SSB Structure · 240 SCs × 4 Symbols
Symbol 0: PSS (127 SCs centered, others zero). Symbol 1: PBCH. Symbol 2: SSS (127 SCs centered) + PBCH on the side bands. Symbol 3: PBCH. Total: 240 SCs × 4 sym = 960 REs per SSB.
PSS · Primary Synchronization Signal
3 sequences (length 127, m-sequence-based). UE correlates incoming signal against all 3 PSS sequences to (a) detect first arrival time, (b) get rough subframe sync, (c) determine N_id^(2) ∈ {0,1,2}.
SSS · Secondary Synchronization Signal
336 sequences (length 127, derived from two M-sequences). Combined with the 3 PSS sequences gives 336 × 3 = 1008 unique Physical Cell IDs (PCI). UE detects SSS by correlation against all 336 candidates conditioned on the PSS-detected N_id^(2).
PBCH · MIB Container
PBCH (Physical Broadcast Channel) carries the MIB (Master Information Block) — 32 bits including SFN (System Frame Number), SCS info for SIB1, k_SSB pointer to Point A, half-frame indicator, and DMRS-position info. Polar-coded for robust decoding.
1008 PCIs · Cell Identity
1008 PCIs is a 3× increase from LTE's 504. The PCI determines pseudo-random sequences for DMRS, scrambling, and resource-element mapping — making each cell look "different" at the PHY level.
SS Burst Set · L = 4 / 8 / 64
In a 5 ms half-frame, NR transmits up to L SSBs as a burst. L = 4 for sub-3 GHz FR1. L = 8 for FR1 above 3 GHz. L = 64 for FR2 mmWave. Each SSB index in the burst corresponds to a different gNB beam direction.
Beam Sweeping in mmWave
In FR2, the gNB transmits each of the 64 SSBs on a different narrow beam. The UE measures all 64; reports the best to the gNB during RACH (preamble allocated per SSB index). This selects the gNB→UE beam pair without needing full channel measurement.
Cell Search Procedure · End-to-End
1) UE sweeps frequency raster, correlates against PSS sequences. 2) On detection, UE detects SSS for full PCI. 3) UE decodes PBCH to extract MIB. 4) UE locates SIB1 (CORESET#0 from MIB) on PDSCH. 5) UE decodes SIB1 for full system info → ready for RACH.
SSref · The Frequency Anchor
SSref (SSB Reference Frequency) is the center subcarrier of the SSB. It's placed on a defined raster (the GSCN). UE knows where to look without trying every frequency.
Frequently Asked Questions
What is SSB in 5G NR?
SSB (Synchronization Signal Block) is a 240-subcarrier × 4-symbol block containing PSS, SSS, and PBCH. It is the first signal a UE looks for during cell search, providing time-frequency synchronization, PCI detection, and MIB content.
How many Physical Cell IDs does 5G NR support?
1008 PCIs (336 SSS sequences × 3 PSS sequences). This is twice as many as LTE's 504 PCIs.
What is k_SSB in MIB?
k_SSB is a pointer in the MIB that tells the UE the offset (in subcarriers) between the SSB's lowest subcarrier and Point A — the absolute frequency anchor of the cell. UE uses k_SSB + offsetToPointA from SIB1 to locate every CRB.
How does mmWave beam sweeping work in 5G NR?
The gNB transmits up to 64 SSBs in a 5 ms half-frame, each on a different narrow beam. The UE measures all 64, selects the best, and tells the gNB its choice via a per-beam PRACH preamble allocation. This locks the gNB→UE beam pair without a full channel measurement.