LTE UE Measurement Reporting Events: A1-A5, B1-B2

In LTE systems, the RRC Connection Reconfiguration message is used to configure UE measurement reporting. This message replaces the Measurement Control message used in UMTS systems.

The 3GPP standard defines 5 intra-system and 2 inter-system measurement reporting events, as described below. LTE Events A1, A2, A3, A4, and A5 are based on either RSRP or RSRQ measurements. For more details on these measurements, refer to documentation comparing LTE RSRP vs. RSRQ.

Inter-system LTE events B1 and B2 are based on CPICH RSCP or CPICH Ec/Io for UMTS systems, RSSI for GSM, and pilot strength for CDMA2000. The criteria for triggering and subsequently cancelling each event are evaluated after layer-3 filtering has been applied. The criteria for each event must be satisfied for at least the duration of the “time to trigger” parameter.

Time to Trigger:

The “time to trigger” parameter can be configured independently for each reporting event. The allowed values are as follows (in milliseconds): {0, 40, 64, 80, 100, 128, 160, 256, 320, 480, 512, 640, 1024, 1280}.

Layer-3 Filtering:

Layer-3 filtering is applied before evaluating whether or not any measurement reporting events have been triggered. The filtering is performed using the following formula:

$F_n = (1 - a) * F_{n-1} + (a * MEAS_n)$

Where:

• $F_n$ = Updated filtered measurement result
• $F_{n-1}$ = Previous filtered measurement result
• $a = (1/2)^{k/4}$, where k is the appropriate filter coefficient
• $MEAS_n$ = Latest measurement result received from the PHY layer

The filter coefficient k can be configured independently for LTE RSRP, LTE RSRQ, UMTS CPICH RSCP, UMTS CPICH Ec/Io, and GSM RSSI.

LTE Events

Event A1: Serving Cell Becomes Better Than a Threshold

The LTE Event A1 is triggered when the serving cell’s signal quality becomes better than a configured threshold.

• Triggering Condition:

  $MEAS_{serv} - Hyst > Threshold$

• Cancellation Condition:

  $MEAS_{serv} + Hyst < Threshold$

The hysteresis value ($Hyst$) can be configured with a value between 0 and 30 dB.

Event A2: Serving Cell Becomes Worse Than a Threshold

The LTE Event A2 is triggered when the serving cell’s signal quality becomes worse than a configured threshold.

• Triggering Condition:

  $MEAS_{serv} + Hyst < Threshold$

• Cancellation Condition:

  $MEAS_{serv} - Hyst > Threshold$

The hysteresis value ($Hyst$) can be configured with a value between 0 and 30 dB.

Event A3: Neighboring Cell Becomes Better Than Serving Cell by an Offset

The LTE Event A3 is triggered when a neighboring cell’s signal quality becomes better than the serving cell’s signal quality by a specified offset. The offset can be either positive or negative.

• Triggering Condition:

  $MEAS_{neigh} + O_{neigh,freq} + O_{neigh,cell} - Hyst > MEAS_{serv} + O_{serv,freq} + O_{serv,cell} + Offset$

• Cancellation Condition:

  $MEAS_{neigh} + O_{neigh,freq} + O_{neigh,cell} + Hyst < MEAS_{serv} + O_{serv,freq} + O_{serv,cell} + Offset$

Event A4: Neighboring Cell Becomes Better Than a Threshold

The LTE Event A4 is triggered when a neighboring cell’s signal quality becomes better than a configured threshold.

• Triggering Condition:

  $MEAS_{neigh} + O_{neigh,freq} + O_{neigh,cell} - Hyst > Threshold$

• Cancellation Condition:

  $MEAS_{neigh} + O_{neigh,freq} + O_{neigh,cell} + Hyst < Threshold$

Event A5: Serving Cell Becomes Worse Than Threshold-1 and Neighboring Cell Becomes Better Than Threshold-2

The LTE Event A5 is triggered when the serving cell’s signal quality becomes worse than a threshold (Threshold-1) while, simultaneously, a neighboring cell’s signal quality becomes better than another threshold (Threshold-2).

• Triggering Condition:

  Both of the following conditions must be true:

  • $MEAS_{serv} + Hyst < Threshold-1$
  • $MEAS_{neigh} + O_{neigh,freq} + O_{neigh,cell} - Hyst > Threshold-2$

• Cancellation Condition:

  Either of the following conditions must be true:

  • $MEAS_{serv} - Hyst > Threshold-1$
  • $MEAS_{neigh} + O_{neigh,freq} + O_{neigh,cell} + Hyst < Threshold-2$

Inter-System Events

Event B1: Inter-System Neighboring Cell Becomes Better Than a Threshold

The LTE Event B1 is triggered when a neighboring cell from a different radio access technology (e.g., UMTS, GSM, CDMA2000) becomes better than a threshold.

• Triggering Condition:

  $MEAS_{neigh} + O_{neigh,freq} - Hyst > Threshold$

• Cancellation Condition:

  $MEAS_{neigh} + O_{neigh,freq} + Hyst < Threshold$

Event B2: Serving Cell Becomes Worse Than Threshold-1 and Inter-System Neighboring Cell Becomes Better Than Threshold-2

The LTE Event B2 is triggered when the serving cell’s signal quality becomes worse than a threshold (Threshold-1) while, simultaneously, a neighboring cell from a different radio access technology becomes better than another threshold (Threshold-2).

• Triggering Condition:

  Both of the following conditions must be true:

  • $MEAS_{serv} + Hyst < Threshold-1$
  • $MEAS_{neigh} + O_{neigh,freq} - Hyst > Threshold-2$

• Cancellation Condition:

  Either of the following conditions must be true:

  • $MEAS_{serv} - Hyst > Threshold-1$
  • $MEAS_{neigh} + O_{neigh,freq} + Hyst < Threshold-2$

REFERENCE: 3GPP TS 36.331 rel.8