PRACH — Physical random access channel — the first thing a device sends
Channels — what carries what
Where it sits
What it is
What it is. The channel a device uses to announce itself. TS 38.211 clause 6.3.3, with the procedure in TS 38.321 clause 5.1 and the resource configuration in TS 38.213 clause 8.
It is the only channel with its own sequence-generation clause (TS 38.211 clause 5.3.2, alongside the general modulation mapper) — because it must work before timing is agreed. Everything else in NR assumes the device is time-aligned to the cell; PRACH is what establishes that alignment, so it cannot depend on it.
Hence the long cyclic prefix and the guard period. A preamble must survive arriving early or late by the round-trip time to the cell edge, which is why preamble formats are far longer than an ordinary symbol and why cell radius is a PRACH configuration question.
Contention is expected. Several devices may pick the same preamble in the same occasion; the procedure resolves it in Msg4, and the losers try again. This is the only place in NR where collisions are part of the design — everywhere else the scheduler prevents them.
Read on
This concept was first written up in ref-channels, which reads the whole group as one argument.
Before this concept, the hierarchy says to learn the following — the full chain, in order: