  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.
