The timing advance is sent by the eNodeB to the UEs so that each UE advances/delays its UL transmission(s) in order to compensate for the propagation delay and thus time-align the transmissions from different UEs.
Timing Advance (TA) in LTE (Long-Term Evolution) is an important parameter used to synchronize the uplink transmissions from user equipment (UE) to the base station (eNodeB). This article explains what TA is, how it works, procedure and its significance in LTE networks.
What is Timing Advance (TA)?
Timing Advance (TA) is a mechanism that ensures uplink signals from different UEs arrive at the eNodeB in a synchronized manner. Since UEs can be located at varying distances from the eNodeB, the time it takes for their signals to reach the eNodeB will differ. TA compensates for this by adjusting the transmission timing of each UE, so their signals reach the eNodeB simultaneously, preventing interference and ensuring efficient communication.
In LTE, the concept of Timing Advance (TA) is crucial for maintaining uplink timing synchronization between the User Equipment (UE) and the eNodeB (eNB).
HUAWEI LTE Timing Advance.
TA is the time by which the UE must advance its uplink transmission to align with the eNodeB’s downlink timing. This ensures that the uplink signals from different UEs arrive at the eNodeB at the correct time, preventing interference.
How TA Works:
- Initial Sync: When the UE sends an initial access request (Preamble) to the eNodeB, it starts a timer.
- Receiving TA: The eNodeB responds with the required TA value for the UE to use in its future transmissions.
Technical Details:
- According to 3GPP 36.213, one TA unit equals 0.52 microseconds (µs).
- 1 TA = 16Ts = 16 (0.5ms/15360) = 0.52μs
- 1 TA time advance maps to the distance between eNodeB and UE is: (300,000km/s*0.52μs)/2 = 78m
- This translates to a distance of 78 meters because the signal travels at spread speed of electric wave (300,000 km/s), and the 2 value accounts for the round-trip travel time of the signal.
The distance between the UE and the eNodeB can be calculated as 78 meters multiplied by the TA value. The accuracy of TA is 78 meters, meaning it can determine the UE’s distance from the eNodeB to within 78 meters. Huawei and other equipment manufacturers have counters to track TA values when receiving random access requests from UEs.
Below is a detailed explanation of the timing advance procedure:
LTE Timing Advance (TA) Procedure.
Initial TA Command:
When a UE wants to initiate communication, it sends a Random Access Channel (RACH) preamble. The eNB responds with a TA command in the RACH Response (Message 2) to adjust the UE’s transmission timing to compensate for the propagation delay.
Ongoing TA Adjustments:
The eNB continuously monitors the uplink timing and sends periodic TA updates to the UE. These updates are differential, meaning they represent changes relative to the last TA command sent.
eNodeB Side Mechanism.
TA Command Timer (timingAlignmentCommandTimer):
At the creation of a UE connection, a TA command timer is started. When the timer expires, a new TA command is sent, and the timer is restarted. The Layer 1 (L1) UL indicates to the DL MAC when there is a change in TA, prompting a new TA command. If a TA command PDU fails to transmit after the maximum number of HARQ attempts (8), it is rescheduled for transmission after a short delay (TTI k+2).
If four consecutive TA command transmissions fail, the process is considered failed. Only one TA command is active per UE at a time, and new commands preempt ongoing transmissions.
UE Side Mechanism.
Time Alignment Timer:
Upon receiving a TA command, the UE adjusts its transmission timing and restarts its Time Alignment Timer. The Time Alignment Timer is specific to the cell and is configured via broadcast (SIB2) or dedicated RRC signaling.
If the Time Alignment Timer expires, the UE:
- Flushes all HARQ buffers.
- Notifies RRC to release PUCCH/SRS.
- Clears any configured downlink assignments and uplink grants.
- Initiates a new Random Access procedure to regain uplink synchronization.
Timer Configurations.
timeAlignmentTimerCommon: Configures the Time Alignment Timer value broadcast on SIB2.
timeAlignmentTimerDedicated: Configures the Time Alignment Timer value sent over dedicated RRC signaling.
If the Timing Advance Command was received in a Random Access Response Message and the contention resolution fails, the Time Alignment Timer stops.
Starting from LA5.0, timingAlignmentCommandTimer is no longer configurable and is derived as rounddown(timeAlignmentTimer / 2.5) in milliseconds.
If the UE’s Time Alignment Timer is set to “infinity,” periodic TA commands are not transmitted.
Out-of-Sync Handling.
DL_OUT_SYNC:
- The DL scheduler considers the UE out of sync if notified by the UL scheduler.
- If TA failure occurs (TAFailureTimer expires without a successful TA PDU HARQ ACK), the UE transitions to DL_OUT_SYNC.
- In DL_OUT_SYNC, the DL scheduler stops scheduling the UE’s data, except for RRC signaling messages (SRB1 and SRB2).
Understanding and managing these timing mechanisms ensure reliable and synchronized uplink transmissions, essential for maintaining the overall performance and efficiency of the LTE network.
How Does Timing Advance Work?
1. Distance Calculation
The eNodeB calculates the distance between itself and the UE based on the time delay of the signal received from the UE. This time delay is measured during the Random Access Procedure or through regular TA updates.
2. TA Command
Once the eNodeB determines the necessary timing adjustment, it sends a TA command to the UE. This command specifies how much earlier or later the UE should transmit its uplink signals.
3. Adjustment
The UE adjusts its transmission timing based on the TA command. The TA value is typically expressed in terms of time units (Ts), where each unit corresponds to a specific duration. In LTE, one TA unit represents 16 Ts, which equals approximately 0.52 microseconds.
4. Continuous Updates
TA is not a one-time adjustment. The eNodeB continuously monitors and updates the TA values to account for any changes in the UE’s position or network conditions.
Significance of Timing Advance
1. Synchronization
TA ensures that all uplink signals from different UEs arrive at the eNodeB in a synchronized manner. This prevents signal overlap and interference, which can degrade network performance.
2. Efficient Resource Utilization
By synchronizing uplink transmissions, TA helps in efficient utilization of the uplink frequency resources. This maximizes the network’s capacity and improves overall performance.
3. Improved Quality of Service (QoS)
Proper TA adjustments enhance the quality of service by reducing errors and improving the reliability of data transmission. This is crucial for maintaining high data rates and low latency in LTE networks.
4. Handover Performance
During handovers, accurate TA calculations ensure seamless transitions between cells, minimizing the risk of dropped calls or data sessions.
Practical Considerations.
TA Range
In LTE, the TA value can range from 0 to 1282, which corresponds to a distance range of approximately 0 to 78 km. This wide range accommodates various cell sizes and UE locations.
TA Grouping
For efficient management, UEs are often grouped based on their TA values. This grouping helps the eNodeB in handling UEs with similar timing requirements more effectively.
Challenges
- Mobility: Rapidly moving UEs, such as those in vehicles, can pose challenges for TA adjustments due to frequent changes in distance.
- Environmental Factors: Obstacles and varying terrain can affect signal propagation and thus the accuracy of TA calculations.
Final Thoughts:
Timing Advance (TA) is a vital mechanism in LTE networks that ensures synchronized uplink transmissions, efficient resource utilization, and improved quality of service. By continuously adjusting the transmission timing of UEs, TA maintains network performance and reliability, even as UEs move and network conditions change. Understanding and managing TA is essential for optimizing LTE network operations and delivering a seamless user experience.