SS/PBCH block — The object a device with no knowledge finds first
Reference signals
Where it sits
What it is
What it is. The bundle of PSS, SSS and PBCH with its DM-RS, transmitted together as one small rectangle. TS 38.211 clause 7.4.3, on antenna port 4000.
Four symbols by 240 subcarriers — 20 resource blocks wide (Table 7.4.3.1-1). Small on purpose: a device that knows nothing must find it by brute force, so it has to be searchable.
PSS — one of three length-127 sequences. Finding it gives symbol timing, a coarse frequency correction, and $N^{(2)}_{\text{ID}}$ (clause 7.4.2.2).
SSS — one of 336, at a known offset from the PSS. Together they give the physical cell identity, $N^{\text{cell}}_{\text{ID}} = 3N^{(1)}_{\text{ID}} + N^{(2)}_{\text{ID}}$, so $3 \times 336 = 1008$ cells are distinguishable (clause 7.4.2.1).
PBCH and its DM-RS — the MIB, described in ref-channels §5.
SSBs are transmitted in a burst set, and the burst is a beam sweep. Several SSBs are sent in
different directions within a half-frame; a device reports which one it heard best, and that index
tells the network which beam to use for everything that follows. This is where beam management
starts, and it is why ssb-PositionsInBurst appears in the PDSCH rate-matching rules of
PDSCH §7.2 — a PDSCH must not be mapped over an SSB that is
actually being transmitted.
Read on
This concept was first written up in ref-signals, which reads the whole group as one argument.
Before this concept, the hierarchy says to learn the following — the full chain, in order: