GSM BSS Network KPI (SDCCH Call Drop Rate) Optimization Manual
Product Name
Confidentiality Level
G3BSC
INTERNAL
Product Version
Total 26 pages
INTERNAL
GSM BSS Network KPI (SDCCH Call Drop Rate)
Optimization Manual
(For internal use only)
Prepared by
Date
2008-8-25
Reviewed by
Date
yyyy-mm-dd
Reviewed by
Date
yyyy-mm-dd
Approved by
Date
yyyy-mm-dd
Du Jian, WCDMA & GSM Network
Performance Research Dept.
Huawei Technologies Co., Ltd.
All rights reserved
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Contents
1 Overview of SDCCH Call Drop Rate ................................................................................. 6
1.1 Definition ......................................................................................................................................................... 6
1.2 Recommended Formula ................................................................................................................................... 6
1.3 Signaling Procedure and Measurement Points ................................................................................................. 7
2 Factors That Affect SDCCH Call Drop Rate ...................................................................... 8
2.1 Hardware Failure .............................................................................................................................................. 8
2.2 Transmission .................................................................................................................................................... 8
2.3 Version Upgrade ............................................................................................................................................... 8
2.4 Parameter Setting ............................................................................................................................................. 8
2.5 Intra-Network and Inter-Network Interference ................................................................................................9
2.6 Coverage Problem ............................................................................................................................................ 9
2.7 Antenna System .............................................................................................................................................. 10
2.8 Imbalance Between Uplink and Downlink..................................................................................................... 10
2.9 Repeater.......................................................................................................................................................... 10
3 Analysis of and Solutions to High SDCCH Call Drop Rate ........................................... 11
3.1 Analysis Process ............................................................................................................................................. 11
3.2 Solutions to High SDCCH Call Drop Rate .................................................................................................... 13
3.2.1 Checking the Hardware ......................................................................................................................... 14
3.2.2 Checking the Transmission ................................................................................................................... 15
3.2.3 Checking the BSC and BTS Version Upgrade ...................................................................................... 16
3.2.4 Checking the Parameter Settings........................................................................................................... 16
3.2.5 Checking the Interference ..................................................................................................................... 18
3.2.6 Checking the Coverage, Antenna System, and Balance Between Uplink and Downlink ..................... 18
3.2.7 Checking the Repeaters ......................................................................................................................... 19
4 Test Method ........................................................................................................................ 20
5 Remarks About the Signaling Analysis of the SDCCH Call Drop Rate........................ 21
6 Cases for SDCCH Call Drop Rate Optimization ............................................................. 23
6.1 Case 1: SDCCH Call Drop in a Synchronous Network ................................................................................. 23
6.2 Case 2: Call Drop Due to Imbalance Between Uplink and Downlink ........................................................... 23
6.3 Case 3: Call Drop Due to Antenna System Problem ...................................................................................... 24
6.4 Case 4: Call Drop Due to Transmission Problem ........................................................................................... 25
7 Feedback Form for SDCCH Call Drop Rate..................................................................... 26
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Figures
Figure 1-1 Number of successful SDCCH seizures............................................................................................... 7
Figure 1-2 Number of successful SDCCH seizures in the signaling channel handover ........................................7
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Revision Record
Date
Revision Version
Change Description
Author
2008-8-25
V1.0
Draft completed.
Du Jian
References
SN
Document
Author
Date
1
G-Guide to Eliminating Interference - 20050311-A-1.0
Chen Baolin
2005-3-11
2
GSM BSS Network KPI (Network Coverage) Optimization Manual
Xie Haibin
2008-6-18
3
GSM BSS Network KPI (SDCCH Call Drop Rate) Baseline
Wu Zhen
2007-6-22
4
GSM BSS Network KPI (Uplink and Downlink Balance)
Optimization Manual
Yang Jixiang
2008-3-26
5
Guide to Solving Call Drop Problems
Yang Bin
2002-3-7
6
GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual
Su Shi
2008-6-20
GSM BSS Network KPI (SDCCH Call Drop Rate) Optimization
Manual
Keywords: SDCCH call drop rate, KPI
Abstract : This document describes the definition, test method, and optimization method of
the SDCCH call drop rate.
Acronyms and Abbreviations:
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Acronym and Abbreviation
Full Spelling
SDCCH
Standalone Dedicated Control Channel
MS
Mobile Station
BSC
Base Station Controller
KPI
Key Performance Index
TCH
Traffic Channel
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1
INTERNAL
Overview of SDCCH Call Drop Rate
1.1 Definition
The SDCCH call drop rate indicates the probability of call drops when the MS occupies the
SDCCH. The SDCCH call drop rate is one of accessibility KPIs. This KPI reflects the seizure
condition of signaling channels. If the value of this KPI is high, user experience is adversely
affected.
1.2 Recommended Formula
The SDCCH call drop rate is obtained on the basis of the traffic measurement results. The
recommended formula is as follows:
Call Drop Rate on SDCCH = (Call Drops on SDCCH/Successful SDCCH Seizures +
Successful SDCCH Seizures in the signaling channel handover) x 10 0%
Compared with the formula of the BSC32, the formula of the BSC6000 adds the measurement
of the number of call drops due to release indication received on the SDCCH in stable state.
As few call drops due to release indication occur in the existing network, the measurement
values of the BSC32 and BSC6000 are the same in the formula.
For details, see the GSM BSS Network KPI (SDCCH Call Drop Rate) Baseline.
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1.3 Signaling Procedure and Measurement Points
Figure 1-1 Number of successful SDCCH seizures
Figure 1-2 Number of successful SDCCH seizures in the signaling channel handover
The measurement points illustrated in Figure 2 are described as follows:
A indicates the number of successful SDCCH seizures.
B indicates the number of successful SDCCH seizures in the signaling channel handover
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2
INTERNAL
Factors That Affect SDCCH Call Drop
Rate
According to user complaints and network optimization experience, the major factors that
affect the SDCCH call drop rate are as follows:

Hardware failure

Transmission

Version upgrade

Parameter setting

Intra-network and inter-network interference

Coverage, antenna system, and imbalance between uplink and downlink
2.1 Hardware Failure
When a TRX or a combiner is faulty, seizing the TCH becomes difficult, and thus the SDCCH
call drop rate increases.
2.2 Transmission
The SDCCH call drop rate increases in any of the following conditions: (1) The transmission
quality is poor on the A or Abis interface d ue to var ious reasons. (2) Transmission links are
unstable.
2.3 Version Upgrade
After the BTS version or BSC version is upgraded, the BTS version may be incompatible
with the BSC version, and the parameters and algorithms in the new version may be chan ged.
In this case, the SDCCH call drop rate increases.
2.4 Parameter Setting
The settings of some parameters on the BSC and MSC sides may affect the SDCCH call drop
rate. If the following situations occur, the SDCCH call drop rate may increase:
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Radio link and cell parameters:
SACCH Multi-Frames (SACCH period (480ms)) and Radio Link Timeout(SACCH
period (480ms)) are set to too small values.
RACH Min.Access Level(dBm) is set to a too small value.
T200 SDCCH(5ms) and N200 of SDCCH are set to too small values.
MAIO is set improperly.
Handover parameters:
SDCCH HO Allowed is set to Yes.
T3103A(ms) and T3103C(ms) are set to too small values.
T3109(ms) is set to a too small value.
T3111(ms) is set to a too small value.
The length of timer T305/T308 is set to an invalid or too great value.
Disconnect Handover Protect Timer is set to a too small value.
The network planning is improper after the Um inter face is synchronized.
Software Parameter 13 and MAX TA are set to too small values.
2.5 Intra-Network and Inter-Network Interference
If inter-network interference and repeater interference exist, or if severe intra -network
interference occurs because of tight frequency reuse, call drops may occur on SDCCHs due to
poor QoS. This affects the SDCCH call drop rate.
The following types of interference may occur:
1.
Inter-network interference from scramblers or privately installed antennas
2.
Interference from the CDMA network
3.
Repeater interference
4.
Intermodulation interference from BTSs
5.
Intra-network co-channel and a djacent-channel interference
2.6 Coverage Problem
The following coverage problems may affect the SDCCH call drop rate.
1. Poor indoor coverage
Densely distributed buildings and th ick walls cause great attenuation and low indoor signal
level, which causes call drops.
2. Coverage failure
If the signal from an antenna is blocked or the BCCH TRX is faulty, call drops may occur.
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2.7 Antenna System
The following antenna system problems may affect the SDCCH call drop rate
1.
If the transmit antennas of two cells are misconnected, the uplink signal level in each cell
is much lower than the downlink signal level in the cell. Therefore, call drops are likely
to occur at a place far away from the BTS.
2.
If a directional cell has main and diversity antennas, the BCCH and SDCCH of the cell
may be transmitted from different antennas. If the two antennas have different pitch
angles or azimuths, the coverage areas of the two antennas are different. In this case, the
following result may occur: An MS can receive the BCCH signals from one antenna;
when a call is made, the MS cannot seize the SDCCH transmitted by the other antenna
and thus a call drop occurs.
3.
If the feeder is damaged, if water runs into the feeder, or if the feeder and the connector
are not securely connected, both the transmit power and receiver sensitivity of the
antenna are reduced. Thus, call drops probably occur.
2.8 Imbalance Between Uplink and Downlink
The difference between the uplink signal level and the downlink signal level may be great in
the following conditions: The transmit power of the BTS is high; t he tower mounted amplifier
(TMA) or BTS amplifier does not work properly; the antenna and the connector are not
securely connected. As a result, call drops may occur at the edge of the BTS coverage area.
2.9 Repeater
If a cell is installed with a repeater, BTS coverage problems may occur in the case that the
repeater is faulty or that the uplink and downlink gain is inappropriately set. Therefore, the
call drop rate increases.
If a wide-frequency repeater is used and the gain is set to a great value, strong interference
may be caused. As a result, the network quality is adversely affected and the call drop rate
increases.
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3
INTERNAL
Analysis of and Solutions to High
SDCCH Call Drop Rate
3.1 Analysis Process
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开始
确定掉话率高小
区
否
传输或硬件是
否存在问题
是
解决硬件或传输
问题
否
是否由于版本
升级
是
更换版本或打补
丁
检查版本BUG
否
数据配置是否
存在问题
是
调整切换功控等
相关参数
是
是否是网内干
扰
否
是否存在干扰
否
是
否
覆盖是否存在
问题
是
根据覆盖优化指
导书优化覆盖
是
调整天馈
是
根据指导书优化
上下行不平衡问
题
是
解决直放战问题
排查外部干扰
检查频率规划情
况
否
天馈是否存在
问题
根据指导书优化上下行不平衡问题：
Optimize the imbalance between uplink and
downlink
直放站是否存在问题：The repeater is faul
解决直放站问题：Rectify the repeater faul
掉话率问题是否解决：Check the call drop
rate
结束：End
是：Yes
否：No
否
是否上下行不
平衡
否
直放战是否存
在问题
否
掉话率问题是
否解决
结束
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Comment [x1]: 开始：Start
确定掉话率高小区：Determine the cell wit
high call drop rate
传输或硬件是否存在问题：Transmission
hardware failure?
解决硬件或传输问题：Rectify the
transmission or hardware fault
是否由于版本升级：The problem is caused
version upgrade?
检查版本 BUG：Check version informatio
更换版本或打补丁：Replace the version o
install patches
数据配置是否存在问题：The data
configuration is improper?
调整切换功控等相关参数：Adjust related
parameters such as handover and power
control
是否存在干扰：The interference exists?
是否是网内干扰：Intra-network interferen
检查外部干扰：Check external interferenc
检查频率规划情况：Check frequency
planning
覆盖是否存在问题：The coverage proble
occurs?
根据覆盖优化指导书优化覆盖：Optimize
coverage according to the coverage
optimization guide
天馈是否存在问题：The antenna system is
faulty?
调整天馈：Adjust the antenna system
是否上下行不平衡：ImUnbalance betwee
uplink and downlink?
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3.2 Solutions to High SDCCH Call Drop Rate
Before analyzing the causes of high SDCCH call drop rates, you sh ould find out the
difference between the actual SDCCH call drop rate and the expected value. You should also
find out the influence of the problems and the related KPIs.
You can ana lyze the distribution of call drops based on the related traffic measurement results.
If a certain type of call drop accounts for a large proportion of call drops, you can locate the
fault by performing the corresponding procedure. The following table lists the distribution of
call drops based on the type of call drop and the cause for call drop in the traffic statistics.
Analyzing the traffic statistics based on the cause for call drop
Type of Call Drop
Cause for Call Drop
Measurement
Code
Interface
Distribution
Call Drops due to CONN FAIL Received
on SDCCH in Stable State
Radio Link Failure
M3001A
HO Access Failure
M3001B
OM Intervention
M3001C
Radio Resource
Unavailable
M3001D
Other Causes
M3001E
T200 Expired
M3000A
The sum of the
counter and the
number of call
drops due to
SDCCH
handover failure
is the number of
call drops on
radio interface
(SDCCH).
Unsolicited DM
Response
M3000B
Sequence Error
M3000C
Release Indication
M3002
Call Drops due to ERR IND Received on
SDCCH in Stable State
Call Drops due to REL IND Received on
SDCCH
Call Drops due to No MRs from MS for a
Long Time
M302
As the seizure
duration of the
SDCCH is short,
the call drop
may not occur.
Call Drops due to Abis Terrestrial Link
Failure
M303
Call Drops Due to Equipment Failure
M304
Call Drops due to Forced Handover
M305
Number of call
drops on the
SDCCH due to
transmission and
equipment
causes
Call Drops due to Resource Check
M306
Clear Requests Sent on the A Interface
M309
According to the traffic statistics, you can obtain the distribution of SDCCH call drops d ue to
Um and non Um causes. For Um causes, you need to check configuration parameters and
network interference. For hardware causes, you need to check hardware, transmission, and
equipment failure. If no obvious causes are found, you can perform the procedures shown in
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the preceding figure. The following table lists the mapping between the traffic measurement
counters and the troubleshooting procedures.
Traffic Measurement Counter
Troubleshooting Procedure
M3000A (T200 expired)
3.2.4 (check whether the T200 and N200 are set
properly)
M3001A (radio link failure)
3.2.4 (check whether the radio link failure counter
and the number of SACCH multi-frames are set
properly)
M3001D (radio resource
unavailable)
3.2.1 (check hardware failure)
M303 (Abis terrestrial link failure)
3.2.2 (check the transmission)
M304 (equipment failure)
3.2.1 (check hardware failure)
The following sections describe the solutions to h igh SDCCH call drop rates.
The traffic measurement counters and parameters in this document are the same as those in
the BSC6000V9R8C01B051 version.
3.2.1 Checking the Hardware
If a TRX or a combiner is faulty or if an R F cable is incorrectly connected, seizing the
SDCCH becomes difficult, and thus t he SDCCH call drop rate increases. You can c heck
whether hardware is faulty by viewing BTS alarms or viewing the hardware state on the Site
Device Panel tab page of the LMT. The following table lists the major BSC alarms related to
hardware failures.
Alarm ID
Alarm Name
1000
LAPD OML Fault
2204
TRX Communication Alarm
4414
TRX VSWR Alarm
3606
DRU Hardware Alarm
In addition, you can locate the fault by checking the traffic measurement related to hardware
failures.
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Cause
BSC-Level
Cell-Level
Equipment
Faults
BSC Measurement -> Access
measurement per BSC ->
Congestion Ratio on SDCCH per
BSC
Call Drop Ratio on SDCCH per
BSC
SDCCH Availability per BSC
Channel Measurement ->
INTERNAL
Analyzed Measurement of Available
Channels (SDCCH)
Call Drop Measurement per Cell in Call
Measurement:
Call Drops due to Equipment Failure
(Signaling Channel)
3.2.2 Checking the Transmission
Poor transmission quality, unstable transmission links, or insufficient resources on the Abis or
A interface may lead to the inc rease of the SDCCH call drop rate . You can check t he
transmission conditions by viewing the alarms related to transmission. If a large number of
transmission alarms are generated, you can infer that transmission failures occur. Then, you
should check the transmission connections.
The following table lists the BSC alarms related to transmission failures.
Alarm ID
Alarm Name
1000
LAPD OML Fault
11270
LAPD Alarm
11278
E1 Local Alarm
11280
E1 Remote Alarm
20081
Loss of E1/T1 Signals (LOS)
20082
Loss of E1/T1 Frames (LOF)
In addition, you can locate the fault by checking the traffic measurement related to
transmission failures.
Cause
BSC-Level
Cell-Level
Transmis
sion
failure
BSC Measurement -> LAPD
Measurement
Call Measurement: -> Channel Activation
Measurement per Cell
CHAN ACTIV NACK Messages Sent by BTS
Channel Activation Timeouts
Call Measurement -> :Call Drop Measurement
per Cell
Measurement of Call Drops Due to Abis
Terrestrial Link Failure
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3.2.3 Checking the BSC and BTS Version Upgrade
If the SDCCH call dro p rate increases after the B SC version or BTS version is u pgraded, You
should check whether the BTS version is compatible with the BSC version and whether the
parameters and algorithms in the new version are changed.
To locate the p roblem, you can check the version description document and the related
documents, or provide the feedback for the R&D department to learn whether the new version
has known defects. If the new version has defects, you should replace it with another version
or install the patch.
For details, see the BSC6000 Upgrade Guide.
3.2.4 Checking the Parameter Settings
The parameter settings on the BSC side and MSC side may affect the SDCCH call drop rate.
You shou ld check the settings of the following parameters for a cell with a high SDCCH call
drop rate.

SACCH Multi-Frames (SACCH period (480 ms))
This parameter determines whether an uplink radio link is faulty. Each time the BTS fails
to decode the measurement report on the SACCH from the MS, the counter decreases by
1. Each time the BTS successfully decodes the measurement report on the SACCH, the
counter increases by 2. When the value of this parameter is 0, the BTS regards the radio
link as faulty. In the traffic measurement, if there are many call drops (M3001A) related
to radio link failure, you can infer that the radio propagation conditions are poor. In this
case, you can set this parameter to a greater value.

Radio Link Timeout (SACCH period(480ms))
This parameter determines whether a d ownlink radio link is fa ulty. Each time the BTS
fails to decode the measurement report on the SACCH from the MS, the counter
decreases by 1. Each time the BTS successfully decodes the measurement report on the
SACCH, the counter increases by 2. When the value of this parameter is 0, the BTS
regards the radio link a s faulty. In the traffic measurement, if there are many ca ll drops
(M3001A) related to radio link failure, you can infer that the radio propagation
conditions are poor. In this case, you can set this parameter to a greater value.

RXLEV_ACCESS_MIN
This parameter specifies the minimum receive level of an MS to access the BSS. If this
parameter is set to a too small value, some MSs with low receive levels may access the
network and call drop s are likely to occur. You can set this parameter to a great value to
reduce the SDCCH call drop rate. The counters related to traffic volume, ho wever, are
accordingly affected.

RACH Min.Access Level
This parameter determines whether an MS can access the network over the RACH. If
this parameter is set to a too small value, some MSs with low signal levels may access
the network and ca ll drops are likely to occur. You can set this parameter to a great va lue
to reduce the SDCCH call drop rate. The counters such as call setup success rate and
paging success rate, however, are affected.

Min DL Power on HO Ca ndidate Cell and Min Access Level Offset
The sum of the va lues of the two parameters specifies th e minimum downlink receive
level of a candidate neighboring cell for a handove r. If this parameter is set to a t oo great
value, some desired cells may be exc luded from the candidate cells; if this parameter is
set to a too small value, an unwanted cell may become the candidate cell. Both
conditions may lead to the increase of call drops.
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
INTERNAL
T3103A(ms) and T3103C(ms)
Timer T3103 series consists of T3103A, T3103C , and T8. These timers are started to
wait for a handover complete message. If the lengths of the timers are set to small values,
probably no message is received when timer T3103 series expires. In this case, the BSC
considers that the radio link in the originating cell is faulty. Then, the BSC releases the
channel in the originating cell. Thus, call dro ps occur. In the traffic measurement, if
many call drops are related to handovers (CM361: Call Drops on Radio Interface in
Handover State), you can set this parameter to a greater value. If this parameter is set to a
too great value, channel resources are wasted and TCH congestion occurs.

T3109(ms)
This parameter specifies the period for waiting for a Release Indication message after the
BSC sends a Channel Release message to the BTS. If this parameter is set to a too small
value, the link may be released before the Release Indication message is received. As a
result, a ca ll drop occurs. You can set this parameter to a greater value to reduce the
SDCCH call drop rate. It is recommended that timer T3109 be set to 1 –2 seconds longer
than timer Radio Link Timeout.

T3111(ms)
This parameter specifies the interval between the time that the main signaling link is
disconnected and the time that a channel is deactivated. The purpose is to reserve a
period of time for repeated link disconnections. If this timer is set to a too small value, a
channel may be deactivated too early. Thus, the ca ll drop rate increases.

Timers T305 a nd T308
Timers T305 a nd T308 are used o n the MSC side. Timer T305 specifies the pe riod
during which the MSC monitors the on-hook procedure. Timer T308 specifies the period
during which t he MSC monitors the resou rce release procedure. You should set the t wo
parameters when adding BSC data. Note that the modification of the data in the timer
table does not take effect. If timers T305 and T308 are set to invalid or great values, the
MSC clears the call a long time after the MS hangs up. After the T3103 and Radio Link
Timeout timers expire, the number of call drops is increased and th us the call drop rate
is significantly affected.

T200 SDCCH(5ms) and N200 of SDCCH
If T200 SDCCH(5ms) and N200 of SDCCH are set to too small values, data links are
disconnected too early. Thus, call drops a re likely to occur. If call drops occur because of
T200 expiry (M3400A), you can increase the values of T200 and N200 properly.
Neighboring relationship (affecting call drops in SDCCH handover when the signaling
channel handover is enabled)
If the neighboring cells configured in the BA2 table are incomplete, call drops are likely
to occur in the case of no suitable neighboring cell for handover and progressive
deterioration in the voice quality. Neighboring relationships should be configured
completely on the basis of the drive test data and electronic map (for example, Nastar) to
minimize the call drops due to no available neighboring cells.

Disconnect Handover P rotect Timer
This parameter is a so ftware parameter of the BSC. After receiving a DISCONNECT
message from an MS, the BSC cannot hand over the MS within the period specified by
this parameter. Therefore, the following ca se can be avo ided: After being handed over to
the target cell, the MS cannot be put on hook because it does not receive a release
acknowledgement message. You are advised to set this parameter properly.

MA and TSC settings in Um sync hronization
The settings involve the AISS function. If the TSC is planned improperly for a cell in the
synchronous network, a great error decoding probability occurs in the area covered by
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the cells with the same BCCH or MA. When the multi-frame link is set up or the
signaling is transmitted at layer 2, the error decoding cau ses ERR_IND reporting by the
BTS. In this case, call drops occur. If the value of "Call Drops due to E RR IND Received
on SDCCH in Stable State" is great, you need to optimize TOP cells.

MAIO
If FH is used in a cell, and MAIO is set improperly (for example, the FH offsets of
different TRXs in a cell are set to the same value), the frequencies collide during the FH.
In this case, the call drop rate increases.

Software Parameter 13 an d MAX TA
When Software P arameter 13 is enabled and MAX TA is set to a too small value, the
channel is released when t he TA of a call exceeds the value of MAX TA. In t his case,
call drops occur. It is recommended that Software Parameter 13 should not be enabled.
3.2.5 Checking the Interference
If inter-network interference and repeater interference exist, or if severe intra -network
interference occurs because of tight frequency reuse, call drops may occur on SDCCHs due to
poor QoS. This affects the SDCCH call drop rate.
The information on UL interference can be obtained by checking the interference bands in the
traffic statistics. The h igher percentage of band level 3-5 indicates t hat serious UL
interference exists. In this case, you need to view the distribution of interference bands in the
SDCCH measurement report of the TRX.
The interference elimination can be classified into intra-network interference elimination and
inter-network interference elimination. For d etails about interference elimination, see the
G-Guide to Eliminating Interference.
Cause
TRX-Level
Interference
MR Measurement ->
Interference Band Measurement per TRX
Mean Number of SDCCHs in Interference Band 1
Mean Number of SDCCHs in Interference Band 2
Mean Number of SDCCHs in Interference Band 3
Mean Number of SDCCHs in Interference Band 4
Mean Number of SDCCHs in Interference Band 5
3.2.6 Checking the Coverage, Antenna System, and Balance
Between Uplink and Downlink
For a cell with a high SDCCH call drop rate, you can check its coverage by the DT and CQT.
If the cases (suc h as too low DL receive level, great difference between the UL and DL levels,
poor level quality, lost DL measurement reports, and long call access time) occur, it indicates
that the problems with the coverage and imbalance between uplink and d ownlink occur in the
cell.
For the problematic cell, the call setup success rate and handover success rate slightly fall and
the receive quality becomes poor. In addition, you can analyze the cell coverage according to
the DT route and geographical conditions and then check the antenna system. Some problems
with the coverage and imbalance between uplink and downlink are caused by the reasons such
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as the antenna c overage direction, down tilt, damaged feeder, water running into the feeder,
and insecurely connected c onnector.
For solutions for the problems, see the GSM BSS Network KPI (Network Coverage)
Optimization Manual and the GSM BSS Network KPI (Uplink and Downlink Balance)
Optimization Manual.
3.2.7 Checking the Repeaters
Check whether Directly Magnifier Site Flag is set to Yes and SDCCH HO Allowed is set to
Yes in the data configuration on the L MT. If the parameters are set t o Yes, you can infer that
the cell is configured with repeaters. If the parameters are set to No, you should check
whether other operators' repeaters are installed near the cell.
If repeaters are installed, you should check whether they are wide-frequency repeaters, and
check whether the uplink/downlink amplification coefficient is too great. Ensure that the
amplification coefficient is properly set. If a repeater has an impact on the SDCCH call drop
rate, you should sw itch off the repeater.
In addition, you should check whether a repeater is faulty and whether the uplink/downlink
gain is set to a t oo great/small value. If such problems exist, the c overage area of the BTS
changes. Thus, the SDCCH call drop rate increases.
If repeater problems exist in a cell, the TA distribution varies greatly in t he traffic
measurement results. The following table lists the traffic measurement counters related to
repeaters.
Cause
Cell-Level
TRX-Level
Repeater
None
MR Measurement ->
Number of MRs Based on TA
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Test Method
The SDCCH call drop rate is one of accessibility KPIs, which can be measured through
registration or reporting of the related traffic measurement KPIs. Currently, vendors and
mobile operators use different formulas to calculate the SDCCH call drop rate, thus leading to
different values of this cou nter. In actual measurement, you should register the specific
counters and use an appropriate formula to calculate the value of this counter.
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Remarks About the Signaling Analysis of
the SDCCH Call Drop Rate
Trace the RSL signaling on the Abis interface. Then, generate the signaling trac ing file on the
LMT or through the Signal Analyze Tool. Obtain ”CO NN_FAIL” and “ERROR_IND”
signaling, and then choose Call Trace from the shortcut menu, as shown in the following
figure.
Then, right-click a piece of signaling of a call and choose Show Chart from the shortcut
menu, as shown in the following figure.
From the following figure, you can view such information as the uplink and downlink receive
level of the serving cell, uplink and downlink signal quality of the serving cell, downlink
receive level of ne ighboring cells, TA, MS powe r, and BTS power. Based on the information,
you can find out the causes for call drops, such as insufficient downlink coverage and
interference.
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Cases for SDCCH Call Drop Rate
Optimization
6.1 Case 1: SDCCH Call Drop in a Synchronous Network
Description: The 1x3 radio FH mode is used in a BTS. After the synchronization, the
SDCCH call drop rate increases. According to the t raffic measurement, the co unter "Call
Drops due to ERR IND Received on SDCCH in Stable State (Sequence Error)" increases. The
counter item is M3000C.
Cause analysis and handling: According to t he call drop signaling, the BTS reports an Error
Indication message when the MS is authenticated. In this case, an error occurs when the BTS
and MS transmit the information with serial numbers to be acknowledged on the Um interface.
According to the analysis of the layer 2 procedure, if the serial number V(r) carried in the I
frame received by the receive end is different from the expected value, an error is reported
after the specified threshold is e xceeded. According to the analysis of a TOP cell, the cell and
another cell (cross coverage) in the synchronous area use the same main BCCH and BCC
settings. In this case, the error decoding probability greatly increases when the signa ling is
transmitted at layer 2 in a synchrono us network. After the TSC is modified for cross coverage
signals, the SDCCH call drop rate reaches a normal value.
Fault point: The TSC is planned improperly in a synchronous net work.
6.2 Case 2: Call Drop Due to Imbalance Between Uplink
and Downlink
Description: The following symptoms occur during the DT: After the test MS camps on a cell,
it cannot make calls; the MS can only receive calls; call drops occur frequently at a certain
distance from the antenna.
Cause analysis and handling: When the uplink signal level is much lower than the downlink
signal level, the power may not be balanced. In this case, the authentication or assignment
procedure cannot be completed.
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The tracing result (as shown in the previous figure) shows that the uplink signal level is much
lower than the downlink signal level. Therefore, call drops occur.
6.3 Case 3: Call Drop Due to Antenna System Problem
Description: A new BTS3012 is de ployed at a site and the cell configuration is S2/2/2. After
the BTS3012 is put into operation, the number of TCH call drops in cell 1 and ce ll 2 in busy
hours reaches 20 and the number of SDCCH call drops reaches 1,000. These counters in cell 3
are normal.
Cause analysis and handling: According to the analysis of the traffic measurement results of
TRX-level radio link performance in busy hours, the number of abnormal radio links on both
TRX 3 (TRX 2 and TRX 3 are configured for cell 1) and TRX 7 (TRX 6 and TRX 7 are
configured for cell 2) is great. TRX 3 and TRX 7 are the seco nd TRX in cell 1 and cell 2
respectively; therefore, they are connected to the TXB channel of the DDPU in the
corresponding cell. The jumpers of the two non- main BCCH TRXs may be misconnected.
According to the analysis of the traffic measurement results related to the uplink and
downlink balance performance, the proportions of the TRX 3 in cell 1 and TRX 7 in cell 2 at
level 1, level 2, and level 3 a re large, which indicates that the downlink loss is too great and
the downlink transmit power is too small. Be cause TRX 2 (main BCCH TRX) in cell 1 and
TRX 6 (main BCCH TRX) in cell 2 transmit the signal throu gh the TXA port o f the DDPU in
the sector independently. When the ca ll is assigned on the channel in the non BCCH TRX, the
transmit power decreases sharply and incurs the call drop due to the inverse transmission of
TRX 3 and TRX 7. After the wrong connection between TRX 3 and TRX 7 is adjusted, the
number of call drops on t he TCH and the SDCCH in two cells is normal.
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6.4 Case 4: Call Drop Due to Transmission Problem
Description: At a certain site, the MOTO BTS is replaced with Huawei BTS and the cell
configuration is S2/2/2. At the night of the replacement, calls are made normally and drive
tests show that all performance counters are normal. The traffic measurement results within a
measurement period of 15 minutes show that MS-originated and MS-terminated calls are
made normally and handovers are performed normally. After a week o f operation, the traffic
measurement results show that the value of the counter SDCCH Seizure Request is not
normal: The maximum number of SDCCH seizure requests reaches 9,000, the number of
Successful SDCCH Seizure Requests is over 7,000, and the number o f Failed SDCCH
Seizures due to Busy SDCCH is over 900. Compared with the similar SDCCH counters, the
TCH traffic volume is small and the TCH call drop rate is high.
Cause analysis and handling: According to t he check of the ha rdware on site, making a call
is difficult on site. In addition, some subscribers complain that it is difficult to make calls after
the replacement. After obtaining the consent from the customer, the on-site engineer powers
off the BTS and loads the data again. During the initialization of the BTS, a message is
displayed, indicating that the process is disrupted and that the communication expires. Some
parameters of the BTS cannot be initialized. The BTS hardware is normal and the cable
connections are p roper.
Before the replacement, the transmission is normal. After the replacement, most of the
transmission cables are the original ones. Huawei engineers replace only the transmission
cable between the transmission equipment room and the Huawei BSC and use a new E1
connector to fix the DDF transmission cable to the E1 port on top of the BTS cabinet. The
possible cause is that the E1 connector is made improperly. In this case, the transmission bit
error rate is high and the BTS cannot be completely initialized. As a result, when a subscriber
makes a call, assigning a TCH is difficult. A detailed check sho ws that the E1 connector on
top of the BTS cabinet is made improperly. After a proper E1 connector is used, t he problem
is solved.
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Feedback Form for SDCCH Call Drop
Rate
If the SDCCH call drop rate is high and technical support is required, you need to fill in t he
following form.
Check Item
Remarks
Purpose
Software version
Software versions of the BSC and
BTS
Check whether the software version is faulty.
Data
configuration
*.dat file
Check the network optimization parameters and
power configuration.
Alarm
information
Hardware, clock, and transmission
alarms (self-check)
Check whether these alarms are generated in the
cell. Clear the alarms if they are generated.
Traffic
measurement
Cause for call drop
Measure SDCCH call drops.
Traffic
measurement
Traffic measurement results related
to the voice quality and the balance
between uplink and downlink
Check whether interference and imbalance between
uplink and downlink exist.
Signaling
RSL signaling tracing data
Check the causes for call drops.
DT data
*.log (*.cell site) or *.ant file
Based on the DT data, determine whether
interference or coverage problems exist.
Others
Engineering parameter table and
electronic map
Check the geographical information through the
Nastar software.
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