PRACH
Physical Random Access Channel: the uplink channel used by UEs to initiate random access procedures for initial access, handover, scheduling request, and beam failure recovery.
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.
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