When PDSCH is transmitted with different rank?
Or what is the conditions when PDSCH is transmitted with different rank?
UE reports Rank Indicator so gNodeB will use that rank indicator.
For example, if UE reports RI = 3 then gNodeB will use 3 PDSCH layers.
A UE selects the rank with the highest spectral efficiency based on the downlink channel measurement and receiver capability. The RI report from UE may be not optimal due to channel fluctuation and slight difference between different spectral efficiencies. In this case, the performance can be enhanced if eNodeB overrides the reported RI.
For example, for closed-loop MIMO, when a UE reports rank = 2 but rank 1 is scheduled, the eNodeB selects a PMI corresponding to a codeword with good performance for scheduling.
How does rank adaptive work?