5G • RAN

TAC

Timing Advance Command

Control message conveying timing adjustment: 12 bits in Msg2 RAR and 6 bits in MAC Control Elements.

Technical Explanation

Initial Timing Advance is signaled during random access in the 12-bit TAC field of the Msg2 Random Access Response (RAR), providing an absolute timing value (0 to 3846). During connected mode, the gNodeB tracks uplink DMRS/SRS and sends 6-bit relative adjustments (-32 to +31 steps) in Timing Advance Command MAC CEs.

Key Functions

Provides 12-bit absolute timing advance in Msg2 RAR
Sends 6-bit relative updates (-32 to +31 steps) in periodic MAC CEs
Maintains uplink synchronization within fraction of cyclic prefix duration

Specifications

3GPP TS 38.213, TS 38.321

Interfaces

Related 5G Concepts

Timing Advance (TA)Timing Advance Timer

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