Timing Advance (TA) in 4G and 5G
- Venkateshu
- Jun 30
- 7 min read
Introduction
Timing Advance (TA) is a critical concept in 4G Long-Term Evolution (LTE) and 5G New Radio (NR) systems, ensuring synchronized uplink transmissions from User Equipment (UE) to the base station (eNodeB in 4G, gNodeB in 5G). This article provides a comprehensive explanation of TA, its calculation, scenarios where TA is sent, and the parameters involved as per 3GPP specifications, with practical examples.
What is Timing Advance?
Timing Advance is a mechanism to compensate for propagation delays in uplink transmissions. In cellular networks, UEs are located at varying distances from the base station, causing different signal propagation delays. To ensure that uplink signals from all UEs arrive at the base station within the same time slot, the base station instructs each UE to adjust its transmission timing using TA.
TA is a correction value (in time units) that the UE applies to advance its uplink transmission relative to the downlink reception timing. This ensures that uplink signals align with the base station’s receiver window, preventing inter-symbol interference and maintaining orthogonality in Orthogonal Frequency-Division Multiple Access (OFDMA) or Single Carrier FDMA (SC-FDMA) systems.
Key Objectives of TA
Synchronization: Align uplink transmissions from multiple UEs to avoid overlap.
Interference Mitigation: Prevent collisions in time-domain resources.
Efficient Resource Utilization: Ensure accurate scheduling in time-sensitive systems.
Scenarios Where TA is Sent by eNB/gNB
The eNodeB (eNB) in 4G or gNodeB (gNB) in 5G sends TA commands to the UE in various scenarios, as defined by 3GPP standards (TS 36.213 for LTE, TS 38.213 for NR). Below are the primary scenarios:
Initial Access (Random Access Procedure):
Description: During the Random Access Channel (RACH) procedure, the UE sends a preamble to the eNB/gNB. The base station measures the time offset of the received preamble relative to its reference timing and calculates the initial TA.
Message: The TA command is sent in the Random Access Response (RAR) message (Msg2).
Example: A UE located 1 km from the gNB experiences a propagation delay of approximately 3.33 µs (1 km / speed of light). The gNB sends a TA command to advance the UE’s uplink transmission by twice this delay (round-trip time, ~6.67 µs).
Periodic TA Updates:
Description: As the UE moves (e.g., due to mobility), the propagation delay changes. The eNB/gNB periodically monitors uplink signals (e.g., Sounding Reference Signals or PUSCH) and sends TA updates to adjust for these changes.
Message: TA commands are sent via Medium Access Control (MAC) Control Elements (CE) in 4G/5G.
Example: A UE moving at 60 km/h away from the gNB requires frequent TA updates to account for increasing propagation delay.
Handover:
Description: During handover between cells, the target eNB/gNB may issue a new TA command to align the UE’s uplink timing with the new cell’s reference.
Message: TA is included in the handover command or subsequent MAC CE.
Example: A UE moving from Cell A to Cell B may receive a new TA command if the distance to the target gNB differs significantly.
Uplink Synchronization Re-establishment:
Description: If the UE loses uplink synchronization (e.g., due to a long period of inactivity), the eNB/gNB may request the UE to reinitiate the RACH procedure, resulting in a new TA command.
Message: Sent in RAR or MAC CE after re-synchronization.
Example: A UE in idle mode for an extended period may need to re-align its timing before resuming uplink transmission.
Beam Switching in 5G NR:
Description: In 5G, beam-based communication may require TA adjustments when the UE switches between beams with different propagation paths (e.g., due to reflections or line-of-sight changes).
Message: TA updates are sent via MAC CE or as part of beam management signaling.
Example: A UE switching from a direct beam to a reflected beam may experience a slight change in propagation delay, triggering a TA update.
Carrier Aggregation and Dual Connectivity:
Description: In scenarios involving multiple carriers (CA) or dual connectivity (DC), different TA values may be assigned for different carriers or cells due to varying propagation delays or frequency-dependent path characteristics.
Message: TA commands are sent per Timing Advance Group (TAG) via MAC CE.
Example: A UE connected to a macro cell and a small cell in DC may receive separate TA commands for each cell.
Parameters Involved in Calculating TA
The TA value is calculated based on several parameters, as specified in 3GPP TS 36.213 (LTE) and TS 38.213 (NR). Below is a detailed explanation of these parameters, their roles, and examples.
1. Propagation Delay (Tp)
Definition: The time taken for the signal to travel from the UE to the eNB/gNB (one-way delay).
Calculation: Tp = Distance / Speed of Light (c = 3 × 108 m/s).
Role: The round-trip delay (2 × Tp) is the primary component of the TA value, as the UE must advance its transmission to compensate for both uplink and downlink delays.
Example: For a UE 3 km from the gNB, Tp = 3,000 / 3 × 10^8 = 10 µs. The round-trip delay is 20 µs, so the TA is approximately 20 µs.
2. Timing Advance Command (NTA)
Definition: The TA command is an integer value sent by the eNB/gNB to instruct the UE how much to advance its uplink transmission.
Calculation:
In LTE: TA = NTA × Ts, where Ts = 1 / (15000 × 2048) seconds (~0.0326 µs, based on a 30.72 MHz sampling rate).
In 5G NR: TA = (NTA + NTA,offset) × Tc, where Tc = 1 / (Δf_max × Nf) seconds, Δf_max = 480 kHz, Nf = 4096, and Tc ≈ 0.509 ns.
Role: NTA is a quantized value representing the time adjustment in multiples of Ts (LTE) or Tc (NR).
Example:
LTE: For a 20 µs TA, NTA = 20 µs / 0.0326 µs ≈ 614 (sent as an 11-bit value in RAR, 0–1282).
5G NR: For a 20 µs TA, NTA = 20 µs / 0.509 ns ≈ 39,292 (sent as a 12-bit value in RAR, 0–3846 for initial TA).
3. Timing Advance Offset (NTA,offset)
Definition: A fixed offset added to the TA to account for the base station’s processing time and switching delays (e.g., between uplink and downlink).
Values:
LTE: Configurable (0, 624, or 2570 Ts, depending on duplex mode and frame structure).
5G NR: Specified in TS 38.133, typically 0, 39936, or 13792 Tc for different frequency ranges (FR1, FR2) and duplex modes (FDD, TDD).
Role: Ensures proper alignment between uplink and downlink frames.
Example: In 5G TDD (FR1), NTA,offset = 39936 × 0.509 ns ≈ 20.34 µs, added to the calculated TA.
4. Subcarrier Spacing (SCS) in 5G NR
Definition: In 5G, the subcarrier spacing (15 kHz, 30 kHz, 60 kHz, or 120 kHz) affects the granularity of TA.
Role: Higher SCS reduces the slot duration, requiring finer TA adjustments to maintain synchronization.
Example: For 120 kHz SCS (slot duration ~0.125 ms), a small timing misalignment (e.g., 1 µs) is more critical than for 15 kHz SCS (slot duration ~1 ms).
5. Timing Advance Group (TAG)
Definition: A group of carriers or cells sharing the same TA value, used in CA or DC scenarios.
Role: Allows the eNB/gNB to manage TA independently for different groups of carriers with similar propagation characteristics.
Example: A UE in CA with two carriers in different frequency bands may belong to two TAGs, each with a distinct TA value.
6. Timing Advance Command Format
LTE:
Initial TA (RAR): 11 bits, NTA = TA × 16 (0–1282), corresponding to 0–66.7 µs.
TA Update (MAC CE): 6 bits, TA = 0–63, adjusting NTA by ±(TA × 16).
5G NR:
Initial TA (RAR): 12 bits, NTA = TA × 16 × 15 / (2μ), where μ is the SCS index (0 for 15 kHz, 1 for 30 kHz, etc.).
TA Update (MAC CE): 6 bits, adjusting NTA by ±(TA × 16 × 15 / 2μ).
Example: For 30 kHz SCS (μ=1), an initial TA command of 100 corresponds to NTA = 100 × 16 × 15 / 2^1 = 12000 Tc ≈ 6.11 µs.
Practical Example: TA Calculation in 5G NR
Scenario: A UE is 2 km from a gNB operating in FR1 with 30 kHz SCS (μ=1) and TDD mode (NTA,offset = 39936 Tc).
Propagation Delay:
Distance = 2 km, Speed of light = 3 × 10^8 m/s.
Tp = 2,000 / 3 × 10^8 = 6.67 µs.
Round-trip delay = 2 × Tp = 13.34 µs.
TA Calculation:
Tc = 0.509 ns.
Total TA = Round-trip delay + NTA,offset = 13.34 µs + (39936 × 0.509 ns) ≈ 13.34 µs + 20.34 µs = 33.68 µs.
Convert to NTA: NTA = (33.68 µs / 0.509 ns) ≈ 66149 Tc.
For 30 kHz SCS, TA command = NTA / (16 × 15 / 2^1) ≈ 551 (12-bit value).
TA Command:
The gNB sends TA = 551 in the RAR, instructing the UE to advance its uplink transmission by ~33.68 µs.
Challenges and Considerations
Mobility: High-speed UEs (e.g., in vehicles or trains) require frequent TA updates to account for rapidly changing propagation delays.
Multi-Path Propagation: In 5G, especially in mmWave (FR2), multi-path effects may require TA adjustments for different beams.
Granularity: Higher SCS in 5G NR demands more precise TA values due to shorter slot durations.
Inter-Cell Interference: In TDD systems, incorrect TA can cause uplink-downlink interference between neighboring cells.
Conclusion
Timing Advance is a cornerstone of uplink synchronization in 4G and 5G networks, ensuring efficient and interference-free communication. By compensating for propagation delays, TA enables precise alignment of uplink signals at the eNB/gNB. The TA mechanism is triggered in scenarios like initial access, mobility, handovers, and beam switching, with calculations based on parameters like propagation delay, NTA, NTA,offset, and SCS, as defined by 3GPP. Understanding these parameters and their interplay is essential for optimizing network performance and supporting advanced use cases in 5G NR.
References:
1. ETSI TS 138 133 V15.3.0
2. 3GPP TS 38.321 version 16.1.0 Release 16
3. TS 36.213 for LTE, TS 38.213 for 5G
Comments