  Acquiring downlink channel state information (CSI) is crucial for optimizing
performance in massive Multiple Input Multiple Output (MIMO) systems operating
under Frequency-Division Duplexing (FDD). Most cellular wireless communication
systems employ codebook-based precoder designs, which offer advantages such as
simpler, more efficient feedback mechanisms and reduced feedback overhead.
Common codebook-based approaches include Type II and eType II precoding methods
defined in the 3GPP standards. Feedback in these systems is typically
standardized per subband (SB), allowing user equipment (UE) to select the
optimal precoder from the codebook for each SB, thereby reducing feedback
overhead. However, this subband-level feedback resolution may not suffice for
frequency-selective channels. This paper addresses this issue by introducing an
uplink CSI-assisted precoder upsampling module deployed at the gNodeB. This
module upsamples SB-level precoders to resource block (RB)-level precoders,
acting as a plug-in compatible with existing gNodeB or base stations.
