omd6068 case analysis--call drop issue1.1

8/13/2019 OMD6068 Case Analysis--Call Drop ISSUE1.1

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OMD6068Case Analysis--Call Drop

ISSUE1.1

Wireless Curriculum

Development Section


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Summary

Principle of call drop rate calculationThe cause analysis of call dropTypical call drop cases


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Principle of Call Drop Rate Calculation

P key points:

Master the calculation formula of TCH call drop rate and

statistic points.Master the calculation formula of SDCCH call drop rate and

statistic points.


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Overview

There are two types of call drops: 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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General Concepts of TCH call drop

Formula of TCH call drop rate

TCH call drop rate=Number of TCH call drops/Number of

TCH occupied successfully 100%


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General Concepts of TCH call drop Rate

Statistic point of TCH call drops

When BSC originates CLEAR_REQ message to MSC, the channeloccupied is TCH.


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Usually,The typical cause values for sending the Clear_Request message

are:

Radio interface message failure

O&M intervention

Equipment failure

Protocol error between BSS and MSC

Preemption


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Analysis of TCH call drop RateStatistic Points

Statistic Point of TCH Occupied Successfully

(a)when CH_ACT_ACK message is received in the immediate

assignment process. Since there is no available SDCCH channel,

TCH channel is directly assigned.

(b) When CH_ACT_ACK message is received when the calling

status is CS_WAIT_RR_EST(Waiting RR setup status), and the

present channel is TCH.

(c) When Assignment complete message is sent in process of

assignment.


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Analysis of TCH call drop Rate StatisticPoints

Statistic Point of TCH Occupied Successfully

d When the MSG_ABIS_HO_DETECT message is received during inter-

coming BSC handover, and the handover type is non-SDCCH handover;

e When the MSG_ABIS_HO_DETECT message is received during intra-

BSC handover, and the handover type is non-SDCCH handover;

f When the CLEAR-CMD message with the cause values of HO_SUCC or

CALL_CTRL is received from the MSC during inter-BTSC handover , and thehandover cause is directed retry;


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Analysis of Call Flow Chart and TCH call dropStatistic Points (I)

Immediate assignment flow chart:


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Analysis of Call Flow Chart and TCH call dropStatistic Points (II)


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Analysis of Call Flow Chart and TCH call drop Statistic Points(III)

Assignment flow chart

MS BTS BSC MSC

Est_Ind

Ch_Act

Ch_Act_Ack

Ass_Req

Ass_Cmd

Ass_Cmp

Ass_Cmd

Ass_Cmp Ass_cmp


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Analysis of Call Flow Chart and TCH call drop Statistic Points(IV)

Intra-BTSC handover flow


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Analysis of Call Flow Chart and TCH call drop Statistic Points(V)

Inter-BTSC handover flow


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Analysis of Call Flow Chart and TCH call drop Statistic Points(VI)

Directional re-try flow

When the CLEAR-CMD message with the cause values of

HO_SUCC or CALL_CTRL is received from the MSC during

inter-BSC handover , and the handover cause is directed retry,one statistic point is counted and added to the times of TCH

occupation success.


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General Concepts of SDCCH call dropRate

Formula of SDCCH call drop rate

SDCCH call drop include:Number of SDCCH lost connections (connect failure)Number of SDCCH lost connections (error indications)Number of unsuccessful SDCCH seizures due to terrestrial (ABIS)

SDCCH call drop rate=Number of SDCCH call drops/Totalsuccessful SDCCH seizures*100


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Analysis of SDCCH call drop Rate StatisticPoint (I)

Statistic point of SDCCH call dropsWhen the CLEAR_REQ and ERR_IND messages are originated to the MSC,

the channel currently seized is the SDCCH.

Statistic point of successful SDCCH seizures (1) CH_ACT_ACK is received in the immediate assignment process. Thechannel type is SDCCH(2) CH_ACT_ACK message is received in CS_WAIT_RR_EST status and thecurrent channel is SDCCH(3) HO_DETECT is received when incoming SDCCH handover is performed.(4) HO_DETECT is received when intra-BTSC SDCCH handover isperformed.


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Analysis of SDCCH call drop Rate StatisticPoint (II)


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Analysis of SDCCH call drop Rate StatisticPoint (III)

The following conditions can lead to SDCCH call drop:When the HO_DETECT message is illegal during incoming SDCCH

handoverWhen the HO_CMP message is illegal during incoming SDCCH

handoverWhen HO_CMP message transfer fails during incoming SDCCH

handoverWhen TN_WAIT_HO_DETECT, and TN_WAIT_HO_CMP (SDCCHhandover) are overtime

When TN_WAIT_INTER_HO_CMP(SDCCH handover) is overtimeWhen TN_T8 (Out-BTSC handover complete) is overtimeWhen internal clearing is caused by other causes


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Overview of Causes of call drop

According to the definition of call drop statistic point, call drop is usuallycaused by the following:

Radio link fault. During the communication, messages can not be receivedcorrectly. T3103 counter gives no response after timeout Other system faults (for example, the matching between BSC timer and MSCtimer.

Timers that may cause call drops (BSC timer):T3109 T3103

T3103Time counting from sending up HANDOVER CMD start to receive HANDOVERCMP end or overtime

T3109

Time counting from sending up CHAN REL to receive REL IND end or overtime


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Wireless Link Fault

Signaling flow chart of radio link fault:


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Wireless Link Fault

Diagram of radio link timeout


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T3103 Counter Has No Response After Timeout

Handover flow (T3103 time counting):


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Analysis of Causes of Wireless Link Fault---Interference

Interference

Co-channel interference

Adjacent-channel interference

Intermodulation interference and other external

interferences


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Analyze and Locate TCH call drop Problem (Interference)

Solution

First check equipment problems.

Make an actual 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 rapidly lower the

internal cross-interference of the system


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Analyze and Locate TCH call drop Problem (Interference)

Process of judgment

1. Analyze the occurring laws of interference band in the

traffic measurement.

2. Observe the receiving level performance3. Find the poor quality handover ratio

4. Observe receiving quality performance

5. Observe call drop performance

6. The handover fails, calling re-establishment

also fail too many times.


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Analysis of Causes of Wireless Link Fault---Coverage

Coverage:

1. Cross-cell coverage

2. Blind area coverage

3. Signal attenuation

4. Incomplete definition of adjacent cells

5. Imbalance of uplink/downlink


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Analyze and Locate TCH call drop Problem (Coverage)

Process of judgment

Power control performance measurement

Receiving level performance measurement

Cell performance measurement/inter-cell handover performancemeasurement

call drop performance measurement

Adjacent cell performance measurement

Measurement of undefined adjacent cell average levelPower control performance measurement

Out-cell handover performance measurement

Uplink/downlink balance performance measurement


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Analyze and Locate TCH call drop Problem (Coverage)

Solution

Adjust network parameter

Add BTS


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Analysis of Causes of Wireless Link Fault---ParameterSetting

Main parameters that may affect the call drop rate:1. Radio link timeout counter and number of SACCH multi-frames2. RACH busy threshold and RACH minimum access level.3. MS minimum receiving signal level4. Call re-establishment permission.5. Nation color code (NCC) permission

6. Frequency planning parameters7. Handover related parameters. Refer to Case 6 8. Power control related parameters. Refer to Case 5 9. Version related parameters. Refer to Case 7


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Analyze and Locate TCH call drop Problem (Radio Parameter Setting)

Locate and Clear Fault:

System message data table

Cell attribute table

Radio link connection counter

max. repeat times of Physical information

call drop rate performance measurement

Judge from the cause of call drop: error indication and connection failure.


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Analyze and Locate TCH call drop Problem (Handover ParameterSetting)

Process of Judgment

Inter-cell handover performance measurement: it occurs frequently thatthe handover fails and the calling re-establishment also fails.

Inter-cell handover performance measurement: handover occurs many

times and re-establishment succeeds many times.

Measurement of undefined adjacent cell performance: level of the

undefined adjacent cells and number of reports.

Out-cell handover performance measurement: the success ratio of out-

cell handover is low (for a specific cell). Find the adjacent cell where the

handover success ratio is low and find the cause.

In-cell handover success ratio is low. The handover verdict parameter

setting of the opposite cell is not proper.

TCH performance measurement: handover times are not in proportion tothe success times of TCH call occupation.(handover/call>3)


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Analyze and Locate TCH call drop Problem (Handover ParameterSetting)

Solution:

Add adjacent cells as necessary

Adjust improper handover parameters

Hando

ver

Parameter name

Value before

change

Value after

change

PBGT watch time 5 4

PBGT valid time 4 3

PBGT HO threshold 72 68

UL qual. threshold 70 60

Min. DL level candidate cell 10 15


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Analyze and Locate TCH call drop Problem (Power ControlParameter Setting)

Power

control

Parameter nameValue before

change

Value after

change

UL Qual. Or RX_LEV

compensation50 80

PC interval 8 5

filter length for stable 10 6


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Analysis of Causes of Wireless Link Fault---Equipment Problem

call drop arising from equipment problem

Hardware problem

Transmission problem

Antenna & feeder fault

Other causes


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Analyze and Locate TCH call drop Problem by TrafficMeasurement (Equipment Fault)

Process of judgment

TCH performance measurement

Excessive failure times of A interface when TCH is occupied

TCH availability abnormal.

Excessive call drop and disconnection times of terrestrial links.

If a cell always suffers from high call drop rate and congestion rate, some

equipment in this cell may be faulty.


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Typical call drop Cases

Case 1

l Fault Description

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

numbers stand for BCCH frequency. No hopping, DTX). Some users

complained that call drop in cell 2 of base station C is serious.

(hardware fault is ruled out).Step 1: please

confirm whetherthe frequency

distribution in theBTS cells are

correct?


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Case Analysis

Here is the conclusion from the analysis of BTS topology: the frequency are well

planned.

Next step: View the interference band of traffic measurement:

09:00~10:00 IB1 IB2 IB3 IB4 IB5

cell 1: 2.85 14.25 1.14 0.27 0.54

cell 2: 4.09 12.57 3.14 0.03 0.01

cell 3: 0 2.92 13.27 0.25 0.37

03:00~04:00 IB 1 IB2 IB3 IB4 IB5

cell 1: 2.85 4.28 0.00 0.00 0.00

cell 2: 4.09 2.89 0.00 0.00 0.00

cell 3: 0 2.12 0.00 0.00 0.00

Is there anything abnormalindicated by the interference?

What does it imply?


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Troubleshooting Procedure

Finally, lower the power of the repeater and it works. The

interference band is lowered immediately from IB2, IB3 to IB1.

Now the high call drop rate problem at site C is solved.


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Summary

Discussion

Common methods and procedures ofChecking and clearing call drop due to interference.


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Case 2

Fault Description

1 3 RF hopping is used in a specific site. After expansion, TCH channel allocation

failure rate is continuously high (due to radio link fault), accompanied by high TCH

call drop rate and incoming handover failure rate. SDCCH call drop rate is normal.

Since the channel allocation failure rate and

Incoming handover failure rate are high, what are the causes?


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Case Analysis

Since assignment failure is accompanied with high call drop

rate and incoming handover failure rate, basically the causes may

be as follows:

A problem occurs when TCH channel is assigned

The band or timeslot occupied by this communication is

not stable or affected by interference

Since SDCCH call drop rate is normal, it is unlikely that the

interference comes from the carrier of BCCH band and the BCCH

band. Accordingly, the carrier of non-BCCH band and hopping band

may attribute to the interference.


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Case Analysis

Find the problem:1. Check the equipment hardware, antenna & feeder and the

transmission stability. No problem is found.2. In the drive test, it is found that high level and low quality

problem is very serious.3. Make a site calling test to find that the communication quality is

poor.4. In checking the parameter, it is found that the MAIO of the new

carrier is the same as that of another carrier.The fault: There exists conflict of hopping bands.


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Case Analysis

Troubleshooting:

Set a new value for the MAIO of the new carrier. The related

indexes such as call drop rate are normal.

What other hopping parameters may cause theco-channel or adjacent-channel conflict?


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Case III

Fault description

In a drive test, it is found that an MS occupies a cell but can not

call out. Communication is unidirectional. call drop occurs

frequently at a specific distance from the cell and after

frequently handover.

AnalysisIt may be caused by ?


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Case Analysis

The drive test phenomena as mentioned above is usuallycaused by:Uplink/downlink imbalanceIt is possible that the power is unbalanced because uplink signalis much lower than the downlink signal.

Make a driving testDuring the test, move the MS to the cell boundary. At the sametime, trace and capture data at the BTS side with an MA10signaling analyzer. (see the figure below).

l


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Case Analysis

Capture data with MA10:


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Case Analysis

Judge, check and clear the problem of unlink/downlink

imbalance

1. Check whether the service area of the cell is too large.2. When the unlink power control is enabled, improper settingOf power control parameters will also cause obvious unbalancedPower.


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Summary

call drop due to unbalanced uplink/downlink

1. If the cell coverage is tooLarge, reduce the BTS transmit power or increase MS access threshold

And handover threshold of the cell.

2. AdjustHigher the uplink compensation factor and shorten the stable signalFilter.


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Case Analysis

Fault analysis and clearing1. The congestion rate remains high even if the RC channel of the cell is

blocked in rotation.

2. Check and analyze the traffic measurement task data, interference band

and traffic and call drop rate. They are all regular.

3. Change band. Change the band of CELL3 to 1M away from the previous

band. But the problem persists.

4. Judge whether The equipment itself is faulty.

5. Locate external interference.


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Case Analysis

5. Make a scanning test with a spectrum analyzer.

A suspect signal with 904.14 center frequency, 300K spectrum

bandwidth is found. It is similar to an analog spectrum and it exists

continuously.

At the distributor output port of CELL3, the signal intensity is 27dBm. CELL2 is 40dBm, CELL3 is 60dBm. It accord with the

degree of interference.

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

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


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Case Analysis

Perform a drive test with a spectrum analyzer to locate the problem.

Perform all tests on top of buildings.

Acknowledge interference signal by interrupting test.


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l


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Case Analysis

Step 1: Perform site test

There is call drop and noise on the site

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

is always in a service zone of a remote BTS-A.

Step 2: Check traffic measurementMake sure the cell in the BTS-B is 3-4 kilometers from this

building. It is determined that the BTS cell signal received in this

area is the reflection of a obstacle . Thus an isolated island

coverage is formed in this area.

C A l i


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Case Analysis

Step 3: Check data configurationIn BSC data configuration, only cell 2 but not cell 3 of BTS-A is configuredas the adjacency of BTS-B

Analyze the call dropWhen the MS uses the signal of cell 2 of BTS-B in this area, the signal ofcell 3 of BTS-A is strong. But cell2 of BTS-B and cell3 of BTS-A are notadjacent, therefore, handover fails.

The signal in cell 2 of BTS-B is the result of multiple reflections. When thesignal of BTS-B received by the mobile phone is weakened suddenly, anemergency handover is needed. However, cell2, cell3 of BTS-A are notbest candidate cells for cells of BTS-B, so the signal may be handed overother BTS-C. But the mobile phone can not receive signals from BTS-C,hence the call drop occur.

C A l i


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Case Analysis

SolutionModify the data in BA1 (BCCH) table, BA2 (SCCH) table and cell

adjacency relation table in BSC data configuration.

Set cell 3 of BTS-A as an adjacent cell of cell 2 of BTS-B.

Optimize the network parameter to eliminate the island effect.

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

S


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Summary

Commentary

Two methods to solve island problem:

Adjust the antenna of the isolated cell, to eliminate the island problem.

Define new adjacent cells for the isolated cell.

T i l ll d C


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Case 6 Optimize handover parameter to reduce call drop

In a drive test from A to B, it is found that the call drop occurs many

times at the tunnel near the BTS due to the failure of handover

immediately.


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C A l i


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Case Analysis

Modify related parameter table as follows:

hand

over

Parameter name Value before

change

Value after

change

PBGT watch time 5 3PBGT valid time 4 2

PBGT threshold 72 68

C A l i


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Case Analysis

Hand

over

Parameter name Value before

change

Value after

change

UL qual. threshold of

emergency handover

70 60

Min. DL level on candidate cell 10 15

Modify related parameter table as follows:

S


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Summary

Optimize and adjust handover parameter to reduce call drop

On condition that there is no ping-pong handover and excessive voice

interruption, PBGT handover take places easily, so as to resist

interference and lower call drop rate.Set emergency handover trigger threshold properly, make sure the

emergency handover is triggered correctly before the call drop so as to

reduce call drops.



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Case 7 call drop due to faulty TRX board

Fault description

In the calling test, it is found that call drop occurs frequently in cell 2.

Fault analysis

Check the traffic measurement to find that TCH congestion ratio of this cell is

over 10% and inward handover failure rate is high. Check the remote

maintenance console to find that one TRX board of this cell is abnormal. A

preliminary conclusion is that board problem causes the call drop.



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Fault clearingLock the band with a test mobile phone and perform dial test formany times. It is found that call drop only occurs in timeslots 1, 3, 5,7while communication is timeslots 2, 4, 6, 8 are normal. Move thisboard to another slot, and the problem exists. Move other goodboards to this slot, and the communication is normal.Move this

defective board to other cabinet, the problem arises. This TRXproves defective. When it is replaced with a standby board, thecommunication is recovered.



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Comments:The BTS side test should guarantee that communication should

be successful not only in each RC but also in each timeslot of

each RC. It must ensured that each TCH channel provides

bidirectional communication and the communication is of highquality.



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Case 8 call drop due to clock problem

Fault description

A cell 1800A in an area is connected to the network. When

communication is set up in a cell of Site B, the call is handed over

to the 900M cell located the same site address. After 2~5

seconds, call drop is found in 900M, and at a high call drop rate.


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Comments

Dual frequency requires that the clocks of 900 and 1800

should be exactly synchronous.


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omd6068 case analysis--call drop issue1.1

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