Handover Parameters: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 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 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) > RSRP (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.

Time to Trigger Event a3  

As explained in part 1 of these blogs, if the RSRP of a neighbor cell is a3offset+ hysteresisa3 dB stronger than the serving cell for a time period equal to timetotriggera3 then the UE sends the first measurement report to the eNodeB indicating that eventa3 has occurred. Timetotriggera3 typical values are [0, 40, 64, 80, 100, 128, 160, 256, 320, 480, 512, 640, 1024, 1280, 2560, 5120] milliseconds.

Clearly, the utilization of timetotriggera3 is highly dependent on the parameters a3offset and hysteresisa3.However, some general troubleshooting guidelines are provided here to minimize ping pong effects.

Rules:

a) If a3offset+ hysteresisa3 is relatively large (i.e.: 6dB or stronger), then a value of timetotriggera3 under 100 ms is acceptable.

Explanation: Since the RSRP of the neighbor cell is already stronger than the value of the source cell, the time to trigger should not be large.

b) If a3offset+ hysteresisa3 is relatively small (i.e.: 2dB), then a value of timetotriggera3 should be around 320 to 640 ms.

Explanation: Since the RSRP of the neighbor cell is not much stronger than the value of the source cell, the time to trigger should not large to ensure the value remains the same for a long period of time.

c)     If a3offset = hysteresisa3, see b)

d)    If a3offset > hysteresisa3, see a)

e)     If a3offset < hysteresisa3, see a)

However, these recommendations depend much on the speed of the mobile and the coverage scenarios.

The value allocated to timetotriggera3, hence, depends on:

Parameter setting of a3offset and hysteresisa3, Morphology (dense urban, urban, suburban, rural) Speed of UE in the cells (freeways and or suburban roads).

So far, we have discussed two methods for optimizing event A3. In out next blog we will talk about the benefits of optimizing another parameter called, filtercoefficient for event A3 that will allow us to eliminate some of the effects of fast fading in the UE measurements.

Filter Coefficient for Event a3

Once the UE is configured to do measurements, the UE starts measuring reference signals from the serving cell and any neighbors it detects. The next question is whether the UE should look at just the current measurement value, or if the recent history of measurements should be considered. LTE, like other wireless technologies, takes the approach of filtering the currently measured value with recent history. Since the UE is doing the measurement, the network conveys the filtering requirements to the UE in an RRC Connection reconfiguration message.

The UE filters the measured result, before using for evaluation of reporting criteria or for measurement reporting, by the following formula:

 

Where

Mn is the latest received measurement result from the physical layer; Fn is the updated filtered measurement result, that is used for evaluation of

reporting criteria or for measurement reporting; Fn-1 is the old filtered measurement result, where F0 is set to M1 when the first

measurement result from the physical layer is received; and a = 1 / 2(k/4), where k is the filterCoefficent for the corresponding measurement

quantity received by the quantityConfig.

Then, the UE adapts the filter such that the time characteristics of the filter are preserved at different input rates, observing that the filterCoefficent k assumes a sample rate equal to 200 ms.

The parameter “a” defines the weight given to current value and (1-a) (i.e., the remaining weight is given to the last filtered value). For example, if filter coefficient k = 4, then a = ½^ (4/4) =1/2. This means that new measurement has half the weight and the last filtered measurement gets the other half of the weight.

Example of Filter coefficient values are:

Case 1: value k = 8 , a = ¼, Fn = ¾ Old + ¼ New Case 2: value k = 4, a = ½, Fn = ½ Old + ½ New

Optimization Rules:

a)     A high value of the parameter filtercoefficient will provide higher weight to old measurements (more stringent filter) (the opposite is true)

b)    The higher the values of filtercoefficient the higher the chances of eliminating fast fading effects on the measurement reports

1. This eliminates reporting a cell which RSRP was suddenly changed due to multipath or fast fading

2. Which in turns eliminates the chances to handover to a cell which RSRP was strong for some milliseconds

3. Therefore reducing the chances for Ping-Pong effects

c)     A value of 8 is typically used in the network although a value of 16 might also be used in dense urban areas.