 base station answers the mobile on the beam selected by the mobile (message 2); - the mobile UE acknowledges the beam selected by message 3; - the base station confirms the configuration of the radio link. The random access procedure allows the mobile to notify the radio node of the best radio beam. This beam is dedicated for future exchanges of control messages and data. Beam management is suitable for CPE (Customer Premise Equipment) equipment such as a 5G/Wi-Fi router (FWA: Fixed Wireless Access). Mobility requires constant adjustment of the direction of the beams. NG-RAN and 5G-NR Synchronization Signal Block - SSB sweeping Msg 1 PRACH - Preamble (best beam) UE choose the best beam Specific Msg 2: PDCCH (RA-RNTI) PDSCH/MAC RAR (TA, UL Grant, TC-RNTI, RAPID) Msg 3: PUSCH / MAC RRC SetupRequest Msg 4: PDCCH/MAC RRC Setup Message (Signaling and Data) Beam Failure(CSI-RS) Beam Switching sweeping Beam Recovering Figure 6.17. Beam selection procedure The beam switching procedure only applies when the mobile is in the intra-cell connected state. This procedure can be transparent to the mobile. The base station requests the mobile to periodically generate CSI measurement reports from the measurements of the CSI-RSs. The base station can therefore switch from one beam to another without informing the mobile. Beam failure detection is performed by the mobile when measuring the CSI-RSs and/or the SSB if the latter is present in the active band partitioning (BWP). The MAC layer of the UE mobile has been configured by an RRC message in order to save the beam loss recovery parameters: - the duration of detection of the loss of the beam; - the power threshold Q below which the mobile considers the signal to be insufficient; - the maximum number of measurements for which the measured signal is below the power threshold; - the power ramp to reach the radio node; - the maximum number of random access attempts after the beam is lost; - the preambles for the random access procedure after the beam loss. 5G-NR Radio Interface - M