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5G Measurement Events

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5G Measurement Events

In any telecom technology (2G, 3G, 4G or 5G), mobility (handover) decision whether mobile device will be handover or not is taken by by base station based on measurement reports from the mobile device. There are multiple measurement items (RSRP, RSRQ, SINR) and multiple ways (periodic, event triggered) to measure the signal quality of the serving cell and neighbor cells.

In Ideal case, a base station shall allow UE to report serving cell and neighbor cell signal quality and trigger the handover with single measurement, but in practice it can create overload conditions due to unnecessary ping pong handovers. As solution to avoid such situation, 3GPP specifications have proposed a set of predefined set of measurement report mechanism to be performed by UE. These predefined measurement report type is called “Event“. The type of “event” a UE have to report is specified by RRC signaling message sent by the base station.

3GPP specification 38.331 specified following events defined for 5G NR.

If we closely  observed these event, we can categories them A1-A6 as Same RAT and B1-B2 as Inter RAT Events. UE keeps on measuring serving cell and neighbors report quantity and validate it with the threshold or offset defined in report configuration. The report quantity/the trigger for event can be RSRP, RSRQ or SINR.

Below table summarizes the parameters and their ranges used in different events

Event Parameter Range Value
A1, A2, A4, A5, B1 RSRP threshold 0 127 -156 dBm -31 dBm
RSRQ threshold 0 127 -40 dB  20dB
SINR threshold 0 127  -23 dB  40 dB
All Hysteresis 0 30   0 dB  15 dB
A3, A6 Offset -30 30 -15 dB +15 dB
A3,A4,A5,A6,B1,B2 Cell Specific Offset  -24 dB  +24 dB
B1, B2 LTE RSRP 0 97 -140 dBm  -44 dBm
LTE RSRQ 0 34  -19.5 dB  -3 dBm
LTE SINR -23 40  -23 dB   40dB

Measurement Event A1 (Serving becomes better than threshold)

Event A1 is triggered when the serving cell becomes better than a threshold. It is typically used to cancel an ongoing handover procedure. This may be required if a UE moves towards cell edge and triggers a mobility procedure, but then subsequently moves back into good coverage before the mobility procedure has completed.

Following figure and equations shows the trigger and cancel condition.

The variables used in the equations above are defined as follows:

Measurement Event 2 (Serving becomes worse than threshold)

Event A2 is typically used to trigger a mobility procedure when a UE moves towards cell edge. Event A2 does not involve any neighbor cell measurements so it may be used to trigger a blind mobility procedure or it may be used to triggers neighbor cell measurements which can then be used for a measurement based mobility procedure.

For example, the gNB may configure measurement gaps and inter-frequency or inter-system measurements after Event A2 has been triggered. This approach means that the UE only needs to complete the intra/inter frequency or Inter system measurements where coverage conditions are relatively poor and there is a high chance that a handover will be required.

Following figure and equations shows the trigger and cancel condition.

The variables used in the equations above are defined as follows:

Measurement Event A3 (Neighbor becomes offset better than SpCell)

Event A3 is triggered when a neighbor cell becomes better than a special cell (SpCell) by an offset. A special cell is the primary serving cell of either the Master Cell Group (MCG) or Secondary Cell Group (SCG)). The offset can be either positive or negative.

This event is typically used for intra-frequency or inter-frequency handover procedures. When Evenet A2 is triggered  the UE may be configured with measurement gaps to measure the inter frequency objects and Event A3 for inter-frequency handover. Event A3 provides a handover triggering mechanism based upon relative measurement results, e.g. it can be configured to trigger when the RSRP of a neighbor cell is stronger than the RSRP of special cell

Following figure and equations shows the trigger and cancel condition.

The variables used in the equations above are defined as follows:

Measurement Event A4 (Neighbor becomes better than threshold)

Event A4 is triggered when neighbor cell becomes better than defined threshold. This event can be used for handover procedures which does not depend upon the coverage of the serving cell. For example, in load balancing feature take the decision to handover a UE away from the serving cell due to load conditions rather than radio conditions. In this case, the UE only needs to verify that the target cell is better than certain signal level threshold and can provides adequate coverage.

Following figure and equations shows the trigger and cancel condition.

The variables used in the equations above are defined as follows:

Measurement Event A5 (SpCell becomes worse than threshold1 and neighbor becomes better than threshold2)

Event A5 is triggered when a special cell becomes worse than threshold 1, while a neighboring cell becomes better than threshold 2. You can thing Event A5 is combination of Event A2 and Event A4.

This event  is typically used for intra-frequency or inter-frequency handover procedures. After Event A2 is trigger the UE may be configured with measurement gaps and an Event A5 for inter-frequency handover. Event A5 provides a handover triggering mechanism based upon absolute measurement results. It can be used to trigger a time critical handover when a current special cell becomes weak and it is necessary to change towards another cell which may not satisfy the criteria for an event A3 handover.

Following figure and equations shows the trigger and cancel condition.

Event Trigger Condition

Event Cancellation  Condition

The variables used in the equations above are defined as follows:

Measurement Event A6 (Neighbor becomes offset better than SCell)

Event A6 is triggered when a neighbor cell becomes better than a secondary cell by an offset. The offset can be either positive or negative. This measurement reporting event is applicable to Carrier Aggregation, i.e. connections which have secondary serving cells in addition to a primary serving cell.

Following figure and equations shows the trigger and cancel condition.

The variables used in the equations above are defined as follows:

Measurement Event B1 (Inter RAT neighbor becomes better than threshold)

Event B1 can be used for inter-RAT handover procedures which does not depend upon the coverage of the serving cell. For example, in load balancing feature take the decision to move a UE away from NR due to load conditions rather than radio conditions. In this case, the UE only needs to verify that the target cell in other RAT (e.g. LTE) is better than certain signal level threshold and can provides adequate coverage.

Following figure and equations shows the trigger and cancel condition.

 

The variables used in the equations above are defined as follows:

Measurement Event B2 (PCell becomes worse than threshold1 and inter RAT neighbor becomes better than threshold2)

Event B2 is triggered when a primary serving cell becomes worse than threshold1, while a neighbor inter-RAT cell becomes better than threshold2. This can be used to trigger inter-RAT mobility procedures when the primary serving cell becomes weak. Inter-system neighbor cell measurements arc used to ensure that the target cell provides adequate coverage.

Following figure and equations shows the trigger and cancel condition.


Event Trigger Condition

Event Cancellation  Condition

The variables used in the equations above are defined as follows:

Related Post:

  1. 5G NR Measurement – Serving Cell and Neighbor Cell
  2. 5G NR Network Relationship – Neighbor Planning

Reference:

  1. 3GPP TS 38.331 5G; NR; Radio Resource Control (RRC); Protocol specification Release 15
  2. http://niviuk.free.fr/lte_event.php



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