Description
The Time Alignment Command (TAC) is a fundamental mechanism in the LTE and 5G NR physical layer for maintaining uplink synchronization. In OFDMA (Orthogonal Frequency Division Multiple Access) and SC-FDMA (Single Carrier FDMA) systems used in the uplink, precise timing alignment of signals from all User Equipments (UEs) is essential to preserve orthogonality between subcarriers and prevent inter-symbol interference (ISI) and inter-carrier interference (ICI). The TAC is a parameter sent via the Medium Access Control (MAC) layer in a MAC Control Element (MAC CE) to instruct a specific UE to advance or delay its uplink transmission timing.
The process works as follows: The base station (eNB in LTE, gNB in NR) continuously measures the timing of received uplink signals from each UE, such as during the transmission of Sounding Reference Signals (SRS) or the Physical Uplink Shared Channel (PUSCH). It calculates the timing error, which is the difference between the ideal reception time and the actual arrival time of the UE's signal. This error is quantized and mapped to a TAC value. The TAC is then transmitted to the UE in a downlink control message. Upon receiving the TAC, the UE adjusts its uplink transmission timing by a corresponding amount, typically in steps of a fraction of the basic time unit (e.g., Ts in LTE, Tc in NR). The adjustment range is defined by the standard, and the UE maintains a Time Alignment Timer (TAT); as long as this timer is running, the UE considers itself uplink-synchronized.
Key components involved are the base station's uplink scheduler and timing measurement unit, the MAC layer for generating the MAC CE, and the UE's physical layer and timing advance control mechanism. The TAC is part of a closed-loop control system. Its role is absolutely critical for mobility, especially as UEs move and their propagation delay changes. Without continuous time alignment, the carefully constructed orthogonality of the uplink would break down, leading to increased interference, reduced data rates, and degraded overall system capacity. In 5G NR, the concept remains fundamentally the same but operates within the new NR frame structure and supports wider carrier bandwidths and more diverse numerologies.
Purpose & Motivation
The Time Alignment Command mechanism was introduced to solve the fundamental problem of uplink synchronization in cellular OFDMA/SC-FDMA systems. In earlier CDMA-based systems like UMTS, precise power control was the primary method for managing multiple access interference, but timing alignment was less critical. With the shift to OFDMA in LTE, orthogonality in the frequency domain became paramount. If uplink signals from different UEs do not arrive at the base station within the cyclic prefix (CP) duration, their orthogonality is lost, causing severe interference that cannot be filtered out.
Before a standardized, dynamic TAC mechanism, maintaining uplink synchronization for moving UEs would be nearly impossible, severely limiting cell sizes and mobility support. The TAC provides a fast, network-controlled method to compensate for varying propagation delays as UEs change their distance from the base station or due to multipath effects. It addresses the limitations of a simple initial random access procedure, which only provides coarse timing alignment. The continuous fine-tuning enabled by TACs is what allows LTE and NR to support high-speed mobility, large cell radii, and efficient uplink resource sharing among many users. Its creation was motivated by the need to achieve the high spectral efficiency targets of 4G and 5G, making the uplink as robust and efficient as the downlink.
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (131 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-5, normative work from Rel-15.
In Release 15, a specific correction was made to the Time Alignment Command (TAC) for NG-RAN cells prior to the Non-Standalone (NSA) ASN.1 freeze, ensuring proper alignment in the specification. This update was part of broader alignment and correction efforts across various protocols, including security mode command procedures and CONFIGURATION UPDATE COMMAND handling. The change ensured the TAC function was correctly specified for the new 5G radio access network architecture.
- Alignment and correction of mapped security context creation at S1 to N1 mode HO TS 24.501CR0037
- Alignment CR for storing CE mode B UE capability in MME TS 23.401CR3378
- Alignment with stage-3 reporting and access restriction for unlicensed spectrum TS 23.401CR3403
- Alignment with CT1/RAN on handling of S1AP CONNECTION ESTABLISHMENT INDICATION TS 23.401CR3410
- Alignment with CT WG1 for handling of Connection Resume Requests at Service Gap TS 23.401CR3413
- UE radio capabilities - alignment with rSRVCC from GERAN TS 23.401CR3451
+ 27 more changes
In Release 16, the TAC function was updated by removing the previous requirement that a TAC combined with an SV (Software Version) is used to identify the UE model within the manufacturer-assigned ID. This change was implemented alongside other alignment corrections, such as those for the DAPS Handover procedure and the handling of the PDU session release command, as part of broader specification maintenance.
- Alignment CR for DAPS HO TS 23.401CR3602
- UE Handling upon receipt of PDU session release command TS 24.501CR1977
- Alignment with stage-2 on PEI for 5G-RG and FN-RG TS 24.501CR1279
- removing requirement that TAC+SV is used to identify UE model in manufacturer assigned ID TS 23.401CR3549
- Alignment of abnormal case handling in case of attach for access to RLOS not accepted by the network TS 24.301CR3244
- Alignment for the notification procedure TS 24.501CR0797
+ 23 more changes
In Release 17, specific alignment was made for Time Alignment (TA) handling for moving cells in Non-Terrestrial Networks (NTN) for IoT. This was detailed in the CR titled "IoT NTN: Alignment of TA Handling for moving cells with NR NTN," ensuring procedures were consistent for these specific network scenarios. The update focused on aligning the mechanisms for managing the Time Alignment Command in these unique, cell-movement conditions.
- Configuration updates for 5GS/EPS alignment TS 24.484CR0205
- MC Credentials for DN and NS AA & NS alignment TS 24.484CR0218
- Alignments for the introduction of SOR-CMCI TS 24.501CR3365
- Alignment to KI#2 conclusions on EPLMN list TS 24.501CR3606
- UUAA-MM completion alignment TS 24.501CR3868
- UUAA revocation alignment TS 24.501CR3867
+ 30 more changes
In Release 18, the updates to the Time Alignment Command (TAC) function focused primarily on procedural alignments and error corrections within specific control messages. Key changes included corrections and clarifications to the UE behavior upon receiving a CONFIGURATION UPDATE COMMAND message, particularly when it indicates a registration request. Additionally, alignments were made for the handling of this command during PDU session modification when semantic errors are present in packet filters or QoS operations.
- Discontinuous coverage update to remove EN and alignments TS 23.401CR3744
- Discontinuous coverage overload control priority users term alignment TS 23.401CR3757
- EMM context storage when emergency attached, 24.501 alignment TS 24.301CR3958
- Alignment of <MigratablePartnerMCPTTSystemInfo> element with <MigratablePartnerMCVideoSystemInfo> element and <MigratablePartnerMCDataSystemInfo> element TS 24.484CR0255
- Alignment of term re-NSSAA TS 24.501CR4529
- Corrections for UE behaviour upon receiving CONFIGURATION UPDATE COMMAND message that indicates registration requested TS 24.501CR4612
+ 18 more changes
In Release 19, the TAC function saw updates primarily focused on alignment and correction across various procedures and interfaces. These included specific alignments for the Attach and Service Request procedures in Satellite access, updates to RAT utilization control, and corrections for PEIPS and Service-level authentication messages. The changes ensured consistency with the latest stage-2 specifications and clarified terminology for functionalities like ATSSS-LL steering.
- Alignment of EMM Transport message TS 24.301CR4459
- Alignment of estimated uplink delivery time with latest stage-2 updates TS 24.301CR4311
- Alignments in S&F in Satellite TS 24.301CR4415
- Alignment of Attach procedure and Service Request procedure in S&F for Satellite TS 24.301CR4450
- Update and alignment of RAT utilization control in UCU TS 24.501CR6475
- Correction, clarification and alignment of PEIPS TS 24.501CR6792
+ 3 more changes
Explore further
Broader topics and technologies where TAC plays a role.
Defining Specifications
3GPP specifications that define or reference TAC, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TS 23.401 vj50 | Evolved Packet System (EPS) Stage 2 Description | Rel-19 |
| TS 24.229 vj50 | IMS call control protocol based on SIP and SDP | Rel-19 |
| TS 24.301 vj60 | NAS protocol for Evolved Packet System | Rel-19 |
| TS 24.484 vj30 | MCS Configuration Management | Rel-19 |
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |
| TS 24.801 v810 | CT1 SAE NAS Aspects for EPC | Rel-8 |
| TS 24.890 vg00 | 5G NAS Protocol for 5GS Stage 3 | Rel-16 |
| TS 28.875 vj00 | Study on IAB Node Management | Rel-19 |
| TS 32.836 vc00 | NM Centralized Coverage and Capacity Optimization Study | Rel-12 |
| TS 36.300 vj00 | E-UTRAN Radio Interface Protocol Architecture Overview | Rel-19 |
| TS 36.413 vj10 | S1 Application Protocol (S1AP) | Rel-19 |
| TS 36.423 vj10 | X2 Application Protocol (X2AP) Specification | Rel-19 |
| TS 36.463 vj00 | XwAP Protocol Specification | Rel-19 |
| TR 36.763 vh00 | NB-IoT/eMTC Support for Non-Terrestrial Networks | Rel-17 |
| TS 37.473 vj00 | W1 Application Protocol (W1AP) Specification | Rel-19 |
| TS 38.401 vj10 | NG-RAN Architecture Specification | Rel-19 |
| TS 38.413 vj10 | NG Application Protocol (NGAP) | Rel-19 |
| TS 38.423 vj10 | Xn Application Protocol (XnAP) specification | Rel-19 |
| TS 38.473 vj10 | 5G F1 Application Protocol (F1AP) | Rel-19 |
| TS 48.061 vj00 | BTS-TRAU Protocol for HR Speech/Data | Rel-19 |