These events are helpful to trigger Handovers. This event is used to trigger intra-LTE mobility. By default, user equipment in the LTE network detect suitable neighbor cells without the assistance of a list of neighbor cells sent by the serving RBS. User equipments are expected to blindly detect suitable handover candidates and the subsequent handover evaluation is undertaken using generic offset values. The exception to this occurs when specific handover offset values are defined between neighboring cells by the network operator. When specific cell offset relationships exist, this information is sent to the user equipment in RRCConnectionReconfiguration messages by the RBS.
The RRCConnectionReconfiguration message can also contain a list of cells for which handover is not allowed. These cells are identified by their Physical Cell Identity or by a range of Physical Cell Identities.
The user equipment uses parameters sent by the RBS to determine when to perform handover measurements. Measurements commences on the serving and neighboring cells when the RSRP of the serving cell falls below the value defined in the sMeasure parameter. The user equipment detects neighboring cells via intra frequency searches.
First of all the user equipment uses either RSRP or RSRQ measurements to determine whether to enter the EventA3 condition. The triggerQuantity A3 parameter is used to configure whether RSRP or RSRQ values are used to trigger EventA3. Measurements of RSRP and RSRQ are performed on the serving and detected neighboring cells. The measured values of RSRP and RSRQ can then be filtered based upon the settings of the parameter filterCoefficientEUtraRsrp and filterCoefficientEUtraRsrq parameters. The filter averages a number of measurements in order to filter out the impact of large scale fast fading.
The user equipment then uses an offset value, a3offset, and a hysteresis value, hysteresisA3, to determine whether to trigger the EventA3. Non default offset relationships use the value cellIndividualOffsetEUtran instead of a3offset for the particular cell relationship.
The formula used by the user equipment for evaluating entry to EventA3 is shown below:
Mn − HysteresisA3 > Ms + a3offset
Mn = measured value of the neighboring cell (either RSRP or RSRQ)
Ms = measured value of the serving cell (either RSRP or RSRQ)
Once EventA3 is triggered, the user equipment will wait a predetermined time (timeToTriggerA3) before it commences sending measurement reports to the serving RBS. These measurement reports contain measurements for the serving cells and up to three detected intra frequency neighbor cells. The reportQuantityA3 parameter indicates whether RSRP or RSRQ measurements, or both, are to be included in the measurement reports.
Measurement reports are sent periodically whilst the EventA3 condition is active. The parameter reportIntervalA3 determines the time interval between measurement reports. The parameter reportAmountA3 indicates how many reports to send; a value of 0 indicates that the reports should be sent indefinitely whilst the EventA3 condition is active.
The user equipment uses the same offset and hysteresis values to determine when to leave EventA3 when the serving cell improves in RSRP or RSRQ relative to the neighboring cells. The formula used by the UE is shown below:
Mn + HysteresisA3 < Ms + a3offset