Event A3 is defined as a triggering event when a neighbor cell becomes an offset better than the serving cell. The UE creates a measurement report, populates the triggering details and sends the message to the serving cell.
The parameters that define the trigger are discussed below. The role of the Offset in Event A3 is to create inertia for the UE by making the serving cell look better than its current measurement in comparison to the neighbor.
The role of the Hysteresis in Event A3 is to make the measured neighbor look worse than measured to ensure it is really stronger before the UE decides to send a measurement report to initiate a handover.
The role of timeToTrigger (ttt) in Event A3 is to avoid a ping-pong effect.
In summary, the whole process is defined to keep a UE on the serving cell until the UE has really found a new cell that will offer a better radio environment than its serving cell for a while. The following condition needs to be fulfilled before an Event A3 is triggered.
Mn + Ofn + Ocn – Hys > Ms + Ofs + Ocs + Off
The variables in the formula are defined as follows:
- Mn is the measurement result of the neighboring cell, not taking into account any offsets.
- Ofn is the frequency specific offset of the frequency of the neighbor cell.
- Ocn is the cell specific offset of the neighbor cell, and set to zero if not configured for the neighbor cell.
- Ms is the measurement result of the serving cell, not taking into account any offsets.
- Ofs is the frequency-specific offset of the serving frequency.
- Ocs is the cell-specific offset of the serving cell, and is set to zero if not configured for the serving cell.
- Hys is the hysteresis parameter for this event.Mn and Ms are expressed in dBm in case of RSRP, or in dB in case of RSRQ. Ofn, Ocn, Ofs, Ocs, Hys and Off are expressed in dB.