GSM BSS Network KPI CSSR Optimization Manual

GSM Call Drop
Problem Analysis
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Copyright © 2011 Huawei Technologies Co., Ltd. All rights reserved.

Foreword
 A too high call drop rate adversely affects the user’s experience.
 Hence call drop analysis is at the heart of optimization.
 The following training addresses the call drop analysis in depth
from the relevant KPIs over the influential factors to practical case
studies.
Page2
Objectives
 Upon completion of this course, you will be able to:
 Describe the definition of call drop
 Outline the analysis procedure of call drop
 Troubleshooting cause of call drop
 Main solution methods to call drop
 Call drop cases analysis
Page3
Contents
1. Call Drop Definition and Classification
2. Specific Causes of Call Drops and its Solutions
3. Call Drop Cases
Page4
Call Drop Definition
One of accessibility KPIs.

SDCCH SDCCH Call Drop Rate indicates the probability
call drop of call drops when the MS occupies the SDCCH.
One of retainability KPIs.

TCH Call Drop Rate refer to the ratio of call drop to
TCH successful TCH seizures after the BSC successfully

call drop assigns TCHs to MSs.
TCH Call Drop Rate (Including Handover)
TCH Call Drop Rate (Excluding Handover)
Page5
Measurement Point of SDCCH Call Drop
 Measurement point of SDCCH call drop during IMM ASS
K3003:Successful SDCCH Seizures (Call Type)
M3000A~C:Call Drops due to ERR IND

CM30A:Call Drops on
M3001A~E:Call Drops due to CONN FAIL Radio Interface (SDCCH)
M3002:Call Drops due to REL IND
M302:Call Drops due to No MRs from MS for a Long Time (SDCCH)
M303:Call Drops due to Abis Terrestrial Link Failure (SDCCH)
M304:Call Drops Due to Equipment Failure (SDCCH)
M305:Call Drops due to Forced Handover (SDCCH)
CM400~CM404:Number of Clear Requests Sent

on the A Interface
Page6
Measurement Point of SDCCH Call Drop
 Measurement Point related with IMM ASS channel active
MS BTS BSC
CHAN REQ
CHAN RQD
CHAN ACT
R3300A:Channel Activation Attempts in Immediate
Assignment Procedure (SDCCH)
CHAN ACT ACK
R3300C:Channel Activation Timeouts in Immediate
Assignment Procedure (SDCCH)
CHAN ACT NEGATIVE ACK
R3300B: CHAN ACTIV NACK Messages Sent by BTS in
IMM ASS CMD Immediate Assignment Procedure (SDCCH)
Page7
Formula of SDCCH Call Drop Rate
ZTR104C:Call
CM30:Call Drops on SDCCH
Drop Rate on = x 100%
K3003:Successful SDCCH Seizures
SDCCH
[CM30A:Call Drops on Radio Interface (SDCCH)] +

[M302:Call Drops due to No MRs from MS for a Long Time (SDCCH)] +
[M303:Call Drops due to Abis Terrestrial Link Failure (SDCCH)] +
[M304:Call Drops Due to Equipment Failure (SDCCH)] +
[M305:Call Drops due to Forced Handover (SDCCH)]
= x 100%
[R3300A:Channel Activation Attempts in Immediate Assignment Procedure (SDCCH)]-
[R3300B:CHAN ACTIV NACK Messages Sent by BTS in Immediate Assignment Procedure
(SDCCH)]-
[R3300C:Channel Activation Timeouts in Immediate Assignment Procedure (SDCCH)] +
[H3040:Internal Intra-Cell Handover Detection Messages Received by BSC (SDCCH)] +
[H3240:Incoming Internal Inter-Cell Handover Detection Messages Received by BSC (SDCCH)] +
[H3440:Incoming External Inter-Cell Handover Detection Messages Received by BSC (SDCCH)]
Page8
Definition of TCH Call Drop Rate
 Call Drop Rate on TCH indicates the ratio of the number of call drops to the
number of successful TCH seizures after the BSC successfully assigns TCHs to
MSs.
 Call Drop Rate on TCH contains the following aspects
 The probability of call drops after the establishment of TCHs
 The probability of call drops during call establishment
 Call Drop Rate on TCH is one of retainability KPIs. It reflects the probability of
call drops due to various reasons after the MSs access the TCHs properly. If Call
Drop Rate on TCH is high, user experience is adversely affected.
Page9
Measurement Point during (Immediate) Assignment Procedure
MS BTS BSC MSC
Channel_req Channel_Required
Channel_Active
Channel_Active_Ack
K3023:Successful TCH Seizures (Signaling Channel)
IMMEDIATE ASSIGN COMMAND
First SABM Establish_IND(CM Service Req)
UA CR(Complete_L3_information)
CC
CM Service Accepted
Setup
Call Processing
Channel_Active Assignment_Req
Channel_Active_Ack
ASSIGNMENT COMMAND
First SABM
Establish_IND
UA
ASSIGNMENT CMP
Assignment_CMP
K3013A:Successful TCH Seizures (Traffic Channel)
Alerting
Connect
Connect Ack
Conversation
Error Indication CM3300:Call Drops on Traffic Channel in Stable State (Error Indication) or
CM3301:Call Drops on Traffic Channel in Stable State (Connection Failure) or
Connection Failure Indication
DISC CM3302:Call Drops on Traffic Channel in Stable State (Release Indication)
Release Indication
CM332:Call Drops Due to No MR from MS for a Long Time (Traffic Channel)
CM333:Call Drops due to Abis Terrestrial Link Failure (Traffic Channel)
Clear Request CM334:Call Drops due to Equipment Failure (Traffic Channel)
Page10
Measurement Point during Handover Procedure
 Intra-BSC handover
MS Target BTS1 BSC Original BTS2 MS MSC

Measure Report
Channel Active K3013B:Successful TCH Seizures in
Channel Active Ack TCH handovers (Traffic Channel)]
HANDOVER COMMAND
Handover Access
H3047:Internal Intra-Cell Handover Detection Messages Received by BSC (TCHF)
Handover Detect H3048:Internal Intra-Cell Handover Detection Messages Received by BSC (TCHH)
PHYINFO
PHYINFO H3247:Incoming Internal Inter-Cell Handover Detection Messages Received by BSC (TCHF)
H3248:Incoming Internal Inter-Cell Handover Detection Messages Received by BSC (TCHH)
First SABM
UA Establish_IND
CM335:Call Drops due to Forced Handover (Traffic Channel)]
H3027Ca:Failed Internal Intra-Cell Handovers (Timer Expired) (TCHF) (Traffic Channel)
T3103 H3028Ca:Failed Internal Intra-Cell Handovers (Timer Expired) (TCHH) (Traffic Channel)
Time Out H3127Ca:Failed Outgoing Internal Inter-Cell Handovers (Timer Expired) (TCHF) (Traffic
Channel)
H3128Ca:Failed Outgoing Internal Inter-Cell Handovers (Timer Expired) (TCHH) (Traffic
Clear Request
HANDOVER COMPLETE Channel)
Handover Performed
Page11
Page11
Measurement Point during Handover Procedure
 Inter-BSC handover
MS Original BTS1 Original BSC1 MSC Target BSC2 Target BTS2 MS
Measure Report from MS Handover_ Handover

Required _Request
Channel_Active
Handover_Request_Ack Channel_Active_Ack
Handover_Cmd Handover Access
Handover Command Handover_Detect
H3447:Incoming External Inter-Cell Handover
Detection Messages Received by BSC (TCHF)
H3448:Incoming External Inter-Cell Handover
Handover_Detect Detection Messages Received by BSC (TCHH)
PHY INFO
H3327Ca:Failed Outgoing External Inter-Cell
PHY INFO
Handovers (T8 Expired) (TCHF) (Traffic Channel)
K3013B:Successful TCH
H3328Ca:Failed Outgoing External Inter-Cell First SABM Seizures in TCH handovers
Handovers (T8 Expired) (TCHH) (Traffic Channel) Establish_INO UA (Traffic Channel)]
Handover Complete
T8 Time Out Clear_CMD Handover_Cmp
Clear_CMP
Page12
Formula of TCH Call Drop Rate ZTR304:Call Drop Rate on TCH
per cell (Including Handover) =
(Including Handover) [CM33:Call Drops on Traffic Channel] ×100%
[K3023:Successful TCH Seizures (Signaling Channel)] +
[K3013A:Successful TCH Seizures (Traffic Channel)] +
[K3013B:Successful TCH Seizures in TCH handovers (Traffic Channel)]
CM33:Call Drops on Traffic Channel=

[CM330:Call Drops on Radio Interface in Stable State (Traffic Channel)] +
1
[CM331:Call Drops on Radio Interface in Handover State (Traffic Channel)] +

1
[CM332:Call Drops due to No MRs from MS for a Long Time (Traffic Channel)] +
[CM333:Call Drops due to Abis Terrestrial Link Failure (Traffic Channel)] +
[CM334:Call Drops due to Equipment Failure (Traffic Channel)] +
[CM335:Call Drops due to Forced Handover (Traffic Channel)] +
[CM397:Call Drops Due to Loopback Start Failure] +
[CM385:Call Drops Due to Failures to Return to Normal Call from Loopback]
CM330:Call Drops on Radio Interface in Stable State (Traffic Channel) =

[CM3300:Call Drops on TCH in Stable State (Error Indication)] +
[CM3301:Call Drops on TCH in Stable State (Connection Failure)] +
[CM3302:Call Drops on Traffic Channel in Stable State (Release Indication)]
CM331:Call Drops on Radio Interface in Handover State (Traffic Channel) =

[H3027Ca:Failed Internal Intra-Cell Handovers (Timer Expired) (TCHF) (Traffic Channel)] +
[H3028Ca:Failed Internal Intra-Cell Handovers (Timer Expired) (TCHH) (Traffic Channel)] +
[H3127Ca:Failed Outgoing Internal Inter-Cell Handovers (Timer Expired) (TCHF) (Traffic Channel)] +
[H3128Ca:Failed Outgoing Internal Inter-Cell Handovers (Timer Expired) (TCHH) (Traffic Channel)] +
[H3327Ca:Failed Outgoing External Inter-Cell Handovers (T8 Expired) (TCHF) (Traffic Channel)] +
[H3328Ca:Failed Outgoing External Inter-Cell Handovers (T8 Expired) (TCHH) (Traffic Channel)]
Page13
ZTR304A:Call Drop Rate on TCH
per cell (Excluding Handover) =
Formula of TCH Call Drop [CM33:Call Drops on Traffic Channel] ×100%
[K3013A:Successful TCH Seizures (Traffic Channel)] +
Rate (Excluding Handover) [CH323:Successful Incoming Internal Inter-Cell Handovers] +
1
[CH343:Successful Incoming External Inter-Cell Handovers] –

[CH313:Successful Outgoing Internal Inter-Cell Handover] –
[CH333:Successful Outgoing External Inter-Cell Handovers]
CH323:Successful Incoming Internal Inter-Cell Handovers =

[CH320:Incoming Internal Inter-Cell Handover Requests] -
1
[CH322:Failed Incoming Internal Inter-Cell Handovers]

1
CH320:Incoming Internal Inter-Cell Handover Requests =

[H3200W:Incoming Internal Inter-Cell Handover Requests (SDCCH) (900/850-900/850)] +
[H3200X:Incoming Internal Inter-Cell Handover Requests (SDCCH) (1800/1900-1800/1900)] +
[H3200Y:Incoming Internal Inter-Cell Handover Requests (SDCCH) (900/850-1800/1900)] +
[H3200Z:Incoming Internal Inter-Cell Handover Requests (SDCCH) (1800/1900-900/850)] +
[H3209W:Incoming Internal Inter-Cell Handover Requests (TCH) (900/850-900/850)] +
[H3209X:Incoming Internal Inter-Cell Handover Requests (TCH) (1800/1900-1800/1900)] +
[H3209Y:Incoming Internal Inter-Cell Handover Requests (TCH) (900/850-1800/1900)] +
[H3209Z:Incoming Internal Inter-Cell Handover Requests (TCH) (1800/1900-900/850)]
CH322:Failed Incoming Internal Inter-Cell Handovers =
[H3220W:Failed Incoming Internal Inter-Cell Handovers (SDCCH) (900/850-900/850)] +
[H3220X:Failed Incoming Internal Inter-Cell Handovers (SDCCH) (1800/1900-1800/1900)] +
[H3220Y:Failed Incoming Internal Inter-Cell Handovers (SDCCH) (900/850-1800/1900)] +
[H3220Z:Failed Incoming Internal Inter-Cell Handovers (SDCCH) (1800/1900-900/850)] +
[H3229W:Failed Incoming Internal Inter-Cell Handovers (TCH) (900/850-900/850)] +
[H3229X:Failed Incoming Internal Inter-Cell Handovers (TCH) (1800/1900-1800/1900)] +
[H3229Y:Failed Incoming Internal Inter-Cell Handovers (TCH) (900/850-1800/1900)] +
[H3229Z:Failed Incoming Internal Inter-Cell Handovers (TCH) (1800/1900-900/850)]
Page14
Contents
3. Call Drop Cases
Page15
Call Drop Analysis Procedure
Analyzing a call drop rate
Y Analyzing the frequency

Common
planning, coverage, and
phenomenon
cell parameters
N
Filtering for the cell with a
high call drop rate
N RF call drop Y
rate
Times of A interface Radio problem
failures during TCH Average received level
occupation and quality during the
handover
interference band
TCH assignment analysis Power control average level Outgoing handover
failure (equipment success rate
fault)
TRX received level Average received level
Times of handovers and
and received quality during a TCH call drop
times of call occupation
Times of call drops on success
the terrestrial link Ratio of poor Average distance during a
handovers between TCH call drop Which cell has a lower
the uplink and incoming handover
TCH availability downlink success rate
Outgoing handover success
rate and average level
Equipment fault Times of The ratio of inter-office
reestablishment handovers is high.
failures upon Measurement of
handover failure uplink/downlink balance
performance
Handover
Interference
Coverage
Page16
Specific Causes of Call Drops and its
Solutions
Coverage HO
Parameter
Interference
Antenna
Transmis
sion
Page18
Call Drops Reasons for Coverage
1 2 3 4
Discontinuous coverage
(blind area)
Bad indoor coverage
Cross coverage (isolated island)
Small coverage
Page19
Methods to Identify Call Drop Caused by
Coverage
 Drive Test  Traffic Measurement

 For detail, please refer to the
next page
Call Drop for Coverage
Page20
Traffic Statistic Tasks Related to Coverage Analysis
1 Power control performance measurement
2 Receiving level performance measurement Call

Traffic
Cell performance measurement / inter-cell Drop
Statistic 3
handover performance measurement Result
Tasks
4 Call drop performance measurement from
Coverage
5 Neighbor cell performance measurement Problem
6 Uplink/downlink balance performance

measurement
Page21
Solution to Call Drop Caused by Coverage
Area with insufficient

coverage
Remove Ensure the indoor
hardware communication
problem quality
Eliminate
overshooting
coverage
Page22
Call Drops Caused by Handover
Unreasonable
T3103 timer overflows parameters
Main
BTS clock is out of Incomplete
synchronization Reasons
neighbor cells
Traffic congestion Co-BCCH and Co-BSIC
Page23
Handover
Analyze the
traffic
measurement
Method
to
Identify
Check the
Drive test alarms
Page24
Traffic Statistic Tasks Related to Handover
Analysis
1 Inter-cell handover performance

measurement
Call
Traffic 2 Neighbor cells performance measurement
Drop
Statistic
Outgoing cell handover performance Result
Tasks 3
measurement from
Incoming cell handover success rate is Handover
4
low Problem
5 TCH performance measurement
Page25
Solution to Call Drop Caused by Handover
1 2 3
Check handover
parameters
Adjust the traffic
Provide good clock synchronization
Page26
Call Drops Reasons for Interference
Co-channel
interference
Main
Reasons
Inter-
modulation Adjacent-
interf. and channel
Other Outside interference
interfe.
Page27
Interference
Analyze the traffic Analyze basing

statistic on user complaint
Method
Use spectrum analyzer Check the
to scan the frequencies to
frequency planning
Identify
Eliminate causes Adjust ARFCNs

resulting from equipment
Page28
Traffic Statistic Tasks Related to Interference
Analysis
Interference band in
MR
Many handover Receiving level
failures and performance
reestablishment measurement
failures
Interference
Analysis
Call drop
performance BQ handover ratio
measurement
Receiving quality
performance
measurement
Page29
Solution to Call Drop Caused by Interference
 Interference from outside the network and inside the network are solved by different methods.
 Invalid interference from outside the network is solved by the Radio Committee
 interference from inside the network is solved by adjusting the network.
1 2 3
Drive test Use DTX, FH, PC, Exclude the

and diversity interference caused by
technologies the equipment
Page31
Call Drops caused by Antenna Feeder
Antenna feeders
are wrongly
connected
Antenna Different
Main downtilt angles
feeder
problem
Reasons for two sets of
antenna
Different
azimuth angles
for two sets of
antenna
Page32
Antenna Feeder
Whether there is
alarm related to
antenna unit
Check whether whether the boards are
there is BTS normal from remote
hardware fault maintenance console
Method
to
Identify
whether the
BTS on-site check uplink and
and test downlink are
check whether the balanced in MRs
BCCH of the
serving cell is
consistent with
the planning
Page33
Traffic Statistic Tasks Related to Antenna Feeder
Analysis
Analyze whether
1 Uplink and downlink balance PM the uplink and
Traffic downlink are
Statistic 2 Call drop PM balanced in order
Tasks to identify antenna
feeders problems if
3 Power control PM
any exist
Page34
Call Drops Reasons for Transmission
 Observe the transmission and
board alarms
 Bad transmission
quality and unstable
Main transmission link
Reasons Abis interface and
the A interface links
in
 TCH PM shows ：
 Check  Observe  1 ） A interface failures during TCH seizure
transmission traffic 2 ） Whether the TCH availability is normal
paths statistics
 3 ） Call drops due to interruption of
terrestrial links
Page35
Radio Parameters That Affect Call Drop
1 RLT/SACCH Multi-frames
2 T200 parameter
3 RXLEV_ACCESS_MIN
Main Call
Radio 4 RACH Min Access Level Drop
Parameters Problem
5 NCC Permitted
6 Handover parameters
7 Power control parameters
8 Frequency planning parameters
Page36
Contents
3. Call Drop Cases
Page38
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?
Page39
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.
Page40
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

Page41
0.00
Call Drop Case 1
 Troubleshooting
 Conducting a drive tests, it is found that the quality is bad when the
receiving strength is high.
 Traffic statistic show that during periods of high call drop rate,
handovers are mostly caused by quality reasons, and channel
assignment failure rate is also high.
 Conclusion: Interference is high.
Page42
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.
Page43
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 for characteristic footprints
 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.
Page44
Call Drop Case 2
 Fault description
 The call drop rate in cell 3 of a BTS is 10% accompanied by high
congestion rate, but call drop rate and congestion rate in cell 1 and
cell 2 are normal.
Page45
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
Page46
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.
Page47
Call Drop Case 2
 Troubleshooting
 Make a scanning test with a spectrum analyzer.
 A suspect signal with 904.14 MHz center frequency, 300 kHz bandwidth is found. It
is similar to an analog signal and it exists continuously.
 At the distributor output port of cell 1,2 and 3, the signal strength is 60 dBm, 40
dBm and –27 dBm respectively. It accords with the interference margin.
 Traffic volume is higher in the day time than at night.
 Now the problem is found: 904 MHz external interference source.
Page48
Call Drop Case 2
 Conclusion: Reduce interference
 Reduce internal interference through checking frequency planning.
 After internal interference is excluded, we can locate external
interference with spectrum analyzer.
Page49
Call Drop Case 3
 Subscribers complain that call drops often happen on the 5th floor
and further up in a building.
Subscriber complaint is also an important source of information

about the network quality.
Page50
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.
Page51
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.
Page52
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.
Page53
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.
Page54
Call Drop Case 4
 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.
Page55
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?
Page56
Call Drop Case 4
 Troubleshooting
 The adjusted parameters are listed below.
Value before Value after

Parameter name
change change
PBGT watch time 5 3
PBGT valid time 4 2
PBGT HO threshold 72 68
UL Qual. Thrsh. (Emergency
70 60
handover)
Min. DL level on candidate cell 10 15
Page57
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.
Page58
Call Drop Case 5
 In the dialing 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 incoming 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.
Page59
Call Drop Case 5
 Troubleshooting
 Lock the frequency with a test mobile phone and perform dialing 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.
Page60
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.
Page61
Call Drop Case 6
 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.
Page62
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.
Page63
Call Drop Case 6
 Troubleshooting
 After adjusting GSM900 clock system, the abnormal call drop
problem is solved.
Page64
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.
Page65
Summary
 What is classification of the call drop?
 How to analyze the call drop?
 What is the main reasons of the call drop and its corresponding
solution?
Page66
Thank you
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GSM Call Drop

Copyright © 2011 Huawei Technologies Co., Ltd. All rights reserved.

2. Specific Causes of Call Drops and its Solutions

3. Call Drop Cases

One of accessibility KPIs.

One of retainability KPIs.

TCH successful TCH seizures after the BSC successfully

K3003:Successful SDCCH Seizures (Call Type)

M3000A~C:Call Drops due to ERR IND

CM400~CM404:Number of Clear Requests Sent

[CM30A:Call Drops on Radio Interface (SDCCH)] +

MS Target BTS1 BSC Original BTS2 MS MSC

Measure Report from MS Handover_ Handover

CM33:Call Drops on Traffic Channel=

[CM331:Call Drops on Radio Interface in Handover State (Traffic Channel)] +

CM330:Call Drops on Radio Interface in Stable State (Traffic Channel) =

CM331:Call Drops on Radio Interface in Handover State (Traffic Channel) =

[CH343:Successful Incoming External Inter-Cell Handovers] –

CH323:Successful Incoming Internal Inter-Cell Handovers =

[CH322:Failed Incoming Internal Inter-Cell Handovers]

CH320:Incoming Internal Inter-Cell Handover Requests =

2. Specific Causes of Call Drops and its Solutions

3. Call Drop Cases

Y Analyzing the frequency

Bad indoor coverage

Cross coverage (isolated island)

 Drive Test  Traffic Measurement

Call Drop for Coverage

1 Power control performance measurement

2 Receiving level performance measurement Call

6 Uplink/downlink balance performance

Area with insufficient

Traffic congestion Co-BCCH and Co-BSIC

1 Inter-cell handover performance

5 TCH performance measurement

Adjust the traffic

Provide good clock synchronization

Analyze the traffic Analyze basing

Eliminate causes Adjust ARFCNs

Drive test Use DTX, FH, PC, Exclude the

7 Power control parameters

8 Frequency planning parameters

2. Specific Causes of Call Drops and its Solutions

3. Call Drop Cases

Please confirm whether the

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

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

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

Subscriber complaint is also an important source of information

 Check traffic statistic

 Cause analysis of call drop

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

Value before Value after

Min. DL level on candidate cell 10 15