Call Drop Analysis for TCH and SDCCH-2

Huawei Confidential. All Rights Reserved

OMF000404Case Study – Call Drop

Issue 2.0

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References References

31160978-BSC Traffic Statistic Manual Volume I

31033203-BSS Troubleshooting Manual

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ObjectivesObjectives

Upon completion of this course, you are supposed to be able to:

Understand the principles of call drop.

Analyze and solve call drop problems

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Chapter 1 Chapter 1 Principle of call dropPrinciple of call drop

Chapter 2 Analysis of call drop

Chapter 3 Call drop cases

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Section 1 Definition of call drop

Section 2 TCH call drop

Section 3 SDCCH call drop

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Definition of Call DropDefinition 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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Causes of TCH Call DropCauses of TCH Call Drop

Usually, the typical causes for sending the Clear_Request message are as follows:

Radio interface failureO&M interventionEquipment failureProtocol errorPreemption

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Formula of TCH Call Drop RateFormula of TCH Call Drop Rate

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

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Measurement Point of TCH Call DropMeasurement Point of TCH Call DropMeasurement Point of TCH Call Drop

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Measurement Point of TCH Call DropMeasurement 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 DropMeasurement 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 (due to 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 TCH Call Drop -- AssignmentAssignment

MS BTS BSC

ASSIGN CMMAND

MSC

ASSIGNMENT REQUEST

SABM

UAESTABLISH INDICATION

CHANNEL ACTICATION

CHANNEL ACTIVATION ACK

A1ERROR INDICATION

CONNECTION FAILURE INDICATION

A2

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TCH Call Drop TCH Call Drop –– IntraIntra--BSC HandoverBSC Handover

MS BSC' BSC Ori-Cell

HANDOVER ACCESS

UA

Intercell Handover Request

BSC Tag-Cell BSC''

HANDOVER COMMAND

Intercell Handover Response

HANDOVER DETECT

CH ACT

CH ACT ACK

A1

ERROR INDICATION


A2

SABM

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TCH Call Drop TCH Call Drop -- Incoming BSC Handover Incoming BSC Handover

MS Other BSC

HANDOVER ACCESS

UA

HANDOVER REQUIRED

HUAWEI BSC HUAWEI BTS

HANDOVER COMMANDHANDOVER REQUEST ACK

HANDOVER DETECT

CH ACTCH ACT ACK

MSC

A1ERROR INDICATION


A2

SABM

HANDOVER REQUEST

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Measurement Point of Successful TCH Seizures Measurement Point of Successful TCH Seizures

Measurement point of Successful TCH seizures Upon BSC’s 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

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 BSC receives HANDOVER DETECT message from BTS.In the process of incoming internal inter cell handover and intracellhandover 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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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)

Successful TCH Seizure – Very Early AssignmentSuccessful TCH Seizure Successful TCH Seizure –– Very Early AssignmentVery Early Assignment

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Successful TCH Seizure Successful TCH Seizure –– AssignmentAssignment

Assignment process

MS BTS BSC MSCASS_REQ

CH_ACT

CH_ACT_ACK

ASS_CMDASS_CMD

EST_IND

ASS_CMP ASS_CMPASS_CMP

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Successful TCH Seizure Successful TCH Seizure –– IntraIntra--BSC HandoverBSC Handover

Intra-BSC handover process

MS MSBTS1 BTS2BSCMeasurement Report from MS

Channel_Active

Channel_Active ACKHANDOVER COMMAND

HO_ Access

HO_DetectPHY INFO

First SABMEstablish_IND

PHY INFO

Handover Complete HO_Performed

UA

MSC

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Successful TCH Seizure Successful TCH Seizure –– InterInter--BSC HandoverBSC Handover

Inter BSC handover process

MS MSBTS1 BTS2BSC1 BSC2MSCMeasure Report from MS

HO_RequiredHO_Request

CH_ACTCH_ACT_ACKHO_Request_ACK

HO_CMDHandover Command HO_AccessHO_Detect

HO_DetectPHY INFOPHY INFO

First SABMEstablish_IND

Handover CompleteHO_CMPClear_CMD

Clear_CMP

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Formula of SDCCH Call Drop RateFormula of SDCCH Call Drop Rate

Formula of SDCCH call drop rate: SDCCH call drop rate = SDCCH call drops/ successful SDCCH seizures*100%

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Measurement Point of SDCCH Call DropMeasurement Point of SDCCH Call Drop

This item is measured when BSC receives ERROR INDICATION messagefrom 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 DropMeasurement 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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MS 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 Failure Cell SDCCH Call Drop (Subject to different cases)

Measurement Point of SDCCH Call DropMeasurement Point of SDCCH Call DropMeasurement Point of SDCCH Call Drop

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Measurement Point of Successful SDCCH SeizuresMeasurement 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 intracellhandover in BSC.BSC measures this item in the target cell when receiving HANDOVER DETECTION from BTS.

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Chapter 1 Principle of call drop

Chapter 2 Chapter 2 Analysis of call dropAnalysis of call drop

Chapter 3 Call drop cases

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Analysis of Call DropAnalysis of Call Drop

contentmain causes of high call drop ratetroubleshooting of high call drop rate

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Analysis of Call DropAnalysis 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 bereceived correctly.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.

user

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Radio Link FaultRadio Link Fault

Signaling process chart of radio link fault

MS BTS BSC MSCMeasurement 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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Diagram of radio link timeout

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T3103 Times OutT3103 Times Out

Handover process

MS BTS1 BSC MSCBTS2Handover Indication

CH_ACT

CH_ACT_ACK

Handover CommandHandover Command

Handover 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 FaultCauses of Radio Link Fault

The causes of radio link fault can beInterference

Internal interferenceExternal interferenceEquipment interference

Poor coverage Coverage holeIsolated islandUplink/downlink imbalance

Improper parameter settingRadio link timeout, SACCH multi-framesHandover parametersPower control parameters

Equipment problem (Antenna, feeder, combiner, TRX)Clock problemTransmission problem

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Radio Link Fault Radio Link Fault -- InterferenceInterference

CategoryCo-channel interferenceAdjacent-channel interferenceInter-modulation interference and other external interference

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SolutionFirst 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 spectrum analyzer. Activate hopping, DTX and power control functions to lower the internal interference of the system

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Judgment ProcessAnalyze the occurrence regularity of interference band in the traffic measurement.Observe the receiving level performanceFind the poor quality handover rateObserve receiving quality performanceObserve call drop performanceObserve whether there are many handover failures and reversion failures.

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Radio Link Fault Radio Link Fault -- CoverageCoverage

Coverage:OvershootingCoverage holeSignal attenuationIncomplete definition of adjacent cellsImbalance of uplink/downlink

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Radio Link Fault Radio Link Fault -- CoverageCoverage

Judgment ProcessPower control measurement functionReceiving level measurement functionCell measurement function/inter-cell handover measurement functionCall drop measurement functionDefined adjacent cell measurement functionUndefined adjacent cell measurement functionOutgoing inter-cell handover measurement functionUp-down link balance measurement function

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Radio Link Fault - CoverageRadio Link Fault Radio Link Fault -- CoverageCoverage

SolutionAdjust network parametersAdd BTS

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Radio Link Fault Radio Link Fault -- ParametersParameters

Main parameters that may affect the call drop rate:Radio link timeout and SACCH multi-framesRACH busy threshold and RACH minimum access level.MS minimum receiving signal levelCall re-establishment permitted.

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Radio Link Fault Radio Link Fault -- ParametersParameters

Main parameters that may affect the call drop rate:NCC permittedHandover related parameters. Power control related parameters. Frequency planning parameters

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Judgment processSystem information dataCell dataRadio link connection timer (T3105)Maximum times of physical informationCall drop performance measurement functionJudge from the cause of call drop

error indication connection failure.

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HandoverHandover ProblemProblem

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 Handover ProblemProblem

Judgment processIn outgoing inter-cell handover measurement function, the outgoing inter-cell handover success rate is low (for a certain cell). Find theadjacent 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 in normal proportion to successful TCH seizures for call. (handovers/calls>3)

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Equipment ProblemEquipment Problem

Call drop arising from equipment problemHardware problem Transmission problem Antenna and feeder faultOther causes

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Chapter 1 Principle of call drop

Chapter 2 Analysis of call drop

Chapter 3 Chapter 3 Call drop casesCall drop cases

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Call Drop Case 1Call Drop Case 1

Fault DescriptionThe 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 the frequency distribution in the

cells are resonable?

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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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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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TroubleshootingConducting 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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TroubleshootingA 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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Common methods of checking and clearing call drop due to interferenceRule out the internal interference caused by equipment problems and check the separation of BTS transceivers, antenna feeder installation, and so on.Check the interference bandDrive testCheck traffic statistic of handover causes to get judgmentClear uplink interferenceClear downlink interferenceCheck whether DTX, frequency hopping technology and power control application are reasonableUse PBGT handover algorithm flexibly to avoid co-channel and adjacent-channel interference effectively.

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Fault descriptionThe 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.

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AnalysisCheck 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 testCheck the hardware

user

Highlight

user

Highlight

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TroubleshootingBlock 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.

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TroubleshootingMake a scanning test with a spectrum analyzer.

A suspect signal with 904.14MHz center frequency, 300K bandwidthis 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 of interference.Traffic volume is higher in the day time than that at night.

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

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Conclusion: solution of interferenceSolve internal interference through checking frequency planning.After internal interference is excluded, we can locate externalinterference with spectrum analyzer.

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Fault descriptionSubscribers complain that call drops often happen from on the 5th floor and above in a building.

Subscriber complaint is also an important source of informationabout the network quality.

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AnalysisPerform on-site test

There are call drops and noise on the siteThe 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.

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AnalysisCheck data configuration

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

Cause analysis of call dropWhen 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.

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TroubleshootingModify 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.

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Conclusion：two methods to solve isolated island problemAdjust the antenna of the isolated cell, to eliminate the isolated island problem.Define new adjacent cells for the isolated cell.

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Fault descriptionIn 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.

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AnalysisThe 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?

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Troubleshooting The adjusted parameters are listed below.

1510Min. DL level on candidate cell

6070UL Qual. Thrsh. (Emergency handover)

6872PBGT HO threshold

24PBGT valid time

35PBGT watch time

Value after change

Value before changeParameter name

Call Drop Case 4Call Drop Case 4Call Drop Case 4

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Conclusion: optimize and adjust handover parameters to reduce call dropOn 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.

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

AnalysisCheck the traffic statistic and find out that TCH congestion rate of this cell is over 10% and internal inter-cell handover failure rate 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.

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TroubleshootingLock 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.

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ConclusionThe 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.

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Fault descriptionIn 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.

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AnalysisIn 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.

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TroubleshootingAfter adjusting GSM900 clock system, the abnormal call drop problem is solved.

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ConclusionClocks of GSM900 and GSM1800 should be exactly synchronized witheach other in a dual band network, otherwise there will be call drops and handover failures.

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SummarySummary

Types of call dropMeasurement points of TCH call dropMeasurement points of SDCCH call dropCauses of call dropCases

SummarySummary

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Date post:	26-Oct-2014
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Call Drop Analysis for TCH and SDCCH-2

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