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GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL
Product Name Confidentiality Level
G3BSC INTERNAL
Product Version Total 44 pages
GSM BSS Network KPI (TCH Call Drop
Rate) Optimization Manual
(For internal use only)
Prepared by WCDMA & GSM Network
Performance Research Dept.
Su Shi
Date
2008-6-28
Reviewed by Date
Reviewed by Date
Granted by Date
Huawei Technologies Co., Ltd.
All rights reserved
GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL
Contents
1 Overview of the TCH Call Drop Rate......................................................................6
1.1 Meaning.................................................................................................................................................6
1.2 Recommended Formulas.......................................................................................................................6
1.3 Signaling Procedure and Measurement Points......................................................................................8
2 Factors That Affect the TCH Call Drop Rate.........................................................11
2.1 Hardware Failure.................................................................................................................................11
2.2 Transmission Problem.........................................................................................................................11
2.3 Version Upgrade..................................................................................................................................12
2.4 Parameter Setting.................................................................................................................................12
2.5 Intra-Network and Inter-Network Interference...................................................................................13
2.6 Coverage Problem..............................................................................................................................13
2.7 Antenna System Problem....................................................................................................................14
2.8 Imbalance Between Uplink and Downlink..........................................................................................14
2.9 Repeater Problem................................................................................................................................14
3 Analysis of and Solutions to High TCH Call Drop Rate.....................................15
3.1 Solutions to High TCH Call Drop Rate...............................................................................................18
3.1.1 Checking the Hardware.................................................................................................................18
3.1.2 Checking the Transmission............................................................................................................19
3.1.3 Checking the BSC and BTS Version Upgrade..............................................................................20
3.1.4 Checking the Parameter Settings...................................................................................................20
3.1.5 Checking the Interference..............................................................................................................25
3.1.6 Checking the Coverage..................................................................................................................26
3.1.7 Checking the Antenna System.......................................................................................................27
3.1.8 Checking the Balance Between Uplink and Downlink.................................................................28
3.1.9 Checking the Repeaters.................................................................................................................29
4 Test Methods................................................................................................................30
5 Remarks About the Signaling Analysis of the TCH Call Drop Rate................30
6 Cases for TCH Call Drop Rate Optimization........................................................33
6.1 Case 1: Call Drop Due to Interference................................................................................................33
6.2 Case 2: Call Drop Due to Imbalance Between Uplink and Downlink................................................34
6.3 Case 3: Call Drop Due to Repeater Problem.......................................................................................34
6.4 Case 4: Call Drop Due to Coverage....................................................................................................35
6.5 Case 5: Reduction of Call Drops by Optimizing Handover Parameters.............................................36
GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL
6.6 Case 6: Call Drop Due to Inappropriate Parameter Setting................................................................36
6.7 Case 7: Call Drop Due to TRX Board Fault........................................................................................37
6.8 Case 8: Call Drop Due to Antenna System Problem...........................................................................38
6.9 Case 9: Call Drop Due to Transmission Problem...............................................................................38
6.10 Case 10: Call Drop Rate Doubled in a CoBCCH Network After Upgrade from V9R1 to V9R3.....39
6.11 Case 11: Increase in Call Drop Rate Due to Inactivity of T305 and T308........................................39
6.12 Case 12: Increase in Call Drop Rate Due to Change of TR1N on the MSC Side.............................40
7 Feedback Form for the TCH Call Drop Rate .........................................................41
GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL
Figures
Figure 1.1Immediate assignment procedure................................................................8
Figure 1.2Assignment procedure....................................................................................8
Figure 1.3 Intra-BSC handover procedure....................................................................9
Figure 1.4 Incoming BSC handover procedure............................................................9
Figure 1.5Procedures for analyzing high TCH call drop rate.................................16
GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL
Revision Record
Date Revision Version Change Description Author
2008-6-28 0.8 Draft completed. Su Shi
2008-7-26 1.0 The document is modified according
to review comments.
Su Shi
References
SN Document Name 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 (TCH 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
GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL
GSM BSS Network KPI (TCH Call Drop Rate)
Optimization Manual
Keywords: TCH call drop rate, KPI
Abstract: This document provides the definition and describes the test method and
optimization method of TCH call drop rate.
Acronyms:
The following table lists the acronyms and their expansion:
Acronym Expansion
TCH Traffic Channel
MS Mobile Station
BSC Base Station Controller
KPI Key Performance Indicator

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04 gsm bss network kpi (tch call drop rate) optimization manual

  • 1. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL Product Name Confidentiality Level G3BSC INTERNAL Product Version Total 44 pages GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual (For internal use only) Prepared by WCDMA & GSM Network Performance Research Dept. Su Shi Date 2008-6-28 Reviewed by Date Reviewed by Date Granted by Date Huawei Technologies Co., Ltd. All rights reserved
  • 2. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL Contents 1 Overview of the TCH Call Drop Rate......................................................................6 1.1 Meaning.................................................................................................................................................6 1.2 Recommended Formulas.......................................................................................................................6 1.3 Signaling Procedure and Measurement Points......................................................................................8 2 Factors That Affect the TCH Call Drop Rate.........................................................11 2.1 Hardware Failure.................................................................................................................................11 2.2 Transmission Problem.........................................................................................................................11 2.3 Version Upgrade..................................................................................................................................12 2.4 Parameter Setting.................................................................................................................................12 2.5 Intra-Network and Inter-Network Interference...................................................................................13 2.6 Coverage Problem..............................................................................................................................13 2.7 Antenna System Problem....................................................................................................................14 2.8 Imbalance Between Uplink and Downlink..........................................................................................14 2.9 Repeater Problem................................................................................................................................14 3 Analysis of and Solutions to High TCH Call Drop Rate.....................................15 3.1 Solutions to High TCH Call Drop Rate...............................................................................................18 3.1.1 Checking the Hardware.................................................................................................................18 3.1.2 Checking the Transmission............................................................................................................19 3.1.3 Checking the BSC and BTS Version Upgrade..............................................................................20 3.1.4 Checking the Parameter Settings...................................................................................................20 3.1.5 Checking the Interference..............................................................................................................25 3.1.6 Checking the Coverage..................................................................................................................26 3.1.7 Checking the Antenna System.......................................................................................................27 3.1.8 Checking the Balance Between Uplink and Downlink.................................................................28 3.1.9 Checking the Repeaters.................................................................................................................29 4 Test Methods................................................................................................................30 5 Remarks About the Signaling Analysis of the TCH Call Drop Rate................30 6 Cases for TCH Call Drop Rate Optimization........................................................33 6.1 Case 1: Call Drop Due to Interference................................................................................................33 6.2 Case 2: Call Drop Due to Imbalance Between Uplink and Downlink................................................34 6.3 Case 3: Call Drop Due to Repeater Problem.......................................................................................34 6.4 Case 4: Call Drop Due to Coverage....................................................................................................35 6.5 Case 5: Reduction of Call Drops by Optimizing Handover Parameters.............................................36
  • 3. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL 6.6 Case 6: Call Drop Due to Inappropriate Parameter Setting................................................................36 6.7 Case 7: Call Drop Due to TRX Board Fault........................................................................................37 6.8 Case 8: Call Drop Due to Antenna System Problem...........................................................................38 6.9 Case 9: Call Drop Due to Transmission Problem...............................................................................38 6.10 Case 10: Call Drop Rate Doubled in a CoBCCH Network After Upgrade from V9R1 to V9R3.....39 6.11 Case 11: Increase in Call Drop Rate Due to Inactivity of T305 and T308........................................39 6.12 Case 12: Increase in Call Drop Rate Due to Change of TR1N on the MSC Side.............................40 7 Feedback Form for the TCH Call Drop Rate .........................................................41
  • 4. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL Figures Figure 1.1Immediate assignment procedure................................................................8 Figure 1.2Assignment procedure....................................................................................8 Figure 1.3 Intra-BSC handover procedure....................................................................9 Figure 1.4 Incoming BSC handover procedure............................................................9 Figure 1.5Procedures for analyzing high TCH call drop rate.................................16
  • 5. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL Revision Record Date Revision Version Change Description Author 2008-6-28 0.8 Draft completed. Su Shi 2008-7-26 1.0 The document is modified according to review comments. Su Shi References SN Document Name 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 (TCH 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 INTERNAL GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual Keywords: TCH call drop rate, KPI Abstract: This document provides the definition and describes the test method and optimization method of TCH call drop rate. Acronyms: The following table lists the acronyms and their expansion: Acronym Expansion TCH Traffic Channel MS Mobile Station BSC Base Station Controller KPI Key Performance Indicator
  • 7. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL 1 Overview of the TCH Call Drop Rate 1.1 Meaning The TCH call drop rate refers to the ratio of call drops to successful TCH seizures after the BSC successfully assigns TCHs to MSs. The TCH call drop rate can be measured from the following aspects:  TCH call drop rate (including handover)  TCH call drop rate (excluding handover) The TCH call drop rate, one of the most significant KPIs for telecom operators, is related to retainability. It indicates the probability of call drops due to various reasons after MSs access TCHs. A too high TCH call drop rate adversely affects the user's experience. 1.2 Recommended Formulas BSC32: TCH call drop rate (including handover) = (Number of TCH call drops + Number of TCH call drops during very early assignment)/Number of successful TCH seizures x 100% TCH call drop rate (excluding handover) = Number of call drops on TCH/(Number of successful TCH seizures + Number of successful incoming internal inter-cell handovers + Number of successful incoming external inter-cell handovers – Number of successful outgoing internal inter-cell handover] – Number of successful outgoing external inter-cell handovers) x 100% BSC6000: TCH call drop rate (including handover) = Number of call drops on TCH/(Number of successful TCH seizures (signaling channel) + Number of successful TCH seizures (TCH) + Number of successful TCH Seizures in TCH handovers (TCH)) x 100%
  • 8. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL TCH call drop rate (excluding handover) = Number of call drops on TCH/Number of successful TCH seizures (TCH) x 100% Through the analysis of each pair of formulas, you can find out that the TCH call drop rate (including handover) is lower than the TCH call drop rate (excluding handover) irrespective of the BSC model (BSC32 or BSC6000). The following takes the formulas for the BSC32 as an example. The number of call drops on TCH is small during the very early assignment procedure. Therefore, the difference between the numerator of the formula for the TCH call drop rate (including handover) and that of the formula for the TCH call drop rate (excluding handover) can be omitted. Including the TCH seizures in the case of handovers, the denominator of the formula for the TCH call drop rate (including handover) is greater than the denominator of the formula for the TCH call drop rate (excluding handover). Therefore, the result of the formula for the TCH call drop rate (including handover) is smaller than that of the formula for the TCH call drop rate (excluding handover). For details, refer to the GSM BSS Network KPI (TCH Call Drop Rate) Baseline.
  • 9. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL 1.3 Signaling Procedure and Measurement Points Figure 1.1 Immediate assignment procedure Figure 1.2 Assignment procedure
  • 10. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL Figure 1.3 Intra-BSC handover procedure Figure 1.4 Incoming BSC handover procedure The meanings of the measurement points in the these figures are as follows: TCH-SUCC-A: indicates the number of successful TCH seizures. TCH-SUCC-B: indicates the number of successful incoming internal inter-cell handovers plus the number of successful internal intra-cell handovers.
  • 11. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL TCH-SUCC-C: indicates the number of successful incoming external inter-cell handovers. TCH-SUCC: indicates the number of successful TCH seizures during the very early assignment procedure.
  • 12. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL 2 Factors That Affect the TCH Call Drop Rate According to user complaints and network optimization experience, the major factors that affect the TCH call drop rate are as follows:  Hardware failure  Transmission problem  Version upgrade  Parameter setting  Intra-network and inter-network interference  Coverage problem  Antenna system problem  Imbalance between uplink and downlink  Repeater problem 2.1 Hardware Failure When a TRX or a combiner is faulty, seizing the TCH becomes difficult, and thus the TCH call drop rate increases. 2.2 Transmission Problem The TCH call drop rate increases in the following conditions:  The transmission quality on the A or Abis interface is poor for various reasons.  Transmission links are unstable.
  • 13. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL 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 TCH call drop rate increases. 2.4 Parameter Setting The settings of some parameters on the BSC and MSC sides may affect the TCH call drop rate. If the following situations occur, the TCH call drop rate may increase: 1. The parameters SACCH Multi-Frames and Radio Link Timeout are set to too small values. 2. The parameter RXLEV_ACCESS_MIN is set to a too small value. 3. The parameter RACH Min.Access Level is set to a too small value. 4. The parameters Min DL Power on HO Candidate Cell and Min Access Level Offset are inappropriately set. 5. The length of timer T3103 (this timer is set to wait for a Handover Complete message) is set to a too small value. 6. The length of timer T3109 (this timer is set to wait for a Release Indication message) is set to a too small value. 7. The length of timer T3111 (this timer specifies the connection release delay) is set to a too small value. 8. The length of timer T305/T308 is set to an invalid or too great value. 9. The parameter TCH Traffic Busy Threshold is set to a too small value. 10. The parameter Call Reestablishment Forbidden is set to Yes. 11. The parameters related to edge handover are inappropriately set. 12. The parameters related to BQ handover are inappropriately set. 13. The parameters related to interference handover are inappropriately set. 14. The parameters related to concentric cell handover are inappropriately set. 15. The parameters related to power control are inappropriately set. 16. T200 and N200 are set to too small values. 17. Some neighboring cell relations are not configured. 18. The parameter MAIO is inappropriately set. 19. The parameter Disconnect Handover Protect Timer is set to a too small value. 20. The parameter TR1N is set to a too small value. 21. The parameters Software Parameter 13 and MAX TA are set to too small values. 22. If a repeater is used, the parameter Directly Magnifier Site Flag is set
  • 14. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL to No. 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 TCHs because of poor QoS. This adversely affects the TCH 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 of China Unicom 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 TCH call drop rate. 1. Discontinuous coverage (blind areas) The voice quality at the edge of an isolated BTS is poor and calls cannot be handed over to other cells. In this case, call drops may occur. In complex terrains such as mountainous regions, the signals are blocked and thus the transmission is discontinuous, leading to call drops. 2. Poor indoor coverage Densely distributed buildings and thick walls cause great attenuation and low indoor signal level, which causes call drops. 3. Cross coverage (isolated BTS) The serving cell causes cross coverage due to various reasons (such as excess power). An MS cannot be handed over to another cell due to no suitable neighboring cells. In this case, the signal level becomes low and the voice quality of the MS deteriorates. Thus, call drops occur. 4. Insufficient coverage If the signal from an antenna is blocked or the BCCH TRX is faulty, call drops may occur because of discontinuous coverage.
  • 15. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL 2.7 Antenna System Problem The following antenna system problems may affect the TCH call drop rate 1. If the transmit antennas of two cells are improperly connected, 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 places 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, water leaks in the feeder, or the feeder and the connector are not securely connected, both the transmit power and receiver sensitivity of the antenna are reduced. Thus, call drops may 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 Problem 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 TCH 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 TCH call drop rate increases.
  • 16. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL 3 Analysis of and Solutions to High TCH Call Drop Rate Figure 1.5 shows the procedures for analyzing high TCH call drop rate.
  • 17. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL Figure 1.5 Procedures for analyzing high TCH call drop rate
  • 18. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL
  • 19. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL 3.1 Solutions to High TCH Call Drop Rate Before analyzing the causes of high TCH call drop rates, you should find out the difference between the actual TCH call drop rate and the expected value. You should also find out the impact of the problems and the related KPIs. You can analyze the distribution of call drops based on the traffic measurement. If a certain type of call drop accounts for a large proportion of total call drops, you can locate the fault by performing the corresponding procedure. If no obvious causes are found, you can perform the procedures shown in Figure 1.5. The following table lists the mapping between the traffic measurement counters and the troubleshooting procedures. Traffic Measurement Counter Troubleshooting Procedure M3100A (T200 expiry) 3.1.4Checking the Parameter Settings M3101A (radio link failure) 3.1.4Checking the Parameter Settings M3101D (radio resource unavailable) 3.1.1Checking the Hardware CM333 (Abis terrestrial link failure) 3.1.2Checking the Transmission M314 (device failure) 3.1.1Checking the Hardware The following sections describe the solutions to high TCH call drop rates. The traffic measurement counters and KPIs in this document are the same as those in the BSC6000V9R8C01B051 version. 3.1.1 Checking the Hardware If a TRX or a combiner is faulty or if an RF cable is incorrectly connected, seizing the TCH becomes difficult, and thus the TCH call drop rate increases. See Case 7: Call Drop Due to TRX Board Fault. You can check whether hardware is faulty by viewing BTS alarms or viewing the hardware state on the Site Device Panel of the LMT. Table 1.1 lists the major BSC alarms related to hardware failures. Table 1.1 Major BSC alarms related to hardware failure Alarm ID Alarm Name 1000 LAPD_OML Fault Alarm 2204 TRX Communication Alarm 4414 TRX VSWR Alarm
  • 20. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL 3606 DRU Hardware Alarm In addition, you can locate the fault by checking the traffic measurement related to hardware failures. Cause BSC Level Cell Level Equipmen t failure Access Measurement per BSC in BSC Measurement: Available TCHs per BSC Configured TCHs per BSC Available TCHs per BSC KPI Measurement per Cell: TCH Availability Available TCHs Configured TCHs TRX Measurement per Cell: Number of configured TRXs in a cell Number of available TRXs in a cell Call Drop Measurement per Cell in Call Measurement: Call Drops due to Equipment Failure (TCH) 3.1.2 Checking the Transmission Poor transmission quality, unstable transmission links, or insufficient resources on the Abis and A interface may lead to the increase of the TCH 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 failure has occurred. Then, you should check the transmission connections. See Case 9: Call Drop Due to Transmission Problem. Table 1.1 BSC alarms related to transmission Alarm ID Alarm Name 1000 LAPD_OML Fault Alarm 11270 LAPD Alarm 11278 E1 Local Alarm 11280 E1 Remote Alarm 20081 Loss of E1/T1 Signals (LOS)
  • 21. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL 20082 E1/T1 Frame Out-of-Synchronization (LOF) In addition, you can locate the fault by checking the traffic measurement related to transmission failures. Cause BSC Level Cell Level Transmission failure LAPD Measurement in BSC Measurement Channel Activation Measurement per Cell in Call Measurement: CHAN ACTIV NACK Messages Sent by BTS Channel Activation Timeouts Call Drop Measurement per Cell in Call Measurement: Measurement of Call Drops Due to Abis Terrestrial Link Failure 3.1.3 Checking the BSC and BTS Version Upgrade If TCH call drop 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. See Case 6: Call Drop Due to Inappropriate Parameter Setting and Case 10: Call Drop Rate Doubled in a CoBCCH Network After Upgrade from V9R1 to V9R3. To locate the problem, you can check the version description document and the related documents, or provide 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 requisite patch. For details, refer to the BSC6000 Upgrade Guide. 3.1.4 Checking the Parameter Settings The parameter settings on the BSC side and MSC side may affect the TCH call drop rate. You should check the settings of the following parameters for a cell with a high TCH call drop rate. See Case 5: Reduction of Call Drops by Optimizing Handover Parameters and Case 12: Increase in Call Drop Rate Due to Change of TR1N on the MSC Side. 1. SACCH Multi-Frames
  • 22. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL 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 counter is 0, the BTS regards the radio link as faulty. In the traffic measurement, if there are many call drops (M3101A) 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. 2. Radio Link Timeout This parameter determines whether a downlink radio link is faulty. Each time the BTS fails to decode the measurement report sent over the SACCH by the MS, the counter decreases by 1. Each time the BTS successfully decodes the measurement report sent over 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 (M3101A) 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. 3. 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 TCH call drop rate. The counters such as call setup success rate and the counters related to traffic volume, however, are accordingly affected. 4. 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 TCH call drop rate. The counters such as call setup success rate and paging success rate, however, are affected. 5. 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. 6. Timer T3103 series Timer T3101 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
  • 23. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL call drops are related to handovers (CM331: 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. 7. Timer T3109 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 TCH call drop rate. It is recommended that timer T3109 be set to 1–2 seconds longer than timer Radio Link Timeout. 8. Timer T3111 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, call drops increase. 9. 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 TCH call drop rate is significantly affected. 10. TCH Traffic Busy Threshold If the current channel seizure ratio exceeds the value of this parameter, the BSC preferentially assigns a half-rate channel to a dualrate-enabled call. Otherwise, the BSC assigns a full-rate channel to the dualrate-enabled call. Compared with a full-rate channel, a half-rate channel has weak anti- interference capabilities. Therefore, if a large number of half-rate channels are assigned, the TCH call drop rate increases. It is recommended that this parameter should not be set to a too small value if congestion is unlikely to occur. 11. Call Reestablishment Forbidden This parameter specifies whether to allow call reestablishment. In case of burst interference or radio link failure due to blind areas caused by high buildings, call drops occur. In this case, MSs can initiate the call reestablishment procedure to restore communication. To reduce the TCH call drop rate, you can set this parameter to No to allow call reestablishment. In certain conditions, allowing call reestablishment greatly reduces the TCH call drop rate. Call reestablishment lasts for a long time, and therefore some subscribers cannot wait and hang up. This affects user experience.
  • 24. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL 12. Parameters related to edge handover When the receive level drops greatly, an edge handover cannot be performed in time in any of the following conditions: The parameter Edge HO UL RX_LEV Threshold or Edge HO DL RX_LEV Threshold is set to a small value; the parameter Inter-cell HO Hysteresis is set to a great value; the parameters Edge HO Watch Time and Edge HO AdjCell Watch Time are set to great values; the parameters Edge HO Valid Time and Edge HO AdjCell Valid Time are set to great values. As a result, a call drop occurs. To reduce the TCH call drop rate, you can appropriately set these parameters so that edge handovers can be performed in time to avoid call drops. 13. Parameters related to BQ handover When the signal quality deteriorates, a BQ handover cannot be performed in time in any of the following conditions: The parameters ULQuaLimitAMRFR, ULQuaLimitAMRHR, UL Qual. Threshold, DLQuaLimitAMRFR, DLQuaLimitAMRHR, and DL Qual. Threshold are set to great values; the parameter BQ HO Margin is set to a small value; the parameter Inter-cell HO Hysteresis is set to a great value. As a result, call drops occur. To reduce the TCH call drop rate, you should appropriately set these parameters so that BQ handovers can be performed in time to avoid call drops. 14. Parameters related to interference handover If the parameters RXQUAL1 to RXQUAL12 are set to great values or if the RXLEVOff parameter is set to a great value, strong interference may occur. In this case, if interference handovers are not performed in time, call drops occur. To reduce the TCH call drop rate, you can appropriately set these parameters so that interference handovers can be performed in time to avoid call drops. If the parameters RXQUAL1 to RXQUAL12 are set to small values, the number of handovers due to other causes increases greatly, thus affecting the handover success rate. 15. Parameters related to concentric cell handover A call at the edge of the overlaid subcell cannot be handed over to the underlaid subcell in any of the following conditions: In the case of a normal concentric cell, the parameters RX_LEV Threshold and RX_LEV Hysteresis are set to great values; in the case of an enhanced concentric cell, the parameter OtoU HO Received Level Threshold is set to a great value. As a result, a call drop is likely to occur. If the Call Drop Ratio on TCH on the TRX in the OverLaid Subcell (RM330a) is high, you can appropriately set these parameters so that calls at the edge of the overlaid subcell can be handed over to the underlaid subcell in time. When a call in the underlaid subcell has interference, the call cannot be handed over to the overlaid subcell if the RX_QUAL for UO HO Allowed parameter is set to Yes and the RX_QUAL Threshold parameter is set to a great value. Thus, a call drop occurs. If the Call Drop Ratio on TCH on the TRX in the Underlaid Subcell (RM330) is high, you can set these parameters properly so that the call can be handed over to the overlaid subcell at the earliest.
  • 25. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL 16. Parameters related to power control If the power control level and quality threshold are set to small values, call drops are likely to occur because of low signal level or bad voice quality. 17. T200 and N200 If the parameters T200 FACCH/F, T200 FACCH/H, N200 of FACCH/Full rate, and N200 of FACCH/Half rate are set to small values, data links are disconnected too early. Thus, all drops are likely to occur. If call drops occur because of T200 expiry, you can increase the values of T200 and N200 properly. 18. Neighboring cell relations 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 cell relations 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. 19. MAIO If frequency hopping (FH) is applied in a cell and the MAIO is set inappropriately (for example, different TRXs serving the same cell have the same MAIO), frequency collision may occur during FH. Thus, the TCH call drop rate increases. 20. 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. 21. TR1N This parameter should be set on the MSC side. It is used to avoid the retransmission of short messages. When this parameter is set to a too great value, the MSC does not send a CLEAR CMD message if the MS receives a short message during link disconnection. As a result, the MS sends the BTS a DISC message to disconnect layer 2 connection. After receiving the DISC message, the BTS sends a REL_IND message to the BSC. Then, the BSC sends a CLEAR REQ message to the MSC and the number of call drops is incremented by one. 22. Software Parameter 13 and MAX TA When the parameter Software Parameter 13 is enabled and the parameter MAX TA is set to a too small value, the channel is released when the TA of a call exceeds the MAX TA. In this case, call drops occur. It is recommended that the parameter Software Parameter 13 should not be enabled. 23. Directly Magnifier Site Flag
  • 26. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL If a BTS is installed with repeaters, the handover between repeaters can only be asynchronous because the distance between repeaters is long. If synchronous handovers are performed, the handovers may fail and thus many call drops occur. Therefore, when a BTS is installed with repeaters, the parameter Directly Magnifier Site Flag should be set to Yes to avoid synchronous handovers between cells under the same BTS. You can check whether the data configuration is correct by viewing the traffic measurement results. The following table lists the traffic measurement counters related to repeaters. Cause Cell Level TRX Level Parameters Call Drop Measurement per Cell in Call Measurement: Call Drops on TCH in Stable State (Error Indication) Call Drops on TCH in Stable State (Connection Failure) Call Drops on TCH in Stable State (Release Indication) Call Drop Ratio on TCH on the TRX in the Underlaid Subcell Call Drop Ratio on TCH on the TRX in the Overlaid Subcell KPI Measurement per Cell: Call Drops in TCH Handovers (Traffic Channel) Call Drops on TCH in Stable State (Traffic Channel) Measurement of Power Control Messages in MR Measurement Radio Link Failure Measurement in MR Measurement 3.1.5 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 TCHs due to bad QoS. This affects the TCH call drop rate. See Case 1: Call Drop Due to Interference. The uplink interference information can be obtained on the basis of the interference band distribution in the traffic measurement results. A large proportion of interference levels belong to interference bands 3–5, you can infer that the uplink has strong interference. You can obtain the downlink interference information by performing drive tests or by analyzing the traffic measurement results related to receive quality. For details on how to check the intra-network and inter-network
  • 27. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL interference, refer to the G-Guide to Eliminating Interference. The following table lists the traffic measurement counters related to interference. (If a cell has interference, the TCH call drop rate is high. In addition, the handover success rate decreases and the handovers due to bad quality account for a large proportion of all handovers.) Cause Cell Level TRX Level Interference Outgoing Internal Inter-Cell Handover Measurement per Cell in Call Measurement: Number of Outgoing Internal Inter-Cell Handover Requests (Signal Quality) Number of Outgoing Internal Inter-Cell Handover Requests (Other Causes) Interference Band Measurement per TRX in MR Measurement Mean Number of TCHs in Interference Band 1 Mean Number of TCHs in Interference Band 2 Mean Number of TCHs in Interference Band 3 Mean Number of TCHs in Interference Band 4 Mean Number of TCHs in Interference Band 5 Measurement of Receive Quality in MR Measurement 3.1.6 Checking the Coverage You can check the network coverage by conducting outdoor drive tests or indoor dialing tests. During the tests, you can infer that the network coverage is insufficient if the following conditions occur: The downlink receive level is low (lower than –110 dBm) so that the test MS cannot access the network or the voice quality is bad; a handover cannot be performed because of no suitable neighboring cells and the signal quality gradually deteriorates. See Case 4: Call Drop Due to Coverage. If the network coverage in a cell is insufficient, the TCH call drop rate is high. In addition, the handover success rate is low, and a large proportion of handovers are performed because of weak signal strength. You can check whether a cell has coverage problems by viewing the traffic measurement results. The following table lists the traffic measurement counters related to coverage.
  • 28. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL Cause Cell Level TRX Level Coverag e Outgoing Internal Inter-Cell Handover Measurement per Cell in Call Measurement: Number of Outgoing Internal Inter-Cell Handover Requests (Signal Strength) Outgoing External Inter-Cell Handover Measurement per Cell in Call Measurement: Outgoing External Inter-Cell Handover Requests (Uplink Strength) Outgoing External Inter-Cell Handover Requests (Downlink Strength) TCHF Receive Level Measurement per TRX in MR Measurement TCHH Receive Level Measurement per TRX in MR Measurement If a coverage problem exists, you can solve the problem through the following methods: adjusting the tilt of the antenna, increasing the transmit power, adding repeaters, or changing the combining mode. For details, refer to the GSM BSS Network Performance KPI (Coverage) Optimization Manual. 3.1.7 Checking the Antenna System In the case of dual-transmit antennas, ensure that the tilt and azimuth of one antenna are the same as those of the other antenna. In addition, you should check whether the jumpers are improperly connected (for example, by analyzing drive test data). If a jumper is improperly connected, the uplink signal level in the cell is significantly lower than the downlink signal level, and call drops are likely to occur at a place far away from the BTS. You should ensure that the jumpers are connected correctly. See Case 8: Call Drop Due to Antenna System Problem. If the feeder is damaged, water runs into the feeder, or 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. You can locate these problems by checking the VSWR alarms. If any feeder is faulty, you should replace it in time. If the antenna system has problems, the TCH call drop rate and handover failure rate are high. In addition, the difference between the uplink quality and the downlink quality is great, or both the uplink quality and the downlink quality are bad. You can check whether the antenna system is faulty by viewing the traffic measurement results. The following table lists the traffic measurement counters related to the antenna system.
  • 29. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL Cause Cell Level TRX Level Antenna system KPI Measurement per Cell in Call Measurement: Success Rate of Radio Handover Outgoing External Inter-Cell Handover Measurement per Cell in Call Measurement: Outgoing External Inter-Cell Handover Requests (Uplink Strength) Outgoing External Inter-Cell Handover Requests (Downlink Strength) Measurement of Receive Quality in MR Measurement Uplink-and-Downlink Balance Measurement per TRX in MR Measurement 3.1.8 Checking the Balance 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. See Case 2: Call Drop Due to Imbalance Between Uplink and Downlink. To analyze the balance between the uplink and the downlink, check whether the transmit power of the BTS is too high. Then, you should check whether such components as the TMA, BTS amplifier, and antenna port that affect downlink receive level have problems. For details, refer to the GSM BSS Network Performance KPI (Uplink and Downlink Balance) Optimization Manual. If the uplink and downlink are imbalanced, the following conditions may occur: The difference between the mean uplink receive level and the mean downlink receive level is great; the uplink and downlink balance level is high; the immediate assignment success rate and the assignment success rate are low. The following table lists the traffic measurement counters related to the balance between the uplink and the downlink.
  • 30. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL Cause Cell Level TRX Level Balance between uplink and downlink Assignment Measurement per Cell in Call Measurement: TCH Assignment Success Ratio Success Rate of Call Establishment Immediate Assignment Measurement per Cell in Call Measurement: Success Rate of Immediate Assignments Uplink-and-Downlink Balance Measurement per TRX in MR Measurement TCHF Receive Level Measurement per TRX in MR Measurement TCHH Receive Level Measurement per TRX in MR Measurement 3.1.9 Checking the Repeaters Check whether the parameter Directly Magnifier Site Flag is set to Yes in the data configuration on the LMT. If this parameter is set to Yes, you can infer that the cell is configured with repeaters. If this parameter is 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 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 TCH 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 TCH call drop rate increases. See Case 3: Call Drop Due to Repeater Problem. 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 Repeate r None Number of MRs based on TA per TRX in MR Measurement
  • 31. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL 4 Test Methods The TCH call drop rate is one of retainability KPIs, which can be obtained through registration of or reporting of the related traffic measurement counters. In addition, the TCH call drop rate is one of key drive test counters and it can be obtained through drive tests. At present, the formula for the TCH call drop rate varies with device manufacturer and with telecom operator, thus affecting the value of the TCH call drop rate. During actual tests, you should register the specific counters and use an appropriate formula to calculate the value of the TCH call drop rate. 5 Remarks About the Signaling Analysis of the TCH 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 the CONN_FAIL and ERROR_INC signaling from the file. Then, right-click a piece of signaling and choose Call Trace from the shortcut menu, as shown in the following figure.
  • 32. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL Then, right-click the 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 of call drops, such as insufficient downlink coverage and interference.
  • 33. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL
  • 34. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL 6 Cases for TCH Call Drop Rate Optimization 6.1 Case 1: Call Drop Due to Interference Symptom description: A certain BTS adopted the 1x3 RF FH mode. After the capacity of the BTS was expanded, the TCH assignment failure rate remained high (because of radio link failure), and the TCH drop rate and handover failure rate were high. The SDCCH call drop rate, however, was normal. Cause analysis and handling: In the case of high TCH assignment failure rate, TCH call drop rate, and handover failure rate, you can infer that there are two probabilities: Failure occurs during the TCH assignment, and the frequency or timeslot used by the call has interference or is unstable. In the case of normal SDCCH call drop rate, there is a low probability that the BCCH TRX and the BCCH frequency have interference. Therefore, there is a high probability that the non-BCCH TRX and the FH frequency have interference. The check of the device hardware, antenna system, and transmission stability finds no problems. Through drive tests, you can find a high ratio of high signal level and low voice quality. Dialing tests show that the voice quality is bad. When checking the parameter settings, you can find that the MAIO of the new TRX is the same as the MAIO of another TRX. The cause of the fault is frequency collision because the same MAIO is used.
  • 35. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL 6.2 Case 2: Call Drop Due to Imbalance Between Uplink and Downlink Symptom description: The following symptoms occurred during drive tests: After the test MS camped on a cell, it could not make calls; the MS can only receive calls; call drops occurred frequently at a certain distance from the antenna; a call drop occurred after frequent handovers. Cause analysis and handling: The cause may be the uplink signal level is much lower than downlink signal level. During drive tests, move the test MS towards the edge of the cell, and use the MA10 signaling analysis tool to trace the signaling on the BTS side. 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 Repeater Problem Symptom description: Under a BTS, the TCH call drop rate in cell 3 reached 10%; however, the call drop rate and congestion rate in cell 1 and cell 2 remained normal. Cause analysis and handling: 1. Block the channels in the cell. The congestion rate in cell 3, however, remains high. 2. Check the traffic measurement results. The distribution of interference bands is regular, that is, the interference is high during peak traffic hours and is low during low traffic hours. 3. Change the frequency of cell 3 so that the spacing between the frequency and the original one is at least 1 MHz. The interference, however, persists. Therefore, the probability of co-channel interference and adjacent-channel interference is eliminated. 4. Ensure that the devices are not faulty. 5. Find the external interference. 6. Use a spectrum analyzer to perform frequency scan tests. The signal from a certain frequency (the central frequency is 904.14 MHz and the spectrum bandwidth is 300 kHz) exists continuously and it is similar to the signal from an analog spectrum. The strength of the signal at the divider
  • 36. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL port of cell 3, cell 2, and cell 1 is –27 dBm, –40 dBm, and –60 dBm respectively, and the signal strength is consistent with the interference level. The traffic volume in daytime is greater than that at night, and thus the probability of intermodulation is high. It can be concluded that the 904.14 MHz frequency is the interference source. When a spectrum analyzer is used to perform drive tests, the interference source cannot be located. When tests are performed on a rooftop, it is found that the interference is generated by a small antenna of a repeater. If the signal from the antenna is blocked, all the test results are normal. Therefore, the interference signal is generated by the antenna. 6.4 Case 4: Call Drop Due to Coverage Symptom description: Subscribers complained that call drops occurred frequently when calls were made on the fifth or the higher floors of a building. Cause analysis: Step 1: Perform onsite tests. Call drops occur and interference exists. When a call drop occurs, the MS is located in a cell that does not belong to local BTS A. Step 2: Confirm that the cell belongs to BTS B, which is about three or four kilometers away from the building. Therefore, the signal from a cell under BTS B is reflected by an obstruction and then is received by the MS. A cross coverage area is formed on the fifth or above floor of the building. Step 3: Check the data configuration. In the BSC data configuration, cell 2 of BTS A is configured as a neighboring cell of BTS B, but cell 3 of BTS A is not. When the MS in the area uses the signal from cell 2 of BTS B, the signal from cell 3 of BTS A is stronger than that from cell 2 of BTS B. In this case, handovers cannot be performed because cell 3 of BTS A is not a neighboring cell of BTS B. The signal from cell 2 of BTS B may be reflected multiple times before it is received by the MS. If the signal becomes weak suddenly, an emergency handover is required. In this case, if both cells 2 and 3 are not the best candidate cells for the handover, the MS may be handed over to a cell under BTS C. The MS, however, cannot receive signals from BTS C. Thus, a call drop occurs. Handling: Modify the BA1 (BCCH) table, BA2 (SACCH) table, and neighboring cell relation table in the BSC data configuration. Ensure that cell 3 under BTS A is a neighboring cell of cell 2 under BTS B. Network engineering parameters are further optimized to eliminate the cross coverage problem. Subsequent tests show that the call drop problem is solved. Conclusion: You can use the following methods to solve the cross coverage problem: 1. Adjusting the antenna of the cross coverage cell to eliminate cross coverage 2. Defining new neighboring cells for the cross coverage cell
  • 37. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL 6.5 Case 5: Reduction of Call Drops by Optimizing Handover Parameters Symptom description: During drive tests, call drops occurred frequently at a cave entrance near the BTS because handovers were not performed in time. Before the MS entered the cave, the signal level of both the serving cell and the target cell was high, and thus a handover was not triggered. After the MS entered the cave, the signal level of the target cell was proper (about 80 dBm), but the signal level of the original serving cell rapidly decreased to below 100 dBm. Therefore, a call drop occurred before the measurement period ended. Cause analysis and handling: Modify the settings of the related parameters. Parameter Name Before Modification After Modification PBGT Watch Time 5 3 PBGT Valid Time 4 2 PBGT HO Threshold 72 68 UL Qual. Threshold 70 60 Min DL Power on HO Candidate Cell 10 15 You can adjust the handover parameters to reduce call drops in the following ways: 1. If there is no frequent audio discontinuity or ping-pong handover, set the parameters properly so that PBGT handovers can be easily performed, thus minimizing the interference and reducing the call drop rate. 2. Set the emergency handover threshold properly so that emergency handovers are triggered before call drops occur. 6.6 Case 6: Call Drop Due to Inappropriate Parameter Setting Symptom description: After a cutover of five BTSs for capacity expansion, the TCH call drop rates in the cells under these BTSs were high (reached 5%). The number of call drops in each cell was about 100. Among the five BTSs, one BTS that had no capacity change also had a high TCH call drop rate. The causes of all call drops were related to radio frequency. There was no interference, and
  • 38. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL the BTS hardware was not faulty. Cause analysis and handling: Check the data configuration, frequency planning, BSIC planning, and traffic measurement results. All the interference bands are normal and no interference exists. The handover success rate is over 93% and thus handovers are performed normally. Check the versions of each TRX and FPU. It is found that the TRX version is inconsistent with the FPU version. Upgrade the TRX and FPU to ensure that their versions are compatible, but the problem persists. Check the data configuration again. It is found that the BTS after capacity expansion adopts the 15:1 multiplexing mode, and that the measurement report preprocessing function is enabled for the BTS2X. The BTS2X in some versions, however, do not support the measurement report preprocessing function. Therefore, the TCH call drop rate is too high. After a large-scale adjustment is performed on the system, for example, BTS cutover, BTS capacity expansion, frequency replanning, upgrade, patch installation, the related system parameters should be checked completely and adjusted if required. The following parameters should be checked: neighboring cell relations, frequency interference conditions, FH parameters, and cell parameters. Special attention should be paid to the BTS version. 6.7 Case 7: Call Drop Due to TRX Board Fault Symptom description: During dialing tests, call drops occurred frequently in cell 2 of a BTS. Cause analysis: The traffic measurement results show that the TCH congestion rate in the cell exceeds 10% and that the incoming handover failure rate is high. The remote maintenance terminal shows that one TRX board in the cell is not normal. Thus, the TRX board may be faulty. Handling: Use the test MS to make calls repeatedly on only the frequency of the faulty TRX board. It is found that call drops occur on timeslots 1, 3, 5, and 7 and that calls are made normally on timeslots 2, 4, 6, and 8. Remove the TRX board and then insert it into another slot. The problem persists. Insert a functional TRX board into the slot of the faulty TRX board. Calls are made normally. Then, insert the faulty TRX board into another cabinet. The problem persists. As a conclusion, the TRX board is faulty. Insert a spare board into the slot of the faulty TRX board, calls are made normally. Summary: When tests are performed on the BTS side, each TRX and each timeslot on the TRX should be tested. You should ensure that bi-directional calls can be made on each TCH and that the voice quality is good.
  • 39. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL 6.8 Case 8: Call Drop Due to Antenna System Problem Symptom description: A new BTS3012 was deployed at a site and the cell configuration was S2/2/2. After the BTS3012 was put into operation, the number of TCH call drops in both cell 1 and cell 2 in busy hours reached 20, the number of SDCCH call drops in cell 3 in busy hours reached 1,000. These counters in cell 3 were normal. Cause analysis and handling: Analyze the traffic measurement results of TRX-level radio link performance in busy hours. It is found that 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-BCCH TRXs may be improperly connected. Analyze the traffic measurement results related to the uplink and downlink balance performance. It is found that uplink and downlink imbalance levels 1, 2, and 3 account for a large proportion of all imbalance levels for both TRX 3 and TRX 7. This indicates that the downlink loss is too great or the downlink transmit power is too low. TRX 2 (main BCCH TRX) and TRX 6 (main BCCH TRX) are connected to the TXA channel of the DDPU in cell 1 and cell 2 respectively. When calls are assigned with the channels on the non-BCCH TRX, transmit power decreases sharply because the feeders of TRX 3 and TRX 7 are improperly connected. Thus, call drops occur. Rectify the misconnection and find that the TCH call drop rate and SDCCH call drop rate in both cell 1 and cell 2 become normal. 6.9 Case 9: Call Drop Due to Transmission Problem Symptom description: At a certain site, the MOTO BTS was replaced by the Huawei BTS and the cell configuration was S2/2/2. On the night of the replacement, calls were made normally and drive tests showed that all performance counters were normal. The traffic measurement results within a period of 15 minutes showed that MS-originated and MS-terminated calls were made normally and handovers were performed normally. After a week of operation, the traffic measurement results showed that the value of the counter SDCCH Seizure Request was not normal: The maximum number of SDCCH seizure requests reached 9000, the number of Successful SDCCH Seizure Requests was over 7000, and the number of Failed SDCCH Seizures due to Busy SDCCH was over 900. Compared with the similar SDCCH counters, the TCH traffic volume is small and the TCH call drop rate is high.
  • 40. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL Cause analysis and handling: Check the hardware on site. It is found that making a call is difficult on site. In addition, some subscribers complained that it was difficult to make calls after the replacement. After obtaining the consent from the customer, the onsite engineers power off the BTS and load the data again. During the initialization of the BTS, a message is displayed, indicating that the process is disrupted and that the communication is timed out. 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. Therefore, the E1 connector may be made improperly so that the transmission bit error rate is high and that 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. 6.10 Case 10: Call Drop Rate Doubled in a CoBCCH Network After Upgrade from V9R1 to V9R3 Symptom description: In the Egypt 3rd license project, after the BSC was upgraded from V9R1 to V9R3, the number of call drops in the CoBCCH network doubled. Cause analysis and handling: Compare V9R1 with V9R3. It is found that the function of configuring the BCCH in the overlaid subcell is added to V9R3 and that a new TRX-level parameter HW_Concentric Attribute (with the default value None) is add. Check the code. It is found that if the parameter HW_Concentric Attribute is set to None, the operating frequency band of the MS may be wrongly determined. As a consequence, power control may be performed improperly. For example, the 900 MHz frequency band may be mistakenly regarded as the 1800 MHz frequency band. In this case, if power control is performed, the power control amplitude becomes large and the signal level is adjusted to a low level. Thus, call drops increase. Manually set the HW_Concentric Attribute of the main BCCH to Underlay cell. The problem is solved and the call drop rate becomes normal. 6.11 Case 11: Increase in Call Drop Rate Due to Inactivity of T305 and T308 Symptom description:
  • 41. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL After a replacement was performed in Hainan Mobile project, the TCH call drop rate increased. In urban areas, the TCH call drop rate increased from 0.4% to 0.7%; in suburban areas, it increased from 0.7% to 1.1%. Cause analysis and handling: Analyze the A interface signaling and the version change. A version change is found, that is, timers T305 and T308 must be set during the addition of the BSC data, and the data modification in the timer table does not take effect. Timer T305 and T308 are inactive; therefore, the MSC does not initiate the call release procedure. As a result, the number of call drops increases greatly. After the settings of the two parameters are modified, the call drop rate becomes lower than that in the original network. The problem is solved. 6.12 Case 12: Increase in Call Drop Rate Due to Change of TR1N on the MSC Side Symptom description: The value of the TR1N parameter was changed from 20s to 60s to avoid retransmission of short messages and to improve user experience. After the change, the number of call drops with the cause value Release Indication increased greatly. Cause analysis and handling: Analyze the signaling on the A interface. After the value of TR1N is changed, the following signaling flow takes place: After the MS sends a DISCONNECT message to the network, the MSC does not send a CLEAR CMD message to release the terrestrial resources and the TCH. In this case, the MS sends the BTS a DISC message to disconnect layer 2 connection. After receiving the DISC message, the BTS sends a REL_IND message to the BSC. Then, the BSC sends a CLEAR REQ message to the MSC and the number of call drops is incremented by one. After the TR1N parameter is set to 20s again, the TCH call drop rate decreases greatly and returns normal.
  • 42. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL 7 Feedback Form for the TCH Call Drop Rate If the TCH call drop rate is high and technical support is required, fill in the following form. Check Item Example Description Software version BSC and BTS software versions Check whether the software version is faulty. Data configuration table *.dat file Check whether the network optimization parameters and power settings are proper. Alarm information Hardware, clock, and transmission (self- check) Check whether alarms related to the hardware, clock, and transmission are generated in a cell with a high TCH call drop rate. Traffic measurement Traffic measurement results related to the voice quality and the balance between uplink and downlink Based on traffic measurement results, check whether interference and imbalance between uplink and downlink exist. Signaling RSL signaling tracing data Check the causes of call drops. Drive test data *.log (*.cell site) or *.ant file Based on the drive test data, determine whether interference or coverage problems exist.
  • 43. GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL Check Item Example Description Others Engineering parameter table and electronic map The NASTAR software can be used to import the electronic map to facilitate the geographical information check.