MRO Feature in LTE

MRO(Mobility Robustness Optimization):- MRO is a SON(Self Optimizing Network) feature in which they provide the solution for automatic detection and correction of errors(Radio Link Failure) in Handover.

Scenarios for MRO:- There are 3 scenarios where MRO feature can optimize in case of RLF in Handover.

1. Too Late Handover.
2. Too Early Handover.
3. Handover to Wrong Cell.

We will discuss one by one.

1. Too Late Handover:- In this case the UE does not receive the RRC Handover command, due to weak signal see figure below, the handover procedure in the source cell is initialized too late, since the UE is moving faster than the Handover (HO) parameter settings allow. Hence when the RRC HO command from the serving cell is transmitted the signal strength is too weak to reach the UE, which is now  located in the target cell, connection is lost. The UE attempts a connection re-establishment, containing PCID and C-RNTI belonging to the source cell, but received by the target cell. The target eNB will then inform the source cell about RLF to adjust Handover parameters.

2. Too Early Handover:-  In this scenario the signal strength in the target cell is too weak, and the connection is lost almost immediately after the Handover. see figure below. The UE has successfully been handed over from source cell to target cell , but since it was triggered too early the connection will drop almost immediately due to too poor radio conditions in the target cell . The UE will then re-established the connection in source cell.

3. Handover to Wrong Cell:- In this case RLF occurs in the target cell after a handover has been completed, and the UE attempts to re-establish its radio link in a cell which is not the source cell nor the target cell. See the fig below.

Working of MRO Feature:- Source eNodeB takes some appropriate actions to overcome the above type of failures by changing the threshold at which the handovers are triggered. Source eNodeB can use ‘Mobility Settings Change’ procedure to inform the neighbour eNodeB of the change of threshold it has performed. In this case the ‘MOBILITY CHANGE REQUEST’ message is sent with a cause value ‘handover optimization’ and indicates the change (in dB) of the handover trigger parameter change performed in the source cell. For instance, in the case of repeated handover to an

inappropriate cell, the source eNodeB could modify the way it builds its list of candidate

target cells.

The Parameters that can be optimized in connected mode: The majorly parameter modification are done for A3 events triggering as listed below.

1. A3 offset

2. A3 hysteresis.

3. Cell individual offset(CIO)

4. Time to trigger(TTT).

Ref: 3GPP.ORG