-
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 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.
MarcoOval
MarcoOval
MarcoOval
MarcoOval
MarcoTypewriter1
MarcoTypewriter2
MarcoTypewriter3
MarcoTypewriter4
MarcoHighlight
MarcoArrow
MarcoHighlight
MarcoHighlight
MarcoLine
MarcoLine
MarcoLine
MarcoLine
MarcoTypewriter1
MarcoTypewriter2
MarcoTypewriter3
MarcoTypewriter4
MarcoTypewriter5
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoRectangle
-
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.
MarcoRectangle
MarcoTypewriterP
MarcoTypewriter1
MarcoTypewriter
MarcoTypewriter1
MarcoTypewriter2
MarcoTypewriter2
MarcoTypewriter2
MarcoTypewriter3
MarcoTypewriter5
MarcoTypewriter('2')
MarcoTypewriter('320ms')
MarcoTypewriter('2')
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoTypewriter=1db
MarcoTypewriter=1db
MarcoTypewriter1db
MarcoTypewriter1db
MarcoTypewriter
MarcoTypewriter
MarcoTypewriter6
MarcoLine
MarcoTypewriter6
MarcoTypewriter240ms
MarcoTypewriter240ms
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoTypewriter7
MarcoLine
MarcoTypewriter''infinity''
MarcoLine
MarcoHighlight
MarcoHighlight
MarcoTypewriter0
MarcoTypewriter
MarcoTypewriter5
MarcoTypewriter5
MarcoLine
-
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).
MarcoOval
MarcoOval
MarcoRectangle
MarcoHighlight
MarcoTypewriter5
MarcoRectangle
MarcoRectangle
MarcoHighlight
MarcoArrow
MarcoArrow
MarcoHighlight
MarcoLine
MarcoOval
MarcoOval
MarcoOval
-
In our next blog, we will discuss the parameter timetotriggera3,
which is another tool for optimizing handovers in LTE.
TimetoTrigger 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.
MarcoTypewriter3
MarcoRectangle
MarcoOval
MarcoOval
MarcoOval
MarcoTypewriter1
MarcoTypewriter2
MarcoHighlight
MarcoHighlight
MarcoOval
MarcoOval
MarcoTypewriter3-3=0
MarcoTypewriter3-1=2
MarcoTypewriter1db
MarcoTypewriter1db
MarcoTypewriter2db
MarcoTypewriter2db
MarcoOval
MarcoOval
MarcoOval
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoLine
MarcoLine
MarcoArrow
-
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:
MarcoTypewriter4
MarcoRectangle
MarcoHighlight
MarcoArrow
MarcoHighlight
MarcoTypewriterNxt
MarcoOval
MarcoHighlight
MarcoHighlight
MarcoRectangle
MarcoTypewriterNxt
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoOval
MarcoLine
MarcoLine
MarcoOval
MarcoOval
MarcoArrow
MarcoRectangle
MarcoOval
-
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 = ^(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 = , Fn = Old + New
Case 2: value k = 4, a = , Fn = Old + 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.
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoTypewriter4
MarcoLine
MarcoLine
MarcoTypewriter8
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoOval
MarcoOval
MarcoRectangle
MarcoHighlight
MarcoHighlight
MarcoHighlight
MarcoOval
MarcoOval
MarcoHighlight
MarcoHighlight
MarcoArrow
MarcoArrow
MarcoLine
MarcoLine
MarcoArrow
MarcoArrow
MarcoHighlight
MarcoHighlight
MarcoArrow
MarcoArrow
MarcoRectangle
MarcoOval
MarcoHighlight
MarcoHighlight
MarcoArrow
MarcoArrow
MarcoLine