09 gsm bss network kpi (handover success rate) optimization manual

GSM BSS Network KPI (Handover Success Rate) Optimization Manual

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Product name Confidentiality level

GSM BSS Internal

Product version

Total 30pagesV00R01


For internal use only

Prepared by GSM&UMTS Network Performance

Research Department

Dong

Xuan Date 2008-4-23

Reviewed by Date yyyy-mm-dd

Reviewed by Date yyyy-mm-dd

Granted by Date yyyy-mm-dd

Huawei Technologies Co., Ltd.

All rights reserved

2023-4-10 华为机密，未经许可不得扩散第 1页, 共 30页


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Revision Record

DateRevision

VersionChange Description Author

2008-4-23 0.88 Draft completed Dong Xuan

2008-7-30 1.0 Some flowcharts are updated. Dong Xuan



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Keywords

Handover Success Rate

Abstract

By analyzing the factors that affect the Handover Success Rate (HOSR) on the BSS side, this

document provides a method of quickly locating the cause of low HOSR or slow handover. In

addition, this document provides measures for optimizing the HOSR, thus meeting field engineers'

working requirements for solving handover problems. This document is used for optimizing the

KPIs of network performance and monitoring the network quality.

Acronyms and Abbreviations

Abbreviations Expansion

AMR Adaptive Multi Rate

BCCH Broadcast Control Channel

BER Bit Error Ratio

BQ Bad Quality

BSC Base Station Controller

BSIC Base Station Identity Code

CDU Combining and Distribution Unit

CIC Circuit Identification Code

HOSR Handover Success Rate

KPI Key Performance Index

MR Measure Report

MS Mobile Station

NE Network Element

QoS Quality of Service

RQI Radio Quality Indication

TA Timing Advance



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Table of Contents

1 Basic Principles................................................................................................................... 8

1.1 Definition...................................................................................................................... 8

1.2 Theory......................................................................................................................... 8

1.3 Recommended Formula..............................................................................................8

1.4 Measurement Point.....................................................................................................9

2 Influencing Factors............................................................................................................11

3 Analysis Process and Optimization Method....................................................................11

3.1 Process of Analyzing Handover Problems.................................................................11

3.1.1 General Process of Locating a Handover Problem.........................................11

3.2 Methods for Optimizing Handover Problems.............................................................11

3.2.1 Classification of Handover Problems..............................................................11

3.2.2 Hardware and Transmission Failure...............................................................11

3.2.3 Improper Data Configuration...........................................................................11

3.2.4 Congestion of the Target Cell..........................................................................11

3.2.5 Clock Problems...............................................................................................11

3.2.6 Interference Problems.....................................................................................11

3.2.7 Coverage Problems, and Uplink and Downlink Imbalance..............................11

3.2.8 Failed Inter-BSC/Inter-MSC Handovers..........................................................11

3.2.9 Automatic Neighboring Cell Optimization........................................................11

3.2.10 Testing Tool Selection and Testing Suggestions..........................................11

3.2.11 Configuration Suggestions for Tests on the Existing Network.......................11

4 Optimization Cases............................................................................................................11

4.1 A Handover Fails Because the BSIC Cannot Be Decoded........................................11

4.2 A Handover Fails Because Frequency Sequencing of the MS Is Different from That of

the BSC........................................................................................................................... 11

4.3 A Handover Fails Due to Unreasonable Parameter Configuration............................11

4.4 The Number of Failed Incoming BSC Handovers Increases Because the Handover

Request Does Not Contain Class Mark 3........................................................................11

4.5 An Incoming BSC Handover Fails Because the A Interface Phase Flag Is Set

Wrongly........................................................................................................................... 11

4.6 Because the Idle Burst Is Enabled, the Interference Increases, the Receiving Quality

Decreases, and the HOSR Becomes Low.......................................................................11

4.7 Different HOSRs Resulting from Different Cause Values Contained in the Clear

Command Messages Sent by Different Switches............................................................11

5 Information Feedback........................................................................................................11

5.1 TEMS Log Files About Problem Cells.......................................................................11



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5.2 Requirements of Configuration Data of the Existing Network and Traffic

Measurement Feedback..................................................................................................11



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List of Tables

Table 3-1 List of handover timers commonly used..........................................................11



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List of Figures

Figure 1-1 Signaling process of intra-BSC handover........................................................9

Figure 1-2 Signaling process of inter-BSC handover......................................................10

Figure 3-1 Flow chart of locating a handover problem....................................................11

Figure 3-2 Flow chart and timer description....................................................................11

Figure 3-3 Flow chart of automatic neighboring cell optimization....................................11



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GSM BSS Network KPI (Handover Success

Rate) Optimization Manual

1 Basic Principles

1.1 Definition

Handover is an important function in mobile communication systems. As a means of radio link

control, handover enables users to communicate continuously when they traverse different cells.

The HOSR is the ratio of the number of successful handovers to the number of handover requests.

The major purpose of handover is to guarantee call continuity, improve speech quality, reduce

cross interference in the network, and thus provide better services for mobile station (MS)

subscribers.

1.2 Theory

The HOSR is an important KPI of the call hold type. According to the processes, this KPI can be

divided into two types: Handover Success Rate and Radio Handover Success Rate. According to

the relations between involved network elements (NEs), this KPI can be divided into three types:

Success Rate of Intra-BSC Handover, Success Rate of Incoming BSC Handover, and Success Rate

of Outgoing BSC Handover. The HOSR is an important KPI assessed by operators because the

value of the HOSR directly affects the user experience.

1.3 Recommended Formula

The HOSR is obtained through traffic measurement. The recommended formula for calculating

this KPI is as follows:

Handover Success Rate = Successful Handovers/Handover Requests

Radio Handover Success Rate = Successful Handovers/Handover

Commands

For details, refer to the GSM BSS Network KPI (TCH Call Drop Rate) Baseline.



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1.4 Measurement Point

Figure 1-1 Signaling process of intra-BSC handover



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Figure 1-2 Signaling process of inter-BSC handover

The measurement points illustrated in Figure 1-1 and Figure 1-2 are as follows:

A1——Measurement point of Incoming Internal Inter-Cell Handover Requests and Internal Intra-

Cell Handover Requests

B1——Measurement point of Incoming Internal Inter-Cell Handover Responses (Incoming

Internal Inter-Cell Handovers) and Internal Intra-Cell Handover Commands

C1——Measurement point of Successful Incoming Internal Inter-Cell Handovers and Successful

Internal Intra-Cell Handovers

A2——Incoming External Inter-Cell Handover Requests

B2——Incoming External Inter-Cell Handover Responses (Incoming External Inter-Cell

Handovers)

C2——Successful Incoming External Inter-Cell Handovers

A3——Outgoing External Inter-Cell Handover Requests

B3——Outgoing External Inter-Cell Handover Commands (Outgoing External Inter-Cell

Handovers)

C3——Successful Outgoing External Inter-Cell Handovers

Replaced with corresponding measurement points, the formulas for calculating different types of

HOSR can be as follows:

Success Rate of Handover: (C1<Successful Incoming Internal Inter-Cell Handovers>

+C3)/(A1<Incoming Internal Inter-Cell Handover Requests> +A3)



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Success Rate of Radio Handover: (C1 <Successful Incoming Internal Inter-Cell Handovers>

+C3)/(B1<Number of Incoming Internal Inter-Cell Handover Responses> +B3)

Success Rate of Intra-BSC Handover: C1/A1

Internal Radio Handover Success Ratio per cell: C1/B1

Success Rate of Incoming BSC Handover: C2/A2

Success Rate of Incoming BSC Radio Handover: C2/B2

Success Rate of Outgoing BSC Handover: C3/A3

Success Rate of Outgoing BSC Radio Handover: C3/B3

Note:

If the BSC receives the Clear Command message sent by the MSC during an

inter-BSC handover, the current version does not count this case as a failed

handover. If a subscriber hangs up the phone during an intra-BSC handover, the

current version counts this case as a failed handover.

2 Influencing Factors

According to the cases and experience of the existing network, the factors that influence the

handover include the following types:

Hardware and transmission failures

Data configuration

Congestion

Coverage problems, and uplink and downlink imbalance

Interference

Clock problems

Failed inter-BSC/inter-MSC handovers

For details about all these factors, see section 3.2 "Methods for Optimizing Handover Problems."

3 Analysis Process and Optimization Method

This chapter provides solutions to the problems about the handover when the following conditions

are all met:

The data configuration complies with the baseline of related parameters.

There is no problem about the engineering quality.

The coverage is good.



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3.1 Process of Analyzing Handover Problems

Generally, there are the following types of handover problems:

Call drops due to no timely occurrence of handovers

Failed handovers

Frequent (ping-pong) handovers

Poor downlink quality caused by slow handovers

These problems directly result in poor experience of end users, which is inclined to cause

complaints. Therefore, it is necessary to work out a method for optimizing the HOSR quickly or

even automatically to improve the network quality and user experience.

3.1.1 General Process of Locating a Handover Problem

Figure 1-3 shows the general process of locating a handover problem.



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Figure 1-3 Flow chart of locating a handover problem



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3.2 Methods for Optimizing Handover Problems

Generally, handovers occur between two cells. The relationship between cells may be:

Between different BTSs in a BSC

In the same BTS in a BSC

Between different BSCs

Therefore, after you know how to locate and optimize the handover problems between two cells,

you can solve the handover problems in an entire network.

Handover problems may be caused by:

Hardware and transmission failures (bad TRXs or problems about the

combiner of the feeder and antenna system)

Improper data configuration

Congestion problems

Clock problems

Interference problems

Coverage problems, and uplink and downlink imbalance

If a low HOSR occurs, do as follows:

(2) Identify the problem.

(3) Troubleshoot the problem based on the factors such as hardware, data

configuration, congestion, clock, interference, and coverage

(4) Improve the HOSR according to the automatic optimization of neighboring

cells.

3.2.1 Classification of Handover Problems

I. Classification Description

Before analyzing the problem about the HOSR, determine the following points about handover

classification:

(5) Decide the scope of the failed handover. If the low HOSR occurs in all the

cells, check the problem from such aspects as the handover feature

parameters, the A interface circuits, and the BSC clock.

(6) If the low HOSR does not occur in all the cells, find out the TOP n poorest

cell. Then, proceed with the following steps specific to the cell.

(7) Distinguish whether there is any problem in the wireless interfaces

according to the differences between the HOSR and the Radio HOSR. The

Radio HOSR must be greater than or equal to the HOSR. If the HOSR is

much smaller than the Radio HOSR, analyze the problems about the

terrestrial links and the capacity. If the HOSR is a little different from the

Radio HOSR, consider the problems about the coverage and the

interference.

(8) Query the success rates of outgoing/incoming external/internal inter-cell

handovers in the handover performance measurement to analyze whether a

failed outgoing or incoming handover occurs. Analyze the performance



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measurement of outgoing and incoming external inter-cell handovers of the

faulty cell. From the performance measurement of outgoing external inter-

cell handovers, find out to which cells the handover fails. Analyze counters

of the cells where incoming handovers fail, such as the Failed Incoming

External/Incoming Inter-Cell Handovers (No Channel Available), the Traffic

Volume on TCH, and the Congestion Ratio on TCH(All Channels Busy), to

decide whether the congestion of the target cell causes the failed handover.

(9) Query such counters as the TRX Availability and the TCH Availability of the

target cell to check whether any device is faulty.

(10) Query relevant alarms to analyze whether any terrestrial link device is faulty.

II. Traffic Measurement Analysis

By registering and analyzing the following counters, you can decide the scope and the basic cause

of a handover problem:

Cell Level

Incoming Internal Inter-Cell Handover Measurement

per Cell

Outgoing Internal Inter-Cell Handover Measurement

per Cell

Incoming External Inter-Cell Handover Measurement

per Cell

Outgoing External Inter-Cell Handover Measurement

per Cell

Incoming Inter-RAT Inter-Cell Handover

Measurement per Cell

Outgoing Inter-RAT Inter-Cell Handover

Measurement per Cell

Measurement of MRs upon Handover Initiation per

Cell

Channel Assignment Failure Measurement per Cell

Traffic Volume on TCH

3.2.2 Hardware and Transmission Failure

Symptom: The alarm system reports relevant alarm information. To rectify a hardware fault, clear

the alarms about the hardware failure. If the alarms are cleared, check the traffic measurement

information and analyze handover counters.

A hardware failure may involve the following hardware devices:

BTS transmission management unit

BTS TRXs

BTS combining and distribution unit

BTS feeder and antenna system



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I. Handling Process

(11) Check the data configuration of the hardware. If none of the data

configuration of the faulty cell and its neighboring cells is changed recently,

consider whether the handover problem is caused by a BTS hardware

failure.

If the handover problem occurs in only one cell under the BTS, consider

whether the problem is caused by the hardware failure of the cell. If a TRX is

damaged, a call fails to be handed over to this TRX.

If a similar problem also occurs in a co-site neighboring cell of this cell,

consider whether the problem is caused by the failure of the common hardware

of the cells, for example, the TMU failure.

You can block some TRXs to verify the preceding problems. If the HOSR

returns to normal after a TRX is blocked, check whether this TRX is faulty or

whether the CDU or the antenna related to this TRX is faulty.

If the uplink and downlink signals of a TRX are unbalanced, handover

problems such as frequent handover and lower HOSR often occur.

(12) Trace the Abis interface, and observe whether the signaling of the faulty cell

is normal and whether the uplink and downlink receiving quality in the

measure report is good. For detailed operations, refer to the M900&M1800

BSS Signaling Analysis Manual.

If the receiving level quality of half rate or full rate channel in the measurement

report is poor, the hardware of the cell is faulty or signaling cannot interact

normally due to serious interference in the cell. As a result, a handover problem

occurs.


Omitted.

III. Alarm Analysis

Observe whether any alarms with the following IDs are reported. If yes, refer to the BSS Alarm

Guide to handle the alarms.

Alarm ID and Name4102 TRX LAPD Link Interrupt Alarm

4104 TRX Config Mismatch Alarm

4108 Radio link critical Alarm

4114 TRX Interior I/O Alarm

4136 TRX Hardware Critical Alarm

4144 TRX VSWR alarm

4192 TRX communication alarm

4714 E1/T1 Local Alarm

5286 CDU Level 1 VSWR Critical



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Alarm

5284 CDU Level 2 VSWR Critical

Alarm

5326 Level 1 VSWR alarm

5328 Level 2 VSWR alarm

3.2.3 Improper Data Configuration

I. Handling Process

Symptoms: An MS does not initiate any handover or frequently initiates handovers, which affects

the HOSR.

The handover parameters control the handover decision algorithm. If the handover parameters are

set improperly, the MS may not initiate any handover or frequently initiates handovers. In this

case, consider the cause from the following aspects:

Whether the PBGT HO Threshold in the data configuration is set properly

Avoid difficult handovers due to too great values of the handover thresholds or

frequent handovers due to the too small values. Proper settings can prevent

ping-pong handovers. For detailed settings of the thresholds, refer to the GSM

BSC6000 Performance Parameter Baseline (V900R008) (Chinese/English)

V2.0. Do not set the thresholds to the values deviating greatly from the baseline

values.

Whether the parameters related to the handover candidate cell in the data

configuration are set properly

Avoid the case that the MS cannot be handed over to a neighboring cell due to

the missed setting of the neighboring cell.

Whether the handover hysteresis parameters in the data configuration are

set properly

Avoid difficult handovers due to too large values of the handover hysteresis

parameters or frequent handovers due to too small values.

Whether the N and P counters in the data configuration are set properly

Avoid insensitive handover decision or difficult handovers due to the too large

values of the parameters, or the case that the target cell of a handover is not the

optimal due to the too small values of the parameters.

Avoid configuring the neighboring cells that share the same BCCH or the same

BSIC for a cell.

Abnormal circuit identification code (CIC) circuits may cause failed handovers.

For example, the CIC circuit allocated through a Handover REQ message received by the target

BSC is identified in the BLOCK state in the target BSC. Therefore, the BSC responds to the MSC

with a Handover Failure message whose cause value is Requested Terrestrial Resource

Unavailable. In this case, check the statuses of the circuits at the two sides of the A interface and

ensure that the circuits are in the same state.

You can trace the A interface signaling on the maintenance console to check whether the failed



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handover is caused by the inconsistency of the circuit statuses. Do as follows:

(13) Trace the A interface signaling.

(14) Filter the Handover Failure message.

(15) Check whether the cause value is Requested Terrestrial Resource

Unavailable.

II. Handover timer

When an abnormal handover occurs, promptly check the handover timer and ensure that the

handover timer is not less than the preset default value.

Table 1-1 lists the handover timers commonly used.

Table 1-1 List of handover timers commonly used

Timer

Name

Default Value

(ms)Description

T7 10000Timer for sending outgoing external inter-cell handover

requests and handover commands

T8 10000Timer for running outgoing external inter-cell handover

commands and handover completion or clearance

T3103 10000

From the time when an intra-cell or inter-cell handover

command is executed to the time of an intra-cell or

inter-cell handover completion

T3105 70

During an asynchronous handover, from the time when

the BTS sends the MS a physical message to the time

when the BTS receives the set asynchronous balanced

mode (SABM) from the MS

T3124 320

During an asynchronous handover, from the time when

the MS sends the network a handover access Burst

message to the time when the MS receives a physical

message from the BTS

Figure 1-2 shows the flow chart and the description of the timers.



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Figure 1-2 Flow chart and timer description

III. Traffic Measurement Analysis

Omitted.

IV. Alarm Analysis

Omitted.

3.2.4 Congestion of the Target Cell

I. Handling Process

Symptoms: After an MS initiates a handover request, the handover fails because no channel is

obtained.

The possible causes of cell congestion are:

The number of users in the cell soars and exceeds the designed number.

Improper settings of the network optimization parameters cause redundant

users in the cell.

Improper settings of the handover parameters cause the increase of the

users accessing the cell.



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After a handover fails because congestion occurs in the target cell, penalize the target cell to

prevent the MS from retrying to be handed over to this target cell. It is recommended that Penalty

Allowed be set to Yes.

Check whether the channel the congested cell is normal. If a TRX is faulty or a channel is

abnormal, rectify the relevant faults.

If full rate channels cannot be converted to half rate channels, it is recommended that you change

the channel attributes on the BSC6000 local maintenance terminal (LMT). That is, set the TCH

Rate Adjust Allow of all the TRXs under this cell to Yes. If the full rate channels can be

converted to half rate channels, properly reduce the value of TCH Traffic Busy Threshold(%) to

allocate half rate channels ahead of time and thus increase the system capacity. If the preceding

methods cannot solve the congestion problem, divide the cell or expand the capacity of the cell.

Since capacity expansion cannot be completed in a short time, you can set Channel Type to 1 or 2

to reserve channels for handovers. In this way, the failed handovers caused by congestion can be

reduced, and thus the HOSR improves.


(16) Register the measurement unit Channel Assignment Failure Measurement

per Cell. By analyzing the traffic measurement, you can be familiar with the

number of the times that all the channels are busy or that none of the

channels is configured when the BSC allocates SDCCHs, TCHFs, or

TCHHs in the processes such as immediate assignment, assignment,

internal intra-cell handover, incoming internal inter-cell handover, and

incoming external inter-cell handover.

(17) Change relevant parameters for the target cell according to the cause of the

failed handover.

If the failed handover is caused by the SDCCH congestion, set SDCCH Dynamic Allocation

Allowed to Yes.

If the failed handover is caused by the TCH congestion, reduce the value of the TCH Traffic

Busy Threshold(%) to allocate half rates ahead of time and thus relieve congestion. In addition,

you can set Channel Type to 1 or 2 to reserve channels for handovers.

SN Measurement Counter

1Channel Assignment Failures (All Channels Busy or Channels Unconfigured) in Immediate

Assignment Procedure (SDCCH)


Assignment Procedure (TCHF)


Assignment Procedure (TCHH)

4Channel Assignment Failures (All Channels Busy or Channels Unconfigured) in Assignment

Procedure (TCHF/TCHH)


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5Channel Assignment Failures (All Channels Busy or Channels Unconfigured) in Internal

Intra-Cell Handover Procedure (SDCCH)

6Channel Assignment Failures (All Channels Busy or Channels Unconfigured) in Internal

Intra-Cell Handover Procedure ( TCHF/TCHH )

7Channel Assignment Failures (All Channels Busy or Channels Unconfigured) in Incoming

Internal Inter-Cell Handover Procedure (SDCCH)


Internal Inter-Cell Handover Procedure ( TCHF/TCHH )


External Inter-Cell Handover Procedure (SDCCH)


External Inter-Cell Handover Procedure ( TCHF/TCHH )

11 Channel Assignment Failures (All Channels Busy or Channels Unconfigured) (SDCCH)

12 Channel Assignment Failures (All Channels Busy or Channels Unconfigured) (TCHF)

13 Channel Assignment Failures (All Channels Busy or Channels Unconfigured) (TCHH)

14 Channel Assignment Failures (All Channels Busy or Channels Unconfigured) (TCH)

III. Alarm Analysis

Omitted.

3.2.5 Clock Problems

I. Handling Process

Asynchronization and instability of the BTS clock are major causes of call drops during a

handover. Therefore, keep the BTS clock stable. Otherwise, handovers often fail and call drops

occur frequently.

A 13 MHz unlocked alarm is generated. The BSIC cannot be decoded. The HOSR of the

concerned cells decreases.

The clock source is abnormal and deviation may occur between the BTS clock and other BTS

clocks. As a result, MS abnormalities may occur during handovers.

To solve the problems about the unlocked clock and abnormality of the clock source, do as

follows:

(18) Check alarms. That is, check whether there is a 2214 E1 local alarm or

2216 E1 remote alarm. If there is, follow the concerned alarm handling

manual to handle the alarm. Then, observe the HOSR.


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(19) Check the transmission link clock of the BTS. That is, use a frequency

meter to test the frequency deviation of the transmission link clock of the

BTS. If the frequency deviation is greater than or equal to 0.05 ppm, the

transmission link clock is abnormal, and the E1 or the optical transmission

link may be faulty or the clock source is faulty. Rectify the transmission link

fault through link-by-link self-loop until the alarm handling is complete.

(20) If the clock problem is not solved, reset the BTS (level-4) and observe

alarms and the HOSR.

(21) If the problem remains unsolved, replace the TMU.


Omitted.

III. Alarm Analysis

Observe whether any alarms with the following IDs are reported. If yes, refer to the BSS Alarm

Guide to handle the alarms.

Alarm ID and Name4154 TRX main clock alarm

4156 TRX slave clock alarm

4184 TRX Clock Critical Alarm

4708 Clock Reference Abnormal

Alarm

4732 TMU clock critical alarm

4734 Master TMU clock alarm

4760 13M Maintenance Alarm

3.2.6 Interference Problems

I. Handling Process

Severe interference in the network is inclined to cause the decrease in the receiving quality. As a

result, interference handovers or handovers in poor quality increase, the proportion of the power

budget (PBGT) decreases, and the quality of service (QoS) of the existing network is reduced to

some degree. Thus, user experience and the HOSR are affected.

Currently, the common interferences are intra-frequency and inter-frequency channel

interferences, Unicom CDMA interference, and mass multiplexing of the EGSM. If the idle Burst

function is not manually disabled after it is enabled, the interference of the entire network rises,

the noise floor increases, and the quality of the entire network decreases, thus affecting the HOSR.

The remote source signals of some optical fiber repeaters are inclined to cause intra-frequency

interference. Therefore, during optimization, you need to check the frequency of the source signals

and the frequencies of the cells close to the repeaters so that the frequency space is over 400 kHz.

If there is a repeater in the serving cell, do as follows:



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(22) Choose Cell Software Parameters > Directly Magnifier Site Flag.

(23) Select Yes.

To solve interference problems, do as follows:

(1) Find out the cell or the frequency where large interference exists through

drive tests.

(2) Optimize the radio frequency (RF) by the following regular means:

- Adjust the tilt angle of the antenna.

- Replace the frequency.

- Change the transmit power and the coverage area of the cell.

You can also register the measurement results of interference fringes by auxiliary means to

estimate downlink interferences.

For more details about the solutions of interference problems, refer to the GSM Interference

Analysis Guide.


Omitted.

III. Alarm Analysis

Omitted.

3.2.7 Coverage Problems, and Uplink and Downlink Imbalance

I. Handling Process

Symptoms of signal coverage problems: The HOSR is low. Call drops occur frequently. There are

noises and metallic rings during conversations. The voice quality and the user experience are poor.

There are three types of signal coverage problems:

Low HOSR caused by cross coverage

Low values of the fringe thresholds, the large BTS power, and an improper tilt

angle cause cross coverage, thus forming intra-frequency interference and

affecting the HOSR.

Failed handovers caused by island effects

For example, the coverage area of the serving cell is much larger than that of its

neighboring cells, and the neighboring relation between the serving cell and the

neighboring cells of its neighboring cells is not configured. In this case, failed

handovers easily occur at the fringe of the serving cell.

Loopholes formed due to weak coverage

This section does not describe it in detail.

To solve signal coverage problems, do as follows:

(3) Find out the coverage problems in the existing network through drive test

reports of network optimization.

(4) Optimize the RF.



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The low HOSR caused by uplink and downlink imbalance generally occurs when uplink signals

are weak. For example, there are problems in the hardware such as the CDU combiner, the uplink

channel loss is large, the uplink signals are weak, and the success rate of incoming external inter-

cell handover is low. This low HOSR is generally caused by data problems (such as CGI errors in

the cell description data table, lack of measurement frequencies in BA list 1 and BA list 2, or intra-

frequency and inter-frequency interferences), coverage dead zones in high traffic, or MS access

difficulties due to weak uplink signals. To test and analyze the low HOSR caused by uplink and

downlink imbalance, do as follows:

(1) Check whether the hardware and maintenance boards of the relevant cell

are in the normal state, and whether there are any alarms about hardware

failures and the standing wave ratio (SWR). Refresh the channel status and

check whether the TCHs can be normally occupied.

(2) After hardware and channel problems are solved, check the handover data

configuration and ensure that the handover data complies with the

parameter baseline.

(3) Register the traffic measurement results of cell-level handovers. Check

whether the HOSR between some cells is always low.

1) Make a field test for the cells where the HOSR is always low. That is,

perform a switchover and lock the main BCCH to act as the calling and

called parties respectively. Then decide the uplink and downlink problems

accordingly.

2) If the uplink loss is large, it is recommended that you replace the combiner

to carry out an observation and a test.

Coverage problems and uplink and downlink imbalance are solved through RF optimization. For

detailed analysis, refer to the GSM BSS Network KPI (Coverage Problems) Optimization Manual

(V1.0).


Register the measurement unit Uplink-and-Downlink Balance Measurement per TRX about the

cells where the HOSR is low. Collect the uplink and downlink balance cases and carry out

analysis.

III. Alarm Analysis

Omitted.

3.2.8 Failed Inter-BSC/Inter-MSC Handovers

I. Handling Process

Symptoms: Inter-BSC or inter-MSC handovers fail.

The possible causes are that:

The data of the cells relevant to inter-MSC handovers is set wrongly.

The data of the cells relevant to the inter-BSC handovers is set wrongly.



INTERNAL

The MSC and the BSC have different understandings of A interface

handover signaling. As a result, the cooperation at the A interface fails.

The clocks between BSCs are not synchronous.

To solve this problem, do as follows:

(4) Check whether the MSC data relevant to the cells where handovers fail is

set correctly, for example, the CGIs and the office direction of the cells. If

any data is set incorrectly, correct it and observe whether handovers

succeed.

(5) Check whether the neighboring cells of the source and the destination BSC

are set correctly. If there is any abnormality, correct it and observe whether

handovers succeed.

(6) Trace A interface signaling. Check whether there is any abnormality in the

signaling cooperation of the handover process between the source BSC and

the MSC, and between the MSC and the destination BSC. For example,

check whether such a process that the MSC abnormally releases a

handover exists. If there is any abnormal process, find out the cause and

observe whether handovers succeed after such a problem is solved. For

detailed signaling analysis, refer to the M900&M1800 BSS Signaling

Analysis Manual.

(7) Check whether the source and the destination BSCs relevant to handovers

are locked with the clock of the upper-level MSC. If not, find out the cause

that the clock cannot be locked. Observe whether handovers succeed after

this problem is solved.


Omitted.

III. Alarm Analysis

Omitted.

3.2.9 Automatic Neighboring Cell Optimization

Currently, automatic neighboring cell optimization is the best method for optimizing the HOSR.

The method has been fully verified in the new functions of the MTN project. This optimization

method is to choose optimal neighboring cells for the serving cell through many times'

neighboring cell selection and tailor. This method can make the serving cell have neighboring cells

more close to the traffic model, thus avoiding failed handovers and call drops due to forced

configuration of neighboring cells according to geographical locations.

The premise of automatic neighboring cell optimization is to exclude objective factors such as

hardware problems, cross coverage, and uplink and downlink imbalance. After that, make it clear

to which neighboring cell the success rate of handovers from the serving cell is lower, and then

optimize this neighboring cell. The optimization method includes two types: parameter adjustment

and neighboring cell adjustment.



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The process of automatic neighboring cell optimization is as follows:

(8) According to geographical locations, set as many neighboring cells as

possible for the serving cell. The upper limit is 32.

(9) Register the measurement unit GSM Cell to GSM Cell Outgoing Handover

Measurement, which period is 15 minutes.

(10) Observe the traffic measurement results. Exclude the cells that meet any of

the following conditions from the neighboring cells:

The HOSR is lower than 30%.

The call drop rate is higher than 80%.

The handovers are relatively fewer according to the traffic, for example,

about 30 handovers every hour.

(11) After a neighboring cell is excluded, re-add a new neighboring cell

according to the timing advance (TA) principle from small to large, and

repeat the preceding steps.

Figure 1-1 shows the process of automatic neighboring optimization.

Figure 1-1 Flow chart of automatic neighboring cell optimization

The TA is limited to six times as many as the average distance between sites. Do not consider the

cells with the TA beyond this threshold. You can flexibly set the lower limit of the HOSR and the

fewer limit of handover times.

3.2.10 Testing Tool Selection and Testing Suggestions

Generally, choose the industry-accepted and large-scale used TEMS as the testing tool. For the

cells where the HOSR is low, you need to carry out drive tests. In drive tests, the actual move

modes and habits of terminal users can be simulated. Therefore, drive tests play an important role

in neighboring cell optimization. Drive tests can avoid such risks as few handovers or low HOSR

caused by the addition of improper neighboring cells only according to geographical location



INTERNAL

distribution on a map. Lay stress on and analyze any handover abnormality during a drive test.

Any of these abnormalities may be a cause of low HOSR.

3.2.11 Configuration Suggestions for Tests on the Existing Network

To configure the existing network according to different scenarios, refer to the GSM BSC6000

Performance Parameter Baseline (V900R008) (Chinese/English) V2.0. In the case of the low

HOSR, focus on checking the data configuration greatly different from the parameter baseline.

4 Optimization Cases

4.1 A Handover Fails Because the BSIC Cannot Be Decoded

During a drive test in an office, it is found that an MS cannot decode the BSIC of a neighboring

cell. As a result, the MS cannot initiate a handover even when the MS detects good levels of the

neighboring cell.

After analysis, it is learned that the PTCCH points to a wrong memory zone. As a result, some

MSs misunderstand that this channel is an FCCH, thus fail to synchronize the SCH and fail to

decode the BSIC. This product problem must be solved through version upgrade.

4.2 A Handover Fails Because Frequency Sequencing of the MS Is Different from That of the BSC

In an office, field engineers check the TEMS drive test file and find that the cells of the main

BCCH at the EGSM cannot be handed over to the PGSM. It is checked that the rule of sequencing

frequencies at the MS side is different from that at the BSC side. When the serving cell is at the

EGSM and is configured with 1,800 neighboring cells, the MS sorts the EGSM first and then the

1,800 neighboring cells, whereas the BSC does in the contrary order. Thus, the different sequences

of neighboring cells at the BSC and MS sides cause a failed handover.

To mitigate such a problem, disable the Way of BA Delivery Optimized.

4.3 A Handover Fails Due to Unreasonable Parameter Configuration

It is found in an office of BSC6000V9R8 that no bad quality (BQ) handover can occur. It is

checked that the Inter-cell HO Hysteresis of the serving cell is set to 63. The result of checking

codes shows that the level of the serving cell is equal to a value increased by 63 grades if Inter-

cell HO Hysteresis is set to 63 when BQ HO Margin is set to the default value. Thus, the

calculated level of the serving cell is always larger than the level of any neighboring cell. As a

result, the handover cannot be initiated.

To solve such a problem, increase the value of BQ HO Margin to 127.



INTERNAL

4.4 The Number of Failed Incoming BSC Handovers Increases Because the Handover Request Does Not Contain Class Mark 3

In a cell at the BSC boundary, the frequency of the main BCCH is configured as the PGSM and

that of the other TRXs is configured as the EGSM. Symptoms: The number of failed incoming

BSC handovers increases. The cause value is No Available Channel. The BSC6000 decides the

frequency supporting capability of an accessing MS according to class mark 3. If there is no class

mark 3, the BSC considers that the MS supports only the frequency of the main BCCH. If the

Handover Request does not contain class mark 3 and the frequency of the other TRXs in the cell is

different from that of the main BCCH, accessing MSs are all allocated to the main BCCH. This

causes congestion and thus handovers fail. After the frequency of the other TRXs is changed to the

PGSM, the number of failed incoming BSC handovers caused by no available channel decreases

to 0. The problem is mitigated. According to the protocol, however, class mark 2 also has a field

for identifying whether an MS supports the EGSM or the RGSM (incapable of identifying the

DCS1800). Such a problem often occurs because some MSCs carry only class mark 2 in the

Handover Request or because an MS in a cell with the EGSM enabled does not report class mark

3.

To solve such a problem, upgrade the software version.

4.5 An Incoming BSC Handover Fails Because the A Interface Phase Flag Is Set Wrongly

During an incoming BSC handover, after the BSC returns a Handover Request Ack message to the

MSC, the MSC returns a Clear Command immediately to clear call resources. The clearance cause

is Equipment Failure. The system responds to the MSC with a Handover Request Ack message

containing the speech version IE only when the A Interface Tag in the BSC attributes is

GSM_Phase_2+ and the BSC software parameter SpeechVer Send Flag In Ho Req Ack is set to

Yes. Since the A Interface Tag in the data configuration is GSM_Phase_2, the Handover Request

Ack message does not contain the speech version IE. Therefore, the MSC considers that this

message is illegal and sends a Clear Command.

To mitigate such a problem, set the A Interface Tag in the BSC attributes to GSM_Phase_2+ and

the BSC software parameter SpeechVer Send Flag In Ho Req Ack to Yes.

4.6 Because the Idle Burst Is Enabled, the Interference Increases, the Receiving Quality Decreases, and the HOSR Becomes Low

After an office is cut over, the drive test results show that the network quality decreases obviously

by about 3% to 4%. After it is confirmed that the problem is not caused by the problems about the

hardware, frequency planning, or cross engineering, it is found that the HOSR of the network is

2% to 3% lower than that the original network and other KPIs are normal.

After analysis, it is found that the idle burst function is enabled for testing. This function cannot



INTERNAL

automatically be disabled. As a result, idle timeslots of TRXs are transmitted at full power, the

interference increases, the bit error ratio (BER) rises, and the receiving quality decreases.

Manually disable the idle burst function. The air interface quality of the entire network improves

effectively. The HOSR increases by 2% on the whole, almost the same as that of the original

network.

4.7 Different HOSRs Resulting from Different Cause Values Contained in the Clear Command Messages Sent by Different Switches

There are multiple BSCs in office Z. Some BSCs are connected to Nortel switches and some are

connected to Ericsson switches. The whole HOSR of the BSCs connected to Ericsson switches is

2% to 3% lower than that of the BSCs connected to Nortel switches.

The result of the analysis on traffic measurement shows that the whole number of Failed Incoming

External Inter-Cell Handovers (Others) of the BSCs connected to Ericsson switches is very great

whereas the counter value of the BSCs connected to Nortel switches is 0. As defined in the

protocol, a Clear Command sent by the MSC can contain four cause values:

09: Call Control

0B: Handover Success

0A: Radio Interface Failure, Reverse to old Channel

01: Radio Interface Failure

For the failed handovers caused by MSC clearance, the Clear Command messages sent by

Ericsson switches contain the cause values of 0A and 01, whereas those sent by Nortel switches

contain the cause value of 09. Nortel switches do not send messages in accordance with the

protocol. Under Nortel switches, the failed handovers caused by MSC clearance are not collected

into the statistics. As a result, the HOSR of Nortel switches is higher.

5 Information Feedback

5.1 TEMS Log Files About Problem Cells

Cell information tables of TEMS tests are required to be provided in log files. The tables must be

in the format of *.cel.

5.2 Requirements of Configuration Data of the Existing Network and Traffic Measurement Feedback

The latest data configuration and engineering parameter list are required. The feedback of traffic

measurement counters is required for consecutive two days. The following table lists the counter

types.

Call Drop Measurement per Cell



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Incoming Internal Inter-Cell Handover Measurement per

Cell

Outgoing Internal Inter-Cell Handover Measurement per

Cell

Incoming External Inter-Cell Handover Measurement per

Cell

Outgoing External Inter-Cell Handover Measurement per

Cell

Incoming Inter-RAT Inter-Cell Handover Measurement per

Cell

Outgoing Inter-RAT Inter-Cell Handover Measurement per

Cell

Measurement of MRs upon Handover Initiation per Cell

Channel Assignment Failure Measurement per Cell


09 gsm bss network kpi (handover success rate) optimization manual

Engineering

handover success rate

handover request

slow handover

network quality

existing network

optimization method

kpis of network performance

optimization cases