lte event a3.docx
TRANSCRIPT
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LTE Event A3 (Neighbour becomes offset better than serving)
The most important parameters involved in event a3 reporting are listed below:
- eventA3offset - hysteresis - timeToTrigger - sMeasure- cellIndividualOffset- triggerQuantity - reportAmount- reportInterval- filterCoefficientRsrp
LTE R8 uses hard handover. Therefore, one of the main optimization concerns is to avoid ping pongs between cells. Ping pongs significantly reduce user throughput and increases signaling in the E-UTRAN (in the case of X2 handovers) and in the EPC (in the event of an S1 handover). The table below shows an example with three different combinations for the parameters eventA3offset and hysteresis.
Assuming a cellindividualoffset = 0, then:
Event a3 will trigger when
RSRPsource + eventa3offset +hysteresis <= RSRPtarget + hysteresis
Event a3 will not be further valid when
RSRPsource + eventa3offset +hysteresis > RSRPtarget + hysteresis
Under these circumstances:
a) CASE 1:
a. Event a3 will trigger when the RSRP of the target cell is 2dB stronger than the RSRP of the serving cell
b. The UE will cease sending measurement reports when the RSRP of the target cell is less than 2dB stronger than the RSRP of the serving cell
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b) CASE 2:
a. Event a3 will trigger when the RSRP of the target cell is 2dB stronger than the RSRP of the serving cell
b. The UE will cease sending measurement reports when the RSRP of the target cell is weaker than the RSRP of the serving cell
c) CASE 3:
a. Event a3 will trigger when the RSRP of the target cell is 2dB stronger than the RSRP of the serving cell
b. The UE will cease sending measurement reports when the RSRP of the target cell is -2dB or weaker than the RSRP of the serving cell
Clearly, case 3 could be counterproductive since a candidate can be reported to the source cell when the target is weaker than the source cell!!
A healthier approach is to provide a value of say, 3dB to a3offset and a value of 1 dB to the hysteresis parameter (for core cells). This will ensure that the target cell is at least 4 dB to trigger the event a3 and the handset will not report a candidate when the target is not at least 2dB stronger than the source cell (assuming that the number of measurement reports given by reportamount haven't expired).
Also, in order to ensure that the target cell is strong enough than the source cell for a good amount of time, the parameter timetotrigger should be set to values of 480, 512 or 640 miliseconds. However, a drive test is recommended before and after these parameters have been modified along with the creation of counter reports for X2 and S1 handovers.