Case AnalysisCall Drop

8/11/2019 Case AnalysisCall Drop

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Page1Copyright 2006 Huawei Technologies Co., Ltd. All rights reserved.

References 31160978-BSC Traffic Statistic Manual Volume I

31033203-BSS Troubleshooting Manual


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Objectives Upon completion of this course, you will be able to:

31160978-BSC Traffic Statistic Manual Volume I

31033203-BSS Troubleshooting Manual


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Contents1. Principle of Call Drop

2. Analysis of Call Drop

3. Call Drop Cases


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1.1 Definition of Call Drop

1.2 TCH Call Drop

1.3 SDCCH Call Drop


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Definition of Call Drop There are two types of call drop: TCH call drop and SDCCH

call drop: TCH call drop means TCH channel is released abnormally

after it is occupied successfully.

SDCCH call drop means SDCCH channel is released

abnormally after it is occupied successfully.


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1.1 Definition of Call Drop

1.2 TCH Call Drop

1.3 SDCCH Call Drop


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Causes of TCH Call Drop Usually, the typical causes for sending the Clear_Request

message are as follows:

Radio interface failure

O&M intervention

Equipment failure

Protocol error

Preemption


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Formula of TCH Call Drop Rate Formula of TCH call drop rate

TCH call drop rate=TCH call drops / Successful TCH seizures (all)


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Measurement Point of TCH Call Drop


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Measurement Point of TCH Call Drop TCH serves as traffic channel, BSC receives ERROR INDICATION message from BTS.

TCH is seized as traffic channel, BSC receives CONNECT FAILURE INDICATION

message from BTS.

In assignment procedure and handover procedure, failure of decoding HANDOVER

DETECTION and HANDOVER COMPLETE message.

TCH serves as traffic channel, incoming BSC handover initiated and the timer for the

target cell to wait for HANDOVER COMPLETE message expires.

TCH serves as traffic channel, outgoing BSC handover initiated and the timer for the

source cell to wait for CLEAR COMMAND message from MSC expires (T8 expires).

In intra-BSC handover procedure, the target cell sends Inter Clear Request to the

source cell when the timer for the target cell to wait for HANDOVER COMPLETE from

MS expires, in this case, this item is measured in the source cell.


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Measurement Point of TCH Call Drop In Intra-BSC handover procedure, the source cell, excluding the source cell for directed

retry, measures the item when the timer for the source cell to wait for Inter Clear

Request with cause value HANDOVER COMPLETE from the target cell expires.

In Intra-BSC handover procedure, when the target cell AM/CM net-drive fails (due to

timeout or negative acknowledgement), the target cell sends Inter Clear Request to

source cell, in this case, this item is measured in the source cell.

In the case that MS reverses to the original channel after intra BSC handover fails, the

source cell first releases the terrestrial connection but the AM/CM re-net-drive fails (dueto timeout or negative acknowledgement). In this case this item is measured in the

source cell.

The resource of the lower_priority call will be preempted by the higher_priority call if

MSC and BSC both supports preemption, which will cause call drop.

This item is measured when the RSL link of the TRX that the call is using disconnects,

which will cause call drop.


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TCH Call Drop - Assignment

MS BTS BSC

ASSIGN CMMAND

MSC

ASSIGNMENT REQUEST

SABM

UA

ESTABLISH INDICATION

CHANNEL ACTICATION

CHANNEL ACTIVATION ACK

A1

ERROR INDICATION

CONNECTION FAILURE INDICATION

A2


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TCH Call Drop Intra-BSC HandoverMS BSC' BSC Ori-Cell

HANDOVER ACCESS

UA

Intercell Handover Request

BSC Tag-Cell BSC''

HANDOVER COMMAND

Intercell Handover Response

HANDOVER DETECT

CH ACTCH ACT ACK

A1

ERROR INDICATION


A2

SABM


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TCH Call Drop - Incoming BSC HandoverMS Other BSC

HANDOVER ACCESS

UA

HANDOVER REQUIRED

HUAWEI BSC HUAWEI BTS

HANDOVER COMMAND

HANDOVER REQUEST ACK

HANDOVER DETECT

CH ACT

CH ACT ACK

MSC

A1

ERROR INDICATION


A2

SABM

HANDOVER REQUEST


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Measurement Point of Successful TCH Seizures Measurement point of Successful TCH seizures

Upon BSCs reception of CHANNEL ACTIVATION ACKNOWLEDGE

message from BTS in very early assignment TCH process. In the case the target cell of directed retry is located in other BSC and

directed retry succeeds, MSC sends CLEAR COMMAND message to

the originating BSC to release the original connection. This item is

measured when the originating BSC receives this message. In the case the directed retry target cell is located in the same BSC

and the directed retry succeeds, target cell sends Inter Clear Request

message to the source cell to request to release the resource and the

original connection. This item is measured when the source cell

receives this message.


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Measurement Point of Successful TCH seizures Measurement point of Successful TCH seizures

This item is measured when BSC sends ASSIGNMENT COMPLETE

message to MSC after the assignment procedure is successfully

implemented.

In incoming BSC handover procedure, MS sends HANDOVER

ACCESS message to the BSC. This item is measured when BSCreceives HANDOVER DETECT message from BTS.

In the process of incoming internal inter cell handover and intracell

handover in BSC, MS sends HANDOVER ACCESS message to BSC.

BSC measures this item in the target cell when receiving HANDOVER

DETECTION message from BTS.


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Successful TCH SeizureVery Early Assignment

Very early assignment process

MS BTS BSC MSCChannel Request (RACH) Channel Required

Channel Activation (TCH or SDCCH)

Channel Activation Acknowledge

Immediate Assignment Command

Immediate Assignment (AGCH)


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Successful TCH Seizure Assignment Assignment process

MS BTSBSC MSC

ASS_REQ

CH_ACT

CH_ACT_ACK

ASS_CMDASS_CMD

EST_IND

ASS_CMP ASS_CMPASS_CMP


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Successful TCH Seizure Intra-BSC Handover

Intra-BSC handover process

MS MSBTS1 BTS2BSC

Measurement Report from MS

Channel_Active

Channel_Active ACK

HANDOVER COMMAND

HO_ AccessHO_Detect

PHY INFO

First SABM

Establish_IND

PHY INFO

Handover CompleteHO_Performed

UA

MSC


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Successful TCH Seizure Inter-BSC Handover

Inter BSC handover process

MS MSBTS1 BTS2BSC1 BSC2MSCMeasure Report from MS

HO_Required

HO_Request

CH_ACT

CH_ACT_ACKHO_Request_ACK

HO_CMDHandover Command HO_Access

HO_DetectHO_Detect

PHY INFO

PHY INFO

First SABMEstablish_IND

Handover CompleteHO_CMPClear_CMD

Clear_CMP


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1.1 Definition of Call Drop1.2 TCH Call Drop

1.3 SDCCH Call Drop


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Formula of SDCCH Call Drop Rate Formula of SDCCH call drop rate:

SDCCH call drop rate = SDCCH call drops/ successful SDCCHseizures*100%


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Measurement Point of SDCCH Call Drop

This item is measured when BSC receives ERROR INDICATION

message from BTS due to an abnormal case for a radio link layer

connection.

This item is measured when BSC receives CONNECTION FAILURE

INDICATION message from BTS because an active connection has been

broken for some reason such as SDCCH link failure or hardware failure

(see GSM 0508 for details). In incoming BSC handover procedure on SDCCH, this item is measured

in the target cell in the case of the failure of decoding HANDOVER

DETECTION and HANDOVER COMPLETE message.

In the process of incoming BSC handover on SDCCH, this item is

measured in the target cell when the timer for the target cell to wait for the

HANDOVER COMPLETE message expires.


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Measurement Point of SDCCH Call Drop In the process outgoing BSC handover on SDCCH, this item is

measured when the timer for the source cell to wait for CLEAR

COMMAND message from MSC expires (T8 expires).

In the process of intra BSC handover on SDCCH, this item is

measured in the source cell when the timer for the source cell or

the target cell to wait for HANDOVER COMPLETE message

expires.

This item is measured when the RSL link of the TRX that the call

is running on disconnect, which will cause call drop, this item

measures call drop on SDCCH for RSL disconnection.


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Measurement Point of SDCCH Call DropMS BTS BSC MSC

Or:

Or:

Channel Request Channel Required

Channel Activation (SDCCH)

Channel Activation Acknowledge

Immediate Assignment CommandImmediate Assignment

Establish Indication (L3 Info)

Connection Failure

Error Indication

Abis FailureCell SDCCH Call Drop

(Subject to different cases)


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Measurement Point of Successful

SDCCH Seizures In immediate SDCCH assignment procedure, this item is

measured when BSC receives CHANNEL ACTIVATION

ACKNOWLEDGE message from BTS.

In the process of incoming BSC handover on SDCCH. This

item is measured when BSC receives HANDOVER

DETECTION from BTS.

In the process of incoming internal inter cell handover and

intracell handover in BSC.BSC measures this item in thetarget cell when receiving HANDOVER DETECTION from BTS.


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2. Analysis of Call Drop3. Call Drop Cases


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Analysis of Call Drop content

main causes of high call drop rate

troubleshooting of high call drop rate


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Analysis of Call Drop According to the definition of call drop measurement point, call drop is

usually caused by the following:

Radio link fault. During the communication, messages can not be receivedcorrectly.

Abis link broken during conversation.

Call drop during handover.

Other system faults.

Timers that may cause call drops (BSC timer):

T3103: starting from sending HANDOVER CMD and ending at receiving

HANDOVER CMP. Time out of the timer will cause call drop.

T3109: starting from sending CHAN REL and ending at receiving REL IND.


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Radio Link Fault Signaling process chart of radio link fault

MS BTS BSCMSC

Measurement Report

Measurement Result

Connection Failure

Clear_REQ (Radio Interface Failure)

(1)

(2)

(3)

(1) Dadicated mode is created. (SDCCH/TCH)

(2) Activate Abis monitoring function.

(3)SACCH message block can not be decoded(uplink/downlink),

resulting in radio link timeout.


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Radio Link Fault Diagram of radio link timeout


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T3103 Times Out Handover process

MS BTS1 BSC MSCBTS2

Handover Indication

CH_ACT

CH_ACT_ACK

Handover Command

Handover CommandHandover Access HO_Detect

SABM

Physical Information (TA)

UA

Handover CompleteHandover Complete

EST_IND

Set T3103

Reset T3103


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Causes of Radio Link Fault The causes of radio link fault can be

Interference

Internal interference

External interference

Equipment interference

Poor coverage

Coverage hole

Isolated island

Uplink/downlink imbalance

Improper parameter setting

Radio link timeout, SACCH multi-frames

Handover parameters

Power control parameters

Equipment problem (Antenna, feeder, combiner, TRX)

Clock problem

Transmission problem


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Radio Link Fault - Interference Category

Co-channel interference

Adjacent-channel interference

Inter-modulation interference and other external interference


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Radio Link Fault - Interference Solution

First check equipment problems.

Make an drive test, check the interference area and distribution

of signal quality. Find the interference frequency.

Further search for the interference source with the spectrumanalyzer.

Activate hopping, DTX and power control functions to lower

the internal interference of the system


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Radio Link Fault - Interference Judgment Process

Analyze the occurrence regularity of interference band in the

traffic measurement.

Observe the receiving level performance

Find the poor quality handover rate

Observe receiving quality performance

Observe call drop performance

Observe whether there are many handover failures and reversion

failures.


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Radio Link Fault - Coverage Coverage:

Overshooting

Coverage hole

Signal attenuation

Incomplete definition of adjacent cells

Imbalance of uplink/downlink


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Radio Link Fault - Coverage Judgment Process

Power control measurement function

Receiving level measurement function

Cell measurement function/inter-cell handover measurement function

Call drop measurement function

Defined adjacent cell measurement function

Undefined adjacent cell measurement function

Outgoing inter-cell handover measurement function

Up-down link balance measurement function


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Radio Link Fault - Coverage Solution

Adjust network parameters

Add BTS


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Radio Link Fault - Parameters

Main parameters that may affect the call drop rate:

Radio link timeout and SACCH multi-frames

RACH busy threshold and RACH minimum access level.

MS minimum receiving signal level

Call re-establishment permitted.


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Radio Link Fault - Parameters

Main parameters that may affect the call drop rate:

NCC permitted

Handover related parameters.

Power control related parameters.

Frequency planning parameters


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Radio Link Fault

Judgment process

System information data

Cell data

Radio link connection timer (T3105)

Maximum times of physical information

Call drop performance measurement function

Judge from the cause of call drop

error indication

connection failure.


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Handover Problem

Judgment process :

In inter-cell handover measurement function, it occurs

frequently that the handover fails and the reversion also fails.

In inter-cell handover measurement function, handover failures

with successful reversions occur many times.

In undefined adjacent cell measurement function, observe the

receiving level of the undefined adjacent cells and number of

the measurement reports.


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Handover Problem

Judgment process

In outgoing inter-cell handover measurement function, the

outgoing inter-cell handover success rate is low (for a certain

cell). Find the adjacent cell to which the handover success rate

is low and find the cause.

Incoming inter-cell handover success rate is low. The handover

judgment parameter setting of the target cell may be improper.

In TCH measurement function, handover times are not innormal proportion to successful TCH seizures for call.

(handovers/calls>3)


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Equipment Problem

Call drop arising from equipment problem

Hardware problem

Transmission problem

Antenna and feeder fault

Other causes


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Contents

1. Principle of Call Drop

2. Analysis of Call Drop

3. Call Drop Cases


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Call Drop Case 1

Fault Description

The BTS distribution of an area is as illustrated in the diagram (red numbers

stand for BCCH frequencies. No hopping, no DTX). Some subscribers

complain that call drop in second sector of base station C is serious. Hardware

fault has been ruled out.

please confirm whether thefrequency distribution in the

cells are reasonable?


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Call Drop Case 1

Analysis

From the analysis of BTS topology, it can be concluded that

the frequencies are well planned.

Next, check the interference band of traffic statistic.


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Call Drop Case 1

Analysis

2.92

12.57

14.25

IB2

13.27

3.14

1.14

IB3

0.25

0.03

0.27

IB4

cell 3

cell 2

cell 1

09:00~10:00

0

4.09

2.85

IB1

0.37

0.01

0.54

IB5

2.12

2.89

4.28

IB2

0.00

0.00

0.00

IB3

0.00

0.00

0.00

IB4

cell 3

cell 2

cell 1

03:00~04:00

0

4.09

2.85

IB1

0.00

0.00

0.00

IB5


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Call Drop Case 1

Troubleshooting

Conducting a drive tests, it is found that the quality is bad when

the receiving strength is high.

Check traffic statistic and it is found that when the call drop

rate is high, handovers are mostly caused by quality reasons,

and channel assignment failure rate is also high.

The conclusion is that there is interference from the analysis of

comprehensive traffic statistic and drive test.


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Call Drop Case 1

Troubleshooting

A site investigation shows that the operator has a repeater. It

is a broadband repeater. It transmits the signals from a

remote TACS site. TACS signals are amplified and then there

is interference in second sector of base station C.

Problem has been located: interference causes the call drop.

Finally, lower the power of the repeater. The interference

band reduce to IB1. Now the high call drop rate problem at

site C is solved.


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Call Drop Case 1

Common methods of checking and clearing call drop due to interference

Rule out the internal interference caused by equipment problems and check

the separation of BTS transceivers, antenna feeder installation, and so on.

Check the interference band

Drive test

Check traffic statistic of handover causes to get judgment

Clear uplink interference

Clear downlink interference

Check whether DTX, frequency hopping technology and power control

application are reasonable

Use PBGT handover algorithm flexibly to avoid co-channel and adjacent-

channel interference effectively.

C C


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Call Drop Case 2

Fault description

The call drop rate in cell 3 of a BTS is 10% accompanied with

high congestion rate, but call drop rate and congestion rate in

cell 1 and cell 2 are normal.

C ll D C 2


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Call Drop Case 2

Analysis

Check the related traffic statistic

Check whether there is high interference band in TCH measurement

function.

Check the situation of call drop in call drop measurement function.

Check whether handover of the cell is normal.

Check whether there is interference through checking frequency

planning, moreover confirm whether there is external interference with

spectrum analyzer.

Drive test

Check the hardware

C ll D C 2


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Call Drop Case 2

Troubleshooting

Block TRX in turn and the congestion rate is always quite high no

matter which TRX is blocked.

Check and analyze the traffic statistic, interference band and traffic

volume and call drop rate, and it is found that the interference

becomes more serious as the traffic gets high.

Change frequency. The frequency of cell 3 is changed to 1MHz away

from the original value. But the problem persists.

Judge whether the equipment is faulty.

Locate external interference.

C ll D C 2


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Call Drop Case 2

Troubleshooting

Make a scanning test with a spectrum analyzer.

A suspect signal with 904.14MHz center frequency, 300K bandwidth

is found. It is similar to an analog signal and it exists continuously.

At the distributor output port of cell 3, the signal strength is 27dBm.

cell 2 is 40dBm, cell 1 is 60dBm. It accords with the degree ofinterference.

Traffic volume is higher in the day time than that at night.

Now the problem is found: 904MHz external interference source.

C ll D C 2


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Call Drop Case 2

Conclusion: solution of interference

Solve internal interference through checking frequency

planning.

After internal interference is excluded, we can locate external

interference with spectrum analyzer.

Call Drop Case 3


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Call Drop Case 3

Fault description

Subscribers complain that call drops often happen from on the

5th floor and above in a building.

Subscriber complaint is also an important source of information

about the network quality.

Call Drop Case 3


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Call Drop Case 3

Analysis

Perform on-site test

There are call drops and noise on the site

The test mobile phone shows that before the call drop the serving

cell is BTS-B. But this building should be covered by BTS-A.

Check traffic statistic

BTS-B is about 9 kilometers away from this building. It is

determined that the BTS-B signal received in this area is coming

from some obstacles reflection. Thus an isolated island coverage

is formed in this area.

Call Drop Case 3


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Call Drop Case 3

Analysis

Check data configuration

In BSC data configuration, BTS-A is not configured as the adjacency of

BTS-B

Cause analysis of call drop

When the MS uses the signal of cell 2 of BTS-B in this area, the signal of

cell 3 of BTS-A is strong. But cell 2 of BTS-B and cell 3 of BTS-A are not

adjacent, therefore, handover cannot happen.

The signal in cell 2 of BTS-B is the result of multiple reflections. When the

signal of BTS-B received by the mobile phone gets weak suddenly, an

emergency handover is needed. But there is no adjacent cell of BTS-B, so

call drops will occur.

Call Drop Case 3


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Call Drop Case 3

Troubleshooting

Modify the data in BA1 table, BA2 table and add adjacent cell

relationship, set cell 3 of BTS-A as an adjacent cell of cell 2 of

BTS-B.

Optimize the network parameters to eliminate the isolated island.

The test results show that the call drop problem is solved.

Call Drop Case 3


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Call Drop Case 3

Conclusiontwo methods to solve isolated island problem

Adjust the antenna of the isolated cell, to eliminate the isolated

island problem.

Define new adjacent cells for the isolated cell.

Call Drop Case 4


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Call Drop Case 4

Fault description

In a drive test from A to B, it is found that there are many call

drops at entrance of a tunnel near a BTS due to slow handover.

Call Drop Case 4


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Call Drop Case 4

Analysis

The tunnel is near the BTS. When the MS enters the tunnel, the

power of the target cell is -80dBm. But the signal of source cell goes

down quickly to less than -100dBm. Before the MS enters the tunnel,

the downlink power of the two cells is good and no handover is

triggered. When the MS enters the tunnel, the level of the source cell

goes down rapidly. The call drop occurs before any handover is

triggered.

Think it over: How to solve problems of this type?

Call Drop Case 4


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Call Drop Case 4

Troubleshooting

The adjusted parameters are listed below.

1510Min. DL level on candidate cell

6070UL Qual. Thrsh. (Emergencyhandover)

6872PBGT HO threshold24PBGT valid time

35PBGT watch time

Value afterchange

Value beforechange

Parameter name

Call Drop Case 4


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Call Drop Case 4

Conclusion: optimize and adjust handover parameters to

reduce call drop

On condition that there is no ping-pang handover and

excessive voice interruption, PBGT handover can help to

reduce interference and lower call drop rate.

Set emergency handover thresholds properly, and make sure

the emergency handover can be triggered in time before the

call drop so as to reduce call drops.

Call Drop Case 5


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Call Drop Case 5

Fault description

In the dial test, many call drops are found in cell 2.

Analysis

Check the traffic statistic and find out that TCH congestion rate

of this cell is over 10% and internal inter-cell handover failurerate is high. It is found that one TRX board of this cell is

abnormal in OMC. A preliminary conclusion is that TRX board

problem causes the call drop.

Call Drop Case 5


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Call Drop Case 5

Troubleshooting

Lock the frequency with a test mobile phone and perform dial

test for many times. It is found that call drops only happen in

timeslots 1, 3, 5, 7 while communications in timeslots 2, 4, 6, 8

are normal.

Move this board to another slot, and the problem still exists.

Move another good board to this slot, and the communication

is normal.

Move this defective board to other cabinet, the problem arises.

When it is replaced, the communication is recovered.

Call Drop Case 5


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Call Drop Case 5

Conclusion

The BTS test should guarantee that communication should be

successful not only in each TRX but also in each timeslot of

the TRX.

It must be ensured that each TCH channel provides

bidirectional high quality communication.

Call Drop Case 6


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Call Drop Case 6

Fault description

In dual-band network, when a call is setup in a GSM1800 cell

and being handed over to a GSM900 cell in the same site, the

call drops in the GSM900 cell in 2 to 5 seconds. The call drop

rate in the GSM900 cell is quite high.

Call Drop Case 6


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Call Drop Case 6

Analysis

In the test it is found that the clock of GSM900 cell and

GSM1800 cell are not synchronized.

When a call set up in a GSM1800 cell and is handed over to a

GSM900 cell, the drive test tool shows that FER increases to

the maximum value suddenly and then it goes down to zero

gradually.

It is the same with the handover from GSM900 to GSM1800.

Call Drop Case 6


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Ca op Case 6

Troubleshooting

After adjusting GSM900 clock system, the abnormal call drop

problem is solved.

Call Drop Case 6


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p

Conclusion

Clocks of GSM900 and GSM1800 should be exactly

synchronized with each other in a dual band network,

otherwise there will be call drops and handover failures.

Summary


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y

Types of call drop

Measurement points of TCH call drop

Measurement points of SDCCH call drop

Causes of call drop

Cases


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Case AnalysisCall Drop

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