Measurement report Event A1 in 5G NR

Event A1: Serving becomes better than absolute threshold

Measurement reporting plays a crucial role in the performance and optimization of 5G networks. One such event that network engineers frequently encounter is Measurement Report Event A1.

What is Event A1 in 5G NR?

Event A1 is a measurement reporting event in 5G New Radio (NR). It is triggered when the signal quality of the serving cell exceeds a pre-defined threshold. Simply put, it indicates that the serving cell’s signal quality is good enough, and no additional monitoring or actions are required at that moment.

Measurement Reporting Event A1 in 5G NR, as defined in 3GPP TS 38.331, is used by the User Equipment (UE) to monitor and report the quality of the serving cell based on predefined conditions. Event A1 helps maintain a stable connection by ensuring that the current serving cell offers adequate signal quality. When the signal quality meets the A1 threshold, the network does not need to consider other cells for handover. This reduces unnecessary signaling and keeps the user connected to a strong serving cell.

How Does Event A1 Work?

Triggering Conditions

Event A1 is triggered when the signal quality of the serving cell is better than a set threshold value. This signal quality can be measured using one of the following metrics:

1. RSRP (Reference Signal Received Power): Represents the power level of the signal.
2. RSRQ (Reference Signal Received Quality): Indicates the quality of the signal by considering both signal strength and interference levels.

The network uses these measurements to determine if Event A1 should be triggered.

Example Scenario: Consider a scenario where the network operator has configured the A1 threshold for RSRP to -100 dBm. If the serving cell’s RSRP exceeds -100 dBm, Event A1 is triggered. The user equipment (UE) reports this to the network, indicating that the connection quality is stable.

What Parameters are Used for Event A1 Configuration?

The configuration of Event A1 involves several key parameters:

• Threshold: Defines the signal strength or quality value that the serving cell must exceed to trigger Event A1.
• Hysteresis: A margin that prevents frequent triggering of the event due to minor fluctuations in signal quality.
• Time-to-Trigger (TTT): Specifies the duration for which the signal quality must exceed the threshold before the event is triggered.

Example Configuration Parameters

Let’s say the A1 threshold is set at -105 dBm, with a hysteresis of 3 dB and a TTT of 320 ms. The event would be triggered if:

1. The RSRP of the serving cell is above -105 dBm.
2. The condition persists for at least 320 ms.

Why Would You Use Event A1?

• Reduce Signaling Overhead: Since Event A1 indicates strong signal quality, it helps the network avoid unnecessary monitoring of neighboring cells.
• Improve Battery Efficiency: When Event A1 is triggered, the UE may reduce the frequency of measurement reporting, leading to power savings.
• Enhance User Experience: Staying connected to a strong cell improves the quality of service and minimizes interruptions.

eventA1 SEQUENCE {
    a1-Threshold         MeasTriggerQuantity,
    reportOnLeave        BOOLEAN,
    hysteresis           Hysteresis,
    timeToTrigger        TimeToTrigger
}

Entering Condition (A1-1)

• Condition: The entering condition for Event A1 is satisfied when the measurement result of the serving cell (Ms) minus the hysteresis value (Hys) is greater than a defined threshold value (Thresh).
• Formula: Ms - Hys > Thresh
• Parameters:
• Ms: Measurement result of the serving cell (dBm for RSRP, dB for RSRQ/SINR).
• Hys: Hysteresis value from reportConfigNR (in dB).
• Thresh: Threshold parameter (a1-Threshold) for the event.

Leaving Condition (A1-2)

• Condition: The leaving condition for Event A1 is satisfied when the measurement result of the serving cell (Ms) plus the hysteresis value (Hys) is less than the threshold value (Thresh).
• Formula: Ms + Hys < Thresh
• Purpose: Ensures the UE only reports leaving the event if the quality falls significantly below the threshold level.

Relevance and Usage

• Purpose: Event A1 is primarily used to monitor the quality of the serving cell without considering neighboring cells. It ensures signal strength is adequate to maintain the connection.
• Applications: Useful for intra-frequency handover scenarios where the UE determines whether to stay connected to the current cell or begin evaluating neighbors.

FAQs

1. What is the difference between Event A1 and Event A2? Event A1 is triggered when the serving cell’s signal quality is good, while Event A2 is triggered when the serving cell’s signal quality is poor. Event A1 helps maintain the current cell connection, while Event A2 prepares the network for a potential cell change.

2. Can Event A1 parameters be customized? Yes, network operators can configure the threshold, hysteresis, and TTT values for Event A1. This allows flexibility based on network conditions and performance requirements.

3. How does Event A1 impact network performance? Event A1 helps reduce unnecessary handovers and signaling, contributing to a more stable network with lower signaling overhead.

4. What role does hysteresis play in Event A1? Hysteresis prevents frequent triggering of Event A1 due to minor fluctuations in signal quality, acting as a buffer to avoid unnecessary reporting.

5. What Happens After Event A1 is Triggered? When Event A1 is triggered, the UE stops monitoring neighboring cells for the purpose of handover. It signals the network that the serving cell’s quality is good, and the network maintains the stable connection without initiating handover procedures.

References

• 3GPP TS 38.331