- Full form
- Physical Random Access Channel
- Layer
- 5G NR Physical Layer (uplink)
- 3GPP spec
- 3GPP TS 38.211 §6.3.3, TS 38.213 §8, TS 38.321 (RACH procedure)
What is PRACH?
The Physical Random Access Channel (PRACH) is the first uplink transmission a 5G NR device sends when it wants to connect to the network. Before a UE has a dedicated connection, the gNB does not know it exists and has no uplink timing for it. PRACH solves this: the UE transmits a random-access preamble — a known Zadoff-Chu sequence — that the gNB can detect, use to estimate the UE's round-trip timing, and respond to. This kicks off the Random Access (RACH) procedure that ultimately establishes an RRC connection.
Where PRACH sits in the access procedure
PRACH carries Msg1 in the classic 4-step RACH procedure:
- Msg1 (PRACH): UE sends a preamble on PRACH.
- Msg2 (RAR): gNB replies with a Random Access Response on PDSCH (scheduled by RA-RNTI on PDCCH), granting a timing advance and uplink grant.
- Msg3: UE transmits its RRC request on PUSCH.
- Msg4: gNB resolves contention and completes setup.
In 2-step RACH, Msg1+Msg3 are combined into MsgA (preamble + PUSCH payload) and Msg2+Msg4 into MsgB, cutting latency for capable UEs.
Preamble formats
The preamble format determines the sequence length, subcarrier spacing, and how much cell radius / delay spread the channel can tolerate:
- Long formats (0–3): 839-length Zadoff-Chu sequence, 1.25 or 5 kHz SCS, used in FR1 for larger cells.
- Short formats (A1–A3, B1–B4, C0, C2): 139-length sequence, scalable SCS (15/30/60/120 kHz), used in FR1 and FR2 (mmWave) for smaller cells and beam-based access.
How PRACH is configured
PRACH resources are signalled in SIB1 via the RACH-ConfigCommon IE. Key parameters include the PRACH configuration index (which time-domain occasions are valid), preamble format, root sequence index and zeroCorrelationZoneConfig (which set the cyclic-shift / Ncs and thus how many preambles per root), msg1-FrequencyStart, and the SSB-to-RACH-occasion mapping that ties each beam (SSB) to specific PRACH occasions for beam-correspondent access.
Why PRACH matters for engineers
PRACH configuration directly affects accessibility KPIs. Too few preambles or occasions causes collisions and access failures under load; an Ncs that is too small for the cell radius causes false preamble detection; wrong SSB-to-RO mapping breaks beamformed access in FR2. Optimizing PRACH is a core task in 5G accessibility tuning.
▶ Master the full RACH procedure in the 5G NR PHY Advanced course