How does SSB beam sweeping work?

The gNB transmits SSBs sequentially on different beam directions within a half-frame (5 ms window). Each SSB occupies 4 consecutive OFDM symbols and is transmitted on a different Tx beam. For FR2 (mmWave) with L_max = 64: the gNB can transmit up to 64 SSBs in one burst, each pointing in a different direction, covering the full 120° cell sector with approximately 4° per beam. The SSB burst repeats every SSB periodicity (default 20 ms for initial access). SSB content: Symbol 0 = PSS (127-length m-sequence, cell ID group), Symbol 1 = PBCH + PBCH DMRS, Symbol 2 = SSS (127-length Gold sequence, cell ID sector), Symbol 3 = PBCH continued. The UE does not need prior beam alignment to receive SSBs; it listens omnidirectionally (or sweeps its own Rx beams across SSB occasions) and measures L1-RSRP for each detected SSB index. This is why SSB sweeping is called 'blind' beam discovery.

How long does beam sweeping take?

The sweep duration depends on L_max and SCS: FR1 (sub-6 GHz) with 15 kHz SCS: 4 SSBs × 4 symbols × 66.7 μs = 1.07 ms. Fits within first 2 ms of half-frame. FR1 (sub-6 GHz) with 30 kHz SCS: 8 SSBs × 4 symbols × 33.3 μs = 1.07 ms. Same duration, double the beams. FR2 (mmWave) with 120 kHz SCS: 64 SSBs × 4 symbols × 8.33 μs = 2.13 ms. Fits within 5 ms half-frame with margin. FR2 (mmWave) with 240 kHz SCS: 64 SSBs × 4 symbols × 4.17 μs = 1.07 ms. Fastest sweep. For initial cell search, the UE may need to monitor multiple SSB burst periods (2-4) to try different Rx beam directions, giving total initial access time of 40-160 ms for mmWave. Connected-mode beam sweeping uses the same SSB structure but is monitored with known timing, reducing measurement latency.

Why does FR2 need 64 SSBs while FR1 needs only 4-8?

The number of SSB beams is driven by beamwidth and cell coverage: FR1 (sub-6 GHz): wider beams (30-120° beamwidth) due to lower antenna gain. A 120° sector needs only 4 beams of 30° each (or 8 of 15°) to achieve full coverage. Each beam still provides sufficient link budget because path loss at 2-4 GHz is manageable with moderate antenna gain. FR2 (mmWave, 24-52.6 GHz): narrow beams (3-10° beamwidth) are required because path loss is 20-30 dB higher than FR1. To maintain link budget, the gNB needs antenna gain of 20-25 dBi, which requires beamwidths of 5-10°. Covering a 120° sector with 5° beams requires at least 24 beam positions; 64 SSBs provide margin for overlapping coverage, elevation sectors, and guard beams. The SSB sweep essentially trades time (more sequential transmissions) for spatial coverage density, which is the fundamental tradeoff of narrow-beam systems.

5G NR Initial Access

Beam Sweeping

/beem SWEEP-ing/

The 5G NR P1 procedure where the gNB sequentially transmits SSBs on different beam directions across the cell sector. Each SSB: 4 OFDM symbols × 20 RBs carrying PSS, SSS, PBCH. L_max per 20 ms burst: 4 (sub-3 GHz), 8 (3 to 6 GHz), or 64 (FR2 mmWave). At 120 kHz SCS, a full 64-beam sweep completes in 2.13 ms. UE measures L1-RSRP per SSB index for blind beam discovery. Initial access: 40 to 160 ms (multiple burst periods for UE Rx sweeping).

FR1: 4–8 SSBs

FR2: up to 64 SSBs

Period: 5–160 ms

Understanding Beam Sweeping

Beam sweeping is the foundation of 5G NR beam management. Before any communication can occur, the UE must discover which gNB beam direction provides the best signal. Unlike LTE, where the cell reference signal (CRS) covers the entire sector simultaneously with a wide beam, 5G NR must sequentially transmit narrow beams to maintain sufficient link budget at mmWave frequencies.

The SSB is carefully designed to be self-contained: it carries synchronization signals (PSS/SSS for timing and cell ID) and broadcast information (PBCH for system parameters) in a single 4-symbol block. This allows a UE to synchronize, identify the cell, and determine the best beam direction all from SSB reception, without any prior knowledge of the network.

SSB Sweep Timing

SSB Duration:
4 OFDM symbols × T_symbol
15 kHz: 4 × 66.7 µs = 267 µs
30 kHz: 4 × 33.3 µs = 133 µs
120 kHz: 4 × 8.33 µs = 33.3 µs
240 kHz: 4 × 4.17 µs = 16.7 µs

Full Sweep Duration:
FR1 (15 kHz, L=4): 1.07 ms
FR1 (30 kHz, L=8): 1.07 ms
FR2 (120 kHz, L=64): 2.13 ms
FR2 (240 kHz, L=64): 1.07 ms

Beam Coverage:
120° sector / 64 beams = 1.875°/beam
Beamwidth ~5° with overlap

SSB Configuration by Frequency Range

Parameter	FR1 (<3 GHz)	FR1 (3–6 GHz)	FR2 (mmWave)
L_max	4	8	64
SCS	15 kHz	30 kHz	120/240 kHz
Beamwidth	30–120°	15–30°	3–10°
Sweep time	1.07 ms	1.07 ms	1–2.1 ms
Cell search	20–40 ms	20–40 ms	40–160 ms

Common Questions

Frequently Asked Questions

How does SSB sweeping work?

gNB transmits SSBs sequentially on different beams within 5 ms half-frame. Each SSB: PSS + SSS + PBCH in 4 symbols. FR2: 64 SSBs at 120 kHz SCS in 2.13 ms. UE measures L1-RSRP per index. Blind discovery, no prior alignment needed.

How long?

Single sweep: 1 to 2.1 ms. Initial access: 40 to 160 ms (UE needs multiple bursts to try Rx beam directions). Connected mode: single burst period (5 to 20 ms) since UE timing is known.

Why 64 beams for FR2?

mmWave path loss 20 to 30 dB higher requires 20 to 25 dBi gain (5 to 10° beamwidth). 120° sector / 5° = 24+ positions. 64 SSBs allow overlap, elevation sectors, and guard beams. Trades sweep time for spatial coverage.

Related Terms

← Beam Steering Unit Beam Tilt →

5G NR Systems

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