2g drop analysis
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OMF000404Case Study – Call Drop
Issue 2.0
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References
31160978-BSC Traffic Statistic Manual
Volume I
31033203-BSS Troubleshooting
Manual
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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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Chapter 1 Chapter 1 Principle of call Principle of call
dropdrop
Section 1 Definition of
call drop
Section 2 TCH call drop
Section 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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Chapter 1 Chapter 1 Principle of call dropPrinciple of call drop
Section 1 Definition of call drop
Section 2 TCH call drop
Section 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 (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 - Assignment
MS BTS BSC
ASSIGN CMMAND
MSC
ASSIGNMENT REQUEST
SABM
UAESTABLISH INDICATION
CHANNEL ACTICATION
CHANNEL ACTIVATION ACK
A1
ERROR INDICATION
CONNECTION FAILURE INDICATION
A2
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TCH Call Drop – Intra-BSC 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
CONNECTION FAILURE INDICATION
A2
SABM
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TCH Call Drop - Incoming BSC Handover
MS Other BSC
HANDOVER ACCESS
UA
HANDOVER REQUIRED
HUAWEI BSC HUAWEI BTS
HANDOVER COMMAND
HANDOVER REQUEST ACK
HANDOVER DETECT
CH ACTCH ACT ACK
MSC
A1
ERROR INDICATION
CONNECTION FAILURE INDICATION
A2
SABM
HANDOVER REQUEST
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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
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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Successful TCH Seizure – Very 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 BTS BSC MSC
ASS_REQCH_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_ Access
HO_DetectPHY INFO
First SABMEstablish_IND
PHY INFO
Handover Complete HO_Performed
UA
MSC
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Successful TCH Seizure – Inter-BSC Handover Inter BSC handover process
MS MSBTS1 BTS2BSC1 BSC2MSC
Measure 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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Chapter 1 Chapter 1 Principle of call Principle of call
dropdrop
Section 1 Definition of call drop
Section 2 TCH call drop
Section 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
SDCCH seizures*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 Drop
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)
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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 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 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 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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Radio Link Fault
Signaling process chart of radio link fault
MS BTS BSC MSC
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 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 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 spectrum analyzer.
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 in
normal 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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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 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 resonable?
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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
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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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.
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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.
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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
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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.
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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 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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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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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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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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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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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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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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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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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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Call Drop Case 4
Troubleshooting
The adjusted parameters are listed below.
Parameter name 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
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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.
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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 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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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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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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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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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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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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