39920096 omd6046 case analsyis handover issue1 1

8/3/2019 39920096 OMD6046 Case Analsyis Handover ISSUE1 1

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OMD6046Case Analsyis--Handover

ISSUE1.1

Wireless Curriculum

Development Section


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Outline of Handover Topic Statistics by handover counter

Searching process of handover data

Analysis of causes of handoverproblem

Handover cases


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Statistics of handover counter

Intra-BSC handover

Inter-BSC handover

Statistic Point of Handover Counter


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Statistic Point of Handover CounterIntra-BSC Handover

MS BTS(Source) BSC BTS(Target) MSC

Measurement Report

Measurement Report

Handover algorithm verdict on the cell handover request T09++

(In-cell handover request get to)T12++

Channel Activation

Channel Activation ACK

Handover Command (Old FACCH)

Handover Access (New FACCH)

Handover Complete (New FACCH) T10++ T13++

RF Channel Release

Handover Performed

Attempted incoming internalinter cell handovers

Successful incominginternal in-cellhandover

Attempted outgoing internalinter cell handovers

Successful outgoing internal in-cellhandover


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Possible causes of failure of intra-BSC handover

No available channel in the destination cell

Channel activation negative acknowledgementCHAN ACK NACK Transmitting channel activation message failure

Waiting channel activation timeout

MS access failureRadio interface may be the cause

Max repeat time of physical information*radio link connected withthe timer < time interval of EST IND~HO DETECT (120~180ms)

Waiting to timeout after establish indication (MS has not receivedUA frame or has not sent handover complete message)

Statistic Point of Handover Counter Intra-BSCHandover


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Thesa

measint

ra-MSC

Statistic Point of Handover Counter Inter-BSC HandoverMS BTS(Source) BSC MSC BSC BTS(Target)

Measurement Report

MeasurementReport

Handover Required T17++

HandoverRequest T14++

Channel ACT

Channel ACT ACK

Handover Request ACK

Handover Command

Handover Access

Handover Detect

Handover Complete

Handover Complete T15++

Clear Command(HO successful) T18++

RF Channel Release

Clear Complete

Attempted outgoing inter-BSCinter cell handovers

Attempted incominginter-BSC inter cell handovers

Successful outgoing interBSCinter cell handover

Successful incoming interBSCinter cell handover


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Signaling flow between MSCs

MSC-BMSC-A VLR-BBSC-A BSC-B

HO-RequiredMAP_Prepare_HO

MAP_Prepare_HO_ACK

MAP_Allocate_HO_NUM

MAP_Send_HO_Report

MAP_Send_HO_Report_ACK

MS

HO-Request

HO-Request-ACK

HO-Command

MS

HO-AccessMAP_Process_Access_Signalling

HO-CompleteMAP_Send_End_SignalClear-Command

Clear-Complete

MAP_Send_End_Signal_ACK

Some intermediate steps are omitted

IAIACM

Statistic Point of Handover Counter Inter-BSC Handover


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Success ratio of

inter-BSC handover--------------100Success times of inter-cell handover between BSCsRequest times of inter-cell handover between BSCs

Success ratio of

In-BSC handover----------------100Destination BSC receives HO-COMP from MSAfter destination BSC receives HO-REQ

Success ratio ofOut-BSC handover------------100Source BSC receives Clear-COMD from MSC

After BSC sends HO-Required

Statistic Point of Handover Counter Inter-BSC Handover


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Statistics by handover counter

Searching process of handoverdata


Handover cases

Outline of Handover Topic


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Searching Process of Handover Data

1. BCCH of all adjacent cells in BA2 table are sent to MS on system

message 5.2. MS reports service cell and six adjacent cell where level is the most

powerful (through measurement report) to BSS, including the

BCCH , BSIC and signal level of the adjacent cells and service

cells.

3. When the measurement report is pre-processed, BSC determines themodule ID, cell ID and CGI of all adjacent cells through BCCH

frequency and BSIC in Cell Adjacency Relation Table and Cell

Description Data Table (or External Cell Description Data Table).

4. BSC executes handover verdict flow such as basic cell sorting (on

LAPD board). Once a proper destination is found, the handover

request message of the destination cell CGI will be sent to MPU of

BSC host, and then MPU will confirm the module ID of the cell in

Cell Module Information Table based on the CGI.

5. MPU sends handover request message to the module and makes

statistics of Out-cell handover request.


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6. If the destination cell triggered by BSC is an external cell, the

CGI of the destination cell and service cell is sent to MSC with the

handover request.

7. MSC first goes to Location Area Cell Table to search the cell

that matches the CGI of the destination cell. Once the cell is found,confirm which BSC is belong to. Send the handover request

message to this BSC.

8. If there is no CGI of the destination cell in Location Area Cell

Table, go to adjacent MSC table and find the destination MSC,then send the handover request message to this MSC.

Searching Processing of Handover Data


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Searching process of handoverdata


Handover cases


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Analysis of Handover Problem

Types of Handover Problems

Locating problem

Causes of Handover Problem


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Type of Handover Problem

No handoverDropout occurs Handover failureAffect the communication quality,hence the dropout.

Frequent handoverAffect the communication qualityand increase the system load


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Locating Handover

Problem

Analyze traffic statistics

Measurement of BSC overall performance

Measurement of inter-cell handover performance

Measurement of out-cell/in-cell handover performanceMeasurement of performance of undefined adjacent cells

View alarm: Board fault, transmission, clock, etc.

Drive test

Analyze signaling: A interface, E interface, Abis interface


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Coverage and interference at radiointerface

Antenna & feeder system

BS software/hardware Transmission

BSC software/hardware/ DataConfiguration

A interface problem

Destination cell is busy

Interconnection with equipment of othermanufacturers

Analysis of Handover Problem


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I. Coverage and interference at radio interface

1. Coverage:

Poor coverage: forest, complicated topography,building direction and indoor coverage

Isolated site: no-neighbor cell

Over coverage: island effect result in a no-neighbor

cell

2. Interference: consequently, MS uplink can not beaccessed or the downlink can not receive any signal.

Analysis of Causes of Handover Problem


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Island effect results in handover failure

Adjacent

Cell N3

Adjacent

Cell N2

Adjacent

cell N1

Non-adjacent

cell

Non-adjacent

cell

Non-adjacent

cell

Service cell

There is noadjacent cells,so handoverbecomeimpossible.

Isolated islandresulting fromcross-cellcoverage



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II. Antenna & Feeder System

1. High voltage standing wave ratio(VSWR)

2. Antennae are not properly installed

3. Antennae are not parallel

4. The azimuth and obliquity are not correct

5. Poor antenna isolation

6. RF cables ,connection are loose or incorrectconnection



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III. BTS software/hardware

1. CDU, splitter/combiner failure

2. TRX, RC board failure

3. TMU failure

4. FPU failure

5. Clock failure

6. Internal communication cable

7. BTS software



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IV. Transmission1. Transmission is not stable

2. Serious BER in transmission

V. Fault BSC Software/Hardware

Example for:Clock board: the defective clock board

causes clock inconsistency between BSs.



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Data ConfigurationCause handover failure, delay, frequenthandover, no handover, unreasonable in/out handover ratio Improper setting of handover threshold (higher, lower, the edge handover

threshold is higher than the power control threshold.)

Improper setting of hysteresis handover and priority handover

Improper setting of P, N value of statistic time

Incomplete frequency and adjacency relation configured in BA2 table

There are adjacent cells with the same frequency and the same BSIC

CGI and module ID in Cell Description Data Table are different from those

in Cell Module Information Table.

CGI, BCCH and BSIC in External Cell Description Table are different fromthose in the opposite BSC.

The Destination signaling point of BSC in MSC Location Area Cell Table isincorrect.



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Precautions:

Frequency of Preprocess Measurement Report: when Measurement

Report Preprocess is enable , if the link configuration is 15:1, the

setting is once per second. At this time, for the handovers that needs

P/N verdict, such as boundary handover, layer handover, PBGT

handover and concentric handover, the P/N verdict time should be

shortened.

BSC can not process the CGI in lower case. Therefore it must be input

in upper case, otherwise the handover would fail.

BSC does not recognize CI as FFFF cell, otherwise the handover

would not happen

Transmitting BS/MS Power Level: If the measurement report

preprocess is enabled, this parameter must be set to Yes.



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VI. A interface Problem

Basically the insufficient link resource results in the abnormal handover,

as well as abnormal communication.

VII. Destination Cell Busy

The destination cell is busy and there is no available channel, whichcauses the handover failure. Then the destination cell should be

expanded or reduce its traffic .

VIII. Handover between Equipment of Different

Manufacturers

The signalings at A interface, E interface of the opposite end are not

matching to our equipment and they can not be recognized or support,

which causes the handover failure, such as voice version, handover

number, TUP circuit, addressing mode (CGI or LAI).



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Searching process of handover

data


Handover cases


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Case 1

Fault Description: a 1800 cell of a dual-band network

(all the cells of this are 1800), the success ratio of in-

BSC handover and in-cell handover in BSC are low

from the beginning of the service, while out-cell

handovers inside BSC and between BSCs are normal.

Typical Handover Cases

Typical Handover


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Case 1

Typical HandoverCase

Tips for troubleshooting:

1. Register the traffic statistics and analyze whether the low

handover ratio is due to the failure of handover from all cells to this

cell or from some few cells to this cell.

2. If handover fails from some few cells to this cell, check thehandover data and see whether there is co-channel or same BSIC

problem.

3. If handover fails from all other cells to this cell, check the data of

this cell.

4. If data cause is ruled out, check the hardware carefully. Check the

alarm or perform drive test to locate uplink fault or downlink fault.Check step by step and find out the cause.


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Typical Handover Case

Solution:

1. Register the in-cell handover performance measurement and find that

the success ratio of handover from all other cells to this cell is low,

although it is not always 0 percent. Check the data and the data are

correct.

2. Perform drive test and find that the downlink signal is normal but

almost all handovers to this cell fail. In the BS, the handover is

successful occasionally.Perhaps the problem is with the uplinksignal.

3. Check the uplink, antenna, CDU are no problem. Change the TRX and

everything is normal.

Conclusion: The symptom is that the uplink and downlink atUM port are not balanced so the opposite hears low qualityvoice.


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Typical Handover Case

Fault Description: in a cell of a 900M mono-

frequency network, the success ratio of in-cell

wireless handover in BSC is low (1030), whichis the same as handover success ratio.

Case 2


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Solution:

1. Check data: register In-cell Handover Performance Measurement and

find that the success ratio of handover into all adjacent cells is low, but is not

0. Check the data to find that there is no co-frequency 0f BCCH and co-BSIC

problem. Data cause is ruled out.

2. Check coverage:

The traffic in this cell is relatively low. Check BS Maintenance to find that

TCH channel can be occupied, but occupation times are few.

Perform a drive test. In a place 2 km from the BS, the downlink receiving

signal is85dbm, but in-cell handovers always fail or dropout always occursat handover.

Make dial test in this cell by locking frequency, the caller can not be

created. The called can be created but can not call out. A rough conclusion is

that poor uplink causes the problem.

3. Check the installation of uplink hardware, nothing abnormal is identified.

Maybe a fault has occurred to the uplink path of TRX or CDU. Replace CDU,

the problem is solved.

Conclusion: hardware fault results in poor signal,and the success ratio of wireless handover is low.


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Solution:

1. Check whether it is attributed to LAPD updating: after the upgdating,

not all cells under module 4 are congested and the handover indexes of

some indexes are not lowered. Upgrading is not the cause.

2. Analyze traffic statistics:

Congested cells are mostly at Site A and Site B. Their traffics are

lowered than those before the upgrading. It is a pseudo congestion.

The cells where handover success ratio is lowered are mostly at Site A,

B, C, D and E and the main cause of the failure is timer timeout.

3. Check alarm:

At 19:31 of September 18, the clock systems of Site A and Site B gave

alarms. It was found that 13M loses lock.

the clock problem lowers the handover success ratio of the two sites.

Such impact is spread to the adjacent cells of these two cells and even

to the whole network.


T i l H d C


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Solution:

4. Analyze handover data and traffic statistics:All cells where handover success ratio is lowered are adjacent

to A and B except A and B themselves.

5. Reset Site A and Site B, the clock system is recovered, and

the handover success ratio inter/intra-BSC goes up to 93. Theproblem is solved.

Conclusion: the problem in clock system

will lower the handover success ratio. Pay

attention to the alarm console and the

slow switchover of the clock system.


T i l H d C


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Case 4

Fault Description: Huawei BSC and the another

manufacturerBSC(for short S BSC) are connected to the same

MSC. After the cutover, S BSCBS can not handover into HuaweiBS but Huawei BS can handover into SBSC BS. In other words,

In-BSC handover request times of Huawei BSC is 0.


T i l H d C


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Case 4

Analysis: Since in-BSC handover request

times is 0 but the out handover is normal.

Carefully check the out-cell handover

signaling and handover data at S BSC side


T i l H d C


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Case 4

Solution:

1. Trace interface message. After receiving HO-REQUEST

message, Huawei BSC returns HO-FAILURE message

immediately but Huawei BSC should have returned HO-

REQ-ACK message as in normal conditions.

Carefully analyze HO-REQ message and HO-FAILURE

message


T i l H d C


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T i l H d C


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HO-REQUEST


T i l H d C


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Solution2. HO-FALUER: The cause of the failure is invalid message

content

3. HO-REQthe difference is basically the Address Indicator incomparison with the normal handover request message. HUAWEI

BSC does not recognize case 41. The system thinks that Address

Indicator must be 0x43. inform the owner of network, modify the

relation parameter,and this problem is solved.

Case 4


T i l H d C


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Conclusion:In interworking with equipment of other manufacturers, faults

can be located by analyzing the signaling.

Case 4


T i l H d C


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Fault Description: Huawei MSC interworks with MSC of N(manufacturer) with Ns BSC attached. The communication isnormal. The handovers inside Huawei BSC and in-BSC are

normal, but out-BSC handover success ratio is about 25%.

The handover from the BSC of N to Huawei BSC is successful

and the subsequent handover is also successful (In the same

communication, the handover goes back to BSC of N).

Case 5Typical Handover Cases

T i l H d C


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Analysis: Check handover data, including theexternal cell description table, BA2 table, cell

adjacency relation table inside Huawei BSC

and opposite BSC and CGI at MSC side. If the

data are correct, check out-BSC handover

signaling.




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Solution:

1. Check data: BSC and MSC of Huawei and N. No problem is found.

2. Check alarm: BS maintenance console, No.7 link and A interface circuit are all

normal.

3. Trace Huawei A interface signaling. After HO-Required message is received,

HO-COMD command is not received from MSC.

4. Trace MAP message at E interface (inter-MSC) with a signaling meter. It is found

that after receiving HO-Number, Huawei MSC never sends IAI (Initial Address

Information).

5. Check HO-Number message from the opposite end to find that the handover code

format is 130********(only a number of mobile telephone, no any prefix). But this

mobile company requires that roaming/handover between exchanges should be inthe format: 00+country code+roaming/handover number. In addition, Huawei

equipment does not recognize the handover number without 00+country code.

Therefore the signaling is halted.

6. Coordinate with N, asking them to add 00+country code before the handover

number. Then out-BSC handover is normal.

Case 5




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Conclusion: Problems can be located quickly and accurately

if you are familiar with the signaling flow of handovers.




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Fault Description: In the independent MSC networking of adual frequency network, Huawei 1800M coordinates with

900M of manufacturer A . After the cutover, the success ratioof handover from M1800 to M900 is low (about 60), while thehandover from M900 to M1800 are normal and the handover

data are correct.

Case 6




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Analysis 1: Trace the signalings at A interface and E

interface. Normally:

MSC-BMSC-HW VLR-BBSC-HW BSC-B

HO-RequiredMAP_Prepare_HO

MAP_Prepare_HO_ACK

MAP_Allocate_HO_NUM

MAP_Send_HO_Report

MAP_Send_HO_Report_ACK

MS

HO-Request

HO-Request-ACK

HO-Command

MS

HO-AccessMAP_Process_Access_Signaling

HO-CompleteMAP_Send_End_Signal

Clear-Command

Clear-Complete

MAP_Send_End_Signal_ACK

Intermediate stepsare omitted.

IAIACM




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Analysis 2: Abnormally:

MSC-BMSC-HW VLR-BBSC-HW BSC-BHO-Required MAP_Prepare_HO

MS

HO-REJECT

MS

MAP_Abort




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Analysis 2:

1. Analyze the Prepare-HO message sent from Huawei MSC to the opposite end.

The voice version is full rate version 1, 2 and half-rate version 1. It is a PHASE

2+ version. However there is only full rate version 1 in the message sent from

M900 to Huawei M1800. It turns out that the opposite does not support half-rate

version, so the handover fails.

2. Modify A interface circuit pool table of Huawei MSC data, only using full-rate

version 1.

3. Then the out-BSC handover is normal.

Case 6

Conclusion: There are often many problems in coordination

between the equipment of different manufacturers. However,

problems can be located accurately by tracing signaling.




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Case 7

Fault Description: A specific GSM network, where one MSC is attached with

two BSC, is all configured with Huawei equipment. When the two BSCs are

cut over one after another, the success ratio of handover from BSC1 to

BSC2 is very low, but the success ratio of handover from BSC2 to BSC1 is

normal. Intra-BSC handover inside BSC1 and BSC2 is normal.




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Case 7

Analysis: check handover data and trace signaling.

Solution:

1. Check all data of inter-BSC handover: external cell description data table,

BA2 table, CGI of MSC. No problem is found.

2. Trace A interface message of BSC1 and BSC2. After BSC1 sends HO-

Required, BSC2 has not received HO-Request message, but BSC1 has.

3. The path of data search: MSC goes to Location Area Cell Table according to

the CGI of the destination cell in HO-Required message, refers to the

description of the DSP of the cell given in the table and sends HO request to the

correct BSC.

4. The Ho-request message, which should have been sent to BSC2, is sent to

BSC1, so it is DSP error. After correction, the problem is solved.




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Case 7

Conclusion:

For the data of inter-BSC handover, besides checking

whether CGI at MSC side is consistent with that at the

opposite and at BSC, check whether the DSP is correct. MSCindexes to the CGI of the destination cell with the existing

CGI.. When the DSP is incorrect, HO-Request will be sent to

wrong BSC.




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Case 7

Fault Description: A specific dual frequency network is a mixed

networking of Huawei BSC (M1800) and manufacturer N BSC (M900),

attached to MSC of manufacturer S. After the cutover, the success

ratio of Huawei in-BSC handover is always 8892, while out-BSCand intra-BSC handovers are normal (above 92). Furthermore, thecells with low success ratio of in-BSC handover are randomly

distributed. When there is no fixed BM, there is no fixed LAC.



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Case 7:

Solution:

1. Perform forced handover with a tester between several cells where

handovers have failed. The forced handovers are all successful. Data

problem is ruled out.2. The interference band of cells is ideal, dropout and congestion ratio are

normal. It is unlikely that the coverage and interference at the radio port

causes the problem.

3. Trace A interface signal and compare the failure signaling and success

signaling:




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A interface signaling analysis of inter-BSChandover in MSC:

Failure signaling:

Success signaling:




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Case 7:

Solution:

4. The comparison result is that Huawei BTS has not detected any MS access

information (No HO DETECT signaling occurs). It shows that there is problem when the

MS is accessing cell M1800 or the MS does not access cell 1800.

5. Reconfirm that there is no problem with the handover data, otherwise the handover

success ratio will be very low.

6. Re-trace large number of handover signalings to find that all failures are attributed

to the same cause. In addition, the problem mode is the same: one MS makes multiple

handover requests in one communication and all requests fail.

7. Analyze other similarities of the failed message, to find that the first 6 digits of IMEI

numbers of these MSs are all 449684. It shows that the MSs that have handoverproblem are of the same model.

8. Find the MS owners with the IMSI numbers and find that all MSs with handover

problem are F-MS. Not long before it was proved that this MS has poor insulation

performance between 900 and 1800, so it can not access 1800 network. Now the cause

is found. .




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Case 7:

Conclusion: MS problem will also causes handover failure.

1. In this case the handover success ratio is not very low,

therefore coordination problem is unlikely.

2. Trace the signaling to find the cause of the handover failure:

BTS has not received handover access message from MS.

3. Find the similarity of these MSs through large amount of

signaling tracing: IMEI numbers are the same. Then it is

found that the problem is on F-MS.

4. In this case, the coverage of M900 is very good, F-MS is

always under M900 in idle status. Therefore in a call, TCH

assignment seldom fail, which will not result in high

congestion ratio. But in handover, problem is very likely to

happen.



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39920096 omd6046 case analsyis handover issue1 1

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