PRACH

PRACH is the UE's way of getting the network's attention when it has no scheduled resources — the first uplink transmission in random access. The UE picks a preamble (a Zadoff-Chu sequence) from a configured set and sends it on PRACH; the gNB detects it, estimates the timing advance the UE needs, and replies with a Random Access Response. That timing-advance estimation is a big part of why PRACH exists — until the UE knows it, its uplink isn't aligned with the gNB's frame.

It's used in more situations than just powering on: initial access, re-establishing after radio link failure, handover, requesting uplink resources when there's no other way to ask, and beam failure recovery. NR defines several PRACH formats — long ones (built on 839-length sequences) for big cells and coverage, short ones (139-length, the "format A/B/C" family) for small cells and FR2, where they double as a beam-selection mechanism since the UE sends the preamble on the RACH occasion tied to its best SSB beam. NR also adds 2-step RACH, which collapses the four-message handshake into two to cut access latency.