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.

• Event A1 (Serving becomes better than threshold)
• Event A2 (Serving becomes worse than threshold)
• Event A3 (Neighbor becomes offset better than SpCell)
• Event A4 (Neighbor becomes better than threshold)
• Event A5 (SpCell becomes worse than threshold1 and neighbor becomes better than threshold2)
• Event A6 (Neighbour becomes offset better than SCell)
• Event B1 (Inter RAT neighbour becomes better than threshold)
• Event B2 (PCell becomes worse than threshold1 and inter RAT neighbor becomes better than threshold2)

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.

• • Ms  – Hys >  Thresh (Trigger Condition)
  • Ms  + Hys <  Thresh (Cancellation  Condition)

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

• • • Ms is the measurement result of the serving cell without taking any offsets into account. Ms can be expressed in dBm in case of RSRP, or in dB in case of RSRQ and RS-SINR
    • Hys is the hysteresis parameter for A1 event expressed in dB. Hysteresis is signaled within reportConfigNR value between 0 and 30, Actual value in dB can be obtained by multiplying 0.5 with signal values e.g. signaled value is 5 then Hyst= 5 x 0.5 =2.5dB.
    • Thresh is the threshold parameter for this event i.e. a1-Threshold as defined within reportConfigNR, Thresh is expressed in the same unit as Ms
• Example: Lets consider threshold for Event A1 is configured as -85dBm RSRP, with hysteresis as 1 dB. Suppose Serving cell measured RSRP is -89dBm.
  • • -89 – 1 > -85 is a false condition , UE will not report Event A1, let say now UE moved towards cell center and its RSRP condition is improved and measured value is about -83dBm
    • -83 -1 > -85 is a true condition so UE will report Event A1

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.

• • Ms  + Hys <  Thresh  (Trigger Condition)
  • Ms  – Hys >  Thresh (Cancellation  Condition)

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

• • • Ms is the measurement result of the serving cell without taking any offsets into account. Ms can be expressed in dBm in case of RSRP, or in dB in case of RSRQ and RS-SINR
    • Hys is the hysteresis parameter for A2 event expressed in dB. Hysteresis is signaled within reportConfigNR value between 0 and 30, Actual value in dB can be obtained by multiplying 0.5 with signal values e.g. signaled value is 5 then Hyst= 5 x 0.5 =2.5dB.
    • Thresh is the threshold parameter for this event i.e. a2-Threshold as defined within reportConfigNR,  Thresh is expressed in the same unit as Ms
• Example: Lets consider threshold for Event A2 is configured as -75dBm RSRP, with hysteresis as 1 dB. Suppose Serving cell measured RSRP is -69dBm.
  • • -69 + 1 < -75 is a false condition , UE will not report Event A2, let say now UE moved towards cell edge and its RSRP condition is degraded and measured value is about -78dBm
    • -83 + 1 < -75 is a true condition so UE will report Event A2

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.

• • Mn + Ofn + Ocn – Hys > Mp + Ofp + Ocp + Off           (Trigger Condition)
  • Mn + Ofn + Ocn + Hys < Mp + Ofp + Ocp + Off           (Cancellation  Condition)

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

• • • Mn is the measurement result of the neighboring cell, not taking into account any offsets.
    • Ofn is the measurement object specific offset of the reference signal of the neighbour cell i.e. offsetMO as defined within measObjectNR corresponding to the neighbour cell)
    • Ocn is the cell specific offset of the neighbor cell i.e. cellIndividualOffset as defined within measObjectNR corresponding to the frequency of the neighbor cell, and set to zero if not configured for the neighbour cell.
    • Mp is the measurement result of the SpCell, not taking into account any offsets.
    • Ofp is the measurement object specific offset of the SpCell i.e. offsetMO as defined within measObjectNR corresponding to the SpCell.
    • Ocp is the cell specific offset of the SpCell i.e. cellIndividualOffset as defined within measObjectNR corresponding to the SpCell, and is set to zero if not configured for the SpCell.
    • Hys is the hysteresis parameter for this event i.e. hysteresis as defined within reportConfigNR
    • Off is the offset parameter for this event i.e. a3-Offset as defined within reportConfigNR
    • Mn, Mp are expressed in dBm in case of RSRP, or in dB in case of RSRQ and RS-SINR
• Example: Lets consider a3-offset is set to as 3dB, hys, Ofn, Ofp and Ocp are set to 0. As soon as UE found any neighbor cell whose measurements is 3dB better than serving cell, it should report the Event A3. e.g. Neighbor cell RSRP = -78dB and Serving cell RSRP= -82, here neighbor cell is better and satisfy the Event offset, so UE will report Event A3 to the gNB.

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.

• • Mn + Ofn + Ocn – Hys > Thresh                    (Trigger Condition)
  • Mn + Ofn + Ocn + Hys < Thresh                    (Cancellation  Condition)

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

• • • Mn is the measurement result of the neighboring cell, not taking into account any offsets. Mn is expressed in dBm in case of RSRP, or in dB in case of RSRQ and RS-SINR
    • Ofn is the measurement object specific offset of the neighbor cell i.e. offsetMO as defined within
      measObjectNR corresponding to the neighbor cell
    • Ocn is the measurement object specific offset of the neighbor cell i.e. cellIndividualOffset as defined within measObjectNR corresponding to the neighbor cell, and set to zero if not configured
    • Hys is the hysteresis parameter as defined within reportConfigNR
    • Ofn, Ocn, Hys are expressed in dB.
    • Thresh is the threshold parameter for this event i.e. a4-Threshold as defined within reportConfigNR. Thresh is expressed in the same unit as Mn.
• Example: Lets consider a4-Threshold RSRP = -80 dBm and hys, other offset are set to 0. As soon as UE found any neighboring cell whose RSRP measurement is better than -80 dBm, UE shall report the event. e.g. neighbor Cell “A” measured RSRP = – 76 dBm, and satisfy the Event A4 conditions , UE will report event to gNB.

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

• • Mp + Hys < Thresh1
  • Mn+ Ofn +Ocn – Hys > Thresh2

Event Cancellation  Condition

• • Mp – Hys > Thresh1
  • Mn+ Ofn +Ocn + Hys < Thresh2

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

• • • Mp is the measurement result of the NR SpCell, not taking into account any offsets
    • Mn is the measurement result of the neighboring cell, not taking into account any offsets
    • Mn, Mp are expressed in dBm in case of RSRP, or in dB in case of RSRQ and RS-SINR.
    • Ofn is the measurement object specific offset of the neighbor cell i.e. offsetMO as defined within
      measObjectNR corresponding to the neighbour cell
    • Ocn is the cell specific offset of the neighbor cell i.e. cellIndividualOffset as defined within measObjectNR corresponding to the neighbour cell, and set to zero if not configured
    • Hys is the hysteresis parameter i.e. hysteresis as defined within reportConfigNR
    • Ofn, Ocn, Hys are expressed in dB.
    • Thresh1 is the threshold parameter i.e. a5-Threshold1 as defined within reportConfigNR. It is expressed in the same unit as Mp.
    • Thresh2 is the threshold parameter i.e. a5-Threshold2 as defined within reportConfigNR. It is expressed in the same unit as Mn.
• Example: Lets consider a5-Threshold1 is set as RSRP= -85dBm, a5-Threshold2 is set as RSRP = -80 dB and other offsets, hysteresis is assumed to be 0. The UE report A5 when serving NR cell RSRP become worse than -85 dBm and Neighbor NR cell should be better than RSRP -80, both conditions needs to be satisfied. Let say UE serving RSRP is -90 dBm and neighbor cell RSRP is -78 dBm satisfy both thresholds, so UE can report Event A5 to gNB.

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.

• • Mn + Ocn – Hys  > Ms + Ocs + Off                  (Trigger Condition)
  • Mn + Ocn + Hys < Ms + Ocs + Off                   (Cancellation  Condition)

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

• • • Mn is the measurement result of the neighboring cell, not taking into account any offsets
    • Ocn is the cell specific offset of the neighbor cell i.e. cellIndividualOffset as defined within the associated measObjectNR, and set to zero if not configured
    • Ms is the measurement result of the serving cell, not taking into account any offsets.
    • Ocs is the cell specific offset of the serving cell i.e. cellIndividualOffset as defined within the associated measObjectNR, and is set to zero if not configured
    • Hys is the hysteresis parameter i.e. hysteresis as defined within reportConfigNR
    • Off is the offset parameter i.e. a6-Offset as defined withinreportConfigNR
    • Mn, Ms are expressed in dBm in case of RSRP, or in dB in case of RSRQ and RS-SINR
    • Ocn, Ocs, Hys, Off are expressed in dB

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.

 

• • Mn + Ofn + Ocn – Hys > Thresh
  • Mn + Ofn + Ocn + Hys < Thresh

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

• • • Mn is the measurement result of the inter-RAT neighbor cell, not taking into account any offsets
    • Ofn is the measurement object specific offset of the frequency of the inter-RAT neighbor cell i.e. eutra-QOffsetRange as defined within the measObjectEUTRA corresponding to the frequency of the neighbor inter RAT cell
    • Ocn is the cell specific offset of the inter-RAT neighbor cell i.e. cellIndividualOffset as defined within the measObjectEUTRA corresponding to the neighbour inter-RAT cell, and set to zero if not configured
    • Hys is the hysteresis parameter as defined within reportConfigInterRAT for this event
    • Thresh is the threshold parameter i.e. b1-ThresholdEUTRA as defined within reportConfigInterRAT. It is expressed in the same unit as Mn.
    • Mn is expressed in dBm or in dB, depending on the measurement quantity of the inter-RAT neighbour cell
    • Ofn, Ocn, Hys are expressed in dB

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

• • Mp + Hys < Thresh1
  • Mn + Ofn + Ocn – Hys > Thresh2

Event Cancellation  Condition

• • Mp – Hys > Thresh1
  • Mn + Ofn + Ocn + Hys < Thresh2

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

• • • Mp is the measurement result of the PCell, not taking into account any offsets.
    • Mn is the measurement result of the inter-RAT neighbour cell, not taking into account any offsets
    • Ofn is the measurement object specific offset of the frequency of the inter-RAT neighbour cell i.e. eutra-QOffsetRange as defined within the measObjectEUTRA corresponding to the frequency of the inter-RAT neighbour cell
    • Ocn is the cell specific offset of the inter-RAT neighbor cell i.e. cellIndividualOffset as defined within the measObjectEUTRA corresponding to the neighbor inter-RAT cell, and set to zero if not configured
    • Hys is the hysteresis parameter i.e. hysteresis as defined within reportConfigInterRAT
    • Thresh1 is the threshold parameter i.e. b2-Threshold1 as defined within reportConfigInterRAT. It is expressed in the same unit as Mp
    • Thresh2 is the threshold parameter i.e. b2-Threshold2EUTRA as defined within
      reportConfigInterRAT. It is expressed in the same unit as Mn
    • Mp is expressed in dBm in case of RSRP, or in dB in case of RSRQ and SINR
    • Mn is expressed in dBm or dB, depending on the measurement quantity of the inter-RAT neighbour cell
    • Ofn, Ocn, Hys are expressed in dB

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