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Document No. Product Name Document Users Product Version Written by Document
Version 1.2
Guide to CDMA1X Traffic Statistic
Analysis
Prepared by:
Radio Resource Management Analysis & Application Team
Date: 2002-10-15
Reviewed by: Date: Reviewed by: Date: Approved by: Date:
Huawei Technologies Co., Ltd. All rights reserved
Revision Record
Date Revised version Author Description 2002-11-15 1.00 Tang
Chunmei Complete the first draft
2003-01-13 1.01 Tang Chunmei
Add the inter-BS hard handoff function subset table and supplement the description of call drop ratio
2003-03-15 1.02 Tang Chunmei
Modify the description of call drop ratio and supplement the descriptions of traffic and Eb/Nt
2003-05-10 1.03 Tang Chunmei
Describe the restrictions of the traffic statistic item registration, the RSSI test method and the soft handoff ratio, add the description of disqualified forward transmit power measurement and forward load measurement, and modify the errors in the description of Eb/Nt
Table of Contents 1 Explanatory Notes......................................................................................................................... 2 2 Introduction to Traffic Statistic .................................................................................................... 2
2.1 Function of Traffic Statistic in Network Optimization ............................................................ 2 2.2 Traffic Statistic Function of M2000 ....................................................................................... 2
3 Traffic Statistic Analysis............................................................................................................... 3 3.1 Traffic Statistic Task Registration ......................................................................................... 3
3.1.1 Basic Traffic Statistic Tasks ....................................................................................... 3 3.1.2 Other Traffic Statistic Tasks ....................................................................................... 5 3.1.3 System Tasks............................................................................................................. 5
3.2 Traffic Statistic Analysis........................................................................................................ 5 3.2.1 Acquisition of Basic Network Information................................................................... 5 3.2.2 Major Measurement Items ......................................................................................... 6 3.2.3 Analysis Preparations ................................................................................................ 7 3.2.4 Precautions for Traffic Statistic Analysis.................................................................... 7 3.2.5 Other Auxiliary Methods............................................................................................. 8
4 Introduction to Major Traffic Statistic Items............................................................................. 10 4.1 Call Setup Success Ratio ................................................................................................... 10
4.1.1 Calculation Formula ................................................................................................. 10 4.1.2 Meaning.................................................................................................................... 10 4.1.3 Description of Measurement Items in Function Subset ........................................... 11 4.1.4 Measurement Item Analysis..................................................................................... 11
4.2 Call Drop Ratio.................................................................................................................... 12 4.2.1 Calculation Formula ................................................................................................. 13 4.2.2 Meaning.................................................................................................................... 13 4.2.3 Description of Measurement Items in Function Subset ........................................... 13 4.2.4 Measurement Item Analysis..................................................................................... 13 4.2.5 Possible Causes for a High Call Drop Ratio ............................................................ 16 4.2.6 Examples of Call Drop ............................................................................................. 17 4.2.7 Traffic Call Drop Ratio.............................................................................................. 18
4.3 Traffic Channel Congestion Ratio....................................................................................... 19 4.3.1 Calculation Formula ................................................................................................. 19 4.3.2 Meaning.................................................................................................................... 19 4.3.3 Description of Measurement Items in Function Subset ........................................... 19 4.3.4 Measurement Item Analysis..................................................................................... 19 4.3.5 Possible Causes for Traffic Channel Congestion .................................................... 21
4.4 Traffic .................................................................................................................................. 22 4.4.1 Calculation Formula ................................................................................................. 22 4.4.2 Meaning.................................................................................................................... 22 4.4.3 Description of Measurement Items in Function Subset ........................................... 22 4.4.4 Measurement Item Analysis..................................................................................... 22
4.5 Soft Handoff ........................................................................................................................ 23 4.5.1 Calculation Formula ................................................................................................. 23 4.5.2 Meaning.................................................................................................................... 24 4.5.3 Description of Measurement Items in Function Subset ........................................... 24 4.5.4 Measurement Item Analysis..................................................................................... 24 4.5.5 Possible Causes for Soft Handoff Failure ................................................................ 25
4.6 Hard handoff ....................................................................................................................... 26 4.6.1 Calculation Formula: ................................................................................................ 26 4.6.2 Meaning.................................................................................................................... 26 4.6.3 Description of Measurement Item in Function Subset ............................................. 26 4.6.4 Measurement Item Analysis..................................................................................... 26 4.6.5 Possible Causes for Hard Handoff Failure............................................................... 27
4.7 Carrier Power Control Measurement.................................................................................. 28 4.7.1 Description of Measurement Item ............................................................................ 28 4.7.2 Judgment of Reverse Interference........................................................................... 29
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4.7.3 Querying RSSI in Telnet Mode ................................................................................ 29 5 Analysis of Causes Affecting Measurement Items.................................................................. 30
5.1 Imbalance between Forward Link and Reverse Link.......................................................... 30 5.2 Pilot Pollution ...................................................................................................................... 32 5.3 Improper Setting of Handoff Parameter.............................................................................. 34 5.4 Improper Adjacency............................................................................................................ 35 5.5 Improper Setting of Search Window................................................................................... 36
6 Recording Problems in Traffic Measurement........................................................................... 37 7 References ................................................................................................................................... 37
Guide to CDMA1X Traffic Measurement Analysis _V1.02 Confidential
Guide to CDMA1X Traffic Statistic Analysis
Keywords: M2000, traffic Statistic, measurement item, call drop ratio, call setup success ratio,
congestion ratio, soft handoff and hard handoff
Abstract: This document introduces the methods and procedures to analyze the network
symptoms and carry out the network optimization by using M2000 traffic Statistic
function. It points out the position of the traffic Statistic in the network optimization. It also
gives the signalling measurement points of the major measurement items, optimization
analyses and suggestions. This document provides reference methods or clues for the
network optimization engineers when they solve problems by means of traffic Statistic.
Abbreviations:
References:
References Name Author No.
Release date Source of information
Publisher
Guide to CDMA 1X M2000 Traffic Statistic (V 1.0)
Gu Xinyu
2002/08/22
Specifications for CDMA2000 BSC Performance Measurement Data Collection Requirements
Wang Yifeng, Qiu Jianjun, Cui Yalei and Liu Aihua
2002-06-29
Description of CBSC Traffic Statistic Items
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2.1
2.2
Explanatory Notes
Version of the corresponding M2000 of this first draft: iManager M2000V100R003B01D204SP02. Version of the corresponding BSC: BSC6600V100R001B02D305.
This document introduces how to analyze network symptoms, locate problems and optimize networks by means of the traffic Statistic results. It provides a reference method or clue for network optimizers based on a prior good plan and effective implementation of it.
The compilation of this document is limited to the knowledge available currently. Due to the complexity of networks, there will inevitably exist some deficiencies in this document, but they will be amended in the later versions as the network knowledge gets enriched.
This document is only intended for Huawei engineers.
Introduction to Traffic Statistic
Function of Traffic Statistic in Network Optimization
The traffic statistic data and drive test data are the objective basis of the network optimization, while the human perception is the subjective basis. It is necessary to trace and analyze signalling messages to locate problems before these problems are solved. It is obvious that the traffic statistic data provide a very important means to learn the network performances. Especially when there is traffic in the network, the traffic statistic data provide important references and guidelines for the network optimization. The integrity and accuracy of the measurement items and the convenience of operations will directly affect the efficiency of network optimization. Meanwhile, the grade of measurement items is also a very important factor to measure the effect of network optimization.
On the other hand, network operators set much store by the traffic statistic data. The decision-making level of network operators usually learns and judges the running status of networks according to the visual data obtained from traffic statistic. These visual traffic statistic data also provide an important basis for the future network capacity expansion.
Traffic Statistic Function of M2000
Huawei iManager M2000 is an integrated network management system in the mobile telecommunication field. With an integrated platform, iManager M2000 performs the management of Network Elements (NEs). It also supports the network management function, including centralized configuration management, centralized fault management and centralized performance management. The centralized performance management module provides the network traffic statistic function and is closely related to the network optimization. The centralized performance management module provides a visual, comprehensive operational environment for network
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3.1
3.1.1
operators to perform the system performance management. Network operators can monitor the status of networks, NEs or communication equipment, locate faults and evaluate the status of communication equipment and efficiency of networks or NEs by collecting various traffic statistic data of NEs, such as BSC and MSC. The performance management module provides a basis of network optimization. It consists of the performance adapter, the performance server and the performance client end (PM NWS).
Performance tasks of different NEs can be registered at the centralized performance console (PM-NWS) of the client end (Remote ws) of the M2000 system. The performance tasks are assigned by Adapter to BAM of the corresponding NE and then transferred to the host. The task results are transferred to the performance server via BAM and Adapter and are saved in the database. The task results can be queried by selecting a task and inputting the query condition at the centralized performance console (PM-NWS) of the client end (Remote ws).
Traffic Statistic Analysis
Traffic Statistic Task Registration
Basic Traffic Statistic Tasks
Note: The basic traffic statistic tasks must be registered at the beginning of the equipment deployment and saved during the system optimization period and maintenance period.
Caution ! 1) Be careful that the reporting capability of RMU will overrun so that some measurement items may be lost when too many measurement items are registered, especially when too many carriers are configured in the sector. For the versions below D302, a maximum of 160 carriers (i.e., measurement objects) can be registered in each measurement set if the twelve carrier-level measurement sets are registered. More than 150 carriers can be registered in each measurement set if other measurement sets such as BSC overall performance measurement set are registered. If the four seldom-used carrier-level measurement sets (i.e., the four measurement sets related to the hard handoff) among the twelve measurement sets are not registered, a maximum of 240 carriers can be registered in each measurement set. To solve the above problem, a patch is designed for D302 to improve the reporting capability of RMU, which supports the measurement of 240 carriers or so when the twelve measurement sets are registered. 2) The maximum number of measurement objects in a carrier-level measurement set is 200 carriers. If there are more than 200 carriers, it is necessary to register two tasks.
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The basic traffic statistic tasks are registered at the beginning of the equipment deployment and saved all the time.
1. BSC overall performance measurement and carrier performance measurement, with a measurement period of 1 hour and 24 hours respectively. All function sets and measurement items are selected.
To cope with the unexpected accidents at the stage of network optimization, the BSC overall performance measurement and carrier performance measurement can be registered in a shorter measurement period, such as 15 minutes. But the measurement in a short period is little useful and there are large amounts of data. Therefore, measurement tasks with a measurement period of 15 minutes can be cancelled after the system runs stably.
The call drop ratio, call setup success ratio, congestion ratio and handoff success ratio are the main measurement items that are used to appraise the network quality. This guide mainly analyzes these measurement items. Other measurement tasks, such as carrier power control measurement and carrier channel performance measurement are mainly used for problem analysis.
2. Carrier power control measurement with a measurement period of 5 minutes. The four measurement items are all selected.
The carrier power control measurement can be used to measure the link information, including the parameters of average carrier transmit power, Eb/Nt, RSSI and FER. The RSSI can help to tell the reverse interference. The average carrier transmit power, Eb/Nt and FER can be used to judge whether a balance is achieved between the forward and reverse links, so they are very helpful to the network optimization. For the measurement item RSSI in the task, if the measurement period is too long, the data will be so averaged that it is difficult to observe the exception. Therefore, the shortest measurement period of 5 minutes should be selected. For the introduction of RSSI, please refer 4.7.
3. Channel load measurement in carrier channel performance measurement. The measurement period is 60 minutes. The four measurement items are all selected.
The channel load measurement item can help the network optimizers learn the forward and reverse loads of the network. Currently, only the forward load can be measured, but the reverse load can not yet and it will be “0” in the traffic statistic results.
The channel load measurement covers the measurement of bandwidth occupied by data service. The bandwidth measurement is only based on the forward SCH. The bandwidth is measured at the end of each measurement period. The occupied bandwidth multiplied by the duration of the measurement period is the throughput of the data service. The bandwidth is not an average value.
Specific operations: Select the NE of CBSC, and select overall performance measurement, carrier performance measurement, carrier power control measurement and carrier channel performance measurement from function sets. Select all measurement items in all function subsets for the three measurements other than the carrier channel measurement. Select only the function subset of channel load measurement and all measurement items in it for the carrier channel performance measurement. The measurement time segment is 0 ~ 24 hours. The measurement type is semi-permanent. The measurement mode is measurement by day. The storage period is recommended to be above 7 days.
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Note: 1) The current version supports a maximum of 120 objects in the same traffic statistic task and a maximum of 128 measurement items. 2) If the storage period of the traffic measurement task is 7 days, the data before 7 days can not be viewed. But too long a storage period will increase the load of the system. So it is recommended to output the complete traffic statistic results of the day to an .xls file and save them during the network optimization for the sake of a comparison in the future.
3.1.2
3.1.3
3.2
3.2.1
Other Traffic Statistic Tasks
After the basic traffic statistic tasks are registered, other new traffic statistic tasks can be added according to the actual situation of the network.
The number of channels, total number of carrier control channels and the percentage of traffic channels in good condition can be measured through the carrier channel performance measurement. The carrier channel performance measurement can help to check whether the data configuration is reasonable and to learn the running status of the network. It can also be registered at the beginning of the equipment deployment.
The broken Abis signalling link measurement, A1 interface performance measurement, A2 interface performance measurement, SCCP-related measurement, MTP signalling link measurement, MTP signalling link set measurement, MTP signalling point measurement, PCF call processing measurement, PCF data processing measurement and CPU load measurement of CSPU can help to monitor the links, interfaces and the load of the system for a definite purpose. These measurement tasks can be registered according to the actual situation.
If necessary, the self-defined measurement items can be applied. The new measurement items can be calculated from the existing measurement items of the system.
System Tasks
The system tasks refer to those necessary for the system to run. They are independent of the network optimization. There are four system tasks, which are mainly used to provide performance data for the performance measurement report template of the customized report tool of the M2000 system. Since the data source necessary for the report is the result table of the system task, the user must register the system measurement task before using the prepared template in the intelligent report system. Only the administrator in the entire network and the super administrator can register a system measurement task.
Traffic Statistic Analysis
Acquisition of Basic Network Information
The network problem analysis by means of traffic statistic data is based on the acquisition of the basic information of the network.
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3.2.2
I.
II.
First, it is necessary to get familiar with the early network planning and obtain the documents about the early network planning, such as the planning report, engineering parameter table, network topology and cell parameter design specifications, so as to have a general idea of the network and find some obvious problems.
Secondly, it is necessary to learn the current engineering construction progress, such as, the completion status of BTS installation, the modification of planning, especially the engineering parameter planning, BTS commissioning and simple drive tests. Special attention should be paid to network defects caused by engineering installation quality or the construction progress. For example, the uncompleted or wrong installation of some BTS will cause handoff problems in a large area in a city.
The objective of the network optimization can be defined effectively only after the basic information of network is obtained. Different optimization strategies and optimization objectives should be established for different coverage areas and application environments.
Major Measurement Items
Major appraisal measurement items of China Unicom
Call setup success ratio C2.5: 92% Call drop ratio of radio system C2.4: 1%. Traffic call drop ratio C2.11: 90 Paging success ratio C4.6: 86%. Call completion ratio of system C4.4: 50%. Call completion ratio of MSC C4.5: 80%.
The above appraisal measurement items make sense only when the system is busy. The values of the last three measurement items are measured at MSC. This guide focuses on the first three measurement items. The paging success ratio is not only related to the radio signals, but also the paging strategy of MSC. The problems of low call setup success ratio and high call drop ratio of radio system are solved in this guide. The problems that affect the paging success ratio are also basically solved. Later, emphasis is placed on the cooperation between BSC parameters and the paging strategy of MSC.
It should be noted that the above measurement items are only required by some operator and are given only for reference. These measurement items may vary with the operator and the frequency band. The measurement items of different local networks of the same network operator may also vary and should be optimized according to the local requirements.
Major traffic statistic items
The running status of the network is learnt from the following traffic statistic items. For the explanations of these measurement items, see 4 Introduction to Major Traffic Statistic Items.
Call setup success ratio Learn the number of originated calls, major causes for call failure and call setup success ratio.
Call drop ratio Learn the cause for call drop and the call drop ratio.
Soft handoff
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3.2.3
3.2.4
Learn common soft handoff success ratio, softer handoff success ratio and the total soft handoff success ratio and learn whether the soft handoff ratio is normal.
Traffic Learn the traffic without handoffs.
Congestion ratio Learn whether there exists congestion and the major causes for the congestion.
Hard handoff If the hard handoff takes place between BSCs of different manufacturers, first learn the call handoff (out) success ratio & the cause for handoff failure, and then the call handoff (in) success ratio & the cause for handoff failure.
Analysis Preparations
After the network starts to work, first confirm the following before the network optimization:
1) Check the versions of the software and hardware used by the equipment are correct, the versions of BTSs and BSC are matched, the versions used are the most stable ones, the versions in the entire network are uniform, and all BTSs adopt the same version.
2) Confirm the calibration test and sensitivity test have been carried out for each BTS.
3) Confirm the no-load test and loading test have been carried out for each BTS.
4) Confirm the VSWR test has been carried out for the antenna.
5) Confirm the dialling test has been carried out for each BTS after it is activated, and the engineering installation is correct. During the dialling test, observe whether the call can be accessed normally, the call quality is good, and the handoff can take place normally. Make sure that the antenna feeder is not connected inversely. View the PN from the DEBUG window of the MS under the BTS. (If the PN is consistent with the planned one, the antenna feeder is correctly connected.)
6) After the above problems are all eliminated, check the difference between the actual coverage and the expected coverage of each sector. If the coverage is exceptional, check whether the azimuth and downtilt of the antenna are consistent with the planned ones. If they agree with the planned ones, but the coverage is significantly different from the expected coverage or the coverage is overlapped, it is necessary to adjust the downtilt or azimuth of the antenna. When the antenna is adjusted, do not adjust the coverage of an individual sector only, but the whole area around should be taken into consideration. If necessary, the antennas of several sectors should be adjusted altogether.
7) Check whether any sensitive radio function (such as load control, cell breath, special handoff algorithms, special channel assignment algorithms, special power control algorithms) has been applied. If so, the traffic measurement analysis may be different from below. Therefore, a special analysis is required.
Precautions for Traffic Statistic Analysis
When analyzing the traffic statistic items, first study BSC overall performance measurement items to learn the general situation of the network operation. Then analyze the carrier performance measurement.
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3.2.5
I.
The filtering method is adopted during the traffic statistic analysis. First analyze the cell whose measurement items are obviously exceptional. It may be the version, hardware, transmission equipment, antenna feeder (including GPS) or data that leads to the exception. In combination with the alarm information, check the above-mentioned points. If there is no obvious exception, measure and sort the carriers of each sector according to the measurement items and make a list of cells whose major measurement items are poor so as to analyze them.
Be careful of the parameter modification. Modify the parameters after everything is fully considered. For example, when the timer is modified, be sure that the length of the timer can not be set too large. If so, the system load will become so heavy that other problems will occur.
If it is necessary to adjust the antenna feeder or modify the parameters during the network optimization, it is better to observe the measurement items for a period after a measure is taken. After the effect of the measure is known, go on with the next step. Doing so can provide protection against any accident on the one hand, and help to accumulate experiences on the other hand. In practice, the measurement items of the network fluctuate much, with great randomness. If the measurement items are very poor one hour before the parameter modification, but they become good shortly after the modification, it does not mean the parameter modification is highly effective, because the measurement items may become poor one hour later. It is preferred to observe the measurement items for more than one day after the parameter modification. Compare these measurement items with those in the same time segment before the modification (It is better to compare them with those in the same time segment of the same day in the previous week). Only in this way, can a precise conclusion be drawn. In addition, keep a close eye on the alarm information in this time segment.
When observing the measurement items, you should not only concern the absolute values of the measurement items, but their relative values. Only when the statistical magnitude for a measurement item is big enough, can the measurement item provide guiding significance. For example, a call drop ratio of 50% does not mean a poor network quality. Only when the absolute values of call attempts, call successes and total call drops have the statistical significance, will this call drop ratio make sense
It should be noted that these measurement items do not exist independently, but they relate to each other. For example, the interference and coverage will affect many measurement items. Vice versa, if the problem of a low handoff success ratio is solved, the problem of a high call drop ratio will be improved to some extent. Therefore, you should take other measurement items into consideration when focusing on analyzing a specific measurement item in practice.
Other Auxiliary Methods
The traffic measurement data only provide an important basis for the network optimization, so it is necessary to take other measures and methods to solve network problems.
Drive test
It is a direct, accurate method of learning the network quality and finding network problems. If you want to learn the radio network coverage, then the drive test will have the irreplaceable advantage over the traffic measurement and other methods. You can use the drive test to learn whether the area is covered, whether there is any coverage blind area, whether there is any imbalance between the forward link and reverse link, and whether the antenna feeder is installed inversely that signals of the
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II.
III.
IV.
corresponding PN carrier appear where they should not. Especially after parameters, such as the power ratio, are modified or the coverage area is changed by adjusting the antenna feeder, it is necessary to perform a drive test to learn whether the expected results have been achieved.
The drive test can be used to solve a specific problem, but it has its own limitations. Limited to the test path and time, it is impossible to obtain the complete network data. For example, it is very difficult to find a call drop and to further analyze the cause for the call drop by means of the drive test. Suppose the call drop ratio is 3%, namely, only three calls out of 100 are dropped, it is very difficult to find where a call is dropped, let alone the cause for the call drop.
The drive test can be used to learn the radio network structure and the quality of the engineering installation. The detailed analysis of the measurement items in the traffic measurement will provide a way of improving the measurement items. Only the combination of macro traffic measurement with a particular test can effectively solve the problem.
Signalling message tracing and signalling message printing of debugging console
These methods are usually used to solve difficult problems.
The AirBridge service maintenance system can trace signalling messages of different interfaces. The Um interface and Abis interface can trace signalling messages of a single subscriber. If you encounter a complicated problem, you can trace the signalling messages of interfaces (especially the Um interface) of the test MS as you perform a drive test, and then analyze the signalling flow to locate the problem.
The exceptional signalling message printing of debugging console of CBSC can help you to locate problems. From the signalling message printing of the debugging console, you can find problems, such as, absent configuration of pilot and improper adjacent cell planning.
Alarm information
The alarm information of equipment can reflect the real-time running status of the equipment in the whole-network, so enough attention should be paid to the alarm information. The exception of some measurement item in the traffic measurement may be likely related to the alarm occurrence to the equipment. When different alarms are distinguished and they are related to the measurement items, no time will be wasted blindly. The alarm information of equipment can be viewed in the centralized fault management system of M2000 or at the alarm console of AirBridge.
M2000 also provides the task-based performance alarm function. It defines the performance measurement items. If a measurement item exceeds the set threshold, it will send a performance alarm to the alarm server. The alarm information can be viewed at the centralized alarm workstation.
Traffic statistic function provided by the AirBridge performance management module
The AirBridge performance management module can provide the traffic statistic function, too. New tasks can be created and the measurement results can be queried. The measurement items are just the same as those in M2000. But the measurement results can only be displayed in the form of text, so they are not so visual as those in M2000. The traffic measurement tasks registered in M2000 can also be queried on
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AirBridge. The traffic measurement tasks with “NM task” behind names are those registered in M2000 Network Management (NM) system.
Introduction to Major Traffic Statistic Items
This section introduces the major measurement items in M2000 traffic statistic, and subdivided measurement items and signalling measurement points in the function subsets. It also analyzes the measurement items based on the experiences of a commercial network in Cangzhou, and points out the causes for the exceptional measurement items.
For the analysis of causes for the exceptional measurement items, see 4.1.4.
Note: 1) The causes for the exceptional measurement items and the analyses given in this guide are not very perfect. These causes may vary greatly with operational environments of networks. So we need to accumulate experiences in practice to perfect our theory. 2) For the parameter configurations and modifications involved in this guide, see CDMA1X BSS Network Planning Parameter Configuration Recommendations. 3) The following measurement items except those in 4.7 Carrier Power Control Measurement are all measurement items in the function sets of BSC overall performance measurement and carrier performance measurement. Unless otherwise specified, the subdivided measurement items in the function sets in this guide apply to both the BSC overall performance measurement and the carrier performance measurement. 4) At present there are some indefinite signalling measurement points for the measurement items or their meanings are not clear. They will be improved in later versions.
4.1
4.1.1
4.1.2
Call Setup Success Ratio
Calculation Formula
Call setup success ratio = [Call setup successes/call attempts]*100%
Meaning
Ratio of successful traffic channel (TCH) assignment times of BSC to the total call attempts in mobile originated calls without handoffs, mobile terminated calls without handoffs, and the receiving & transmitting of SMS on TCH.
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4.1.3
4.1.4
I.
II.
III.
Description of Measurement Items in Function Subset
Measurement Item Analysis
When the call setup success ratio is low, find the major cause that leads to the maximum call setup failures. If the number of call setup failures resulting from different causes is basically equal, find the causes for the MS reverse service preamble capture failure and waiting-for-MS-response timeout.
MSC denial
MSC will deny calls when some invalid subscribers continuously make calls. If the number of denials is not too large, they can be ignored temporarily. If the number is exceptionally large, check whether the setting of CCM_T_WT_ASSG_REQ timer is proper, and whether the link connection of A interface is normal. Then make a further analysis of MSC.
Early release
It is normal that this cause value appears, because the measured calls are those that are normally cleared before the assignment is completed. If the early release is exceptional, you can also analyze MSC to locate the problem.
MS reverse service frame preamble capture failure
In terms of the signalling flow, one failure cause may be that the forward signal quality is so poor that the MS fails to receive the Enhanced Channel Assignment Message (ECAM) or the MS can not successfully demodulate the forward traffic channel. Another failure cause may be that the reverse signal quality is so poor that the BTS can not receive the TCH preamble after the MS sends it. The third failure cause may be the wait timeout of the timer of the BTS.
If this failure frequently occurs in the actual network, the possible causes include: Imbalance between the forward link and reverse link
It is necessary to adjust the radio coverage or modify the system parameters in this case (See 5.1).
Improper setting of power control parameters On the one hand, a low initial forward transmit power may make it difficult for MS to access the network and thus lead to the call failure. In this case, check the settings of the initial transmit power and maximum transmit power of the forward traffic channel. Note that the forward power control parameters of IS-95 MSs and CDMA2000 MSs are configured in the PWRPARA table and the FWDPWRPARA table, respectively. On the other hand, the improper setting of the reverse access parameters may cause the access failure. The related reverse access parameters include NOM_PWR, INIT_PWR and PWR_STEP
Improper setting of access parameters The access parameters include NUM_STEP, ACC_TMO, PAM_SZ and MINPAMSZ configured in BTS. For the parameter settings, see CDMA1X BSS Network Planning Parameter Configuration Recommendations.
Improper setting of CCM_T_WT_TCH_PREAMBLE timer Too few assignment message resending times Reverse service frame capture failure due to improper setting of traffic channel research window of BTS
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IV.
V.
4.2
External interference Pilot pollution
See 5.1
Waiting-for-MS response timeout
The causes for waiting-for-MS-response timeout are similar to those for MS reverse service frame capture failure. At this time, the closed power control has started in the reverse link. If the BTS has already demodulated the reverse traffic channel, the power control will also start in the forward link. The possible causes include:
Imbalance between forward link and reverse link Improper setting of power control parameters
The same as that for MS reverse service frame capture failure. Not only the reverse power control step REV_FCH_FER in the reverse link, but also the settings of parameters of power control step and frequency in the forward link can be checked.
Improper settings of timers of CCM_T_WT_MS_ACK_ORD, CCM_T_WT_SRV_CONN_CMP_MSG and CCM_T_WT_STATUS_RSP_MSG Improper setting of traffic channel research window of BTS External interference. Pilot pollution.
The measurement item “traffic channel congestion ratio” is added in the versions above D405. It refers to the congestion ratio of calls.
Others
Causes for the exception of equipment or causes that are not mentioned above. If there is any exception, it is necessary to make an in-depth analysis in combination with the signalling alarm messages.
Besides, the access failure may be caused by the conflict between access and handoff.
If the signal quality in the serving cell is poor during the access, the MS needs to be handed off to a new cell where the signal quality is good. However, no handoff is permitted during the access period. There are two cases during the access.
1) The Ec/Io of the pilot in the serving cell decreases quickly (5-6dB/S) while the Ec/Io of the pilot in the new cell is quite large. In this case, if the handoff region is too small, the MS may be dropped off the network during the access.
2) The access is too slow. The Ec/Io of the pilot in the serving cell decreases very slowly and the Ec/Io of the pilot in the new cell is quite large. If the MS moves quickly, it stays in the good coverage area at the very beginning of access, but it soon enters the coverage border. It will be dropped off the network because the handoff fails.
The parameters ACCESS_HO (access handoff allowed) and ACCESS_PROBE_HO (access probe handoff allowed) are configured to allow a handoff during the access. But these parameters are not recommended.
Call Drop Ratio
The call drop ratio is classified into the call drop ratio of radio system and the call drop ratio of system. The call drops of system refer to the total call drops resulting from different causes in the whole network system, including call drops resulting from the radio system or other causes. Since the majority of call drops are related to the radio
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4.2.1
4.2.2
4.2.3
4.2.4
I.
system, the call drop ratio of radio system should be the highlight. What is mentioned in the traffic measurement is just the call drop ratio of radio system. There should be little difference between the call drop ratio of system and the call drop ratio of radio system. If necessary, the call drop ratio of system can be defined in the traffic measurement.
Calculation Formula
China Unicom: Call drop ratio of radio system = [Total call drops of radio system/Call setup successes]*100%
Huawei: Call drop ratio of radio system = [Total call drops of radio system/Call setup successes + Successful inter-BS hard handoffs (in)]*100%
Meaning
The call drop ratio refers to the ratio of calls exceptionally released due to radio link to the call setup successes. The total call drops of radio system refer to the call drops caused by radio signal quality degrade and handoffs.
Description of Measurement Items in Function Subset
Measurement Item Analysis
When the call drop happens to the link branches of other BSCs without exception, the call drop will not be measured at the local BSC or at the target BSC.
It should be noted that, for the carrier-level call drop ratio measurement in the current traffic measurement, if the call drop takes place in the soft handoff state, it will only be measured for one time and be considered happening to the link branch on which the current call is first established.
It should be noted that the carrier-level call drop measurement in the current traffic measurement is described as below if there are multiple branches: When the original access cell is still in the current active set, the call drop, if it happens, will be considered happening to the access cell. When the access cell is not in the current active set, all branches occupied by the MS can be searched by virtue of a function, and then the call drop will be considered happening to the branch (sector carrier) which is first searched. The call drop measurement is not an average value and the branch search is random.
Radio link causes
One cause is that the BTS is blocked and CCM directly releases the resource. The call drop seldom happens in this case. This cause can be judged and solved by querying the status of the BTS. The other cause is that a high frame error rate (FER) results from the poor radio signal quality. The call drop resulting from a high frame error is very common. FMR will report TCH ERR to CCM due to the high frame error rate and the call drop will happen when CCM releases it. FMR will report TCH ERR with the following cause values. Only after TCH ERR whose cause value is 4, 5 and 6 is reported to CCM, CCM will release the call. The cause value 2 is only for the soft handoff link branch which will be cleared by CCM.
Cause value 5
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After all link branches in FMR are merged, more than 270 out of the 300 reverse frames are erasure frames. For the cause, refer to the analysis below. The Erasure frame ratio and Erasure frame threshold can be modified on AirBridge by using the command MOD SDUMDC. The frame information can be queried by using the command LST FRMINFO.
Cause value 4 300 idle frames are received in the reverse link. In the version above D302, if the delay between two handoff link branches exceeds the length of the reverse frame combination timer, FMR will be unable to combine the frames of the two branches, but send them to the voice subrack. At this time, if the new branch fails to capture the reverse service frame, the CP of BTS will report idle frames and idle frames will be measured by FMR. But all branches share the same idle frame counter and the reading of the counter will be cleared only after the idle frames reach 300 and the counter reports TCH ERR. In this way, a call drop may happen during the soft handoff because of too many idle frames. If a call drop happens during the soft handoff, the exceptional signalling message printing of the debugging console will show too many idle frames. The transmission delay of one branch is so long that the frames of the two branches can not be aligned. Therefore, errors will occur when the frames are combined on FMR. FMR will consider the call drop is caused by the idle frames. The threshold of idle frame counter can be modified on AirBridge by using the command MOD SDUMDC. The frame information can be queried by using the command LST FRMINFO
Cause value 6
Too high a markov FER. The system will compare the received frames with those locally generated. If the received frame is different from the locally generated one, it will be considered as an erasure frame. The markov FER refers to the ratio of the erasure frames to the total received frames. When the system fails to receive the frame sent from the opposite party, this frame will also be considered as an erasure frame. By default, if 95% of 500 frames in 10 seconds are erasure frames, TCH ERR will be reported. The markov FER can be set at the debugging console..
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III.
4) Cause value 2: One branch does not receive any frame within 80ms. The length of the reverse frame receiving timer can be modified on AirBridge by using the command MOD SDUMDC. Note that the length of timer is in millisecond. The frame information can be queried by using the command LST FRMINFO. Due to this cause, the soft handoff link branch is very likely deleted just when the Ec/Io of the pilot of this link branch is very large. After the deletion, the MS will soon report PSMM and request BSC to add the branch, so the link branch is frequently deleted and added during the soft handoff. In this case, the length of the reverse frame receiving timer can be set a little larger, because no signal on one branch will not affect the frame selection of FMR. The possible causes for Cause Value 5 and Cause Value 6 include the following cases:
1) The poor reverse signal quality leads to a high FER.
2) The poor forward signal quality leads to a high FER. From the angle of MS, the mechanism of call drop is described as follows:
A. If the MS receives 12 (N2m) successive erasure frames, the MS will switch off the transmitter, but it still receives frames in the forward link. If two (N3m) successive good frames are received within 5 seconds (Fade Timer), the MS will restart the transmitter. Otherwise, the MS will be reinitialized.
B. The MS fails to receive the response message. If the MS still does not receive any response after N1m successive times of transmission, it will be reinitialized. (N1m is the maximum times that the MS resends a response request message on the reverse traffic channel. It's a protocol value. For IS-95A, it is 3, for IS-95B, it is 9, and IS2000, it is 13.)
If the forward signal quality is poor, the MS will switch off the transmitter. In this case, FMR considers there are erasure frames, so a call drop happens.
3) Abis link fault, such as broken optical fiber. In this case, FMR considers there are erasure frames, so a call drop happens.
For the possible radio link causes for the call drop, see 4.2.5.
Abis interface
1) It can be seen from the signalling measurement point that when Abis interface is exceptionally interrupted (for example, the optical fiber is broken), the call drop will not be considered resulting from Abis interface cause. When Abis interface is suddenly interrupted, BSC does not know it but only knows that FMR does not receive the data frames, so Abis interface cause will be considered as a radio link cause. In this case, it can be easily learnt from the alarm console that Abis interface is exceptionally interrupted.
2) In the case of BTS fault, the BTS will automatically report “Abis BTS release request” only when it finds problems in its internal processing. In this case, the call drop will be considered resulting from this cause and the alarm information of the equipment will also appear at the alarm console.
A interface
The possible A interface causes include:
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V.
4.2.5
I.
II.
1) During the conversation, MSC itself sends or MSC is made to send the A interface reset command.
2) A interface is temporarily faulty or the link is broken. In this case, if FMR does not receive the EVC frame sent from MSC within a certain period, it will send “TRAU ERR” to CCM and CCM will release the resource. Problems on transmission can be solved in combination with the alarm information, such as E1/T1 alarm. If the transmission of A interface is normal, the problem may lie in the equipment, including MSC and BSC.
3) If FMR does not receive the EVC frame, but A interface is normal, the problem may lie in the internal processing or the transmission of the equipment.
Transmission link from BSC to PCF
1) PDSN is abnormal.
2) PPU or A8/A9 interface in BSC is abnormal.
3) When Huawei BSC is connected to PCF of other manufacturers, PCF or A8/A9 interface or the transmission equipment of other manufacturers is abnormal.
Others
It is difficult to locate the other causes for the failure. When there are exceptions, problems can be analyzed from the equipment in combination with the alarm information and the signalling message printing of debug console.
Possible Causes for a High Call Drop Ratio
Low Ec/Io of the forward link
If the forward link can not be demodulated, the MS will switch off the transmitter so as to lead to a call drop.
The Ec/Io of the forward link can be obtained from the MS and the drive test equipment. The receiving level can also be taken into consideration during the cause analysis. If the Ec/Io is small and the receiving level is low, the coverage will be poor. The possible cause may be that the MS is far away from the BTS, or there is a large barrier in the way of the propagation. This problem can be solved by adjusting the antenna of the BTS to improve the coverage. If Ec/Io is small while the receiving level is high, strong interference will exist in the forward link. The forward interference includes the inter-BTS interference and the external interference. The forward interference will make Ec/Io decrease. The forward interference data can be obtained by means of some instruments, such as YBT250.
High FER in the reverse link
The FER data in the reverse link can be obtained from the carrier power control measurement. The possible causes for a high FER in the reverse link include:
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IV.
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4.2.6
I.
II.
1) Too high a propagation attenuation in the reverse link leads to a high FER in the reverse link. If the FER in the forward link is also high at this time, it indicates that the propagation attenuation of the BTS is too large. The cause for this symptom is that the MS is far away from the BTS. To solve the problem, more BTSs should be added.
2) If the signal level in the forward link is high enough, but only the FER in the reverse link is high, it indicates the coverage of the BTS is good. It is possible that the high FER is caused by the low power in the reverse link. To solve the problem, the system parameters, such as the reverse power control threshold Eb/Nt, can be modified. However, the modification of Eb/Nt will only function within a certain range because the maximum transmit power of the MS is limited. If the transmit power of the MS reaches the maximum and the modification of the Eb/Nt is useless, it shows the MS is already on the border of the reverse coverage.
3) The reverse power does not reach the maximum, but the FER in the reverse link increases. The symptom is usually caused by the fast fading. It shows there is no stable master pilot where the MS is located.
4) Dense subscribers and strong reverse interference may also lead to a high FER in the reverse link.
Imbalance between the forward and reverse links
See 5.1.
No master pilot coverage
See 5.2.
Improper setting of handoff parameters
See 5.3.
Improper adjacency
See 5.4 Measurement Item Analysis.
Examples of Call Drop
Call drop caused by conflict between access and handoff
If the system does not support the handoff during the access, but a handoff is required when the MS originates a call on the border, the MS has to wait for the handoff. If it takes a long time for the MS to access the network, the call may be dropped before the handoff process is completed. In this case, the Ec/Io observed on the MS will decrease while the receiver power (Rx) of the MS will increase. That means interference is caused by a new strong pilot and a handoff is necessary. Furthermore, the MS will reselect the new pilot after the call drop.
Call drop caused by traffic channel power restriction
The power of the traffic channel is usually controlled by the settings of Eb/Nt of the forward and reverse links. If the setting of the maximum transmit power of the traffic channel is improper, a call drop may happen when the power is not high enough to maintain the link connection, even though the Ec/Io of the pilot is very large.
Forward link failure
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4.2.7
In this case, it can be seen from the MS that the Ec/Io of the pilot and the receiver power (Rx) of the MS are both above the threshold (for example, Ec/Io is greater than -15dB, and Rx greater than -100dBm). If the TX_GAIN_ADJ of the MS remains unchanged within 5 seconds (Fade Timer of the MS) and the MS then is reinitialized, it shows the MS switches off the transmitter because it fails to receive the forward traffic channel frames. In addition, since the Ec/Io of the original serving pilot is large, the MS will reselect this pilot after the call drop. It shows the call drop is very likely caused by the low power of the forward traffic channel.
Reverse link failure The power of the reverse traffic channel is controlled by the setting of Eb/No. If the power of the reverse traffic channel is not high enough to reach the BTS, the BTS will release the call when it detects 270 erasure frames out of 300.
Call drop caused by forward interference
If it can be seen from the MS that the Ec/Io of the pilot decreases while the receiver power (Rx) of the MS increases, it shows there exists interference in the forward link. Due to the interference, the Ec/Io of the pilot will decrease. If the forward link can not be demodulated, the MS will switch off the transmitter. If the forward interference lasts longer than the length (5s) of the Fade Timer of the MS, the MS will be reinitialized when the Fade Timer is reset.
The possible interference causes include:
1) The CDMA interference may lead to a handoff failure. If the MS selects a new pilot after the re-initialization, the call drop is contributed to the handoff failure. In this case, the call drop is very likely caused by the forward link interference.
2) There is another special case. When a blocked BTS is suddenly unblocked, strong forward interference will occur to the BTSs around, so call drops may happen to those BTSs..
3) External interference. If the MS stays in the search state for a long time (more than 10 second), the call drop may be caused by a high FER due to the existence of an interference source (e.g., AMPS system and microwave emission) that is useless to the MS.
Call drop caused by a heavy forward load
If the load of the BTS is heavy and the transmit power is 20W above the maximum transmit power, the self-protection mechanism of the BTS will automatically decrease the transmit power of each channel so that the Ec/Io of the pilot is lowered and the transmit power of the traffic channel drops. In this way, the forward link can not be demodulated. Especially, it is very likely that call drops will happen to MSs on the border of coverage. Meanwhile, the call setup success ratio will decrease.
Traffic Call Drop Ratio
Traffic call drop ratio = Traffic (without handoff) over traffic channel * 60/total call drops of radio system
China Unicom is much concerned about this measurement item. It is closely related to the traffic. However, for the traffic in a non-populated urban area, the traffic call drop ratio can meet the requirement as long as the call drop ratio is up to standard.
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4.3
4.3.1
4.3.2
4.3.3
4.3.4
I.
II.
Traffic Channel Congestion Ratio
Calculation Formula
Traffic channel congestion ratio = [Traffic channel congestion times/Traffic channel request times]*100%
Meaning
In mobile originated calls, mobile terminated calls, handoffs, and the receiving & transmitting of SMS on TCH, traffic channels can not be successfully assigned due to the insufficient Walsh codes, power, traffic channels, encoders or transmission links from BTS to BSC. The traffic channel congestion ration refers to the ratio of the total traffic channel assignment failures to the total traffic channel requests.
Description of Measurement Items in Function Subset
The traffic channel congestion ratio contains five function subsets, which are call, inter-BS hard handoff (in), intra-BS hard handoff (in), inter-BS soft handoff (in) and intra-BS soft handoff (in), respectively.
Measurement Item Analysis
The congestion ratio is a measurement item that the operator is concerned about. It is also the basis of network capacity expansion.
Insufficient Walsh codes
In the normal case, there are sufficient Walsh codes. The Walsh codes may be insufficient when a data service is requested.
Insufficient forward power
1) The forward common channel occupies too much of the power.
Check the power configuration of the forward channel. The pilot power should not occupy too much of the power (The pilot power usually accounts for 20% of the total forward power) and the initial transmit powers and maximum transmit powers of paging channel, synchronization channel and forward traffic channel should be set in proper proportion to the pilot power.
2) So many subscribers that the forward power is used up.
From the forward and reverse load data in the channel load measurement, the reverse load will be heavy when there are many subscribers or when the forward power is insufficient. In this case, the system capacity will not be large enough if the call quality is guaranteed. If only the capacity of a single sector is not large enough, you can adjust the height and downtilt of the antenna in the sector or change the transmit power of the MS and the sector to share the traffic with adjacent sectors. You can also modify the parameters to relieve the congestion. For example, increase the handoff threshold of the congested cell and reduce the handoff threshold of adjacent cells. If the capacity in the whole coverage area is insufficient in different time segments, it is necessary to expand the capacity.
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IV.
V.
Insufficient reverse power
When the channel is assigned, the system will judge whether the reverse power is sufficient. Since the measurement of the reverse load is not accurate, the admission control will be applied in the reverse channels according to the equivalent number of subscribers.
The different RC voice and data subscribers will be converted into the equivalent RC3 voice subscribers. When the system assigns channels, it will first predict the total equivalent RC3 voice subscribers, and then judge if the number of the total RC3 voice subscribers exceeds REV_MAX_USER (reverse subscriber admission threshold). If so, the channel assignment will fail and the system will be considered the reverse power is insufficient in the traffic measurement. The REV_MAX_USER is configured in the CH_INFO (channel information) table. Other data, such as, equivalent RC3 voice subscribers of RC2 voice subscribers can be configured in this table. For the data configuration, see CDMA BSS Network Planning Parameter Configuration Recommendations.
If the reverse power is insufficient in the traffic measurement, it is possible that there are too many subscribers. If there are too many subscribers, the forward power will be insufficient, too. Besides, you can check whether the parameters of reverse admission control in the CH_INFO table are properly configured.
Insufficient channels
In the normal case, there are sufficient channels unless
1) The channels are not well planned that they can not meet the capacity requirement of the operator. But the probability of this case is very low.
2) The channel planning meets the capacity requirement, but the real number of subscribers goes beyond the capacity. This is a case of insufficient system capacity and the capacity needs to be expanded. This case seldom happens at the early operation stage of the network.
3) The soft handoff ratio is so high that too many CEs are occupied. The soft handoff ratio is by far greater than the planned redundancy. So it should be reduced.
4) For the intra-BS soft handoff and the inter-BS soft handoff, the insufficient channels in the traffic measurement may also result from the CCM wait timer timeout and unsuccessful resource assignment due to other exceptional causes. You can check whether the setting of the CCM_T_WT_RL_CNF timer is proper.
A interface transmission link establishment failure
A interface includes A1/A2 interface between BSC and MSC. For the intra-BS soft handoff, the A interface includes A8/A9 interface between BSC and PCF. According to Description of CBSC Traffic Measurement Items, the intra-BS hard handoff data services are not measured.
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4.3.5
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II.
III.
1) Insufficient capacity of A interface links: When the network load does not exceed the network capacity, this problem can be avoided if the A interface link is properly calculated. If the network load exceeds the network capacity, the capacity of A interface transmission links should be expanded.
2) Transmission fault. You can query the alarm information to solve transmission problems.
3) Timer timeout. The timers of calls and inter-BS hard handoff are CCM_T_WT_BOOK_CIC_CNF, CCM_T_WT_PIE_RES_APPLY_CNF, CCM_T_WT_PIE_RES_AFFIRM_CNF and CCM_T_WT_A9_CONN_A8. The timer of the intra-BS hard handoff is CCM_T_WT_HO_RES_CNF..
4) Other exceptions.
Abis interface transmission link establishment failure
1) Insufficient capacity of Abis interface links.
The same as A interface.
2) Abis transmission link fault. You can query the alarm information to solve transmission problems.
3) The timer expires when CCM requests Abis link resource. Check whether the settings of timers of CCM_T_WT_TCH_SETUP_CNF, CCM_T_WT_BOOK_BIE_CNF, CCM_T_WT_ABIS_BTS_SETUP_ACK and CCM_T_WT_SETUP_LEG_ACK are proper.
A3 interface transmission link establishment failure
During the inter-BS soft handoff, the target BSC needs to connect A3 interface and Abis interface to realize the service transmission between MS and BSC after receiving the A7-Handoff Request message at A7 interface. The failure may be caused by the improper setting of CCM_T_WT_BOOK_TIE_CNF timer or the transmission link fault.
Possible Causes for Traffic Channel Congestion
Insufficient system capacity
Compare the busy hour traffic of the congested cell with the planned capacity. When it is sure that the system capacity is insufficient, capacity expansion is recommended. The probability of insufficient system capacity is very low at the early stage of network optimization.
Interference
If the system capacity is sufficient, but failure often occurs after the command is issued, the failure may be caused by interference. The interference can be analyzed according to the carrier power control measurement. See 4.7.
Insufficient allocated power of traffic channel
The common channel occupies too much of the forward power so that the forward power will be insufficient in the traffic measurement. In consideration of the coverage
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V.
4.4
4.4.1
4.4.2
4.4.3
4.4.4
and the capacity balance, the power reduction of the common channel can relieve the congestion.
Improper setting of handoff parameters
If the handoff threshold of the congested cell is too low while that of the destination cell is too high, the MS can not be successfully handed off to the destination cell. In this case, properly add more adjacent cells according to the actual situation or modify the handoff parameters to relieve the congestion. See 5.3.
Improper adjacency
See 5.4.
Traffic
Calculation Formula
Traffic over TCH (with handoff) = TCH seizure duration (with handoff)/Measurement period (Second)
Traffic over TCH (without handoff) = TCH seizure duration (without handoff)/Measurement period (Second)
Meaning
The measurement item is the traffic over TCH (including traffic with handoff and traffic without handoff). That is, the traffic measurement with handoff is the seizure duration of all call branches, while the traffic measurement without handoff is the actual traffic of the user.
Description of Measurement Items in Function Subset
The BSC overall performance measurement item includes soft handoff ratio. The carrier performance measurement includes the handoff duration and handoff traffic.
Measurement Item Analysis
The traffic without handoff is the essential item for understanding of network bearer. Through the traffic measurement, we can grasp the pattern of subscriber behavior; understand the network position to work out corresponding optimization solutions and strategies. Only the item with large traffic in overall performance measurement item is significant for traffic measurement. The item with small traffic is often unreliable. We should attach importance on the sector with large traffic and take precedence of solving the problems in sector with large traffic.
Soft handoff ratio: At current, the measurement result includes the softer handoff ratio.
Calculation formula = Traffic (with handoff) – Traffic (without handoff)/Traffic (with handoff).
The specification of China Unicom includes 3 types of traffic measurement: actual traffic, CE traffic, and WALSH traffic.
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II.
III.
4.5
4.5.1
Accordingly, other vendors should conform to the traffic measurement of above three types. The formula for soft handoff ratio is (CE traffic—actual traffic)/ CE traffic, and the measurement value obtained is generally within 30% and 40%. However, in Huawei the CE traffic is not independent from the traffic measurement, now the CE traffic and WALSH traffic are the same. Therefore, the soft handoff ratio of Huawei is a little high (i.e. about 50%). Actually, the soft handoff ratio of Huawei is about 33%, i.e., a rational value, if it is calculated in the same way as other vendors (the CE traffic is generally about 75% of the WALSH traffic).
The CE traffic will become independent of the traffic measurement in later versions of BSC ROO2B03D003 so that the soft handoff ratio displayed in traffic measurement equals the actual soft handoff ratio.
The soft handoff makes use of multiple sectors to generate diversity gain for one call to improve the link quality in cell border overlap area. The power control of soft handoff also helps reduce the interference of MS on its adjacent cells. Therefore, appropriate handoff can improve the quality of call, widen the coverage area, expand the system capacity and improve the performance. But high soft handoff ratio will increase the probability of call drops on the one hand, and result in heavy signaling load, occupy CE and impact the forward capacity of the system on the other hand. The venders generally hold different opinions about soft handoff ratio, but it is the unanimous viewpoint that the acceptable soft handoff ratio without softer handoff should be about 30%~40%.
The factors leading to exceptional soft handoff ratio include:
Excessive handoff region
The excessive region with overlapped coverage on the border of adjacent cells leads to frequent soft handoff. The soft handoff region can be located through the drive test. The size of soft handoff region can be controlled by adjusting antenna height, downtilt and azimuth instead of modification of power control.
No master pilot coverage
See 5.2
Improper setting of handoff threshold
See 5.3.
Soft Handoff
Calculation Formula
The soft handoff success ratio= [(Intra-BS soft handoff successes + inter-BS soft handoff successes)/(Intra-BC soft handoff requests + Inter-BS soft handoff requests)]*100%.
Note: The successes and requests here are only related to the soft handoffs (out).
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4.5.2
4.5.3
4.5.4
I.
II.
Meaning
It refers to the intra-BSC and inter-BSC handoff (out) success ratios, including common soft handoff and softer handoff.
Description of Measurement Items in Function Subset
Intra-BS and inter-BS soft handoffs are included.
Perform measurement of softer handoff if softer relation exists between new link branch and original active set branches (i.e. they are located in the same cell), otherwise, perform measurement of soft handoffs.
Measurement Item Analysis
Though soft handoff success ratio aims at the handoff (out), the handoff (in) data used for calculation of congestion ratio is also of great help for analysis of soft handoff success ratio. Both intra-BS and inter-BS soft handoffs are originated by Huawei equipment due to unavailability of A3 and A7 interfaces. The congestion upon handoff (in) corresponds to the soft handoff (out) failure.
Adding a branch
Better carrier After detecting that Ec/Io of certain PN is large, the MS will sends a PSMM to the BSC. The BSC sends an EHDM to the MS to add a soft handoff branch after judging the Ec/Io.
Network optimization The soft handoff due to poor coverage is specified in consideration of the special requirements during network optimization. Currently, measurement of this item is not performed.
Others It refers to manual execution of command of adding soft handoff branch via network management system, or other causes except those above.
Deleting soft handoff branch
Normal handoff branch deletion If the Ec/Io of a certain branch decreases, the MS will send a PSMM to the BSC. The BSC sends an EHDM to the MS to delete a soft handoff branch after judging the Ec/Io.
Forward link quality The precondition for the FMR reporting TCH ERR to the CCM is that this branch fails to receive the frame within 80ms. Upon receipt of the TCH ERR containing the cause value, the CCM will report it to the RMM to request for disconnection of this branch.
A7 interface reset For inter-BSC soft handoff, the call management is performed at the original BSC side. The target BSC should send the received reverse signal to the original BSC for processing via A3/A7 link. A7 interface functions as a signaling interface between the original BSC and target BSC. The measurement of this item can only be performed after A7 interface is reset and "A7 Reset" message is sent to the BSC. That the FMR fails to receive the reverse frame of this branch will be reported to the original BSC in
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4.5.5
case of broken A3/A7 link. In this case, the measurement of branch deletion will fall into the measurement of branch deletion caused by forward link quality. Currently this measurement is unavailable.
Soft handoff failure
Radio resource unavailable It corresponds to radio resource request failure of incoming party, and specific failure cause can be located through analysis of handoff (in) congestion.
Requested terrestrial resource unavailable The establishment of link with BTS terrestrial resource fails, corresponding to handoff (in) Abis interface setup failure.
Radio interface failure Waiting for “Handoff Complete MSG” from the MS times out. The probability of this failure is great.
1) Poor radio link including both forward and reverse links: the cause is that the MS fails to receive the handoff command, or the BTS fails to receive the handoff complete message from the MS. For details, see 4.5.5.
2) Improper timer setting. Please check the CCM_T_WT_MS_HO_CMP timer. MS denial
The MS cannot identify the received handoff command and reject it. This probability is low unless the MS does not comply with protocol specifications to support handoff message. Currently measurement value of it is 0 due to unavailability. If the MS rejects the EHDM indeed, this cause will incorrectly fall into other cause in the traffic measurement.
A3 link setup failure Upon receipt of A7-Handoff Request via A7 interface, the target BSC should establish the connection of A3 and Abis interfaces for service transmission between the MS and original BSC. Analysis of transmission can be performed based on alarm information.
Others On the one hand, there are exceptional causes other than mentioned above, such as code exception and incorrect status, which are hard to locate. Generally the judgment result can be displayed at the debug console.
On the other hand, timer of RRM waiting for handoff completion expires:
1) If the timer of CCM waiting for handoff completion also expires, i.e., a soft handoff failure is counted in the measurement of soft handoff failures caused by radio interface failure, it must be the cause of poor radio link.
2) Check the setting in TT_HO_REQ (timer duration of handoff request), and this value should be larger than the timer duration of CCM waiting for handoff completion. For details, see CDMA1X BSS Network Planning Parameter Configuration Recommendations.
3) If the timer duration of RRM is longer than that of CCM, and CCM timer does not expire, it must be the problem of communication between equipment modules.
Possible Causes for Soft Handoff Failure
No master pilot coverage
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4.6
4.6.1
See 5.2. Incorrect setting of handoff threshold and parameter
See 5.3. Absent configuration of adjacency
See 5.4. Improper adjacent cell priority
See 5.4. Improper setting of search window
See 5.5. Imbalance between forward and reverse links
See 5.1. Congestion Poor coverage
Hard handoff
Calculation Formula:
Hard handoff success ratio= [Intra-BS hard handoff successes+ inter-BS hard handoff successes/ Intra-BS hard handoff requests+ inter-BS hard handoff requests]*100%
Note: The successes and requests are only related to the handoffs (out).
4.6.2
4.6.3
4.6.4
Meaning
Success ratios of intra-BSC and inter-BSC hard handoff (out), including co-frequency and inter-frequency hard handoffs
Description of Measurement Item in Function Subset
Intra-BS hard handoff and inter-BS hard handoff are included.
Measurement Item Analysis
Hard handoff measurement covers the co-frequency, inter-frequency, intra-BSC and inter-BSC hard handoffs. The hard handoff success ratio refers to the success ratio of all hard handoffs (out). The measurement of hard handoff involving multiple equipment suppliers requires the coordination of the client.
As there are many hard handoff algorithms, different optimization targets and measures should be developed for different algorithms during the optimization process. Attention should be given to the co-frequency hard handoff, as it is hard to achieve a satisfying handoff success ratio due to co-channel interference. Take the commercial network in Cangzhou for example, the success ratio of 80% is accepted by the client. The relative low handoff success ratio will definitely affect other measurement items, such as the call-drop rate. With the maturity of technology,
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4.6.5
clients will ask for a higher co-frequency hard handoff success ratio. More experience is also required for the analysis of inter-frequency hard handoff.
A lot of measurement items of hard handoff are similar with those of soft handoff. For the same measurement items, see the relevant description of soft handoff measurement items.
Better pilot When MS detects that the Ec/Io of a certain PN is stronger than the serving one, it will report PSMM message. BSC sends EHDM message according to the Ec/Io
Network optimization It refers to the load-balance hard handoff. At present, this item is not measured.
Border cell It refers to the handoff target cell is border cell. At present, this item is not unavailability.
Pseudo-pilot It is measured when the handoff target is the pseudo-pilot.
Returning to source channel The cause is not unclear yet. At present, it is treated as the successful return to the source channel after the MS fails to be handed off to the target carrier.
Others The hard handoff measurement does not cover the radio interface fault measurement. However, the measurement item includes the radio interface measurement and other causes in the soft handoff.
Possible Causes for Hard Handoff Failure
Improper configuration of adjacency During the preliminary planning of hard handoff adjacency, the adjacency is configured as much as possible. In this way, ping-pong handoff occurs on the border of the cell. In addition, as the system has to monitor the changes of the MS pilot signals in a lot of border cells, the system load increases. The drive test can be carried out to get the information about BTSs really and directly adjacent to Huawei BTSs. In this way, the unnecessary or unimportant adjacency can be removed to reduce the ping-pong handoffs and call drops, and increase the handoff success ratio.
Poor coverage Improper settings of handoff threshold and related parameters
In the co-frequency hard handoff, the signal on the boarder of handoff region fluctuates seriously and fades quickly due to the signal interference of two cells. If the relative hard handoff threshold is too low, it is easy to trigger the handoff, which causes ping-pong handoffs. When a hard handoff occurs, the hard handoff may end up with failure due to the change of signal and fading of target pilot. In this case, the relative hard handoff threshold can be increased to reduce the times of hard handoff attempts and increase the success ratio.
However, the threshold cannot be too high. Otherwise, it could be too late to trigger the hard handoff. When the hard handoff is triggered, the fading of the forward link of Huawei BTS may be too serious for MS to receive the HDM message from the source carrier.
Improper setting of search window Improper setting of access parameters
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4.7
4.7.1
Improper setting of power If the pilot power of destination cell is greater than the source cell, the handoff region will lean towards the source cell. If MS is far away from the destination cell, it is hard for MS to capture the destination cell. This situation can be improved by adjusting the powers of both cells.
If the hard handoff fails because the MS sends a candidate frequency search report to the source carrier and then returns to the source channel, the failure cause is the “failure of capturing target channel”. There are two methods to solve this problem. One is to increase the transmit power of the initial traffic channel of the destination cell. The other is to increase the MS search window.
Carrier Power Control Measurement
Description of Measurement Item
1) It is a sector-specific measurement item. Please note that:
All measurement items are average values of all subscribers in this sector. Take RSSI for instance, the RSSI in the Common Measurement Report sent by BTS is average of RSSI every second. If the measurement period is 5 minutes, the measurement data refers to the average data within five minutes.
2) The units for these measurement items are special. The data measured are not for direct use.
Average Eb/Nt It is used to measure the target Eb/Nt of the reverse outloop power control.
The measurement data is represented by Ec/Io corresponding to Eb/Nt of RC1. Values 0~255 correspond to -63.75~0dB:
Ec/Io = -(255-X)*0.25
Eb/Nt in RC1= Ec/Io + 21
For example, if the measurement value of average Eb/Nt is 195, the actual Eb/Nt is -(255-195)*0.25 +21 = 6dB
Average RSSI
It should be a negative value. But as DB cannot be used for negative values, the average RSSI is represented by its actual value plus -120.
For example, if the measurement value of average RSSI is 28, the actual RSSI should be 28-120=-92dB
Average FER of carrier reverse link
Its unit is 0.1%. For example, if the measured value is 20, the actual FER is 2%. Average carrier transmit power
Its unit is 0.1dB. If the measure value is 386, the actual transmit power is 38.6dB.
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4.7.2
4.7.3
At present, the average transmit power in the traffic measurement is calculated as follows:
The value of transmit power reported by BTS every second is stored in a counter. This counter is reset every five minutes. (The time is counted as per system start time instead of the task registration time.) The final average carrier transmit power reported is the average of the values stored in the counter in a measurement period.
Please note that the forward load is calculated according to the forward transmit power reported by BTS in the last second of the measurement period. If this transmit power does not reach 43dBm, the load calculated will be less than 100%. Hence, it is possible that the measured average transmit power exceeds 43dBm, but the forward load is less than 100%.
Judgment of Reverse Interference
The longer the measurement period is, the harder it is to judge whether there is peak RSSI. The RSSI given in the traffic measurement can be used to check the general situation of interference. If serious interference exists on a certain carrier, the BTS can be accessed in the Telnet mode to check the peak RSSI.
It is better to judge the interference by checking the average RSSI when the system load is almost 0. For a newly deployed network, as the subscriber base is small, the RSSI under no system load is normally less than -105dBm. When there is traffic and several services are in operation, the average RSSI is normally under -95dBm.
The interference can be also judged according to the receiving quality.
For external interference, spectrum analyzer can be used to check out whether there is any interference source.
Drive test can be carried out to check out whether there is cross-cell coverage which also causes interference. Interference can also be judged according to the Ec/Io and MS receiving power.
Querying RSSI in Telnet Mode
TRX resource can be traced via Telnet. Besides the average RSSI, other items such as peak value can be reported. The report period is 1 second and the command is Str infotrace: brdtp=btrm, brdid=XXX, item="rssi", where XXX is the No. of the RM resource allocation board.
In the BTS3612 of B02D006 version, we can start RSSI test via OMC on the BAM. The command used is STR BTSRSSITEST.
After the test is started, the BAM will automatically save the file and write a test record into the file every minute. This function is sector specific.
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5
5.1
Figure 5-1
Analysis of Causes Affecting Measurement Items
The following analysis is based on the assumption that a good preliminary plan is prepared and well implemented, and the signaling flow and the equipment are normal.
Imbalance between Forward Link and Reverse Link
The imbalance between forward and reverse links will seriously affect the whole-network measurement items. For details, see Figure 1 and Figure 2.
Cell B Cell A
It seems possible that MS in this area can be handed from
cell B to cell A. In fact, the reverse link of cell A cannot
support the call.
The coverage of forward link is greater than the reverse link
As Figure 1 shows, the forward and reverse links are balanced in cell B. In cell A, they are not balanced and the coverage of forward link is greater than the reverse link. When an MS moves from cell B towards cell A and into the dashed area in Figure 1, the signal of cell B becomes weaker and weaker while the pilot signal of cell A becomes stronger and stronger. When the pilot strength of cell A exceeds the handoff threshold, the MS is allowed to be handed off from cell B to cell A. However, as the power is not strong enough to support good reverse link, the reverse macro diversity gain and multi-path diversity gain cannot be realized. As a result, the handoff fails. In addition, the MS in the dashed area brings great interference to the cell B, which in turn leads to the drop of capacity of cell B and even call drops of MS.
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Figure 5-2
Cell B Cell AIn this area, MS cannot be handed off
from cell B to cell A. Hence, the MS
in cell A will bring serious inference
to cell B.
The coverage of reverse link is greater than the forward link
In Figure 2, the forward and reverse links are balanced in cell B. In cell A, they are not balanced and the coverage of forward link is smaller than the reverse link. When an MS moves from cell B towards cell A and into the dashed area, MS should have been able to receive the cell A pilot signal whose Ec/Io exceeds the handoff threshold. However, as the coverage of forward link of cell A is not large enough, the Ec/Io of the pilot signal of cell A cannot reach the handoff threshold.
Thus, MS will not report the pilot measurement message and handoff will not occur. On the other hand, MS which is now in the dashed area will bring great forward and reverse interference to the cell B due to the unavailability of macro diversity and multi-path diversity gains that should be available. As a result, the capacity of cell B decreases and call drops may also occur.
Similarly, when MS in the dashed area originates a call, as it cannot obtain the handoff gain or the reverse power is insufficient, the call setup may fail. In the forward link, when the BTS transmit power is specified, the system coverage is determined by the allocation of the pilot power. (The precondition is that the energy of all the forward channels on the border of the cell allows successful demodulation. The energy imbalance between channels does not exist, which depends on the proper setting of forward power.) With the increase of pilot power, the forward coverage increases. The forward coverage radius is determined by the Ec/Io of pilot channel. However, in the whole system, too large a forward pilot Ec/Io will bring interference to other cells and lead to pilot pollution.
Thus the pilot strength should be adjusted according to the requirement of system design. The reverse coverage radius is determined by the link maximum propagation loss. However, as subscribers increase, the system interference increases. The interference will directly affect the system capacity and the coverage radius. In this case, the system load should be under control. A reasonable pilot Ec/Io means favorable quality of forward link. When the MS transmit power reaches its maximum, it means the quality of reverse link is poor. Whether the forward and reverse links are balanced should be determined in consideration of the forward and reverse data.
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5.2
1) The inconsistent of forward link coverage and reverse link coverage will cause MS on the cell border to increase its transmit power. The increased transmit power brings extra interference to the cell. Drive test can be carried out to observe whether the MS transmit power is too large.
Generally, MS transmit power should be kept to a negative value. If the received pilot Ec/Io is good, and the MS transmit power is above 0, it can be decided that the forward coverage is greater than the reverse coverage.
2) Whether the reverse and forward links are balanced can be effectively decided by reviewing the relationship between the power of forward traffic channel and the reverse Eb/Nt threshold. At present, the traffic measurement provides the measurement of the reverse Eb/Nt and the transmit power of forward carrier. Please note that the adjustment of forward channel power is not the most effective way to solve the problem of imbalance between forward link and reverse link. The adjustable range of the parameters should also be noted.
Pilot Pollution
The following is one case. The introduction of soft handoff to the CDMA system makes it possible to apply soft handoff within the handoff region to get the handoff gain and then improve the network performance. On the one hand, the number of Rake receivers is limited (MS can receive three soft handoff branches currently), so the MS will be unable to utilize these signals effectively when the number of signal branches received exceeds the number of the Rake receivers. These signals will bring interference to the desired ones. Further, if the Ec/Io of these signals exceeds the specified threshold, they even bring severe interference. This is the so-called pilot pollution, that is, the number of pilots whose signal strength exceeds the specified threshold is greater than the number of Rake receivers. Tadd is usually set as the value of the specified threshold. At present, the valid branch number for the MS is generally 3. Therefore, when there are four or more branches whose signal strength exceeds Tadd, it will be considered that there exists pilot pollution.
There is another case. The network signal level is rather good, but the Ec/Io is small. That is to say, there is no pilot whose Ec/Io is much larger than that of the others. In other words, the qualities of the pilots are almost the same. As fast fading of signal may cause frequent handoffs between traffic channels in different sectors, call drops will be easily caused. In this case, if there is no external interference, it shows that there are signals from different cells in the area. That is why the Ec/Io is so small and the coverage is so poor. This is also a case of pilot pollution. The Ec/Io of a poor pilot is usually considered smaller than -12dB. Considering that the number of valid Rake receivers for the MS is 3, if the difference between the Ec/Io of the first strongest pilot and that of the fourth strongest one is less than 3dB, this will be considered as pilot pollution.
Where there is pilot pollution, following network problems will rise. High frame error rate (FER)
If there is a high quality pilot, but it cannot be effectively utilized, it will bring interference to the others. As a result, the FER rises, the network quality degrades or the call drop rate increases.
Call drop due to handoff If there are more than three pilots with a high Ec/Io, handoffs will take place among them frequently, resulting in call drops.
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Capacity decrease The interference in the pilot-polluted area will be higher than that in the unpolluted areas, this not only reduces the valid coverage of the system, but affects the system capacity.
Pilot pollution is mostly caused by the interference among the signals from different sectors. As the radio environment (including geography, building distribution, street distribution and water area, etc.) is generally very complex, it is rather hard to control the signals to reach a sound status. Pilot pollution generally occurs in those cities where the BTSs are dense. The typical areas where pilot pollution easily happens include high building, broad street, overbridge, crossroad and the surroundings of water area. The reasons are detailed as below:
Improper cell layout The improper cell layout may cause a blind area in a certain area, but multi-pilot signals in another area. This may cause a large area of pilot pollution or a blind area. Due to the complexity of the geographic environment, some factors are not taken into consideration at the design stage, so modifications are necessary at the network optimization stage.
Too high BTS site or antenna Most surface features around are within the line-of-sight transmission range. Thus, the signals may transmit in a comparatively large area (especially in the outdoors, such as street), this possibly causes interference to other BTSs, resulting in pilot pollution.
Improper antenna azimuth If the antenna is not reasonably designed, some sectors may cover the same area simultaneously, so there may be too many pilots in a certain area. There may also be multiple pilots in a certain area under the influence of the surface features, such as buildings around. In such case, the antenna azimuth should be changed according to the actual propagation. Especially when the antenna faces the street, the coverage will extend along the street to a comparatively long distance. As a result, pilot pollution is caused in the service areas of other BTSs along the street. Then, the antenna azimuth or downtilt should be adjusted.
Improper antenna downtilt The modification to the antenna may bring great effects on the signal on the cell coverage border, and thus the coverage scope of cell. When the design of the antenna is unreasonable, those unwanted strong signals, will be received as well. This brings interference to other areas, resulting in pilot pollution. If the interference is serious, call drop will even be caused.
Improper setting of pilot power When the BTSs are too dense, pilot pollution will also be caused if the required coverage scope is small, while the pilot power is set to be too high.
Too high an altitude If the target coverage is at a high altitude (i.e. in high buildings), target coverage region is within the line-of-sight transmission range of other BTSs around, so pilot pollution is easily caused.
Solutions: Power adjustment
The easiest way is to raise the power of one BTS, while reduce the power of the others, so as to create a master pilot. However, the effect that may be brought to the whole network should be taken into consideration here. If the corresponding
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5.3
strongest pilot varies greatly with the location in the pilot polluted area, this method is not applicable. Generally, this method is mainly applicable to the case that the strength of the corresponding pilot basically remains high.
Antenna adjustment According to the result of drive test, antenna azimuth and downtilt can be modified to change the strength of the pilot signals in the pilot polluted area, and consequently, change the distribution of pilot signals in the area. The principle for the adjustment is to increase the high strength pilot while weakens the low strength pilot. In addition, the modification can also be used together with power modification flexibly.
Modification of BTS configuration If the above mentioned methods cannot be used to solve pilot pollution in certain areas, antenna model, antenna installation location or BTS location should be changed, or BTS can be either added or reduced according to the actual condition. Since the implementation of these measures may involve great engineering change, a careful analysis is recommended.
Adoption of ODU or repeater If pilot pollution cannot be solved through methods such as power modification and antenna feeder modification, then ODU or repeater can be considered. The purpose of adopting ODU or repeater is to introduce a high strength signal to the pilot polluted area, so as to reduce the relative strength of other signals, that is, lower the Ec/Io value of signals of other sectors at a point. However, the effect that may be brought by ODU or repeater to network quality should be taken into consideration in such case.
Adoption of microcell This is an important means to solve pilot pollution. The microcell is mainly applied in the traffic hot spot to increase the capacity and at the same time solves the problem of pilot pollution.
Adoption of distributed antenna This method is used to solve the coverage problem of high buildings.
Drive test and printing data at debugging console This method is utilized to avoid absent configuration of high strength pilot.
Improper Setting of Handoff Parameter
A high call drop ratio is generally caused owing to unsuccessful handoff. Parameters such as T_ADD, T_DROP, T_COMP and T_TDROP are important parameters for the CDMA system to add or delete branch via handoff. If they are set unreasonably, the system call drop ratio will increase and the system capacity will be affected.
If the values of T_ADD and T_COMP are set to be too large, the high strength signals may fail to be added to the active set. This not only brings on interference, but also increases the forward BER, resulting in a call drop. If the values are set to be too small, then soft handoff branch will be added frequently. This not only brings on frequent handoffs, but also increases the system load. So the system capacity is affected, and a call drop is easily caused.
If the value of T_DROP is set to be too small, the MS will fail to delete the soft handoff branch fast, and the high strength signal may fail to be added. This not only brings on interference, but causes call drop. If the value is set to be too large, then soft handoff branch will be added frequently. This not only brings on frequent handoff, but also increases the system load. So the system capacity is affected, and a call drop is easily caused.
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5.4
In addition, the values of T_ADD and T_DROP cannot be the same. A reasonable DB value should be maintained to avoid excessive ping pong handoffs.
If the value of T_TDROP is too large, it will be rather slow for the high strength pilots in the candidate set to be added to the active set. This not only brings on strong interference, but causes a call drop. If the value is too small, the MS will add/delete a branch frequently. This not only increases the system load, but also affects the system capacity.
For details of parameter setting, see CDMA1X BSS Network Planning Parameter Configuration Recommendations.
Improper Adjacency
If the adjacency is improper, the size and shape of the cell will be affected.
On the one hand, the absent configuration of adjacency will affect the following: Call drop rate
1) The signal in Cell A is strong, and the adjacent cell table of Cell B doesn’t contain Cell A. The pilot of Cell A cannot be added to MS active set, and causes interference signal, and the pilot pollution occurs.
2) The signal in Cell A is weak, and the adjacent cell table of Cell A doesn’t contain Cell B. When MSs use A as a serving cell and move towards Cell B, the MSs cannot be handed off to Cell B, and the signal of the original cell becomes weak until the call drop occurs to form the so-called isolation effect.
Congestion If the sector with high traffic is not configured with the adjacent cell, the MSs that should have been handed off to other cells cannot perform handoff, causing cell congestion.
Handoff failure. The MS updates the pilots of the neighbor set through the adjacent cell table. If a certain pilot exceeds NGHBR_MAX_AGE (that is, maximum age of the neighbor set pilots) in MS neighbor set (see CDMA1X BSS Network Planning Parameter Configuration Recommendations), the pilots are removed from the neighbor set to remaining set. The pilots in remaining set may be added to the active set, on the one hand, the speed that the MSs search the pilots in the remaining set one after another is very slow; on the other hand, when the pilots are reported and the corresponding PN is reused in the system, the system cannot determine which BTS signal corresponds to the PN. So the handoff fails.
If the adjacency is not configured, it can be detected through the drive test, and from “Unknown Pilot” of the debugging console.
Add the adjacent cell properly after re-organizing the data.
On the other hand, the improper setting of priority of adjacent cells will affect the handoff success ratio.
The adjacency is very important. But it is not recommended to construct more adjacent cells. Importance should be attached to the proper priority of the adjacent cells. Since the adjacent cells received by the MSs at one time are limited. For a 95 MS, only 20 adjacent cells can be delivered at one time, and for a 2000 MS, 40 adjacent cells can be delivered at one time.
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5.5
Take a 95 MS for example. The MS in null status, only keeps contact with one pilot, and can receives 20 adjacent cells information. In conversation, if the MS keeps contact with three pilots, it can only receive 20 adjacent cells information, and BSC combines the adjacent cells of three PNs, and then delivers them according to the priority, and the adjacent cell with the lower priority will be discarded, and not delivered to the MS.
When the MS receives a new adjacent cell information, the sequence of adjacent cells in the MS is the same as that in the adjacent cell table, and the MS will search the pilot with a highest priority.
If the important adjacent cell is ranked last, the absent configuration of adjacency will occur. So, the cell with a higher handoff probability should be ranked first.
The optimization of adjacency is very important for CDMA. Currently, the system doesn’t support the auto sorting of the adjacent cells priority, which needs the maintenance engineers to spend a lot of time in performing a drive test, and analyze the data to acquire the optimal adjacency.
The adjacent cells can be set and checked by scanning pilots.
Improper Setting of Search Window
The pilot to be detected by the MS seldom reaches at the expected time, so the MS should search it in a rational delay window until the actual time sequence of the pilot is found. Three search window parameters are used when the MS searches the pilot:
SRCH_WIN_A Search the pilot in active set and candidate set
SRCH_WIN_N Search the pilot in neighbor set
SRCH_WIN_R Search the pilot in remaining set
Search window is designated based on PN chip. The principle of setting the search window is as follows:
1) Make a compromise between the size of search window and speed. If the size is large, the MSs need to spend more time, thus, the total number of pilots searched in the fixed period will decrease.
2) The MS cannot search the pilots beyond the search window no matter how strong they are. So, the pilots that are not detected may be strong interference sources.
3) If the pilots are not in the list of candidate set, some equipment may not allow the pilots to enter the active set. We recommend that SRCH_WIN_R be set to 0 after optimization, to prevent the MSs from searching the pilots that are not used for handoff.
For the setting the size of search window, see CDMA1X BSS Network Planning Parameter Configuration Recommendations.
The MSs search the pilots within search window. If SRCH_Win_A is small, the MSs may lose the multi-path signals. If SRCH_Win_N and SRCH_Win_R are small, the MSs may lose the signal of adjacent cell, and the handoff success ratio is reduced.
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Note: When a cell is the donor cell of the repeater or the distributed system, the radius of the cell greatly increases due to the delay of the repeater or the distributed system. The MS search window parameter in both the cell radius parameters and handoff parameters will be enlarged. Meanwhile, the corresponding parameters of the adjacent cell involved in the handoff in the coverage of the repeater or the distributed system in the cell will also be enlarged. If these parameters are not enlarged, the MSs covered by the repeater or the distributed system cannot originate a call or cannot perform any handoff.
The delays of the pilots compared with the reference pilots are shown in CAIT, and you can also see them at debug console and signaling. Be careful to optimize the search window, because, if a large search window is set (for example, SRCH_Win_A is more than 80 chips; SRCH_WIN_N and SRCH_WIN_R are more than 130 chips), the search speed of MSs slows down, which may also affect the handoff and drop calls.
6
7
Recording Problems in Traffic Measurement
M2000 traffic measurement provides a large number of measurement data, which provides the optimization principle and target. The applications show M2000 traffic measurement still has the limitations for network optimization, that is, the network optimization engineers cannot solve the problem directly from the parameters of traffic measurement. Specifically, the measurement function is incomplete; the cause is not very specific; the definitions of some measurement parameters are not correct; the operations are not fully simplified.
To better the traffic measurement function, the network optimization engineers should record the problems during the routine operation, and submit them to Huawei TSD, so that the traffic measurement can really guide the network optimization.
References
1) For M2000 traffic measurement operations, see Guide to CDMA 1X M2000 Traffic Measurement (V1.0).
2) For parameter configuration, see CDMA1X BSS Network Planning Parameter Configuration Recommendations.
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