SSB
SS/PBCH Block (Synchronization Signal Block): a burst of PSS, SSS, PBCH, and DMRS transmitted periodically for cell search, synchronization, and system information acquisition.
The SSB is the first thing a UE ever decodes on an NR cell. Before it knows anything — timing, the cell ID, where to find system information — it sweeps for these blocks. Each one packs the PSS and SSS (which together give the physical cell ID, 1008 possibilities), the PBCH carrying the MIB, and the PBCH DMRS for demodulation, laid out across 4 OFDM symbols and 240 subcarriers.
The detail that trips people up is beam sweeping. In FR2, and often in FR1 too, the gNB doesn't send one SSB — it sends a burst of up to 64 of them in FR2 (4 or 8 in FR1), each on a different beam, so a UE anywhere in the sector catches at least one. The index of whichever SSB the UE locks onto tells the network which beam reached it, which seeds the whole beam management process. Default periodicity for initial access is 20 ms, though it's configurable.
Frequently asked questions
- How is the SSB different from LTE sync signals?
- LTE put PSS, SSS and the PBCH at fixed positions in the centre of the carrier, always at 15 kHz spacing. NR bundles them into a self-contained SS/PBCH block that can be beamformed, repeated as a burst across multiple beams, placed at a configurable frequency (the sync raster), and sent at different subcarrier spacings. That flexibility is what makes beam sweeping and wide-bandwidth NR carriers work.
Related terms
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