02 gsm bss network kpi (sdcch call drop rate) optimization manual

GSM BSS Network KPI (SDCCH Call Drop Rate) Optimization Manual INTERNAL

2012-4-2 Huawei Proprietary and Confidential

Copyright Huawei Technologies Co., Ltd

Page 1 of 26

Product Name Confidentiality Level

G3BSC INTERNAL

Product Version Total 26 pages

GSM BSS Network KPI (SDCCH Call Drop Rate)

Optimization Manual (For internal use only)

Prepared by Du Jian, WCDMA & GSM Network Performance Research Dept.

Date 2008-8-25

Reviewed by Date yyyy-mm-dd

Reviewed by Date yyyy-mm-dd

Approved by Date yyyy-mm-dd

Huawei Technologies Co., Ltd.

All rights reserved




Page 3 of 26

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




Page 4 of 26

Figures

Figure 1-1 Number of successful SDCCH seizures .............................................................................................. 7

Figure 1-2 Number of successful SDCCH seizures in the signaling channel handover ........................................ 7




Page 5 of 26

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:

Acronym and Abbreviation Full Spelling

SDCCH Standalone Dedicated Control Channel

MS Mobile Station

BSC Base Station Controller

KPI Key Performance Index

TCH Traffic Channel




Page 6 of 26

1 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 100%

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.




Page 7 of 26

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




Page 8 of 26

2 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 due to various 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 changed. 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:




Page 9 of 26

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 interface 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 adjacent-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 thick 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.




Page 10 of 26

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; the 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.




Page 11 of 26

3 Analysis of and Solutions to High SDCCH Call Drop Rate

3.1 Analysis Process




Page 12 of 26

BUG

Comment [x1]: Start Determine the cell with a high call drop rate

Transmission or hardware failure?

Rectify the transmission or hardware fault

The problem is caused by version upgrade?

BUGCheck version information Replace the version or install patches

The data configuration is improper?

Adjust related parameters such as handover and power

control

The interference exists? Intra-network interference? Check external interference Check frequency planning

The coverage problem occurs?

Optimize the coverage according to the coverage

optimization guide

The antenna system is faulty?

Adjust the antenna system ImUnbalance between uplink and downlink?

Optimize the imbalance between uplink and

downlink

The repeater is faulty? Rectify the repeater fault Check the call drop rate

End Yes No




Page 13 of 26

3.2 Solutions to High SDCCH Call Drop Rate

Before analyzing the causes of high SDCCH call drop rates, you should 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 analyze 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 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).

HO Access Failure M3001B

OM Intervention M3001C

Radio Resource

Unavailable M3001D

Other Causes M3001E

Call Drops due to ERR IND Received on SDCCH in Stable State

T200 Expired M3000A

Unsolicited DM

Response

M3000B

Sequence Error M3000C

Call Drops due to REL IND Received on

SDCCH Release Indication M3002

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 Number of call

drops on the SDCCH due to

transmission and

equipment causes

Call Drops Due to Equipment Failure M304

Call Drops due to Forced Handover M305

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 due 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




Page 14 of 26

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 high 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 RF cable is incorrectly connected, seizing the

SDCCH becomes difficult, and thus the SDCCH call drop rate increases. You can check 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.




Page 15 of 26

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 ->

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 increase of the SDCCH call drop rate. You can check the 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




Page 16 of 26

3.2.3 Checking the BSC and BTS Version Upgrade

If the SDCCH call drop rate increases after the BSC version or BTS version is upgraded, 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 problem, 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 should 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 downlink 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.

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 drops 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, however, 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 call drops are likely to occur. You can set this parameter to a great value

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 Candidate Cell and Min Access Level Offset

The sum of the values of the two parameters specifies the minimum downlink receive

level of a candidate neighboring cell for a handover. If this parameter is set to a too great

value, some desired cells may be excluded 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.




Page 17 of 26

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 drops 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 call 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 12 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 call drop rate increases.

Timers T305 and T308

Timers T305 and T308 are used on the MSC side. Timer T305 specifies the period

during which the MSC monitors the on-hook procedure. Timer T308 specifies the period during which the MSC monitors the resource release procedure. You should set the two

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 thus 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 are 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 Protect Timer

This parameter is a software 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 case can be avoided: 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 synchronization

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




Page 18 of 26

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 causes ERR_IND reporting by the

BTS. In this case, call drops occur. If the value of "Call Drops due to ERR 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 and MAX TA

When Software Parameter 13 is enabled and MAX TA is set to a too small value, the

channel is released when the TA of a call exceeds the value of MAX TA. In this 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 higher percentage of band level 3-5 indicates that 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 details 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 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 (such 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 downlink 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




Page 19 of 26

as the antenna coverage direction, down tilt, damaged feeder, water running into the feeder,

and insecurely connected connector.

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 LMT. If the parameters are set to 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 switch off the repeater.

In addition, you should check whether a repeater is faulty and whether the uplink/downlink

gain is set to a too great/small value. If such problems exist, the coverage 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 the 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




Page 20 of 26

4 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 counter. In actual measurement, you should register the specific counters and use an appropriate formula to calculate the value of this counter.




Page 21 of 26

5 Remarks About the Signaling Analysis of the SDCCH Call Drop Rate

Trace the RSL signaling on the Abis interface. Then, generate the signaling tracing file on the LMT or through the Signal Analyze Tool. Obtain CONN_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 neighboring cells, TA, MS power, and BTS power. Based on the information, you can find out the causes for call drops, such as insufficient downlink coverage and

interference.




Page 22 of 26




Page 23 of 26

6 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 traffic measurement, the counter "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 the 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 exceeded. 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 signaling is transmitted at layer 2 in a synchronous 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 network.

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.




Page 24 of 26

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 deployed 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 cell 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 second 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 are large, which indicates that the downlink loss is too great and

the downlink transmit power is too small. Because TRX 2 (main BCCH TRX) in cell 1 and TRX 6 (main BCCH TRX) in cell 2 transmit the signal through the TXA port of the DDPU in

the sector independently. When the call 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 the TCH and the SDCCH in two cells is normal.




Page 25 of 26

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 of 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 of 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 the check of the hardware 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 proper.

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 shows that the E1 connector on

top of the BTS cabinet is made improperly. After a proper E1 connector is used, the problem

is solved.




Page 26 of 26

7 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 the 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.

02 gsm bss network kpi (sdcch call drop rate) optimization manual

Documents

high sdcch

dd huawei technologies

overview of sdcch

huawei proprietary

network interference

date yyyy

internal use

version upgrade