Case Study – Call Drop
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Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
References
31160978-BSC Traffic Statistic Manual Volume I
31033203-BSS Troubleshooting Manual
Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Page1
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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Page2
Contents
1. Principle of Call Drop
2. Analysis of Call Drop
3. Call Drop Cases
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Page3
Contents
1. Principle of Call Drop
1.1 Definition of Call Drop
1.2 TCH Call Drop
1.3 SDCCH Call Drop
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Page4
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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Page5
Contents
1. Principle of Call Drop
1.1 Definition of Call Drop
1.2 TCH Call Drop
1.3 SDCCH Call Drop
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Page6
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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Page7
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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Page8
Measurement Point of TCH Call Drop
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Page9
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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Page10
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 (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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Page11
TCH Call Drop - Assignment
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Page12
TCH Call Drop – Intra-BSC Handover
MS
BSC'
BSC Ori-Cell
BSC Tag-Cell
BSC''
Intercell Handover Request
CH ACT
CH ACT ACK
Intercell Handover Response
HANDOVER COMMAND
HANDOVER ACCESS
HANDOVER DETECT
SABM
UA
ERROR INDICATION
A1
CONNECTION FAILURE INDICATION
A2
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Page13
TCH Call Drop - Incoming BSC Handover
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Page14
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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Page15
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 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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Page16
Successful TCH Seizure–Very Early Assignment
Very early assignment process
MS
BTS
Channel Request (RACH)
BSC
MSC
Channel Required
Channel Activation (TCH or SDCCH)
Channel Activation Acknowledge
Immediate Assignment Command
Immediate Assignment (AGCH)
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Page17
Successful TCH Seizure – Assignment
Assignment process
MS
BTS
BSC
MSC
ASS_REQ
CH_ACT
CH_ACT_ACK
ASS_CMD
ASS_CMD
EST_IND
ASS_CMP
ASS_CMP
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ASS_CMP
Page18
Successful TCH Seizure– Intra-BSC Handover
Intra-BSC handover process
MS
BTS1
BSC
BTS2
MS
Measurement Report from MS
Channel_Active
Channel_Active ACK
HANDOVER COMMAND
HO_ Access
HO_Detect
PHY INFO
PHY INFO
First SABM
UA
Establish_IND
Handover Complete
HO_Performed
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Page19
MSC
Successful TCH Seizure – Inter-BSC Handover
Inter BSC handover process
MS
BTS1
BSC1
MSC
BSC2
BTS2
MS
Measure Report from MS
HO_Required
HO_Request
CH_ACT
Handover Command
HO_Request_ACK
HO_CMD
CH_ACT_ACK
HO_Access
HO_Detect
HO_Detect
PHY INFO
PHY INFO
Establish_IND First SABM
HO_CMP
Handover Complete
Clear_CMD
Clear_CMP
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Page20
Contents
1. Principle of Call Drop
1.1 Definition of Call Drop
1.2 TCH Call Drop
1.3 SDCCH Call Drop
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Page21
Formula 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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Page22
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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Page23
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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Page24
Measurement Point of SDCCH Call Drop
MS
BTS
BSC
MSC
Channel Request Channel Required
Channel Activation (SDCCH)
Channel Activation Acknowledge
Immediate Assignment
Immediate Assignment Command
Establish Indication (L3 Info)
Or:
Or:
Connection Failure
Error Indication
Abis Failure
Cell SDCCH Call Drop
(Subject to different cases)
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Page25
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 the
target cell when receiving HANDOVER DETECTION from BTS.
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Page26
Contents
1. Principle of Call Drop
2. Analysis of Call Drop
3. Call Drop Cases
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Page27
Analysis of Call Drop
content
main causes of high call drop rate
troubleshooting of high call drop rate
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Page28
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 received
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.
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Page29
Radio Link Fault
Signaling process chart of radio link fault
MS
BTS
BSC
MSC
(1)
Measurement Report
(2)
Measurement Result
Connection Failure
Clear_REQ (Radio Interface Failure)
(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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Page31
T3103 Times Out
Handover process
MS
BTS2
BTS1
BSC
MSC
Handover Indication
CH_ACT
CH_ACT_ACK
Handover Command
Handover Command
Handover Access
Set T3103
HO_Detect
Physical Information (TA)
SABM
EST_IND
UA
Handover Complete
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Handover Complete
Reset T3103
Page32
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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Page33
Radio Link Fault - Interference
Category
Co-channel interference
Adjacent-channel interference
Inter-modulation interference and other external interference
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Page34
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 spectrum
analyzer.
Activate hopping, DTX and power control functions to lower
the internal interference of the system
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Page35
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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Page36
Radio Link Fault - Coverage
Coverage:
Overshooting
Coverage hole
Signal attenuation
Incomplete definition of adjacent cells
Imbalance of uplink/downlink
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Page37
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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Page38
Radio Link Fault - Coverage
Solution
Adjust network parameters
Add BTS
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Page39
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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Page40
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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Page41
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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Page42
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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Page43
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 in
normal proportion to successful TCH seizures for call.
(handovers/calls>3)
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Page44
Equipment Problem
Call drop arising from equipment problem
Hardware problem
Transmission problem
Antenna and feeder fault
Other causes
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Page45
Contents
1. Principle of Call Drop
2. Analysis of Call Drop
3. Call Drop Cases
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Page46
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 the
frequency distribution in the
cells are reasonable?
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Page47
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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Page48
Call Drop Case 1
Analysis
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
IB1
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
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Page49
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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Page50
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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Page51
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 adjacentchannel interference effectively.
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Page52
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.
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Page53
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
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Page54
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.
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Page55
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 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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Page56
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.
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Page57
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.
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Page58
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.
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Page59
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.
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Page60
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.
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Page61
Call Drop Case 3
Conclusion：two 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.
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Page62
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.
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Page63
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?
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Page64
Call Drop Case 4
Troubleshooting
The adjusted parameters are listed below.
Value before
change
Value after
change
PBGT watch time
5
3
PBGT valid time
4
2
PBGT HO threshold
72
68
UL Qual. Thrsh. (Emergency
handover)
70
60
Min. DL level on candidate cell
10
15
Parameter name
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Page65
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.
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Page66
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 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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Page67
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.
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Page68
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.
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Page69
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.
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Page70
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.
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Call Drop Case 6
Troubleshooting
After adjusting GSM900 clock system, the abnormal call drop
problem is solved.
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Call Drop Case 6
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.
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Page73
Summary
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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Page74
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