Measurement Reporting 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 ,
 hysteresis ,
 timeToTrigger 
 }
 

a1-Threshold	threshold parameter for this event
MeasTriggerQuantity	The IE MeasTriggerQuantity is used to configure the trigger quantity and reporting range for RSRP, RSRQ and SINR measurements.
reportOnLeave	Indicates whether or not the UE shall initiate the measurement reporting procedure when the leaving condition is met for a cell
hysteresis	The IE Hysteresis is a parameter used within the entry and leave condition of an event triggered reporting condition. The actual value is field value * 0.5 dB
timeToTrigger	The IE TimeToTrigger specifies the value range used for time to trigger parameter, which concerns the time during which specific criteria for the event needs to be met in order to trigger a measurement report. Value ms0 corresponds to 0 ms ,a value ms40 corresponds to 40 ms, and so on.

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, expressed in dBm for RSRP, or in dB for RSRQ or RS-SINR.
  • Hys: Hysteresis value defined in reportConfigNR for this event, expressed in dB.
  • Thresh: Threshold parameter for this event (a1-Threshold), expressed in the same unit as Ms.

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: This condition ensures that the UE continues to monitor the quality of the serving cell and only reports leaving the event if the quality falls below a certain level.

1. Hysteresis (Hys): Introduces a buffer or margin to prevent frequent toggling between entering and leaving conditions due to minor variations in measurements.
2. Threshold (Thresh): Represents the quality standard that the serving cell must meet for the event to be triggered.

Relevance and Usage:

• Purpose: Event A1 is primarily used to monitor the quality of the serving cell without considering any neighboring cells. It helps ensure that the serving cell’s signal strength is adequate to maintain the connection.
• Applications: Useful for intra-frequency handover scenarios where the UE needs to determine whether to stay connected to the current cell or trigger an event based on neighboring cell measurements.

3GPP References:

• 3GPP TS 38.331