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Measurement events required for NTN UE handovers.
Hello and welcome. In this article, we will discuss the measurement events required for NTN
UE handovers, including location-based events (Event D1, Event D2, CondEvent D1, and CondEvent D2) and time-based events (CondEvent T1).
Event D1:
This is a location-based measurement event for NTN UEs when the reference location is fixed. It is configured by the network at the time of PDU session creation. When the condition for this event is met, the UE sends the D1 measurement report with the candidate
target cells.
Event definition as per 3gpp 38.331:
The distance between the UE and referenceLocation1 becomes larger than the configured threshold distanceThreshFromReference1, and the distance between the UE and referenceLocation2 becomes shorter than the configured threshold distanceThreshFromReference2.
When both of the conditions below are satisfied, the UE will be considered to be in the situation to report the D1 measurement report.
Entering condition 1:
Ml1 − Hys > Thresh1
Entering condition 2:
Ml2 + Hys < Thresh2
When any of the conditions below are satisfied, the UE will be considered to be in the situation not to report the D1 measurement report.
Leaving condition 1:
Ml1 + Hys < Thresh1
Leaving condition 2:
Ml2 − Hys > Thresh2
Definitions:
• Ml1: Distance between the UE and referenceLocation1 (defined in reportConfigNR for this event). Unit: meters.
• Ml2: Distance between the UE and referenceLocation2 (defined in reportConfigNR for this event). Unit: meters.
• Thresh1: Threshold defined as distanceThreshFromReference1 for referenceLocation1 in reportConfigNR. Unit: meters.
• Thresh2: Threshold defined as distanceThreshFromReference2 for referenceLocation2 in
reportConfigNR. Unit: meters.
For distanceThreshFromReference1 and distanceThreshFromReference2, each step represents 50 meters.
Event D2:
This is a location-based measurement event for NTN UEs when the reference location is moving. It is configured by the network at the time of PDU session creation. When the condition for this event is met, the UE sends the D2 measurement report with the candidate target cells.
Event definition as per 3GPP 38.331:
The distance between the UE and the serving cell’s moving reference location—determined based on movingReferenceLocation and its corresponding satellite ephemeris and epoch time broadcast in SIB19—becomes larger than distanceThreshFromReference1, and the distance between the UE and a moving reference location—determined based on referenceLocation and its corresponding satellite ephemeris and epoch time for the neighbor cell in the associated MeasObjectNR—becomes shorter than distanceThreshFromReference2.
When both of the conditions below are satisfied, the UE will be considered to be in the situation to report the D2 measurement report.
Entering condition 1:
Ml1 − Hys > Thresh1
Entering condition 2:
Ml2 + Hys < Thresh2
When any of the conditions below are satisfied, the UE will be considered to be in the situation not to report the D2 measurement report.
Leaving condition 1:
Ml1 + Hys < Thresh1
Leaving condition 2:
Ml2 − Hys > Thresh2
Definitions:
• Ml1: Distance between the UE and the serving cell’s moving reference location (derived from movingReferenceLocation, epoch time, and SIB19 satellite ephemeris).
• Ml2: Distance between the UE and the neighbor cell’s moving reference location (derived from referenceLocation and ephemeris information in MeasObjectNR).
• Thresh1: distanceThreshFromReference1. Unit: meters.
• Thresh2: distanceThreshFromReference2. Unit: meters.
ASN.1 for Event D2 as per 38.331:
For distanceThreshFromReference1 and distanceThreshFromReference2, each step represents 50 meters.
CondEvent D1:
This is a location-based measurement event for NTN UEs when the reference location is fixed. It is configured by the network for the UE to evaluate and perform handover. When the condition of this event is met, the UE performs handover to the corresponding candidate target cell.
Event definition as per 3GPP 38.331:
The distance between the UE and referenceLocation1 becomes larger than distanceThreshFromReference1, and the distance between the UE and referenceLocation2 of the conditional reconfiguration candidate becomes shorter than distanceThreshFromReference2.
For CondEvent D1, the entering conditions, leaving conditions, and parameter definitions are the same as Event D1.
ASN.1 for CondEvent D1 as per 38.331:
Each step represents 50 meters.
CondEvent D2:
This is a location-based measurement event for NTN UEs when the reference location is moving. It is configured by the network for the UE to evaluate and perform handover. When the condition of this event is met, the UE performs handover to the corresponding candidate target cell.
For CondEvent D2, the entering conditions, leaving conditions, and parameter definitions are the same as Event D2.
ASN.1 for CondEvent D2:
Each step represents 50 meters.
CondEvent T1:
This is a time-based measurement event for NTN UEs. It is configured by the network for the UE to evaluate and perform handover. When the condition for this event is met, the UE performs handover to the corresponding candidate target cell.
Event definition as per 3GPP 38.331:
The time measured at the UE becomes greater than the configured threshold t1-Threshold but is less than t1-Threshold + duration.
When the condition below is satisfied, the UE is considered to be in the situation to perform handover.
Entering condition:
Mt > Thresh1
When the condition below is satisfied, the UE is considered to be in the situation not to perform handover.
Leaving condition:
Mt > Thresh1 + Duration
Definitions:
• Mt: Time measured at the UE (milliseconds).
• Thresh1: t1-Threshold-r17 defined in reportConfigNR.
• Duration: duration-r17 defined in reportConfigNR.
ASN.1 for CondEvent T1 as per 38.331:
t1-Threshold counts the number of UTC seconds in 10 ms units since 00:00:00 on 1 January
1900. For duration each step represents 100 ms.
December 8, 2025