As mentioned in section "Channel Sounding"(page 2), receiver need a feedback about channel ([c]).
In 3GPP, base station control both DL and UL configurations, so the feedback is needed only for DL transmissions.
This is especially true in FDD, where DL carrier frequency is different than UL carrier frequency.
In TDD, base station may decide DL configurations based on UL measurements, assuming channel (experienced) in DL and UL is same.
In DL, base station uses CSI feedback to determine next DL configuration.
CSI feedback is part of Scheduler's PHY signaling.
Fig "DL MIMO feedback loop"
In UL, feedback signaling is not needed; base station determines next UL configuration based on its own SRS based measurements.
