-P1 procedure “SSB beam/ Initial Tx Beam sweeping”:
Before a data flow is enabled in the scheduler, Periodic SSB beam scanning is implemented on the base station side in a certain interval (the SSB periodicity). At the same time, wide beam scanning is implemented on the UE side to determine the optimal receive wide beam.( Optimal SSB/PRACH beams.)
-P2 procedure ”CSI-RS beam/ refinement sweeping”:
Performing a beam sweep in a more narrow angular sector than in P1. The narrow beams closest to the wide beam in the beam grid are selected to be examined using CSI-RS (followed by CSI-report).
-P3 procedure ”CSI-RS beam/ UE Rx beam refinement sweeping”:
The optimal narrow beam is selected from P2 and CSI-RSs are transmitted to the UE ,which updates its Rx beam. In the data transmission,
The base station uses the best BS Tx beam found during P2 and the UE uses the best UE Rx beam found during P3.
Note while data transmission is being performed on an active beam pair link, the UE can monitor PDCCH on another beam pair as a backup link for swift fallback if there a sudden blockage of the active link.