5G • Spectrum
PRACH
Physical Random Access Channel
The physical uplink channel used by the UE for initial access, timing synchronization, and beam recovery.
Technical Explanation
When a UE wishes to connect to a 5G cell, it initiates the Random Access procedure (RACH) by transmitting a random access preamble over the PRACH. 5G NR supports both 4-step RACH (Preamble Msg1, RAR Msg2, Msg3, Contention Resolution Msg4) and 2-step RACH (MsgA, MsgB) for low latency. In 5G, PRACH preambles are mapped to specific SSB beam indices, enabling the gNodeB to immediately know which downlink beam the UE is receiving.
Key Functions
- Initial network access and synchronization of uplink timing advance (TA)
- Beam indication during initial attach via SSB-to-PRACH occasion mapping
- 2-Step RACH (Release 16) for reduced latency and power consumption in URLLC/IoT
- Beam failure recovery and handover execution
Specifications
3GPP TS 38.211 Section 6.3.3, TS 38.213 Section 8
Interfaces
Uu Physical Layer (Uplink)
Related 5G Concepts
Want to memorize 5G concepts like this one?
Study it with SuperMemo SM-2 spaced repetition flashcards.