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How to analyze HandoverIssues in DT
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Handover
What is Handover p rocedure in ne twork? Hando ver Um s igna l ing f low?
Purpos e o f Hando ver Ana lys i s in DT?
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Handover signaling on Um(TEMS Message)
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BCCH of Neighborsto be measured
Serving Cell:DL DTX is ON
RxLev(sub) is -109+ 62 =--49dBmRxQual (sub) is 0
The RxLev ofNeighbors
Tell MS how to access toTarget TCH Channel
Target cell Description
Target channel DescriptionChannel Type, TimeslotTSC
RF hoppingMAIO, HSN
Inter cell SynchronousHandover
Speech version: EFR
Handover Complete Handover Success!!!
Only after SABM & UAin Layer2 Message,handover success!
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The purpose of handover analysis in DT isunderstand the wireless handover performance of network.find out whether the handovers are healthy in this network.what is typical handover failure in this network.find out whether neighbor audit work is needed in this network.
To make optimization of HSR, the most effective way is based ontraffic analysis, combining DT events, neighbor audit, and dataconfiguration audit together.
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The purpose of Handover Analysis
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Handover Types
D i ffe ren t hando ver types by n e two rk topo logy D i fferen t handover types by a lgor i thm PBGT hando ver Qua l i ty hando ver Edge Hand ov er In t racel l Hand ov er O ther types o f hando ver
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Case1 Intra cell HO Case2 Synchronous Inter cell HO Case3 Asynchronous Inter cell HO Case4 Inter BSC HO Case5 Inter MSC HO
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Handover Types(Network Topology)
Operator
CELL CELL CELL CELL
Case 1
Synchronous
Case 2 Asynchronous
Case 3Case 4 Case 5
CELL CELL CELL CELL
MSC MSC
BSC BSC BSC BSC
BTS BTS BTS BTS BTS BTS
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Handover Types(Algorithm)
In DT, the most commonhandover types happens infollowing order of priority
TA
RxQuali ty
RxLevel
PBGT
OL&UL
HO
OM Forced HO
Directed Retry
TA Emergency HO
Bad Quality HO
Rapid Level Drop HO
Load HO
Interference HO
Edge HO
Layer HO
PBGT HO
Fast Moving MS HO
Overlay/Underly HO
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PBGT HO Exp :
When:RXLEV_NCELL(n) (RXLEVEL+PWR_DIFF) > HO_MARGIN MS is handed over to theneighbor in g ce l l .
HO_MARGIN i s u sual ly se t to 3 to6 dB.
HO_MARGIN cou ld be increasedto reduce numb er of handovers .HO_MARGIN shou ld bedecreased i f fas ter handover
decis ion i s wanted .
PBGT Hando ver i s con s idered asmo st healthy hando ver in anetwork.
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PBGT Handover
Distance
ServingCell
TargetCell
RXLEV_NCELL(n)
RX LEVEL + PWR_DIFF
Power-budgetPBGT HO
HO_MARGIN > 0
A typical PBGT HO
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Qual ity Handov er Exp:
When:DL RxQuality >= DL Quality HOThresholdhandov er caused by DLRxQuali ty takes p lace.
When:UL RxQuality >= UL Quality HOThresholdhandov er caused by ULRxQuali ty takes p lace.
DL&UL Qual i ty HOThresholds are general lyset to 6.
UL Quality HO cant bem on itored in DT.
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Quality Handover
Handover was performed to a better qualitycell just after experiencing quality problems.
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EDGE Handover
Edge Handov er Exp:When:
DL RxLevel < Edge HO DLRX_LEV Thresholdhandov er caused by DLRxLevel takes p lace.
When:
UL RxLevel < Edge HO ULRX_LEV Thresholdhandov er caused by ULRxQuali ty takes p lace.
UL EDGE handover cant bem on itored in DT.
Handover was performed to the cell withhigher downlink Rxlev.
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Intracell Handover Based on Interference
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The Intra cel l Handoverfeature a ims to mainta ingood q ua li ty by pe r fo rminga handover to a newchannel wi th in th e samecel l wh en upl ink ordow nl ink in ter ference i sdetected.
Interference is defined bybad RxQual and h ighenou gh Rx LEV.
Intracel l HO also c an be
t r iggered by: Rate change betw een Full rate
and Half rate Handover be tween over la id and
under la id su bcel l . An example of intracell handover afterexperiencing quality problems.
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Handover Issues
What is Dragged Hand ov erPow er Contro l Effec tPing Pong Hando verMiss ing Neighb or Relat ionFake Neighbo rHandov er Fai lure
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Dragged Handover
There will be such cases that you will notice handover process takingplace a little late, which is called dragged handover.
Firs t th ing to check w i l l be handov ermargins be tween the ne ighbors . Ifmargins for level, qual i ty or po werbud get handovers are not se tcorrec t ly, If margins are too m uch ,handov er wi l l h appen la te, v iceversa. A Quality handover
should happen earlier
Dragged handover events of tenocc ur af te r a GSM serving c e l l loses
dom inance for a s igni f icant t imeperiod.
Dragged Handover often cause CallDrop or Handover Failure.
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Power Control Effect
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Power Control Effect sometimes
may mislead us, you might thinkthat handover is happening toolate between two neighbors.
When the call is continuing on atimeslot that belongs toTCHTRX, power control featurewill try to reduce output poweras much as possible until aquality problem occurs. Thatswhy you will see serving cellsignal level is less thanneighbors level. It looks lessbut in reality, the signal level onBCCH TRX is still higher thanneighbors broadcasting level.
Power control
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Missing Neighbor RelationIf a handoff is not performed to a neighbor cell that seems to be bestserver, there is a possibility of a missing neighbor relation. This will
happen with sudden appearance of strong cell in the neighbor list just after a handover.
The shot show s af tersecond handover in
the red box , a muchst rong er Serving Cellappears.
At th is t ime Neighbo raudi t should beperformed.
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Sometimes you will see a
good handover candidate inthe neighbor list buthandover will not take placeand call will drop.
Although that neighboring
cell with a very good signallevel appears to be aneighbor, in reality it is not.Just because the serving cellhas another neighbor withthe same BCCH, this cellappears in the list.
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Fake Neighbor Relation
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Handover Failure
Handover Failure: Handover attempt was failed and the call returned backto its all channel.
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Handover Failure
Target Cell is BCCH is 13
BSIC is 65Target channel is 900M RF hopping Channel. Half Rate MA list is 1, 17, 18, 19, 20, 21, 22, 23, 24 MAIO is 4, HSN is 31, TSC is 5. Channel mode is AMR(SV3)
The handover has notsuccessfully completed yet!
Handover Failure iscaused by T200 ExpiredAnother example: Excessive Number of Handover
Failure due to Hardware Problem
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Proposal of Handover Issue
Dragg ed Hand ov erPing Pong Hando verMissin g Neigh bo r Rela t ionHand ov er Failure
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HUAWEI TECHNOLOGIES CO., LTD. Huawei Confidential Page 22
Solutions of Handover Issues
Issue Proposal
Dragged Handover
Check if the parameter of handover, such as: PBGT, Quality, Interference, Edgehandover threshold, and P/N Value is set too high.Check if there is no dominant coverage. Referring to the solution of Low
Coverage. Check if the traffic volume of target cell at measuring time.
Ping-pong Handover
Check if there is hardware problem. Referring to hardware checking guide Check if there is no dominant coverage. Referring to the solution of Low
Coverage. Check if the parameter of handover, such as: PBGT, Quality, Interference, Edge
handover threshold, and P/N Value is set too low . Check if there is overlapping coverage area, which may also cause ping-pong
handover.
Missing Neighbor
Neighbor Audit. Check if there is neighbor missing according to data
configuration in OMC.
Handover Failure
Find out the cause of Handover failure form Um signaling flow from Layer2 andLayer3 Message.
Check if the HSR between source and target cell is low. If it is, find the rootcause of handover failure reasons by traffic statistic analysis.
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