ain time intervals to allow the UE to test different RX beams for a given TX beam. In this manner, up to NgNB-beam NUE-beam beam pairs can be measured. New Radio Access Physical Layer Aspects (Part 2) 513 Initial gNB beam gNB beam refinement UE beam refinement acquisition Stage 1: Beam alignment (wide- Stage 2: Beam alignment (narrow- beam) beam) - Based on measurement on SS - Based on measurement on narrow- block or wide-beam CSI-RS beam CSI-RS (UE-specific) - Supports robust communications Supports high-rate communications (control channels) (data channels) UE autonomous beam selection in - Beam switching is network idle mode controlled and dynamic - Network-controlled beam switching in connected mode Figure 4.44 Illustration of beam management procedure 26]. The beamformed CSI-RS used for beam management can be transmitted either aperiodically or periodically. When the system is underloaded, beam sweeping across a small number of TX beams over a narrow angular area on an aperiodic basis for a given UE is sufficient. When the system loading increases, it would be more efficient to perform a periodic sweep over a wider angular area covering a larger number of UEs. Despite the fact that NgNB-beam NUE-beam beam pairs correspond to NgNB-beam X NUE-beam beam qualities along with their pair indicators, not all NgNB-beam X NUE-beam beam qualities need to be reported, because for each TX beam, only the measurement quality of one beam pair, that is, the optimal RX beam for the given TX beam, needs to be reported. The optimal RX beam is known at the UE based on the beam measurement, which is not reported. In the subsequent data or control transmission to the UE, the gNB indicates the index of the selected TX beam to the UE. The UE can use the latest optimal RX beam of the indicated TX beam, which is stored in the UE local memory upon beam measurement, for the purpose of reception. Moreover, within the NgNB - beam candi- date TX beams, the qualities and/or indices of 1 < Nbeam < NgNB-beam TX beams can be reported. If Nbea