Handover Parameters (Part 1 of 3)

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Handover Parameters (Part 1 of 3)

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There are three ways of optimizing handovers in LTE:

a) Via the modification of the parameters a3offset and hysteresisa3

b) By changing the parameter timetotriggereventa3

c) Via the modification of the parameter filtercoefficient for event a3.

These set of blogs will dealt with parameter setting for Periodic Reporting of Event A3 only. The intention is to deal with each of the cases mentioned above, one at a time. Hence, this blog will concentrate in case a).

Definitions:

Event A3 is defined as a triggering event when a neighbour 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 include:

  • a3offset: This parameter can be found in 3GPP 36.331. It configures the RRC IE a3-Offset included in the IE reportConfigEUTRA in the MeasurementConfiguration IE. The value sent over the RRC interface is twice the value configured, that is, the UE has to divide the received value by 2.The role of the offset in Event A3 is to make the serving cell look better than its current measurement in comparison to the neighbor.
  • Hysteresisa3: 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.
  • timetoTriggera3: The role of ttt in Event A3 is to avoid a ping-pong effect.
  • CellIndividualoffsetEutran: This parameter is applied individually to each neighbor cell with load management purposes. The higher the value allocated to a neighbor cell, the “more attractive” it will be. This parameter can only be used if the neighbor list is broadcast in SIB4 or in an RRC connection reconfiguration.

 

Based on the picture above, event A3 will trigger when:

RSRP(target) > RSRS(Serving) +a3offset + hysteresisa3 – cellindividualoffsetEutran

And this condition is valid for timetotriggera3.

At the expiration of timetotriggera3, if the UE does not receive an RRC connection reconfiguration message (handover command) from the eNodeB, then it will start a timer called reportingintervala3. At the expiration of this timer, if the conditions for event A3 are still met and the eNodeB has not responded, then another measurement report will be sent to the eNodeB. This process will continue until the eNodeB responds or until a number of measurement reports given by the parameter reportingamount have been sent.

Examples:

The table below assumes that cellindividualoffsetEutran is not used and shows when the eventa3offset is triggered and when the UE ceases sending measurement reports.

 

As it can be seen from the table, eventa3 triggers at a3offset+hysteresisa3

However!!! After the first measurement result, subsequent measurement results can be sent if the RSRP of the neighbor cell is only a3offset-hysterisisa3 dB stronger! Hence, weaker neighbors could be reported in the measurements sent by the UE (this case is very rare but it exists in real systems).

Therefore, it is recommended to follow the optimization rules:

a) a3offset should always be larger than hysteresisa3 if we want UE to handover to cells with an RSRP at least equal to the RSRP value of its serving cell.

b) Ensuring a3offset > hysteresisa3 avoids ping-pongs

c) The higher the value of a3offset+hysteresisa3 the more we drag the calls to neighboring cells. This is very useful where we have coverage holes (not a one to one deployment scenario on top of 3G cells)

d) The smaller the value of a3offset+hysteresisa3 the faster we release the calls to neighboring cells. This is useful in those scenarios where a large number of LTE cells exists in a given geographical area.

e) The higher the value of a3offset+hysteresisa3 the more difficult we make it for calls do handover to other cells.

Remember, eventa3 triggers at a3offset+hysteresisa3. Subsequent message reports are sent when the RSRP of the neighbor cell is a3offset-hysteresisa3 (See figure below).

   

In our next blog, we will discuss the parameter timetotriggera3, which is another tool for optimizing handovers in LTE.

  • Thanks Dr. Lauro,

    It is realy great job.

  • Thank you lauro!!

  • first : I think here you forgot to draw/consider  the CellIndividualoffsetEutran of the serving Cell.( according to TR 36.331 this should be also considered)

    My question is that: is there any standardization  that specifies when the actual HO is performed.

  • Dear expert,

    I have a question about the cellindividualoffsetEutran. For example if BS1 and BS2 have BS3 as neighbor:

    -  can  BS1 and BS2 have a different value for cellindividualoffsetEutran of BS3 . or this is a parameter coming from BS3 and it should be the same in all the network.

    Thank you in advance!