What is RACH?
RACH (Random Access Channel) is a shared uplink channel used by UEs to initiate communication with the base station. It is the first step in the initial access procedure — when a UE wants to connect to the network, perform handover, or request uplink resources, it transmits a random access preamble on the RACH. In 5G NR, the RACH procedure is significantly enhanced compared to LTE to support beamforming, faster access for uRLLC, and 2-step RACH for reduced latency.
How Does RACH Work?
5G NR supports two RACH procedures: 4-step RACH (contention-based, similar to LTE) and 2-step RACH (introduced in Release 16 for lower latency). In 4-step RACH: (1) UE sends preamble on PRACH, (2) gNB responds with Random Access Response (RAR), (3) UE sends Msg3 with identity, (4) gNB resolves contention with Msg4. In 2-step RACH: (1) UE combines preamble + payload in MsgA, (2) gNB responds with MsgB. The PRACH preamble is a Zadoff-Chu sequence with good correlation properties, enabling the gNB to estimate timing advance. In beam-based systems (FR2), RACH is associated with specific SSB beams — the UE transmits the preamble on the beam it received best, informing the gNB which beam to use for subsequent communication.
Use Cases
5G NR initial network access, handover execution, beam failure recovery, scheduling request for uplink resources, and uRLLC low-latency initial access via 2-step RACH.
3GPP / Standards Reference
3GPP TS 38.211 (NR PRACH physical channels), 3GPP TS 38.321 (NR MAC — random access procedure), 3GPP TS 38.213 (NR physical layer procedures for RACH)
Related Terms
PSS | SSS | gNB | UE | Beamforming
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