5G • RAN

Msg2 RAR

Random Access Response (Msg2)

MAC PDU transmitted on PDSCH delivering initial Timing Advance, TC-RNTI, and uplink grant for Msg3.

Technical Explanation

In Step 2, the gNodeB transmits the Random Access Response (RAR) within the configured ra-ResponseWindow. The scheduling PDCCH is scrambled with RA-RNTI. The RAR MAC payload contains: (1) 12-bit Timing Advance Command, (2) 27-bit Uplink Grant for Msg3, (3) Temporary C-RNTI (TC-RNTI), and (4) Random Access Preamble Identifier (RAPID).

Key Functions

Conveys 12-bit absolute Timing Advance Command
Assigns Temporary C-RNTI (TC-RNTI) and uplink PUSCH grant for Msg3
Addressed on PDCCH using RA-RNTI calculated from PRACH time/frequency

Specifications

3GPP TS 38.213, TS 38.321

Interfaces

Related 5G Concepts

Msg1 PRACH Msg3 RA-RNTI TC-RNTI Timing Advance (TA)

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