base station (BS), which potentially also contains calibration errors of their individual radio-frequency chains. This paper models the effects of such calibration errors, underpins the relations between the uplink and downlink channels for repeater-assisted systems with calibration errors clubbed with the over-the-air channel estimation errors, and derives analytical expressions of the downlink spectral efficiency. The presented results can then be simplified to several special cases, underscoring situations wherein such errors can become pronounced.

翻译：基于互易性的下行波束赋形对于可扩展时分双工大规模多输入多输出(MIMO)部署至关重要。具体而言,在双天线中继辅助大规模MIMO系统中,中继处前向与反向路径增益的不匹配会加剧用户设备(UE)与基站(BS)之间的整体校准误差,该误差还可能包含各自射频链路的校准误差。本文建立了此类校准误差的影响模型,明确了中继辅助系统中存在校准误差与空中信道估计误差时上下行信道之间的关系,并推导了下行频谱效率的解析表达式。所得结果可简化为若干特例,从而突出此类误差可能变得显著的场景。