To get channel state information (CSI) at a base station (BS), most of researches on massive multiple-input multiple-output (MIMO) systems consider time division duplexing (TDD) to get benefit from the uplink and downlink channel reciprocity. Even in TDD, however, the BS still needs to transmit downlink training signals, which are referred to as channel state information reference signals (CSI-RSs) in the 3GPP standard, to support spatial multiplexing in practice. This is because there are many cases that the number of transmit antennas is less than the number of receive antennas at a user equipment (UE) due to power consumption and circuit complexity issues. Because of this mismatch, uplink sounding reference signals (SRSs) from the UE are not enough for the BS to obtain full downlink MIMO CSI. Therefore, after receiving the downlink CSI-RSs, the UE needs to feed back quantized CSI to the BS using a pre-defined codebook to support spatial multiplexing. In this paper, possible approaches to reconstruct full downlink MIMO CSI at the BS are proposed by exploiting both the SRS and quantized downlink CSI considering practical antenna structures with reduced downlink CSI-RS overhead. Numerical results show that the spectral efficiencies by spatial multiplexing based on the proposed downlink MIMO CSI reconstruction techniques outperform the conventional methods solely based on the quantized CSI.

翻译：为了在基地站(BS)获得频道状态信息(CSI),大多数关于大规模多投入多输出产出(MIIMO)系统的研究都考虑时间分工(TDD),以便从上链接和下链接频道对等中获益。但是,即使在TDD,BS仍需要传输下链接培训信号,这些信号被称为3GPP标准中的频道状态信息参考信号(CSI-RSs),以支持实践中的空间多路转换。这是因为由于电力消耗和电路复杂问题,传输天线的数量远远少于用户设备接收天线的数量。由于这种不匹配,即使从UDD上链接和下链接频道频道对等,BS仍然需要传输下链接培训信号,这些信号被称为3GPP标准中的频道状态信息参考信号(CSI-RS),以支持实践中的空间多路交。 UE需要用一个预先定义的代码簿向BS提供后向下方端支持空间多路交。在本文中,通过SSI全面重建CSI(CSI)常规智能(SRI)常规结构,通过BS级链路的S,通过S级标准级结构,通过S级结构向下流显示降低标准级结构。