No beam, no link
Above 24 GHz, free-space path loss is brutal and antennas are tiny — so energy is concentrated into narrow beams to close the budget. The price: a beam covers only a sliver of the cell, and any rotation, blockage or movement can drop it. Beam management is the continuous process of keeping the Tx and Rx beams aligned.
Acquisition
Find a usable beam pair at initial access via the periodic SSB beam sweep — coarse, cell-wide.
Refinement
Narrow both sides with CSI-RS sweeps (P-2 on the gNB, P-3 on the UE) for maximum gain.
Recovery
Detect a failed beam fast and recover to a known-good candidate before the connection drops.
P-1, P-2, P-3
Three nested sweeps. P-1 finds a coarse pair using wide SSB beams. P-2 fixes the UE beam and sweeps narrower gNB beams. P-3 fixes the gNB beam and sweeps UE beams. Each step tightens the pair.
SSB beam sweep & selection
The gNB transmits each SS/PBCH block on a different beam within a half-frame burst. The max number of beams L depends on the band: ≤4 below 3 GHz, ≤8 for FR1 3–6 GHz, up to 64 in FR2. The UE measures L1-RSRP per beam and locks the best; its index maps to a RACH occasion.
L1-RSRP beam reporting
The UE reports the best N beams (each as an SSBRI or CRI) with their L1-RSRP — the strongest absolute, the rest as 4-bit differentials. With group-based reporting the UE reports beams it can receive simultaneously (two Rx chains), enabling fast switching and diversity.
TCI beam indication
Once a beam is chosen, the gNB tells the UE which one to use for PDCCH/PDSCH via a TCI state (a QCL Type-D = spatial-Rx reference). MAC-CE activates up to 8 TCI states; DCI's TCI field points to one. After a beam application time, the UE switches its Rx beam.
Beam-failure detection & recovery
The UE monitors BFD-RS hypothetical BLER. Each time it crosses the threshold a beam-failure instance is counted; reaching beamFailureInstanceMaxCount declares failure. The UE then finds a candidate beam above an RSRP threshold and starts a contention-free PRACH recovery. Run it: