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7/29/2019 Case Analysis Handover Huwaie 4
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www.huawei.com
Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Case Analysis Handover
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Page2Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
References
z BSC Counter Reference(V900R008C12)
z BSS KPI Reference(V900R008C12)
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Page3Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Objectives
z Upon completion of this course, you will be able to:
Understand the Measurement Points and Performance Index
of handover
Analyze, locate and solve handover problems
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Page4Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Contents
1. Handover Process Flow
2. Measurement Points and Performance Index of Handover
3. Analysis of Handover Problem
4. Handover Cases
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Page5Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Process Flow
BA2 table1
MR2MS
BTS1
BSC1
Preprocessed MR3
Handoverjudgement4
Channel active5
MSC1
CGI6
BTS2
MSC2
BSC2
H.O request7
Inter-MSC H.O8
Search targetcell7
BCCH frequencies of all adjacent cells in BA2 table are sent to MS on system
message.
MS reports measurement report to BSC. It includes the BCCH , BSIC and signallevel of the adjacent cells and serving cells.
When the measurement report is preprocessed, BSC identifies the CGI of all
adjacent cells through BCCH frequency and BSIC .
BSC executes handover judgment flow such as basic cell ranking. Once a proper
target cell is found, the handover request message which includes the target cell
CGI will be sent to BSC.
If the target cell is an internal cell, BSC send the channel active to the BTS.
If the target cell is an external cell, both the CGI of the target cell and that of
service cell are sent to MSC via the handover required.
By matching the CGI of the target cell, MSC searches the target cell. Once the
cell is found, MSC will confirm which BSC is belonged to, and send the handover
request message to this BSC.
If there is no CGI of the target cell in local MSC, MSC will check Adjacent
MSC Table" and search the target MSC, then send the handover request message
to that MSC.
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Page6Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Contents
1. Handover Process Flow
2. Measurement Points and Performance Index of
Handover
3. Analysis of Handover Problem
4. Handover Cases
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Page7Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
CH300: Internal Intra-Cell Handover Requests
Measurement Points of Intra BSC Handover
MS BTS1(target) BSC BTS2 or BTS1(original) MS MSC
Channel_Acive
Channel_Active_Ack
Measure Report from MS
Handover Command (Old FACCH)
Handover Access (New FACCH)Handover_Detect
PHYINFO
PHYINFO
First SABMEstablish_IND
Handover Complete (New FACCH)
Handover_Performed
UA
CH320: Incoming Internal Inter-Cell Handover Requests
CH310: Outgoing Internal Inter-Cell Handover Requests
CH321: Incoming Internal Inter-Cell Handover Responses(Incoming Internal Inter-Cell Handovers)CH311: Outgoing Internal Inter-Cell Handover Commands
CH313:Successful Outgoing Internal Inter-Cell HandoverCH323: Successful Incoming Internal Inter-Cell Handovers
CH301: Internal Intra-Cell Handover Commands
CH303: Successful Internal Intra-Cell Handovers
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Page8Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
CH323: Successful IncomingInternal Inter-Cell
Handovers
Performance Index of Intra BSC Handover (Cell)
100
RH303B:Internal Handover Success Ratio per Cell
CH303: Successful InternalIntra-Cell Handovers
CH323: Successful IncomingInternal Inter-Cell
Handovers+
CH300: Internal Intra-CellHandover Requests
CH320:Incoming InternalInter-Cell Handover
Requests+
100
RH303C: Intra-BSC Radio Handover Success Rate
+
+CH301: Internal Intra-Cell
Handover CommandsCH321:Incoming Internal
Inter-Cell HandoverResponses
Internal Handover Success Ratio per Cell = intra-cell/inter-cell cell
handover commands or responses, so Internal Handover Success Ratio per CellInternal Handover Success Ratio per Cell
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Page9Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
ZCH323: Successful InternalIncoming Cell Handovers
per BSC
Performance Index of Intra BSC Handover (BSC)
100
ZK3174: Internal Handover Success Ratio per BSC
ZCH303:Successful InternalIntra-Cell Handovers
per BSC
ZCH323: Successful InternalIncoming Cell Handovers
per BSC+
ZCH300: Internal Intra-CellHandover Requests
per BSC
ZCH320: Internal Incoming CellHandover Requests per
BSC+
100
ZK3175: Radio Handover Success Ratio per BSC
+
+
ZCH303:Successful InternalIntra-Cell Handovers
per BSCZCH301:Internal Intra-Cell
Handover Commandsper BSC
ZK3190:Internal IncomingCell Handover
Commands per BSC
Internal Handover Success Ratio per BSC
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Page10Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Measurement Points of Inter-BSC Handover
MS BTS(Original) BTS(Target)BSC(Original) BSC(Target)MSCMeasurement Report
Measurement ReportHandover Required
Handover Request
Channel_Active
Channel_Active_ACK
Handover_Request_ACKHandover Command
Handover Access
Handover Detect
Handover CompleteHandover Complete
Clear Comm and (HO Successful)
RF Channel ReleaseClear Complete
CH330: Outgoing ExternalInter-Cell Handover Requests CH340:Incoming External Inter-Cell Handover Requests
CH341:Incoming External Inter-Cell Handover Responses
CH343:Successful Incoming External Inter-Cell Handovers
CH333:Successful OutgoingExternal Inter-Cell Handovers
CH331:Outgoing External Inter-Cell Handover Commands
MS
The key measurement points are as follows, same as those of intra BSC handover:
1. After sending "HO-REQUIRED", the original BSC measures "Attempted outgoing inter BSC inter cellhandovers".
2. After receiving "HO-REQUEST", the target BSC measures "Attempted incoming inter BSC handovers".
3. After sending "HO-REQUEST ACK", the target BSC measures "incoming inter BSC handovers".
4. After receiving "HO-Command" , the original BSC measures "outgoing inter BSC handovers".
5. After receiving "HO-Complete", the target BSC measures "Successful incoming inter BSC handover"
6. After receiving "Clear-COM" and the cause value is "HO-Successful", the original BSC measures"Successful outgoing inter BSC inter cell handover".
The difference between "handover times" and "handover request times":
z Handover times - After "HO-COM" is received or "HO-REQ-ACK" is sent
z Handover request times - After "HO-REQUIRED" is sent or "HO-REQUEST" is received
Therefore, inter BSC inter cell radio handover success rate >=inter BSC inter cell handover success rate.
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Page11Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Performance Index of Incoming External Inter-Cell
Handover(Cell)
CH343:Successful Incoming External Inter-Cell Handovers100
CH340:Incoming External Inter-Cell Handover Requests
CH341:Incoming External Inter-Cell Handover Responses100
RH303D: External Incoming Radio Handover Success Ratio per cell
Success Rate of Incoming External InterSuccess Rate of Incoming External Inter--Cell Handovers
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Page12Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Performance Index of Incoming External Inter-Cell
Handover(BSC)
ZCH343:Successful External Incoming Cell Handovers per BSC100
ZCH340:External Incoming Cell Handover Requests per BSC
ZK3191:External Incoming Cell Handover Commands per BSC
=
N
i 0
ResponsesHandoverCell-InterExternalIncomingBSCperCommandsHandoverCellIncomingExternal
ZK3178: External Incoming Cell Handover Success Ratio per BSC
ZCH343:Successful External Incoming Cell Handovers per BSC
100
ZK3179: External Incoming Cell Radio Handover Success Ratio per BSC
External Incoming Cell Handover Success Ratio per BSCExternal Incoming Cell Handover Success Ratio per BSC
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Page13Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Performance Index of Outgoing External
Inter-Cell Handover(Cell)
CH333: Successful Outgoing ExternalInter-Cell Handovers
CH330:Outgoing External Inter-CellHandover Requests
CH331:Outgoing External Inter-Cell Handover Commands
100
TH333: Success Rate of Outgoing External Inter-Cell Handover
100
RH303E: External Outgoing Radio Handover Success Ratio per cell
Success Rate of Outgoing External InterSuccess Rate of Outgoing External Inter--Cell HandoverCell Handover
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Page14Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Performance Index of Outgoing External
Inter-Cell Handover(BSC)
ZCH333:Successful External OutgoingCell Handovers per BSC
ZCH330:External Outgoing CellHandover Requests per BSC
ZCH331:External Outgoing CellHandover Commands per BSC
100
ZK3176: External Outgoing Cell Handover Success Ratio per BSC
100
ZK3177: External Outgoing Cell Radio Handover Success Ratio per BSC
External Outgoing Cell Handover Success Ratio per BSC
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Page15Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Performance Indices of BSC Overall Handover
[(ZCH313: Successful Internal Outgoing Cell Handovers per BSC+
ZCH333:Successful External Outgoing Cell Handovers per BSC )/
(ZCH310: Internal Outgoing Cell Handover Requests per BSC +
ZCH330:External Outgoing Cell Handover Requests per BSC )]*100%
ZK3180: Handover Success Ratio per BSC
Handover Success Ratio per BSCHandover Success Ratio per BSC
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Page16Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
MSC-BMSC-A VLR-BBSC-A BSC-B
HO-Required Prepare_HO
Prepare_HO_ACK
Allocate_HO_NUM
Send_HO_Report
Send_HO_Report_ACK
MS
HO-Request
HO-Request-ACK
HO-Command
MS
HO-AccessProcess_Access_Signalling
HO-Complete
Send_End_SignalClear-Command
Clear-Complete
Send_End_Signal_ACK
IAIACM
Measurement Points of Inter-MSC Handover
Note: Signaling flow of A interface and Abis interface of inter MSC handover is the same
as that of intra MSC handover, only the signaling between two MSC is particular for the
inter-MSC handover. As shown in above figure the signaling with "MAP" is of the MAP
layer, and signaling of A and Abis interfaces are omitted.
zAfter receiving "HO-REQUIRED" of BSC-A (the request message includes CGI of the
original cell and target cell), if MSC-A finds that LAC of the target cell doesnt belong to
this MSC, MSC-A will query the "REMOT LAC Table(including the LAC and route of the
adjacent MSC), and send "Prepare-HO" message to MSC-B according to the route. CGI
of the target cell and the indicator of whether to allocate the handover number are
included in this Prepare-HO message.
zAccording to the received "Prepare-HO" message, if the handover number needs
allocating, MSC-B will request the local VLR to allocate the handover number. If VLR has
the free handover number, the handover number will be sent to MSC-B through "Send-
HO-Report". If no handover number is needed, proceed to the next step.
zAfter SCCP link between MSC-B and BSC-B is established, MSC-B sends "HO-
REQUEST" to BSC-B. After that, BSC-B activates the target cells channel, and returns
"HO-REQUEST ACK" to MSC-B after receiving the channel activation acknowledgement.
According to this message, MSC-B sends "Prepare-HO ACK" to MSC-A.
zMSC-A establishes the route to MSC-B according to the handover number, and sends
"Initialize-Address" (IAI) to MSC-B to help the latter to identify which voice channel is
reserved for MS. While MSC-B returns "Address-Complete" (ACM) to MSC-A.
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Page17Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Contents
1. Handover Process Flow
2. Measurement Points and Performance Index of Handover
3. Analysis of Handover Problem
4. Handover Cases
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Page18Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Analysis of Handover Problem
z Types of handover problems
z Locating handover problem
z Causes of handover problem
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Page19Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Types of Handover Problems
z Types Possible Results
No handover Result in call drop
Handover failure Affect the conversation quality, and call drop
Frequent handover Affect the conversation quality and increase
load of the system
Frequent handover: Handover in GSM is hard handover, it is meaning, before handover
to the new channel, MS must release the original channel firstly. So frequent handover
will result in word-loss in handover and break-make of conversation, thus affecting theconversation quality.
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Page20Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Method of Locating Handover Problem
z Traffic statistics analysis
Handover Failure/Attempt/Success Measurement per BSC
Incoming/Outgoing Internal/External Inter-Cell Handover Measurement per Cell
GSM Cell to GSM Cell Incoming/Outgoing Handover Measurement
Neighbor Cell Level Measurement per Cell
z View alarm
Board fault, transmission, clock, etc.
z Drive test
z Signaling analysis Abis interface, A interface, E interface
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Page21Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Causes of Handover Problem
z Coverage
z Interference
z Antenna and feeder system
z BTS hardware
z Transmission
z BSC hardware
z Data Configuration
z A interface
z
Target cell congestionz Cooperation with equipment of other manufacturers
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Page22Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Causes of Handover Problem
H.O between equipment ofdifferent manufactures
Target cell congestion
A interface
Data configuration
BSC hardware
Transmission
BTS hardware
Antenna and feeder system
Interference
Coverage
Frequent H.OH.O FailureNo H.OTypesCauses
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Page23Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Problem in Coverage and Interference
Coverage
Poor coverage: forest, complicated topography, building direction and
indoor coverage
Isolated site: no adjacent cell
Over shooting: island effect result in no adjacent cell
Interference
MS can not access network or receive any signal.
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Page24Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Adjacent
Cell N3
Adjacent
Cell N2
Adjacent
cell N1
Non-adjacent
cell
Non-adjacent
cell
Non-adjacent
cell
Service cell
There is noadjacent cell, sohandover becomesimpossible.
Isolated islandresulting fromover shooting
Island Effect Results in Handover Failure
In isolated cell coverage area, when MS moves towards the cell edge, the signal
becomes weaker and weaker, and since there is no adjacent cell available around,
handover cannot be triggered, thus call drop will occur.
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Page25Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Problem in Antenna and Feeder System
High Voltage Standing Wave Ratio (VSWR)
Antenna is not properly installed
Antenna is not parallel
The azimuth and downtilt are not correct
Poor antenna isolation value
RF cables, connectors are loose or incorrect
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Page26Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Problem in BTS Hardware
DDPU,DCOM, splitter/combiner failure
TRX failure
DTMU failure
Clock failure
Internal communication cable failure
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Page27Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Problem in Transmission and BSC Hardware
z Transmission
Transmission is not stable
Serious BER in transmission
z Fault of BSC Hardware
Clock board: the faulty clock board causes clock inconsistency
between base stations.
For BTS clock, BSC clock, and MSC clock, the following standards are used to judge
whether they are out of synchronization:
MSC: f/f (frequency deviation) 1E-8
BSC/BTS:f/f (frequency deviation) 5E-8
Inaccurate clock will cause that MS cannot decode BSIC of the adjacent cell correctly.
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Page28Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Problem in Data Configuration
Unsuitable setting of handover hysteresis and handover priority.
Unsuitable setting of P, N value of statistic time
Unsuitable frequency and adjacent relationship configured in
BA1/BA2 table
CGI, BCCH and BSIC in "External Cell Description Table" are
different from those in corresponding BSC.
The DPC of BSC in MSC "LAI and GCI Table" is incorrect.
For service cell: Outgoing inter cell handover will fail.
For illusory target cell: Incoming inter-cell handover will fail, and successful rate of
handover = successful rate of radio handover.
Notes:
zCI can not be configured as "FFFF".
z"Transmitting BS/MS Power Level": If the measurement report preprocess is
enabled, this parameter must be set to "Yes".
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Page29Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Problem in A Interface and Target Cell
z A interface problem
Basically, the insufficient link resource results in the abnormal
handover, as well as abnormal communication.
Circuit pool numbers are different ,causing the handover failure.
z Target cell congestion
The target cell is congested, which causes the handover failure.
Then the target cell should be expanded or reduce its traffic .
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Page30Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Problem in Cooperation between manufactures
z Handover between equipment of different manufacturers
The signalings at A interface, E interface of the opposite
equipment dont matched with our equipment and can not be
recognized or supported, which causes the handover failure, such
as voice version, handover number, addressing mode (CGI or LAI).
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Page31Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Contents
1. Handover Process Flow
2. Measurement Points and Performance Index of Handover
3. Analysis of Handover Problem
4. Handover Cases
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Page32Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Case 1
z Fault Description
A 1800 cell of a dual-band network (all the 1800 cells belong to one BSC),
the incoming handover success rate of intra-BSC and inter-BSC are low
from the beginning of the service, while the outgoing handover success
rate of intra-BSC and inter-BSC are normal.
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Page33Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Case 1
z Analysis
Register the traffic statistics and analyze whether the low handover success
rate is due to the failure of handover from all cells to this cell or from
some few cells.
If handover fails from some few cells to this cell, check the handover data
and confirm whether there is co-channel and co-BSIC problem.
If handover fails from all other cells to this cell, check the data of this cell.
If data problem is excluded, check the hardware carefully. Check the alarm
or perform drive test to locate uplink fault or downlink fault. Check step bystep and find out the cause.
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Page34Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Case 1
z Solution
Register the incoming inter cell handover measurement function and find that the
successful rate of handover from all other cells to this cell is low, although it is not
always 0 percent. Via careful data checking, confirm that the data of this cell is
correct.
Perform drive test and find that the downlink signal is normal but almost all
handovers to this cell are failure. But near the BTS, the handover is successful
occasionally, perhaps the problem is with the uplink signal.
Check the uplink channel, antenna, DDPU, there are no problem. Change the TRX,
everything is normal.
z Conclusion
The symptom is that the uplink and downlink at Um interface are unbalanced so
uplink voice quality is bad.
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Page35Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Case 2
z Fault Description
A subscriber complains that the signal in a place on his way to office is
not good. Call drops occurs several times when he drives his car. The
phenomenon is that the conversation is suddenly interrupted. The
handset signal bar indicates no signal, and then normal again after
several seconds.
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Page36Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Case 2
z Analysis
Neighbor cell relationship
Isolated island effect
Geographical factors of buildings and mountains result in the weak
signal
The signal of a cell suddenly fades drastically due to buildings
BTS equipment or intermittent transmission
zThe following possibilities can lead to the problem that a call is dropped, that
there is no signal during call drop, and that the signal is recovered after several
seconds:
The most common possibility is that there is no neighbor cell handover relation, so an MS
cannot be handed over to the neighbor cell, hence the call drops. After the call drops, the
MS re-selects a cell and resides in a good cell, so the signal is recovered.
Isolated island caused by cross-coverage also leads to this problem. When a signal
exceeds the coverage, an MS uses the signal even when the MS is far away from the cell
that provides the signal, but at this time there is no cell that is a neighbor of the cell. When
the cell signal becomes weak, handover is impossible, leading tothe call drop. After the
call drop, the MS re-selects a good cell and displays signal again.
At that place, the signal is indeed too weak to maintain conversation due to geographical
factors of buildings and mountains. When an MS goes to that place, call drop occurs. After
the MS passes the place, the signal is recovered again. In the urban area where there are
many BTSs, this problem does not happen. In the suburb area where only one BTS is used
for coverage, this problem sometimes occurs.
At that place, due to buildings, the signal of a cell suddenly fades drastically, and the MS
has not time to hand over, hence the call drop. After the call drop, the MS resides in a good
cell again.
Signal is interrupted due to the BTS equipment or intermittent transmission.
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Page37Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Case 2
z Solution
Via DT, it shows that the signal of several cells is
strong and is -70 to -80 dBm, so it is unlikely that the
problem is caused by bad signal.
We obtain further information from the subscriber
and know that the driver at that time drive very fast.
Therefore it supposed that this problem may occur
under high speed movement. Then perform drive test
again under high speed for several times and the calldrop recurs. C
B
1
3
A
2
D
3
z The customer service technicians go to the site to perform drivetest and discovers
that the road from south to north is covered by the signals fromfour cells, which are
cell1 (northward) of BTS C, cell 3 (southwestward) of BTS B, cell 2 (southeastward)of BTS A, and cell 3 (southwestward) of BTS D.
z In the complained place the signal of several cells is strong and is -70 to -80 dBm. It
is unlikely that the problem is caused by bad signal. The drive test shows that the
handover relation is normal. The drive route is such that cell 1of BTS C is first
occupied; then the vehicle drives northward and the MS can hand over to cell 3 of
BTS B and cell 2 of BTS A. As the vehicle drives on northward, the MS hands over
to cell 3 of BTS D. Multiple drive tests along the road show normal result and the
problem complained by the subscriber does not occur.
z To investigate the problem, we obtain further information from the subscriber and
know that the driver at that time drove very fast. Therefore we think this problem
may occur under high speed movement. We perform drive test again under high
speed for several times and the call drop recurs.
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Page38Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 2
z Solution
Cell 1 of BTS C is far away from cell 3 of BTS D and the
two cells are not defined as neighbor cells. Thus an
isolated island is generated, handover cannot be
performed, and call drop arises. The special thing
about this isolated island effect is that it occurs at high
speed. Therefore troubleshooting is difficult.
After a neighbor relation is added, this problem is
solved. On the other hand, because this problem
occurs due to insufficient time to hand over, the
handover decision time is reduced so that the
handover can happen timely.C
B
1
3
A
2
D
3
z Cell 1 of BTS C provides coverage along the road and the signal reaches far
away. When a vehicle drives very fast, sometimes the MS has not time to
hand over to cell 3 of BTS B or cell 2 of BTS A. Cell 1 of BTS C is far away
from cell 3 of BTS D and the two cells are not defined as neighbor cells.
Thus an isolated island is generated, handover cannot be performed, and call
drop arises. The special thing about this isolated island effect is that it occurs
at high speed. Therefore troubleshooting is difficult.
z After a neighbor relation is added, this problem is solved. On the other hand,
because this problem occurs due to insufficient time to hand over, the
handover decision time is reduced so that the handover can happen timely.
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Page39Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 3
z Fault Description
This case occurs during the drive test at a new site.
Even when the receive level of serving cell B is higher than the receive level
of neighboring cell M, handover is triggered from cell B to cell M.
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Page40Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 3
z Analysis
The possible causes of the handover from a cell with a higher level to a cell
with a lower level cell are as follows:
1. The PBGT handover threshold for traffic adjustment is lower than 64.
2. The handover is emergency handover due to the heavy load or low quali
ty of the original cell with the higher level.
3. The priority of the cell with the higher level is lower than the priority of t
he cell with the lower level.
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Page41Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 3
z Solution
Analyze the drive test data and find that the downlink receive quality and l
evel of the serving cell are normal. Thus, the handover is not associated wit
h the downlink receive quality and level.
Check the data configurations and find that the handover due to heavy loa
d is disabled. Thus, the handover is not due to heavy load.
Check the BSC parameter configurations. The result shows that the thresho
ld for the PBGT handover from cell B to cell M is 68 and that the handover
hysteresis is 4. Thus, the handover is not PBGT handover.
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Page42Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 3
z Solution
Check the priority settings of the cells. The result shows that cell B is at level 2 on l
ayer 3 and that cell M is at level 1 on layer 3. Check the inter-layer
handover threshold and hysteresis. The result shows that the inter-
layer handover threshold is 20 and that the inter-
layer handover hysteresis is 5. Thus, when the receive level of cell B is higher than -
85 dBm, handover is initiated from cell B to cell M.
Modify the priority level of cell B to level 1. In this way, the cells are at the same pr
iority level. The handover problem is solved.
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Page43Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 4
z Fault Description
Suburb of city A, Huawei BTS3012 M-2 cell, incoming handover success
rate lower than 70%, number of incoming handover failures at busy
hours greater than 35.
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Handover Cases 4
z Analysis
The hardware such as clock board is checked and no problem is found.
Register the traffic measurement of the cell incoming handover, it is found that
most of the handover failures occur from the external N-1 cell of the BTS of
company S.
The field drive test shows that in times of handover failure, there occurs a neighbor
frequency in the neighbor cell frequency list. It is suspect that the handover failures
are caused by downlink inter-frequency interference.
We trace and analyze the handover failure signaling and finds that sometimes the
handover failures occur when the MS receives the handover command and sends
access pulses to the target cell and sometimes the handover failures occur whenthe MS reports the SABM frame.
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Handover Cases 4
z Solution
Therefore the conclusion is that the MS fails to correctly decode the
physical message due to the downlink inter-frequency interference, or
the MS fails to receive the UA frame from the target cell, resulting in
handover failure.
The cell frequency is planned again and is changed from 113 to 121. The
problem is solved.
Signaling tracing is very helpful if there are problems in cooperation with other
manufacturers equipment.
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Page46Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 4z Handling Suggestions
The BTS hardware alarms are checked.
The cell incoming handover traffic measurement is registered.
Drive test to trace the signaling.
The cell frequency is modified.
The traffic measurement is checked and it is confirmed that the number of
incoming handover failures decreases and the incoming handover success rate
reaches 92%.
z Suggestion and Summary
The downlink interference is not represented in the interference band and the
range affected by the partial downlink interference is small. We may miss the
faulty place during the drive test due to selection of the drive route. It is desirable
to perform careful drive test on the abnormal cells.
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Page47Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 5
z Fault Description
BSC6000 has very low HO attempt & during many drive test no HO at all.
KPI indicate that there is very low HO attempt with good success rate.
From Drive test using TEMS, we found out Missing Neighbor Detection
message appears in neighbor list when MS still try to make HO to serving cell.
And also T200 Expired appears.
The figure below and the corresponding KPI statistics table is presented as
below.
Corresponding
KPI Statistics Table
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Page48Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 5
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Handover Cases 5
z Analysis
Neighbors cell configuration is incorrect
Data configuration has some problem, such as NCC Permitted
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Handover Cases 5
z Solution
Compared with other BSC, found out the handover attempts are less than
other BSCs even under the same condition.
Checked the neighbor cell configuration, for some cells, there were some
neighboring cells missing, but after adding, the problem still existed.
Checked "NCC permitted" , we found that the configuration in the problem
BSC is set to be 1111000, that is, enabling NCC 4~7 handover while
disabling NCC0~3, but the neighboring cells' NCC is 0~3.
Changed NCC Permitted to be 11111111, the problem is solved.
To solve the problem of low handover attempts, we firstly compared the handover times withother BSCsunder the same scenario and found out the attempts are less than other BSCsevenunder the same condition, in addition, we also checked the neighbor cell configuration to see
whether there are enough neighbors to ensure the handover proper and prompt, for some cells,there were some neighboring cells missing, but after adding, theproblem still existed, and wechecked the parameters, found out There is a parameter "NCC permitted" relative to the lowhandover times, the definition of this parameter is as follows:
NCC permitted
Range: 0 allowed~7 allowed
Default: 1111111
Description: Network color code permitted. The value 1 stands for permitted and 0 for forbidden.This parameter is sent in system information 2 and 6. When the cell's NCC is consistent with thevalue of NCC permitted, then this cell will be measured by MS. And MS will report the
measurement report to BTS. This parameter consists of one byte (8bit). Each bit is correspondingto an NCC (0~7) and the last bit is corresponding to NCC 0. If bit N is 0, then MS will notmeasure the cell level with NCC being N.
Note:AsMS cannot report the adjacent cell information where NCC is setto 0, the incorrectsetting of this parameter will cause MS to be unable to hand over during conversation. SeeProtocol 0508.
This parameter can be used to make MS 's measurements on some adjacent cells optionally.
We found that the configuration in the problem BSC is set to be 1111000, that is, enabling NCC4~7 handover, but from the feedback, the neighboring cells' NCC is 0~3.
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Handover Cases 5
z Suggestions and Summary:
NCC Permitted parameter is an important parameter to impact on
handover but always been neglected, and if the similar phenomenon
happened, this is a clue to solve this problem.
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Page52Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 6
z Fault Description
A GSM network, where one MSC is attached with two BSC, is all
configured with Huawei equipment. When the two BSC are cutovered,
the successful handover rate from BSC1 to BSC2 is 0, but the opposite is
normal. Intra-BSC1 and intra-BSC2 handover are normal.
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Page53Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 6
z Analysis
Register outgoing inter-cell handover measurement function of
BSC1 and incoming inter-cell handover measurement function of
BSC2.
Check the data related to handover of BSC1 and BSC2.
Analyze signaling of handover failure.
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Page54Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 6
z Solution
Check all data of inter-BSC handover: external cell description data table,
BA2 table, CGI of MSC. No problem is found.
Trace A interface message of BSC1 and BSC2. After BSC1 sends "HO-
REQUIRED", BSC2 has not received "HO-REQUEST" message, but opposite
is normal.
The path of data search: MSC goes to "LAI and GCI Table" according to
the CGI of the target cell in "HO-REQUIRED" message, and sends HO
request to the correct BSC base on the description of the DPC of the cell
given in the table.
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Page55Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 6
z Solution
The HO-REQUEST message, which should have been sent to BSC2, is sent
to BSC1. It is DPC error. After correction, the problem is solved.
z Conclusion
For the data of inter-BSC handover, besides checking whether CGI at MSC
side is consistent with that of opposite and BSC, check whether the DPC is
correct. MSC look up target DPC based on the CGI of the target cell.
When the DPC is incorrect, "HO-REQUEST" will be sent to wrong BSC.
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Page56Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 7
z Fault Description
After all BTSs of a certain BSC32 are swapped to the BSC6000, the PBGT handover
proportion decreases from the average value 60% to 40%. The number of bad
quality handovers increase. It thinks that the problem is caused by the interference
and coverage.
Note:
The PBGT handover proportion is one of the key indexes that the operator uses to
evaluate the network coverage quality.
PBGT Handover Proportion = PBGT Handover Requests/Total Handover Requests
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Page57Copyright 2009 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 7
z Analysis
The possible causes of the decrease in the PBGT handover proportion
are as follows:
The number of other handovers with a higher priority than that of the
PBGT handover increases. The possible causes are the interference or
coverage problems.
The data configurations of PBGT handover change, thus causing an
increase in the condition of the PBGT handover and a decrease in the
number of PBGT handover attempts.
16-bit value affects the priorities of many neighbor cells with good
levels.
(1) The PBGT handover algorithm is used to find a cell that meets the
following conditions on a real-time basis and determine whether to perform a
handover:
The cell has less path loss.
The level of the neighbor cell is higher than the threshold for the local cell.
The cell meets the system requirements.
PBGT Handover Proportion =PBGT Handover Requests/Total Handover
Requests
(2) The possible causes of the decrease in the PBGT handover proportion areas follows:
The number of other handovers with a higher priority than that of the
PBGT handover increases. The possible causes are the interference or coverage
problems.
The system parameters that the PBGT handover requires change, thus
causing an increase in the number of requirements for the PBGT handover and a
decrease in the number of PBGT handover attempts.
(3) The requirements of the PBGT handover are as follows:
The PBGT handover is implemented only between the cells with the same
layer and same level.
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Handover Cases 7
z Solution
The problem may be caused by interference and coverage. Check the related traffic measurement.
The traffic indexes of the interference band before and after the swap are low. The BTS hardware is
operational.
Check the parameters related to the PBGT handover, PBGT Handover Threshold and Decision Time.
The parameters of other handovers with higher priories than PBGT handover are proper.
Check the settings of the 16-bit priority. The inter-layer handover threshold and
hysteresis are greatly different from those configured on the original BSC. For the
existing network, the initial threshold is set to 25 and the hysteresis is set to 3. For the
original BSC, the initial threshold is set to 40 and the hysteresis is set to 1.
(1) The problem may be caused by interference and coverage. Check the related traffic
measurement. The interference handover proportion before the swap is low. The traffic indexes of
the interference band before and after the swap are low. The BTS hardware is operational.
The OMC data shows that not all parameters are mapped according to the original network during
the swap from the BSC32 to the BSC6000. All BTSsare configured according to the initial template
of the BSC6000.
(2) Check the parameters related to the PBGT handover, PBGT Handover Threshold and
Decision Time. The parameters of the handovers with higher priories than PBGT handover are
proper.
(3) Check the settings of the 16-bit priority. The inter-layer handover threshold and hysteresis
are greatly different from those configured on the original BSC. For the existing network, the initial
threshold is set to25and the hysteresis is set to3. For the original BSC, the initial threshold is set to
40and the hysteresis is set to1.
If the following conditions are met, the priority of the neighbor cell is lower than that of the local
cell. Even the level of the neighbor cell is higher than that of the local cell, the PBGT handover failsto be triggered.
The threshold for the receiving level is set to25.
The level of the neighbor cell is lower than the value of threshold +hysteresis.
The level of the local cell is not lower than the value of threshold - hysteresis.
In addition, the sites covered by the BSC are located in the suburb areas. There is a great probability
that the receiving level of these sites is 25. Therefore, the settings of 16-bit priority have great
impacts on the PBGT handover.
After the threshold is set to40and the hysteresis is set to1 in batches on the BSC, the PBGT
handover proportion increases to 55%-60%, which is close to the original BSC index value.
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Handover Cases 7
z Suggestion and Summary
When analyzing the handover traffic measurement, the impacts of the
factors in the handover algorithm on the handover priority and handover
triggering conditions should be taken into account.
The mapping of all parameters at the cell level is essential to the BSC
swap. If parameters are not configured according to the original network,
the network indexes may be affected. There are multiple parameters at
the cell level.
(1) When analyzing the handover traffic measurement, you need to
take the impacts of the factors in the handover algorithm on thehandover
priority and handover triggering conditions into consideration.
(2) The mapping of all parameters at the cell level is essential tothe
BSC swap. If parameters are not configured according to the original
network, the network indexes may be affected. There are multiple
parameters at the cell level. Therefore, the workload of analyzing and
checking the parameters is large if the indexes related to the traffic
measurement have problems.
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Handover Cases 8z Fault Description
Huawei BTSs belong to Huawei BSC independently are inserted into a GSM900
network. After the cutover, the user complains that Huawei BTS handover is not
good and that the BSC outgoing handover success rate is below 85%. The drive test
shows that the serving cell is Huawei cell A and that then drive toward cell B. The
receiving level of cell A is -80 dBm (no downlink power control), while the receiving
level of the neighbor cell B of another BSC is -60 dBm. Handover does not happen
until MS passes across the cell B.
HuaweiCell A Cell B
-80dBm -60dBm
No handover for
a long time
Huawei BSCNo Huawei BSC
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Handover Cases 8
z Analysis
The reason that the receiving level of the neighbor cell is strong enough
to prevent handover is as follows:
The BA2 table has the BCCH frequency of cell B but the handover data,
including parameters such as external cell CGI, BCCH, BSIC, and
"whether share MSC", are wrong, and "minimum downlink power of
the handover candidate cell" is high.
Congestion of cell B results in no handover.
Other parameters are set incorrectly, resulting in no handover.
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Handover Cases 8
z Solution
Drive test shows that handover does not happen. To verify reasonableness of the
data, test MS is used to perform the forced handover. Handover succeeds.
The traffic statistic of the no-Huawei BSCs is checked and confirm that there is no
congestion in the target cell B.
Via drive test, it is found that all the new cells of Huawei BSC have this problem. It is
suspected that the problem is caused by a common parameter.
The traffic measurement is analyzed and it is found that no PBGT handover happens
in the new BSC, while only edge handover happens when the edge handover
condition is satisfied, resulting in serious handover delay.
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Handover Cases 8
z Solution One of PBGT handover condition is target cell must be the first position in the
16bit criterion.If Co-BSC/MSC Adj is set toYES, the cells that do not share
BSC/MSC are sorted at the last positions. Therefore the PBGT handover of the BSC
outgoing handover does not happen. After Co-BSC/MSC Adj is set to NO, the
problem persists.
The data is further checked and it is found that the default parameter
configuration shows that internal cells of Huawei BSC are "layer 3, priority 1" and
the external cells are "layer 3, priority 2". Because PBGT handover only happens at
the same layer and the same priority, the default configuration does not trigger
PBGT handover. After changing all internal cells priority of Huawei BSC from 1 to 2, it shows that
PBGT handover happens by analyzing the traffic measurement. Drive test shows
that BSC outgoing handover is not delayed.
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Handover Cases 8
z Suggestion and Summary
Note that the following two default parameters need to be modified in
the BTS and BSC insertion plan
1. Set Co-BSC/MSC Adj to NO2. If there is no requirement for traffic balance, set the layer and priority of the
internal cells and external cells to the same layer and the same priority.
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Handover Cases 9z Fault Description
The drive test of a network in an office shows that in the drive test in a
county, there is a strange phenomenon: when the receiving level of the
current serving cell (cell A) is as high as 45 dBm, the call still hands over
to a cell (cell B) with low level of 78 dBm. Viewed from geographical
location, this place should not be covered by cell B. The frequency
planning of the network is tight frequency reuse. Therefore the
conversation quality becomes worse after handover.
Cell A Cell B
-45dBm -78dBm
Handover occur fromhigh level to low
level
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Handover Cases 9
z Analysis
There are the following reasons that can lead to handover from a
high-level cell to a low-level cell.
To adjust the traffic, the PBGT handover threshold is set to a value less than 64.
When emergent handover occurs, such as BQ handover (Because the uplink quality
is bad for interference, BQ handover occurs).
Though the downlink signal level of the current cell is strong, the uplink signal level
is weak and already reaches the handover threshold.
The layer and priority of the low-level cell is higher than those of the high-level cell.
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Handover Cases 9z
Solution1. Check BSC data configuration
It is found that the [PBGT HO Threshold]=70 and [Inter-cell HO
Hysteresis]=3. Therefore the handover is not a PBGT handover.
2. The drive test data shows that when a handover is triggered, both the
downlink receiving quality and the downlink receiving level of MS in the
serving cell are good.
This excludes handover caused by downlink receiving quality and level.
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Handover Cases 9
z Solution
3. Check the cell priority
It is found that cell B is at layer 3 with priority 1 while cell A is at layer 3 with
priority 2. [Inter-layer HO Threshold]=25 and [Inter-layer HO Thysteresis]=5.
Therefore when the receiving level of cell B is greater than 80 dBm, handover is
triggered. The handover is a better cell handover from a cell in a high receiving
level to a cell in a low receiving level.
In the initial data configuration, the BSC internal cells are designed at layer 3 with
priority 1 and the external cells are at layer 3 with priority 2. The engineer ever
changed data during routine maintenance.
The priority of cell A is changed to 1, and the priority of cell B is changed to 2, thehandover becomes normal.
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Handover Cases 10
z Fault Description Huawei mini BTSs and NT BTSs are in mixtured networking, that is to say,
few Huawei BTSs are located in a great many NT BTSs.
One day the customer complains that during a drive test, handing over
from a Huawei mini BTS to a neighbor cell (NT BTS) is very difficulty, and
that the handover request is not sent even when the receiving level of the
neighbor cell (NT BTS) is much stronger than that of the serving cell
(Huawei mini BTS). Sometimes handover does not happen until the call
drop.
All the cells are in the same layer and the same priority. [Co-BSC/MSC Adj]
is YES. [Inter-layer HO Threshold]=25, and [Inter-layer HO Thysteresis]=3.
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Handover Cases 10
z Analysis
Firstly, suspected that the problem of failure to send handover request is
caused by improper neighbor cell relationship of the serving cell or by
incorrect frequencies in the BA1 table and BA2 table.
The check result shows that the neighbor cell relationship is right and that
the frequencies are correct.
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Handover Cases 10
0001Yes or No< 82 dBmNeighbor Cell
0010No>= 82 dBmNeighbor Cell
0000Yes>= 82 dBmNeighbor Cell
0001/< 88 dBmServing Cell
0000/>= 88 dBmServing Cell
In the same
BSC
Rx_Lev 10th ~5th bit13th ~ 12th bit14th bit
. All the cells are in the same layer and the same priority, and in the same MSC.
. [Co-BSC/MSC Adj] is YES
. [Inter-layer HO Threshold]=25(-85dBm), and [Inter-layer HO Thysteresis]=3dB
z Analysis
Secondly, Since [Co-BSC/MSC Adj] is set to YES, compare the 16bit value
between serving cell and neighbor cell.
Since Co-BSC/MSC Adj is set to YES, of the 16bit sequence number,
.If the serving cell level >=inter-layer handover thresholdhysteresis, which is >=88 dBm, bit14 is 0, bits
5~10 of the serving cell are the same as those of the neighbor cell, so bits 12, 13 and 14 are all 0.
.If the serving cell level =82 dBm, bit 14 is 0, bits
5~10 of the internal cells are the same as those of the serving cell, so bits 12, 13, and 14 of the internal cells
are all 0, while bits 5~10 of the external cells are the same asthose of the serving cell, so bit 12 is 1, and bits
13 and 14 are 0.
.If neighbor cell level
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Handover Cases 10
z Analysis
Thirdly, the geographical environment around the mini BTS is complex and
P/N = 5/4s, respectively.
These values are large. Therefore sometimes when the MS receiving level is
lower than 88 dBm, the level decreases so rapidly due to the complex
terrain that call drop happens before handover succeeds.
Therefore P/N are changed to 3/2s, respectively. In this case, handover is
triggered very quickly at low level.
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Handover Cases 10z Solution
The handover performance measurement is registered and it is found
that the cell has handover request and that the handover success rate is
high.
The BSC incoming and outgoing handover performance measurement is
registered and handover happens normally between the cell and a NT
cell.
Drive test shows that when serving cell is Huawei mini BTS and the
serving level is below 80 dBm, handover does not happen even when
the signal of the neighbor cell is much stronger than that of the serving
cell. But when the signal of the serving cell is 90 dBm, sometimeshandover happens and succeeds, sometimes due to terrain reason, the
level decreases so rapidly that handover does not happen in time and
result in call drop at last.
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Handover Cases 10z Solution
It is confirmed that all the neighbor cells have configured in the BA1/BA2
table and the corresponding frequency is correct.
The handover-related parameter setting of the cell is configured to be
the same as that of other cells. The parameters that affect inter-BSC cell
handover are mainly checked and it is found that [Co-BSC/MSC Adj] is set
to YES and that P/N time of the edge handover and PBGT handover are
5s/4s, respectively.
The [Co-BSC/MSC Adj] is modified to NO and the P/N time of the edge
handover and PBGT handover are changed to 3s/2s, respectively. DT shows that handover happens when the MS receiving level is high.
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