LTE Handover Optimization
This is the second part of the LTE handover concept post. Here we will see some
practical examples of handover optimization. Handover may take different forms,
intra-cell, inter-cell, inter-frequency, inter-RAT and also can be triggered by
coverage, frequency priority and load balancing algorithms. I’ll stick in this blog to
inter-cell, intra frequency coverage based handover cases.
A3 Event
Imagine that for any reason an UE begins to move from a cell we will call “source
cell” towards another cell we will call “target cell”. Let assume that coverage is
overlapped at the cell border of both cells. It should looks something like this:
So we expect that the serving cell started to be worse in terms
of RSRP/RSRQ while the neighbor cell is getting better. If we plot the RSRP /
RSRQ against time for both cells the graph will look something like this:
Note that there is an offset and a hysteresis line. Offset (on the serving cell) and
hysteresis (on the target cell) are used to prevent handover back and forth or “ping
pong handover” (that’s the the scientific term). So the condition that has to be met
for triggering a HO due to an event 3A is the following:
RSRP Neighbor eNB + Offsetn– hysteresis > RSRP Serving eNB + A3 Offset
And this equation has to be true for the time defined in the Time to Trigger timer
for HO to be ordered. Another thing to take in mind is that this event can be
triggered by RSRP or RSRQ.
Remember I’m just listing conditions on intra -frequency handover, which is
govern by the A3 event. If this was an inter-frequency handover, conditions that
have to be met are of some of the following events: A1, A2, A3, A4 and A5
A3 Time to Trigger: This timer speeds up or delays the handover command once
the conditions are met. The default value is 320 ms but is commonly set higher to
prevent too many handovers. This parameter is only available at lncell level.
A3 Offset: Increasing the offset can delay the handover. The default value is 3, but
it might be set higher too if you want to delay handovers.
Offsetn: This is the so called cell individual offset. The default value is 0, but if the
value is different you should take care of it sign. A positive value will speed up the
Handover while a negative value will delay it.
A3 Hysteresis: A higher value speeds up the triggering point. This value is set at
lncell level.
A3 Handover Trigger Quantity: Here you can choose between RSRP or RSRQ to
be the quantities that are being taken into consideration for HO evaluation. The
default is RSRP, but depending on your radio environment, RSRQ might be a
better choice.
UE Signalling
Using a qualipoc I did a short walk through my neighborhood to catch a handover
event, specifically I was interested in see all the signalling back and forth the UE:
The sequence starts with the measurement report sent by the UE to the eNB. The
last measurement report “1” shows that the source cell (PCell) is worst in terms of
RSRP/RSRQ that the best neighbour cell. The neighbor is better enough during at
least the amount of time defined in the A3 time to trigger so Handover is requested.
What happened next is a negotiation between source eNB and target eNB
(preparation phase). If succeed the handover will be requested from the source eNB
to the UE, and this is what we see in the next RRC Reconfiguration message “2”.
Note that in my previous blog “LTE Handover Concept” I show a different RRC
Reconfiguration message which pases to the UE all the measurement information
(what to measure and how often) . That RRC Reconfiguration message is called
Measurement Configuration. This one instead is called Mobility Control
Information, and is actually the Handover command. In the red square I highlighted
some information I think is important; like for example the details of the source cell
the UE has to handover too. Also how the UE will access is detailed here: the initial
preamble power and if there are reserved preambles for contention free Random
Access which in this case there are 8 (Number of random access preambles: 40 –
sizeOfRA_PreamblesGroupA: n32)
In “3” we see the confirmation of a succeed handover through the
rrcConnectionReconfigurationComplete message from the UE to the eNB. Later,
the formerly called “target eNB” (now source eNB) will provide the UE with all the
information it needs to continue the service on this cell (Master Information block
and the RRC Reconfiguration message “measurement control”).
Handover Optimization
Handover success rate on a mature network should be greater than 99%. The
following are the most common issues that could affect the handover performance:
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PCI confusion: This happened when two neighbor cells have the same PCI.
In this scenario the UE will report a PCI with a RSRP/RSRQ that belongs to
two different cells. You can have more information in this topic with my
post “PCI confussion or PCI Collision?”
Missing neighbor: In this case, there will be many measurement reports but
no Mobility Control Information message (no handover command) even
after A3 condition is met. This should be a problem not very often as most
operators implements SON (self optimizing network) solutions that deal with
the neighbor definitions.
Wrong parameters: What is also called “fat finger error” is where in my
experience you can find most of the major handover success rate
affectations. The options are endless: from wrong offsets, to incorrect cell
radius, or wrongly defined TAC (a ghost TAC that points nowhere) among
others are things that you might find. All have in common that suddenly the
number of handover attempts increase a lot (in a drive test it might look like
a ping pong handover) while the success rate decreases. If you have a
consistency check tool it may be time to run it, if not get one
Poor radio environment: This includes areas of low RSRP and cross
coverage or lack of dominance. Here the solution is to improve the radio
conditions by tilts, change of azimuths or new sites. If that is not possible
you should change handover parameters to reduce the number of handovers.
Conclusion
Handover performance is a very important KPI in an LTE Network. It might
indirectly affect throughput and latency, which at the end affects the service
perceived from the end user, which is in my opinion the most critical KPI. This
blog is not covering all the handover process but I hope it gives you tools to work
with. Finally I strongly recommend a book that is on my references that goes very
deep and extensive on handover.
Cheers!
Diego Goncalves Kovadloff
References:
LTE – The UMTS Long Term Evolution: From Theory to Practice Stefania Sesia,
Issam Toufik and Matthew Baker, 2009 John Wiley & Sons