The SSB burst set
Within one SSB burst period (20 ms default), the gNB transmits a set of SSBs — each in a different beam direction using a different precoding vector. The maximum number of SSBs per burst set is:
Our cell is at 3498.24 MHz (between 3 and 6 GHz) → L_max = 8. The gNB can transmit up to 8 SSBs in different directions within one 5 ms half-frame, then repeat every 20 ms.
SSB time positions
The 8 SSBs within a half-frame occupy specific OFDM symbol positions. Each SSB occupies 4 consecutive symbols (PSS, SSS, PBCH×2) within specific slots and subframes.
// Case C: 30 kHz SCS, FR1 (3–6 GHz) // First SSB group (n=0,1): slots 2 and 8 of subframe 0 SSB 0: slot 2, symbols {4,5,6,7} SSB 1: slot 2, symbols {8,9,10,11} // Second SSB group (n=2,3): slots 2 and 8 of subframe 0 continued SSB 2: slot 8, symbols {4,5,6,7} SSB 3: slot 8, symbols {8,9,10,11} // Third group (n=4,5): subframe 1 SSB 4: slot 16, symbols {4,5,6,7} SSB 5: slot 16, symbols {8,9,10,11} // Fourth group (n=6,7): subframe 1 continued SSB 6: slot 22, symbols {4,5,6,7} SSB 7: slot 22, symbols {8,9,10,11} // Our UE detected SSB index 3 (strongest beam) SSB 3 position: slot 8, symbols {8,9,10,11} SSB 3 beam direction: ~ 30° off broadside (example)
How the UE identifies the best beam
The UE scans all GSCN frequencies and at each one performs a PSS correlation sweep. When it finds a cell, it receives all 8 SSBs in the burst period and measures the RSRP of each. The SSB with the highest RSRP is the best beam — that beam direction aligns best with the UE's location.
The SSB index is conveyed in two places: the PBCH DMRS pattern (3 bits, implicitly) and the PBCH payload overhead bits (additional bits for large L_max). For L_max=8, the 3-bit i_SSB field in the overhead gives the index directly.
Beam management — keeping track after connection
After initial access, the gNB continues to manage beams using three procedures defined in TS 38.331:
| Procedure | Purpose | Trigger |
|---|---|---|
| P1 | Beam sweeping — find best TRP beam and UE beam | Periodic CSI-RS or SSB measurement |
| P2 | Beam refinement — narrow beam within best direction | After P1, triggered by beam-specific CSI-RS |
| P3 | UE-side beam selection — rotate UE's own receive beam | Only for FR2 (mmWave) UEs with antenna arrays |
Beam failure recovery
If the serving beam quality degrades below a threshold (e.g., due to the UE moving or an obstruction), the UE declares a beam failure and must quickly find a new beam. This is faster than a full handover — the UE sends a RACH preamble on the PRACH occasion associated with a candidate new beam.