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

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

2011-8-4

Huawei Proprietary and Confidential Copyright © Huawei Technologies Co., Ltd

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

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Contents 1 Overview of SDCCH Call Drop Rate.................................................6 1.1 Definition.................................................................................................................................................................. Definition.................................................................................................................................................................. 6 1.1 Definition.................................................................................................................................................................. Definition.................................................................................................................................................................. 6 1.2 Recommended Formula............................................................................................................................................6 1.2 Recommended Formula............................................................................................................................................6 1.3 Signaling Procedure and Measurement Points.........................................................................................................7 1.3 Signaling Procedure and Measurement Points.........................................................................................................7

2 Factors That Affect SDCCH Call Drop Rate.......................................8 2.1 Hardware Failure......................................................................................................................................................8 2.1 Hardware Failure......................................................................................................................................................8 2.2 Transmission............................................................................................................................................................. Transmission............................................................................................................................................................. 8 2.2 Transmission............................................................................................................................................................. Transmission............................................................................................................................................................. 8 2.3 Version Version Upgrade............................................................................................................................................ Upgrade............................................................................................................................................ ........ ...........8 ...8 2.3 Version Version Upgrade............................................................................................................................................ Upgrade............................................................................................................................................ ........ ...........8 ...8 2.4 Parameter Setting................................................................... S etting......................................................................................................................................................8 ...................................................................................8 2.4 Parameter Setting................................................................... S etting......................................................................................................................................................8 ...................................................................................8 2.5 Intra-Network and Inter-Network Interference........................................................................................................9 2.5 Intra-Network and Inter-Network Interference........................................................................................................9 2.6 Coverage Problem....................................................................................................................................................9 2.6 Coverage Problem....................................................................................................................................................9 2.7 Antenna System......................................................................................................................................................10 2.7 Antenna System......................................................................................................................................................10 2.8 Imbalance Between Uplink and Downlink.............................................................................................................10 2.8 Imbalance Between Uplink and Downlink.............................................................................................................10 2.9 Repeater..................................................................................................................................................................10 2.9 Repeater..................................................................................................................................................................10

3 Analysis of and Solutions to High SDCCH Call Drop Rate................11 3.1 Analysis Process.....................................................................................................................................................11 3.1 Analysis Process.....................................................................................................................................................11 3.2 Solutions to High SDCCH Call Drop Rate............................................................................................................13 3.2 Solutions to High SDCCH Call Drop Rate............................................................................................................13 3.2.1 Checking the Hardware....................................................................................................................................14 2011-8-4


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3.2.1 Checking the Hardware....................................................................................................................................14 3.2.2 Checking the Transmission...............................................................................................................................15 Transmission...............................................................................................................................15 3.2.2 Checking the Transmission...............................................................................................................................15 Transmission...............................................................................................................................15 3.2.3 Checking the BSC and BTS Version Upgrade.................................................................................................16 3.2.3 Checking the BSC and BTS Version Upgrade.................................................................................................16 3.2.4 Checking the Parameter Settings......................................................................................................................16 3.2.4 Checking the Parameter Settings......................................................................................................................16 3.2.5 Checking the Interference.................................................................................................................................18 3.2.5 Checking the Interference.................................................................................................................................18 3.2.6 Checking the Coverage, Antenna System, and Balance Between Uplink and Downlink................................18 3.2.6 Checking the Coverage, Antenna System, and Balance Between Uplink and Downlink................................18 3.2.7 Checking the Repeaters....................................................................................................................................19 3.2.7 Checking the Repeaters....................................................................................................................................19

4 Test Method.......................................................................... Method................................................................................20 ......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 6.1 Case 1: SDCCH Call Drop in a Synchronous Network.........................................................................................23 6.2 Case 2: Call Drop Due to Imbalance Between Uplink and Downlink...................................................................23 6.2 Case 2: Call Drop Due to Imbalance Between Uplink and Downlink...................................................................23 6.3 Case 3: Call Drop Due to Antenna System Problem..............................................................................................24 6.3 Case 3: Call Drop Due to Antenna System Problem..............................................................................................24 6.4 Case 4: Call Drop Due to Transmission Problem................................................................................................... Problem................................................................................................... 25 6.4 Case 4: Call Drop Due to Transmission Problem................................................................................................... Problem................................................................................................... 25

7 Feedback Form for SDCCH Call Drop Rate ....................................26

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Figures Number of successful SDCCH seizures...............................................7 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

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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.

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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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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 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:

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Radio link and cell parameters: 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 interference exist, or if severe intra-network interference occurs 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-net Inter-network work interfere interference nce from scrambler scramblerss or privately privately installed installed antennas antennas

2.

Inte Interf rfer eren ence ce from from the the CDMA CDMA net netwo work rk

3.

Repe Repeat ater er inte interf rfer eren ence ce

4.

Inte Interm rmod odul ulat atio ion n inter interfe fere renc ncee from from BTSs BTSs

5.

Intra-net Intra-network work co-channe co-channell and adjacent-c adjacent-channe hannell interfere interference nce

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 2011-8-4


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If the signal from an antenna is blocked or the BCCH TRX is faulty, faulty, call drops may occur.

2.7 Antenna System The following antenna system problems may affect the SDCCH call drop rate 1.

If the transmit transmit antenna antennass of two two cells are miscon misconnect nected, ed, the the uplink uplink signal signal level 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 directi directiona onall cell has has main and and diversi diversity ty antenn antennas, as, the the BCCH and and SDCCH SDCCH of the cell 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 feeder is damage damaged, d, if water water runs runs into into the feeder feeder,, or if the the feeder feeder and and the connec connector tor are not securely securely connected, connected, both the transmit power and receiver receiver sensitivity sensitivity of the antenna are reduced. Thus, call drops probably occur.

2.8 Imbalance Between Uplink and Downlink The difference between 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, 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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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否是否由于版本升级否

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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 difference between the actual SDCCH call drop rate and the expected value. You 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

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 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 2011-8-4


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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)

M300 M3001A 1A (radi radio o link link fail ailure ure)

3.2. 3.2.4 4 (check heck wheth hetheer the the radi radio o lin link failu ailure re count ounteer and the number of SACCH multi-frames are set properly)

M3001D (radio resource unavailable)

3.2.1 (check hardware failure)

M303 M303 (Abi (Abiss terr terres estr tria iall link link fail failur ure) e)

3.2. 3.2.2 2 (che (check ck the the tran transm smis issi sion on))

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 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.

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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 ->

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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

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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 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 the BSC6000 Upgrade Guide. Guide .

3.2.4 Checking the Parameter 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, th e 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 g reater 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. occur. You You can set this parameter to a great value to reduce the SDCCH call drop rate. The counters related to traffic volume, however, however, are accordingly affected. 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. occur. You 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, however, are affected. 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.

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

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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. 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 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 d eactivated. 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. 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 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. 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. 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. quality. Neighboring relationships should be configured completely on the basis of the d rive 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. 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 You are advised to set this parameter properly. 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

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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 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 v alue), 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 interference exist, or if severe intra-network interference occurs 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 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 interference elimination and inter-network inter-network interference elimination. For details about interference elimination, see the GGuide to Eliminating Interference. Interference .

Cause

TRX-Level

Interference

MR Measurement -> Interference 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. 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 as the antenna coverage direction, down tilt, damaged feeder, water running into the feeder, feeder, 2011-8-4


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and insecurely connected connector. connector. For solutions for the problems, see the GSM BSS Network KPI (Network Coverage) Optimization Manual and 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 coefficient is too great. Ensure that the amplification coefficient 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

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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, 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.

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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 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 th e uplink and downlink receive 2011-8-4


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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.

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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 According to the traffic measurement, the counter "Call Drops due to ERR IND Received on SDCCH in Stable State (Sequence Error)" increases. The 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 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 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.

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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 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 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. 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. 2011-8-4


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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 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 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. 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. improperly. After a proper E1 connector is used, the 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 the following form.

Check Item

Remarks

Purpose

Soft Softwa ware re vers versio ion n

Soft Softwa ware re vers versio ions ns of of the the BSC BSC and 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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02 GSM BSS Network KPI (SDCCH Call Drop Rate) Optimization Manual

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