Optimisation OF GSM

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Lucent Technologies ProprietaryThis document contains proprietary information of

Lucent Technologies and is not to be disclosed or usedexcept in accordance with applicable agreements

Copyright 1996 Lucent TechnologiesUnpublished and Not for Publication

All Rights Reserved.

WL9016Version 01 eIssue cFebruary 1998

COURSE WL9016

GSM Mobile NetworkOptimization

STUDENT GUIDE


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Lucent Technologies ProprietarySee notice on rst page

This material is protected by the copyright and trade secret laws of the United States and other countries. Itmay not be reproduced, distributed or altered in any fashion by any entity, including other LucentTechnologies Business Units or Divisions, without the expressed written consent of the Customer Trainingand Information Products organization.

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Table of Contents

WL9016 - ST.e.3

GSM Mobile Network Optimization

_ _____________________________________________________________________________LESSON TITLE TAB V.L.I. _ _____________________________________________________________________________ _ _____________________________________________________________________________

WL9016 Table of Contents 00 ST.e.3.toc

9W530 Cellular Network Optimization Overview 01 01.e.d

6W002 OMC-2000 Functional Description 02 03.e.c

9W535 BSS Parameters and Functions 03 01.e.c

6W003 OMC-PMS Functional Description 04 03.e.b9W531 Using Network Performance Monitoring Guide 05 01.e.a

9W533 Drive Testing and Analysis 06 01.e.c

9W537 Optimization Casework 07 01.e.b

9C500 Erlang Tables 08 01.e.a

9W534 GSM System Network Performance Monitoring Guide 09 01.e.a


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Contents

Version 01 e issue d 1-i


1CELLULAR NETWORKOPTIMIZATION OVERVIEW

INTRODUCTION TO NETWORK OPTIMIZATION 1-1s LESSON INTRODUCTION 1-1s LESSON OBJECTIVES 1-1

OPTIMIZING THE NETWORK 1-3s WHAT IS OPTIMIZATION 1-3s WHY OPTIMIZATION 1-5

Accuracy of Radio Planning 1-6

Implementation 1-7

Environment 1-9

NETWORK OPTIMIZATION PROCESS 1-11s DEFINITION 1-11s PHASES 1-11

Phase I - Preparation 1-13

Phase II - Definition of Sub-Networks 1-13

Phase III - Quality Assessment 1-15

Phase IV - Optimization 1-16

NETWORK OPTIMIZATION PARAMETERS 1-19s ENABLE/DISABLE GSM FEATURES 1-21

Discontinuous Transmission 1-21

Frequency Hopping 1-23

Power Control 1-23s BSS PARAMETERS 1-25s NEIGHBOR CELL LISTS 1-27

Definition 1-27

Optimization Impact 1-27

Scenarios 1-27


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1-ii Version 01 e issue d


Contents

s ANTENNA PARAMETERS 1-29s FREQUENCY CHANGES 1-31


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Version 01 e issue d 1-1


1CELLULAR NETWORKOPTIMIZATION OVERVIEW 1

INTRODUCTION TO NETWORKOPTIMIZATION 1

LESSON INTRODUCTION 1

This lesson discusses the principles of cellular network optimization by giving thefollowing topics:

s The denition of cellular network optimization (or asking the question: whatis cellular network optimization?)

s The reason for optimizing the cellular network (or asking the question: whyis it necessary to optimize the cellular network?)

s The network optimization process and its phases

s The tools which are used to optimize the networks The optimization parameters, which are the parameters that can be

adjusted to improve the performance of the cellular network, including sitere-congurations and frequencies

LESSON OBJECTIVES 1

At the end of this lesson, participants will be able to:

s Know what cellular network optimization is.

s Know the reason for cellular network optimization.

s List the different phases of the network optimization process.

s List the tools which are used to optimize the network.

s Identify the parameters that can be adjusted in order to improve theperformance of the network.


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1-2 Version 01 e issue d


CELLULAR NETWORK OPTIMIZATION OVERVIEW 9W530

Figure 1-1. ENDLESS CYCLE OF NETWORK PLANNING ANDOPTIMIZATION

Design

PlanningOptimization

Implementation

Coverage denition Trafc forecast

Cell sizes (coverage)Number of BSSs Cell structure Cell sizes (capacity)TRX dimensioning

Coverage Interference Handover behavior Trafc distribution


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CELLULAR NETWORK OPTIMIZATION OVERVIEW



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OPTIMIZING THE NETWORK 1

WHAT IS OPTIMIZATION 1

Cellular network optimization is the activity of achieving and maintaining therequired quality targets in the existing network without changing the number andlocation of base stations in the network. The quality targets are determined duringthe Design phase of the cycle of cellular network planning and optimization as isdepicted in the gure on the opposite page. Targets are normally changed oncethe network starts to mature. The following phases are shown in this gure:

Design The dimensioning parameters of the network are determined,which are:

Coverage denition , including: mobile classes (forexample, 2W or 8W), mobile environment (outdoor or

indoor), coverage probability (for example, 90% of thearea), area denition, and special coverage objects(for example, airport or shopping mall).

Trafc forecast , including: number of subscribers,target groups (business or private), and trafcdistribution over the country and within cities.

Planning The dimensioning parameters of the design phase aretranslated into a real network consisting of, for example,TRXs. Note that planning is also known as engineering. Thefollowing mapping is done in the planning phase:

Implementation The equipment is installed in the cellular network (includingmasts, antennas and radios).

Optimization Activities are performed to meet the required quality targetsin the cellular network. The following quality parameters haveto be considered:

Coverage (across the whole cell)

Minimization of interference

Handover behavior

Trafc distribution and congestion

RX quality

Design Phase Planning PhaseCoverage definition - Cell sizes (coverage)

- Number of BSSs

Traffic forecast - Cell structure- Cell sizes (capacity)- TRX dimensioning related to growth


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REASONS FOR OPTIMIZATION:

s Deviations between plan and reality

s Deviations due to:

Reason Deviation Examples of Error Sources1. Inaccuracy of

radio planning- Statistical variations in the path loss

characteristics- Finite terrain data base resolution

2. Implementation - Antenna radiation pattern and effectiveradiated power

- Antenna pattern distortion3. Environment - Seasonal environmental changes

e.g.trees and leaves- Environmental changes such as newhighways, raised rail systems and newbuildings, because each environmentalchange is different we normally optimizefor street level


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Note that a poor design, planning and maintenance cannot be compensated byoptimization.

WHY OPTIMIZATION1

When translating the cell design into engineering parameters for cell planning,deviations can exist. The deviations between the plan and the reality frequentlyexist due to:

1. Inaccuracy of the radio planning

2. Implementation

3. Environment

In practice, the complexity of the real life environment has meant that manygeneralizations and approximations have to be made in the design and planning

process. Inevitably, this will lead to depar ture of the actual performance from thedesign objective at least in isolated areas or under specic conditions.

Many of the shortcomings in the radio planning process will not be obvious untilthe part of the network becomes life. This will lead to customer dissatisfaction andan increase in the volume of complaints to an operator. It is, therefore, thatnetwork optimization is a necessary and inevitable activity.


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Accuracy of Radio Planning 1

There are many factors which affect the accuracy of cell planning. These may bedue to the resolution and the accuracy of the planning tool and the input data.

Some of the key reasons are highlighted in the following table.

Frequently the deviation is not due to a single factor but a myriad of convolvedreasons. For instance, the characterization of the radiowave environment is

statistical rather than deterministic. With statistical radiowave propagation models,the prediction of radio coverage has to be associated with a probability. Theuncertainty is further increased by the nite resolution of and the inevitableapproximation made in the terrain and morphology databases. Naturally, thephysical environment is not static and is constantly evolving over time. Inparticular, the morphology database will rarely be able to keep up with thechanges of man-made structures due to continual construction and demolition ofbuildings across the service area. Even if the database is up-to-date, there isalways a time lapse between successive cycles of network replanning andretuning. The seasonal variations in the vegetation and foliage as well as the moreunpredictable changes in the atmospheric conditions are yet adding furtheruncertainties in the precision of the cell plan. For these reasons, a signicantplanning margin has to be incorporated in order to protect the networkperformance. For a network with near uniform trafc distribution, such as across acity centre, the outage statistics across the area can almost be directly translatedto the proportion of dissatised customers. For instance, a ve percent coverageoutage can result in ve percent dissatised customers in the network.

Source Reason for incurring error

Path loss modeling Statistical modeling by linear regression and approximationin diffraction modeling

Signal fading modeling Statistical characterization of the mobile environment byknown probability functions

Terrain database Finite resolution of the terrain maps and the accuracy ofdigitizing contours and terrain features

Morphology database Finite resolution and categorization of land use

Calculation intervals Finite resolution of the incremental steps in the calculationprocess

Antenna radiation pattern Finite resolution in the digitalization of the antenna radiationpatterns and the extrapolation of the three-dimensionalradiation patterns from the vertical and the horizontal planepatterns


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Implementation 1

Apart from inherent error sources due to the cell planning tool, practicalconstraints in the physical implementation will also lead to fur ther sources of error.

Some of these are illustrated in the following table. As we can see, many of thesedeviations are beyond control of cell planners and once again, a sufcientplanning margin should be allowed in the planning process to protect inadvertentperformance degradation due to implementation uncertainties and practicalities.

Sources Reasons for incurring errors

Antenna radiationpattern and effectiveradiated power

The variation of the antenna gain due to tolerance in themanufacturing processes and the aging of the antennaconstruction over time

Antenna radiationpattern distortion Distortion of the radiation pattern due to the physical mountingarrangement and the physical location of the mounting structure(e.g. mounting against a water tank at a roof top).

Antenna height andground height

The accuracy of the antenna height estimation above meanground level and the spot height of the location where theantenna structure is installed

Antenna orientation anddowntilt

The accuracy of aligning the azimuthal and the vertical angles ofthe antenna. For the azimuthal angle, this is due to the alignmentprocess, while for the vertical angle, this is due to the precisionof the downtilt bracket

Cable loss and

connector loss

The accuracy of cable loss and connector loss estimation and

the accuracy of the measurement process if the insertion loss ismeasured physically on site

Combiner loss The adoption of averaged loss value as provided by the vendor

Base site and mobilereceive sensitivity

The adoption of reference sensitivity for link budget calculationsand the variation of the performance of individual receiver due totolerance in the manufacturing process


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Environment 1

A third key source of error is due to the dynamic nature of a mobile environment.This is depicted in the following table. In particular the position of the handset as

adopted by individual customers in sending and receiving calls, the body loss, thebuilding and vehicle penetration loss as well as the efciency of the handsetantennas (retracted versus extended position) can only be estimated by a cellplanner. A large standard deviation can exist among the value of theseparameters for individual customer which can lead to a variation in the perceptionof service quality.

Sources Reason for incurring errors

Physical environment Due to new buildings, highways, trafc patterns and foliage

Body loss The shielding and detuning by the head and the distortion of themobile antenna radiation pattern due to the dielectric andconductive nature of a human body

Building penetration loss The signal attenuation due to the construction material of abuilding as well as the aspect of the building in relationship to thebase site

Vehicle penetration loss The location of the handset inside the vehicle and the aspect ofthe vehicle relative to the base site

Diversity gain The diversity gain is related to the scattering radius within anenvironment. For a more scattering area, the diversity gain ishigher

Frequency hopping gain The signal to noise gain from frequency hopping is related to themobile speed. For slow moving mobiles, the gain is higher.

Handset antenna efciency The physical construction of the handset antenna and whether acustomer extends or retracts the antenna when placing orreceiving a call.

Handset position Handset not located at an optimal position, e.g. lying at on adesk top or placed inside a pocket

NOTE:This is a source that falls outside the scope ofoptimization, but can cause problems in the

network


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Figure 1-2. NETWORK OPTIMIZATION PROCESS

Quality AssessmentPhase

OptimizationPhase

ProblemSolved

EvaluateResults

RepeatMeasurements

Implementationof Solution

Chosen

PreparationPhase

CheckNetwork

Administration

Sub-NetworkDefinition Phase

PerformMeasurements

PerformFunctionalTests

IdentifyStable

Regions

IdentifyOptimization

Problem

Suggest Solutionsand

Predict Effects

Perform DetailedMeasurements inthose Locations

no

yes

Identify Locations with e.g.High Call Failure Rate, CallSetup Problems or Bad RX Quality


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9W530

NETWORK OPTIMIZATIONPROCESS 1

DEFINITION 1

Network optimization is a continuous process that generally consists of thefollowing two steps:

s Measure the cell performance of the operating cellular network todetermine its current status, and

s Improve the cell performance of the network using the measurements asinput, if applicable.

The network optimization process starts before the cellular network comes into

(commercial) operation. However, network optimization is much more importantwhen the network is in full operation (after the commercial launch). For example,the following effects can only be properly evaluated when the amount of trafc inthe network is growing:

s Capacity bottlenecks,

s Uplink interference level, and

s Down-link interference level due to additional TRXs (transceivers) in eachcell as the capacity of the network is increased.

PHASES 1

The network optimization process consists of the following phases:

1. Preparation

2. Denition of sub-networks

3. Quality assessment

4. Optimization

5. Re-assessment

The network optimization process is illustrated in the gure on the opposite page.

The different phases of the network optimization process are discussed in thefollowing sections.


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Preparations in phase I:

s Create a database for optimization problems.

s Retrieve cell site information.

s Select the measurement devices.

Requirements in phase II:

s All BSSs reached the operational status.

s

All neighbor cells are in place and operational.s The nearest co- and adjacent channel cells are

in place and operational.


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Phase I - Preparation 1

The network optimization requires controlled and documented planningprocedures and a disciplined approach to network operation and quality. For this

purpose a data base has to be available to log progress of each optimizationcase. For example, the following information has to be stored for eachoptimization case: optimization problem, date and time, suggested solutions andresults of solutions implemented.

A requirement which must be met before the actual network optimization canstart, is that the data bases related to the cellular network are properlymaintained; the data bases must reect the current state of the network. Thefollowing information must be properly available: cell sites, frequencies used,handover parameters, coverage, interference levels and cell boundaries.

In order to survey the network, output reports of the following devices can be

used:s The GSM OMC (Operations and Maintenance Center)

s Test mobile vehicles

s Hand-portable terminals

This is only possible when sufcient test mobile vehicles and hand-portableterminals are available.

Phase II - Denition of Sub-Networks 1

In the second phase of the network optimization process, the stable sub-networks(or regions) of the cellular network are identied.

A stable sub-network is that part of the cellular network that meets the followingrequirements:

s All BSSs in that particular sub-network have reached the operationalstatus,

s All neighbor cells are in place and operational, and

s The nearest co- and adjacent channel cells are in place and operational.

Note that a co-channel cell of a particular cell is a cell that uses the samefrequencies. An adjacent cell of a particular cell is a cell that uses one or

more adjacent frequencies.

Typically, a sub-network consists of 10 to 12 BSSs with in total approximately 30cells. The area covered by a sub-network can be, for example, a small town in alow density rural area or a business center in an urban area.

The reason for dividing the cellular network into sub-networks is to reduce thecomplexity of a performance problem by, for example, ignoring the inuence ofinstable regions.


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Means in phase III:

s Functional system checks.

s OMC-PMS statistics.

s Drive testing.

s

Customer feedback.

Steps in phase IV:

s

Analyse the retrieved data.s Identify the problem.

s Suggest solutions and predict side-effects.

s Implement solution.

s Perform measurements to evaluate the solution.

s Log the solution, and results in the optimizationdatabase.


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Phase III - Quality Assessment 1

In the quality assessment phase the performance of the cellular network isevaluated in order to identify sub-networks causing problems (i.e. sub-networks

with a high call failure rate). The means to perform quality assessment are:s Functional system checks

The functional system checks are used to test if the BSS equipment isproperly functioning. This kind of network observation is normallyperformed by the OMC.

s OMC-PMS statisticsThe OMC-PMS (Operations and Maintenance Center - PerformanceManagement Subsystem) collects measurements data from the connectedBSSs and MSCs. This performance evaluation tool can be used to analyzethe measurements data.

During analyzing of the BSS measurements data, the following parametersare in particular of interest:

Downlink quality of the signal measured by the mobile station(RXQUAL DL)

Downlink signal level measured by the mobile station (RXLEV DL)

Mobile station transmit power

Call set-up time

Number of handovers per call

Time between handovers

Reason for handovers

Number of dropped calls

These parameters can, for example, be compared to thresholds specied.

Note that it not possible to determine all problems using the OMC-PMS. Itis, for example, not possible to detect an interference problem in thenetwork, you only can get an indication of the presence of a possibleinterferer by looking to the reasons for handovers.


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s Drive testingDrive tests are performed using a trafc mobile system. A trafc mobilesystem can consist of test mobile vehicles and/or hand-portable terminals.These pieces of equipment are used to generate (automatically) calls and,

optionally, to store the measurements in details and subsequently analyzethem. Especially, measurements related to handovers and received signallevels are of interest.

In comparison to using the OMC-PMS statistics, drive testing has thedisadvantage that it is more time consuming, more labor intensive and canincrease the network loading (because test calls are usually generatedduring working hours).

s Customer feedbackCustomer complaints or other sort of feedback can be used to evaluate theperformance of the network. Customers can complain about, for example,a poor speech quality, a high rate of dropped calls, failed call set-ups and a

poor coverage in a particular cell or area.Note that the value of customer feedback should be assessed carefully.The feedback analyst need to be aware that customer perception isnormally subjective and could be dependent on many factors including thequality of the handset and specic pattern of the handset utilization by anindividual customer.

Phase IV - Optimization 1

The result of the previous phase (quality assessment) is a priority-list with one or

more sub-networks causing problems. In the optimization phase, the performanceof each of these sub-network is evaluated and is improved.

The following steps are performed to optimize a cellular sub-network:

s Perform detailed measurements to diagnose the cause of the problem. Forthis purpose the OMC-PMS statistics, test mobile vehicles and hand-portable terminals can be used.

s Identify the optimization problem. For example, the problem of a poorspeech quality in a particular area of a cell is a coverage hole caused by alarge building.

s Suggest/Identify solutions and predict side-effects. Every optimization

solution has side effects, either at the location within the same cell or onneighbor cells.


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s Implement the solution chosen. A solution can be, for example, modifyingan BSS parameter (at the OMC) or tilting an antenna (at the base station).Only one optimization solution should be taken at a time so that its effect can be measured.

s Measure the sub-network to verify the predictions.

s Evaluate the solution chosen using the measurements as input. If theoptimization problem is not solved, another solution has to beimplemented.

The solution chosen, the evaluation and the result of the optimization caseshould be logged in the progress data base.


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PerformanceDeterminants

Adjustments

EnableGSM

Features

BSSPara-

meters

NbourCellList

AntennaTilt,etc.

FrequencyChanges

Coverage Interference HandoverBehavior

Traffic

Distribution

Table 1-1. NETWORK PARAMETER AND PERFORMANCEDETERMINANTS


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NETWORK OPTIMIZATIONPARAMETERS 1

During the network optimization process, the quality or the performance of the air-interface in the cells of a sub-network is determined and, when necessary,improved by adjusting network parameters. The factors that determine thisperformance are called performance determinants. The following performancedeterminants should be taken into account:

s Coverage

The quality of the air-interface in a cell with respect to coverage is goodwhen there are no coverage holes. This means that the signal level is goodenough to receive across the whole cell.

s Interference

The quality of the air-interface in a cell with respect to interference is goodwhen the interference in the cell is at a reasonable level.

s Handover behavior

The quality of the air-interface in a cell with respect to handover behavior isgood when no unnecessary handovers are performed and the signalquality received by the mobile stations is at a reasonable level, and mobileand BTS can use minimum power.

s Trafc distribution

The quality of the air-interface in a cell with respect to trafc distribution isgood when a maximum amount of trafc can be handled. In order to handlemore trafc, the cell size is made smaller.

The following parameters can be adjusted in order to solve problems that occur inthe performance determinants mentioned above:

s Enable or disable GSM features

s BSS parameters

s Neighbor cell lists

s Antenna tilt, height and direction (antenna parameters)

s Frequency changes

The table on the opposite page gives the relation between the parameters thatcan be adjusted and the performance determinants.

The different parameters that can be adjusted in order to solve the problems in asub-network, are discussed in the following sections.


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Discontinuous Transmission:

Advantages:

s Decreases interference levels Saves battery power (in uplink mode)

Disadvantage:

s Deteriorates transmission quality

Frequency Hopping:

Advantages:

s Decreases the probability of interferences Suppresses the effect of Rayleigh fading

Power Control:

Advantages:

s Saves battery power (when enabled to MS)s Decreases interference level (when enabled to

MS and BS)


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ENABLE/DISABLE GSM FEATURES 1

In a GSM cellular network optional features can be enabled or disabled. The GSMfeatures which can inuence the performance of the air-interface are:

s DTX (Discontinuous Transmission)

s Frequency Hopping

s Power Control

Discontinuous Transmission 1

DTX inhibits the transmission of the radio signal when not required from aninformation point of view. In the DTX mode, speech is encoded at 13 Kbit/s whenthe user is effectively speaking, but in a speech pause information is transmittedat a bit rate around 500 bit/s. This low rate ow is sufcient to encode thebackground noise, which is re-generated to ensure that the listener does not thinkthat the connection is broken (comfort noise).

DTX may be applied independently to each direction, so that the control of DTXmust take into account two components:

s The uplink mode

s The downlink mode

When DTX is applied, actual transmission on the radio path is reduced. This willcause a decrease of the interference level in co-channel cells (using the samefrequency). Another advantage will appear when using DTX in the uplink mode: it

saves battery power for the mobile station. However, a disadvantage of the DTXmode is that it slightly deteriorates the quality of transmission.

Note that transmitting in DTX mode does not save time slots on the air-interface.


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Figure 1-3. IMPROVEMENT SCHEME ASSOCIATED WITH ENABLE/ DISABLE GSM FEATURES

Local Signalfluctuations

Solution:

Enable Frequency Hopping

Enable DTX

Enable Frequency HoppingEnable Power Control

Interference

PerformanceDeterminants:


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Frequency Hopping 1

The Frequency Hopping feature changes the frequency used by a channel on theair-interface every new TDMA frame in a regular pattern. The advantages of using

Frequency Hopping are:s Decreasing the probability of interference

Frequency Hopping will spread the annoyance of interference over differentmobile stations in a particular cell.

s Suppressing the effect of Rayleigh fading

Rayleigh fading (or multipath fading) is caused by different paths followedby the radio signal. Rayleigh fading can cause coverage holes.

Rayleigh fading is location and frequency dependent. When the mobilestation is stationary or moves at a slow speed, Frequency Hopping willsignicantly improve the level of the air-interface performance. However,when the mobile station moves at a high speed, Frequency Hopping doesnot harm, but does not help much either. The more frequencies are used ina particular cell, the more Frequency Hopping can gain in suppressing theeffect of Rayleigh fading.

Power Control 1

Power Control enables the mobile station and/or the base station to increase ordecrease the transmit power. It can be enabled for the mobile station and/or thebase station.

The mobile station adopts power according to the base station power controlcommands. This will save mobile station battery power. However, the main reasonfor Power Control is reducing the interference level within the cellular network.Reducing power on the base station or the mobile station, while keeping similarsignal quality received, decreases interference caused on the other calls in thesurrounding area.

If Power Control is disabled for the mobile station, the mobile station will alwaystransmit at maximum power level. The same is applicable for the base station: ifPower Control is disabled for the base station, it will always transmit at maximumpower level.


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Figure 1-4. IMPROVEMENT SCHEME ASSOCIATED WITH BSSPARAMETERS

Solution:

InterferenceHandover behavior

Traffic distribution

Adjust BSS parameters



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BSS PARAMETERS 1

The quality of the air-interface can be improved by adjusting the BSS parameters.BSS parameters are user-controllable parameters which are used to dene theconguration, system services and characteristics of the cellular network. Theyconsist of thresholds, margins, timers and other types of parameters. The BSSparameters are normally modied at the OMC.

The BSS parameters signicantly control the network behavior with respect to thehandover performance and the power control performance.

Examples of BSS parameters are:

s Power Control thresholds, when exceeded, transmit power increase orreduction is initiated

s Power Control reduction or increase steps, indicating the number of dBmthat the transmit power is reduced/increased when Power Control isinitiated

s Signal level and quality handover thresholds, when exceeded, a handoverto another cell is performed

Note that the BSS parameters can only be used for ne tuning of the performance of the network.


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Figure 1-5. EXAMPLES RELATED TO NEIGHBOR CELL LISTS

Figure 1-6. IMPROVEMENT SCHEME ASSOCIATED WITH NEIGHBORCELL LISTS

MS

MS A'

CB

A

D

Interference

PerformanceDeterminants: Solution:

Handover behavior

Traffic distribution Add/remove neighbor cell definition

Add/remove neighbor cell definition

Add neighbor cell definition


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NEIGHBOR CELL LISTS 1

Denition 1

The neighbor cell list contains the cell identiers to which a handover is allowed. Itis kept in the BSC of a particular cell. The list is transferred to the mobile station inthe BCCH during the registration phase of a wireless call. The mobile station usesthe neighbor cell list by only measuring the signals from the base stations locatedin the cells that are on the list. The measurements are used to decide a handoveror Power Control adjustment.

Optimization Impact 1

It is essential to limit the neighbor cell list only to those cells that are reallynecessary (approximately 6 to 8). The reason for this is that the mobile station canonly measure a limited number of signal level samples of the neighbor cell in areport interval (480 ms). This maximum number of samples measured is about100. The more neighbor cells are measured, the less precise are themeasurements. For example, when the neighbor cell list contains 6 cell identiers,16 to 18 samples can be taken by the mobile station. When it contains 32 cellidentiers, three to four samples can be taken. When the measurements are notprecise enough, the probability of an incorrect handover decision will increase.

However, a too short neighbor cell list can lead to more dropped calls, because ofmissing potential best handover-candidate cells. Another consequence of a tooshort neighbor cell list can be a higher interference probability. This is caused byan unnecessary increased transmit power of the currently serving base station.

Scenarios 1

The gure on the opposite page depicts two scenarios related to the neighbor celllist.

If the mobile station moves from cell A to cell B , while cell B is not on the neighborcell list of cell A and cell B would be the best candidate cell, then base station Acould continue sending at maximum transmit power, until for example the cell willbe dropped or taken over by base station D with a low signal quality.

In area A , base station A produces the best signal level, because the signal from

base station C is shadowed by a large obstacle and is therefore weaker than thesignal from base station A. However, the signal from base station C in area A issufcient to do the job. The mobile station moves from cell C via area A back tocell C . To avoid unnecessary handovers, cell A is not included in the neighbor celllist of cell C . Note that it is possible to put cell C in the neighbor cell list of cell A(this implies an asymmetric neighbor relationship).


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Figure 1-7. IMPROVEMENT SCHEME ASSOCIATED WITH ANTENNAPARAMETERS

Coverage

Solution:

Traffic distributionand congestion

Antenna tilt

Antenna heightAntenna direction

Interference



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ANTENNA PARAMETERS 1

Also the antenna parameters can inuence the performance of the air-interface.The antenna parameters are comprised of:

s Antenna tilt

s Antenna height

s Antenna direction

s Antenna radiation pattern

Tilting an antenna reduces the power transmitted in the horizontal direction. Itdecreases the distance between the base station and the cell edge, and increasesthe signal strength near the base station site. This results in an improvedinterference situation in this particular cell.

The antenna parameters can also be used to change the coverage in theparticular cell by, for example, tilting the antenna, positioning the antenna higher inthe antenna mast or change the transmit/receipt direction of the antenna.


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Figure 1-8. FREQUENCY CHANGES

Figure 1-9. IMPROVEMENT SCHEME ASSOCIATED WITH FREQUENCYCHANGES

f1 f2

f1 f2

A

B

f3 f2

f1 f2

A

B

Solution:

Change frequenciesInterference



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FREQUENCY CHANGES 1

In order to improve the performance of the air-interface in a sub-network, thefrequencies used by the base stations in that sub-network can be changed. Re-tuning of the frequencies used may be necessary due to unexpected interference.

Refer to the gure on the opposite page. A mobile station positioned in cell A isinterfered by frequency f1 in cell B for some reason (Terrain or low servingRXlevel). When cell A will change this frequency to frequency f3 , the interferenceprobability of cell B in cell A may decrease.

.


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Contents

Version 03 e issue c 2-i


2OMC-2000 FUNCTIONALDESCRIPTION

INTRODUCTION TO THE OMC-2000 FUNCTIONALDESCRIPTION 2-1

s LESSON INTRODUCTION 2-1s LESSON OBJECTIVES 2-1s DOCUMENTATION 2-1

OVERVIEW 2-3s THE OMC-2000 IN THE GSM NETWORK 2-3

THE OMC INFORMATION MODEL 2-5s OBJECT RELATIONSHIPS 2-5

OMC-2000 FUNCTIONALITIES 2-7s CONFIGURATION MANAGEMENT 2-7s FAULT MANAGEMENT 2-9s PERFORMANCE MANAGEMENT 2-9s SYSTEM ADMINISTRATOR FUNCTIONS 2-11

OMC-2000 MAIN SYSTEM PARTS 2-13s OMC HARDWARE 2-13s OMC SOFTWARE 2-15

OMC-2000 USER INTERFACES 2-17s

GUI (GRAPHICAL USER INTERFACE) 2-19OMC Network 2-19

Performance Management 2-19

Fault Management 2-19

Configuration Management 2-19s AUI (ASCII USER INTERFACE) 2-20

EXTERNAL OMC-2000 INTERFACES 2-21

CAPACITY AND CONFIGURATION 2-22


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2-ii Version 03 e issue c


Contents


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Version 03 e issue c 2-1


2OMC-2000 FUNCTIONALDESCRIPTION 2

INTRODUCTION TO THE OMC-2000FUNCTIONAL DESCRIPTION 2


This lesson gives an introduction of the functions and capabilities of the OMC(Operation and Maintenance Center).

LESSON OBJECTIVES 2

At the end of this lesson students are able to:

s Explain the position of the OMC in the GSM network

s

Describe the OMC functionalities and taskss Identify the OMC hardware, LAN and I/O components

s Identify the different User Interfaces

DOCUMENTATION 2

The documentation for this lesson is:

s The SOG (Lucent Technologies GSM System Operations and Maintenance Center 2000 System Operators Guide).

s The AG ( Lucent Technologies GSM System Operations and Maintenance

Center 2000 Administration Guide )


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2-2 Version 03 e issue c


OMC-2000 FUNCTIONAL DESCRIPTION 6W002

.

Figure 2-1. THE OMC-2000 IN THE GSM NETWORK


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OMC-2000 FUNCTIONAL DESCRIPTION



6W002

OVERVIEW 2

This chapter explains the position of the OMC in the GSM network. Also the main

interfaces to and from the OMC are discussed.

THE OMC-2000 IN THE GSM NETWORK 2

The OMC (Operations and Maintenance Center) manages the BSS (Base StationSystem) in a GSM network. The BSS provides digital cellular call processing andswitching between mobile stations and between mobile stations and the publicswitched network and consists of the BTS (Base Transceiver Station), BCE (BSSCentral Equipment) or BCF (Base Station Controller Frame), the TCE(TransCoding Equipment) or STF (Speech Transcoder Frame).

Through the OMC full access to the operations and maintenance functions of the

BSS can be obtained via an O-interface.

The OMC provides conguration management, fault management andperformance management for the BSS and the 5ESS-2000 Switch MSC,including GSM specied subscriber databases.

The gure on the opposite page shows the relationships between the OMC, theBSS, and the public telephone network.


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Figure 2-2. OMC INFORMATION MODEL

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6W002

THE OMC INFORMATION MODEL 2

The OMC is based on the management information model. This model represents

all of the objects in the BSS system and their relationships to each other.

Understanding the OMC information model is important. Certain objects arecreated automatically, while you must create others manually. Also, not all objectscan be modied and/or deleted. Much of this stems from their position in thehierarchy or tree.

The gure on the opposite page illustrates the OMC information model. In thegure, italicized terms indicate virtual objects. For example, the objects POWER,HOCTRL (handover control), ADJCELL (adjacent cell), ARU (alarm relay unit),and ADJRES (adjacent cell reselect) are virtual objects within the BTS physicalobject. The importance of understanding how the system components interrelatebecomes clear, for example, when handling BTS error messages andtroubleshooting. In such cases, it helps to know that an error could be comingfrom a POWER or HOCTRL object within the BTS.

OBJECT RELATIONSHIPS 2

Through the OMC you cannot only view objects, but you can also create, delete,and modify them. Looking at the previous gure you can see that objects can berelated in either of two ways: by a containment relationship or by a reference relationship .

s A containment relationship exists when one object contains another, in aparent-child hierarchy. An example of this is when a BSS has one or moreBase Transceiver Stations (BTSs) under it as child objects. Containment isreected in the full distinguished name of an object. An example of such apath name is BSS:10-BTS:14-BTC:0 , which shows BTC:0 within BTS:14,and BTS:14 within BSS:10.

s A reference relationship, sometimes called a service provider relationshipexists when one object relies on the services of another. An example of areference relationship is when a Radio Terminal (RT) refers to LAPDLink.In some cases, a service-providing object may need to be created beforeyou create the service user. For example, there should be a B Connection(BCON) object available before creating a Base Transceiver Station (BTS)object.

The design of the OMC allows you to view the containment and referencerelationships between objects.


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OMC-2000 FUNCTIONALITIES

s Conguration Management

s Fault Management

s Performance Management

s Graphical User Interface

s System Administration Tasks

CONFIGURATION MANAGEMENTs Viewing BSS conguration and status

s Creating, modifying, and deleting BSS objects

s State management

s Software administration

s Audits


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6W002

OMC-2000 FUNCTIONALITIES 2

The OMC provides operation and maintenance control capabilities from one

central location based on the management information model. To perform dailyoperational routines the OMC provides the following tasks on the BSSs:

s Conguration management

s Fault management

s Performance management

s System Administration

The rst two tasks can be performed either from AUI (ASCII User Interface) orGUI (Graphical User Interface) terminals of the OMC. The performancemanagement tasks can be done only from the GUI. Besides, performancemanagement resides on a different platform, the OMC Performance ManagementSubsystem.

CONFIGURATION MANAGEMENT 2

The OMC provides the following BSS conguration management capabilities:

s Viewing BSS conguration and status

s Creating, modifying, and deleting BSS objects

s Managing object states

s Software administration for BSS loadable units

The OMC allows remote BSS conguration. A BSS is initially congured using abulk load tape containing the BSS software and its conguration. As the networkgrows, additions and provisioning changes are done from the OMC. Provisioningmay also be based on performance and fault information.

s Audits to synchronize data in both OMC and BSS MIB

The MIB (Management Information Base) stores all system information on allsystem objects, their attributes, and their states. All BSS objects are stored in thesystem twice: once in the OMC MIB and once in the BSS MIB.An audit function automatically correlates the MIBs in both the OMC and BSS.The audit data from the BSS is compared with the conguration in the OMC

database. The OMC MIB automatically updates its information and an auditmismatch record is generated.

By using various combinations of these conguration management operations,you can:

s Install new cell sites

s Install new trafc channel capacity

s Perform maintenance functions such as loading new software or restartinga piece of equipment that has failed


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FAULT MANAGEMENT

s Alarms (event reports) indicating abnormal conditionsfor the BSS

s Alarm management functions: acknowledging andclearing alarms

s Alarm correlation

s Fault tracking records

s Support for external alarms

PERFORMANCE MEASUREMENTS

s Gathering Performance Measurements

s Storing Performance Measurements

s Analyzing Performance Measurements


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6W002

FAULT MANAGEMENT 2

The Fault Management area of the OMC handles event and alarm messagesreceived from the BSSs connected to the OMC. The OMC collects and classies

these events and alarm messages, and displays the appropriate status to OMCoperators.

The idea behind having an integrated Fault Management subsystem is to increasesystem availability by providing the OMC with the ability to monitor the networkand detect faulty equipment or objects as soon as possible. Specically, the BSSdetects faults and state changes within the network by monitoring the networkelements. When the BSS detects problems, it reports them to the OMC, whichuses the information to display status reports to network operators. These repor tscontain information about the causes of errors, the equipment that is faulty, andthe defensive actions the OMC will take to resolve the condition(s).

Whenever possible, errors are corrected by the BSS itself. In some cases,defensive actions are implemented not only to correct the error, but to minimizethe impact of the error on the network. If the BSS cannot correct itself, theoperator notication includes information about the maintenance procedure thatmust be performed to eliminate the problem(s).

Monitoring of BSS alarms and management of alarms are included in the faulttracking system. Management capabilities include:

s Alarms (event reports) indicating abnormal conditions for the BSS.

s Alarm management functions: acknowledging and clearing alarms.

s Alarm correlation.

s

Fault tracking records.s Support for external alarms.

PERFORMANCE MANAGEMENT 2

The Performance Management capabilities of the OMC let you collect, store, andanalyze measurements that the Base Station System (BSS) collects fromspecied Base Transceiver Stations (BTS). Measurements consist of calculationsor data collected about the performance of different aspects of the BSS and itsassociated BTSs. The OMC stores the measurements for a period of time duringwhich the data can be used to create a performance prole of the network.

Performance Management data allows you to measure such things as quality ofservice, plan network conguration and growth, and improve network availabilityand reliability. Performance measurements capabilities include:

s Gathering Performance Measurements

s Storing Performance Measurements

s Analyzing Performance Measurements


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6W002

SYSTEM ADMINISTRATOR FUNCTIONS 2

Certain tasks within the OMC environment are the responsibility of the OMCsystem administrator. Although a typical user can access most of the OMCs

capabilities, only the system administrator has the authority to perform thefollowing tasks:

s Workstation administration (adding, deleting, and modifying workstationinformation)

s User Administration (adding, deleting, and modifying user accounts)

s Loading error denition les

s Maintaining the network clock

Each of these tasks, as they apply to the system administrator, is explained in theSystem Operators Guide.


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Figure 2-3. HARDWARE OF OMC-2000


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6W002

OMC-2000 MAIN SYSTEM PARTS 2

The Lucent Technologies GSM OMC server provides the OMC applicationservices. All of the system processes execute on this node.

The gure on the opposite page shows the relationship between the componentsof the OMC System. The system is under control of a UNIX server. Theworkstations, both AUI and GUI, can be assigned with partitioning in eitherfunction control or limits, or geographical location control, or a mix of both. TheOMC administrator assigns these limits to the operator associated with aparticular login.

OMC HARDWARE 2

The OMC consists of an HP server system and HP workstations and X terminals

connected to the server via local and remote LANs. Operators may also connectto the OMC from local or remote ASCII terminals through a terminal server.

The server is capable of supporting a large amount of main memory and severalgigabytes of disk storage. The server has a Lucent Technologies GSM OMC LANinterface and X.25 interfaces.

The OMC consists of an HP server (D350) with:

s 512-MB on-board RAM

s Five (5) 2-GB external disks, mirrored, total 20 GB

s Two (2) 4-GB external disks, mirrored, total 16 GB

s Two (2) SCSI interface cards

s 4-8 GB DAT drive

s Console, keyboard

s Two (2) ACC card (each card supports up to eight X.25 links).

s One (1) CD ROM drive

The OMC operator workstation consists of:

s 128-MB on-board RAM

s 2-GB disk

s 19" color monitor, keyboard, and mouse

s A CD ROM (optional)


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OMC HARDWARE

The Server consists of:s 512-MB on-board RAM

s Ten (10) 2-GB external disks

s Four (4) 4-GB external disks

s 2 Tape drives (DAT and cartridge)

s Console and keyboard

The Operator workstation consists of:s 128-MB on-board RAM

s 2-GB disk

s 19" color monitor, keyboard, and mouse

The Network consists of:s A Terminal Server

s Routers, Hubs, X.25 PAD

OMC SOFTWARE

s OMC Application

s BaseWorX

s SYBASE

s DataViews, FrameViewer, SwitchTerm


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6W002

The network consists of:

s The HP DTC-16TN Terminal Server that supports local and remote ASCIIterminals and access to external systems through serial ports.

s The HP SR Routers The HP Ethernet Hub Plus

s Communications facilities for the terminal server and routers.

s Modems

s X.25 PAD

OMC SOFTWARE 2

There are four software applications that comprise the OMC Server software.They are:

s Operating System

The server software consists of the HP-UX 10.0 operating system with:

s Platform Applications

s OMC Application

s The server third party applications:

SYBASE * - This software package supplies the Relational DatabaseManagement System. The OMC MIB is contained in the SYBASEDatabase.

BaseWorX 6.1.- This Lucent Technologies software package isused to build the OMC application, and to support OMC functions,such as OA&M (operations, administration, and maintenance), andcommunications.

FrameViewer 4.0 - This desktop publishing system is used tosupport the OMC HELP function.

DataViews 9.5 - This software package allows the Graphical UserInterface (GUI) to display maps and graphical views.

SwitchTerm 4.1 - This software package supports the connection tothe MSC.

For more information about the software refer to the Installation manual.

* SYBASE is a registered trademark of Sybase, Inc. BaseWorX is a trademark of Lucent Technologies. FrameViewer is a registered trademark of Frame Technology Corporation. DataViews is a registered trademark of V.I. Corporation.


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GUI (Graphic User Interface)s X Windows/Motif based point and click interface

s A graphical display

s

On-line help function

AUI (ASCII User Interface)s An MML based command line interface

s Batch script capabilitys On-line help function


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6W002

OMC-2000 USER INTERFACES 2

One of the main functions of the OMC is to provide access to the BSS network.The BSS network can be accessed via:

s The GUI (Graphical User Interface)

s The AUI (ASCII User Interface)

When you log in to the OMC from a workstation, you typically access the systemthrough the GUI. The GUI has the following features:

s X Windows/Motif based point and click interface

s A graphical display

s On-line help function

The AUI provides a command line interface for operators accessing the OMC viaan ASCII terminal. The AUI can also be started via a window on the workstation.The AUI has the following features:

s An MML based command line interface

s Batch script capability

s On-line help


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Figure 2-4. GRAPHICAL USER INTERFACE (GUI) DESKTOP

Figure 2-5. EXAMPLE OF A LOGICAL VIEW OF A BSS


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6W002

GUI (GRAPHICAL USER INTERFACE) 2

When you start up the GUI the OMC Desktop appears on the screen (see gure).From the OMC desktop the following functional areas can be accessed:

s OMC Network

s Performance Management

s Fault Management

s Conguration Management

OMC Network 2

From the OMC Network icon you can access the following areas:

s the Topographical view of the BSS network.

s the BSS browser view which shows all connected BSSs.s the OMC detailed view.

s the Error Records browser.

Performance Management 2

From the Performance Management icon you can access the following areas:

s Measurement Scheduling

s Measurement Records

s Performance Management Subsystem.s Access the Basic Performance Management Report.

Fault Management 2

From the Fault Management icon you can:

s Display the alarm browser.

s Display the Fault Tracking Records Browser.

s Display the Error Denition Browser.

s Access the Locked or Disabled Object Report.

Conguration Management 2

From the Conguration Management icon you can:

s Congure the GSM network through the logical view (see gure).

s Access the Audit Records Browser.

s Access the Log les.


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AUI (ASCII USER INTERFACE) 2

The AUI is a non graphical interface which can be used for direct commands andto execute commands stored in a script le. The bulk load script les provided by

Lucent Technologies are especially useful during the initial conguration stage ofyour GSM network.

Batch operations are ordered sequences of New, Modify or Delete requests, thatyou assemble into a batch, which you can then execute as a single request. Thistool is useful to experienced users who want to create a number of similarrequests, and then run them as part of a batch. Once the batch request is made,the OMC automatically executes the items in the group in sequence.

You can only execute batch operations from the ASCII User Interface (AUI) mode.However, the modication requests and objects created are visible in GUI modewhile batch operations are executing from the AUI.

Use a text editor to create script les. It is not possible to edit script les within theAUI. Once you create and save the scripts in a directory, you can run them fromthe AUI.

The AUI uses a standard MML (huMan Machine Language) as dened by theITU-T. An MML command consists of:

s A command verb (denes the action)

s An object (denes the (logical) unit)

s Parameters (object attributes)

For a detailed description of the MML commands refer to the System OperatorsGuide.


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6W002

EXTERNAL OMC-2000 INTERFACES 2

The OMC interfaces to the BSS through the O interface . This allows the BSS to

communicate with an OMC in a DTE conguration. The interface is dened by theX.25 interface specication.


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CAPACITY AND CONFIGURATION 2

The OMC-2000 hardware platform is powered by a 204 to 264 V (240 V AC

nominal) single phase power supply of 48 to 62 Hz and dissipates roughly 150 W.Terminals such as Notebook PCs or Workstations or Printers dissipate between40 W and 120 W.

The OMC standard conguration consists of one Operations and MaintenanceProcessor (OMP) with one ACC card and two local workstations. One ACC cardcan support eight direct connections (to 8 BSSs), or up to 48 virtual circuitconnections to the BSSs (supported with 4600 Transceivers)

Optional components for the OMP are:

s Additional disks for mirroring

s Additional local workstations for a maximum of 16

s Additional remote workstations for a maximum of 16 (local and remote)

s Additional ACC cards for a total of six

s Additional X.25 links and other associated components to supportmanagement.

The OMC Performance Management Subsystem (OMC-PMS) is a highly-recommended option. Additional disks for mirroring and additional localworkstations (3 more, for a total of 4) are an optional feature.

Other capacities are as follows:

s 3 MSCss 48 users (16 simultaneous logins)


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Contents

Version 01 e issue c 3-i


3BSS PARAMETERS ANDFUNCTIONS

INTRODUCTION TO BSS PARAMETERS ANDFUNCTIONS 3-1

s LESSON INTRODUCTION 3-1s LESSON OBJECTIVES 3-1

GROUPS OF PARAMETERS 3-3

CELL (RE)SELECTION 3-5s WHAT IS CELL (RE)SELECTION 3-5s CELL PRIORITY 3-5s C1 CRITERION 3-7

Calculation of C1 3-7

Operational Aspects 3-7s EXERCISE CELL SELECTION 3-9s C2 RESELECTION CRITERION 3-11

C2 Criterion 3-11

Calculation of C2 3-11

Cell Reselection For Fast Moving MS 3-13

Cell Reselection For Slow Moving MS 3-15s TIMING FOR CELL RESELECTION 3-17s C2 EXERCISE 3-19s CELL RESELECT HYSTERESIS 3-21

Example With Reselect Hysteresis 3-21

Operational Aspects 3-21s PARAMETERS 3-23

POWER CONTROL RELATED PARAMETERS 3-25s WHAT IS POWER CONTROL 3-25s POWER CONTROL PROCESS 3-25

RXLEV 3-27


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3-ii Version 01 e issue c


Contents

RXQUAL 3-27Averaged Parameter Calculation 3-29

Example with Quality Averaged Parameter 3-29

Power Control Evaluation 3-31

Increase 3-31

Decrease 3-31

Uplink and Signal Level 3-31

Signal Level and Signal Quality 3-33

Time between Power Control Commands 3-35

Acknowledge Timer 3-35Interval Timer 3-35

s PARAMETERS 3-37

HANDOVER CONTROL RELATED PARAMETERS 3-41s WHAT IS HANDOVER CONTROL 3-41s HANDOVER PROCESS 3-41

Distance 3-43

Averaged Parameters 3-45

Cell Structures 3-45s HANDOVER CONTROL EVALUATION IN A NON-HIERARCHICAL

NETWORK 3-47

Power Budget Handover 3-49

Intra-cell Handover 3-49

Interference Boundaries 3-49

Traffic Load Criteria 3-53

Calculation of the Traffic Load Criteria 3-53

Traffic Load Example 3-53s TRAFFIC LOAD EXERCISE 3-55

Handover Control Evaluation in a Network Consisting of a

Multiple Cell Layer 3-57s PARAMETERS 3-59

RADIO LINK 3-63s INTRODUCTION 3-63s PAGING CHARACTERISTICS 3-63s DISCONTINUOUS RECEPTION 3-63s PAGING CHANNEL CONFIGURATION 3-63s PCH CONFIGURATION PARAMETERS 3-65


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Version 01 e issue c 3-iii


Contents

Downlink Signaling Failure Counter 3-67RANDOM ACCESS 3-69s TRIGGERS 3-69s RANDOM ACCESS SCHEME 3-69s RACH CONTROL PARAMETERS 3-71

Example 3-71s RADIO LINK FAILURE 3-73

Radio Link Time Out 3-73

Radio Link Failure Warning 3-73

s PARAMETERS 3-75BSS TIMERS 3-77s BSSMAP TIMERS 3-77s Um TIMERS 3-81s ANSWERS 3-82

C1 EXERCISE 3-82

C2 EXERCISE 3-82

TRAFFIC LOAD EXERCISE 3-82


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3-iv Version 01 e issue c


Contents


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3BSS PARAMETERS ANDFUNCTIONS 3

INTRODUCTION TO BSSPARAMETERS AND FUNCTIONS 3


This lesson discusses the Base Station System (BSS) parameters which can beadjusted in order to improve the quality or performance of the U m- or air-interface.

BSS parameters are user-controllable parameters which are used to dene theconguration, system services and characteristics of the cellular network. Theyconsist of thresholds, margins, timers and other types of parameters. The BSSparameters are normally modied at the Operation and Maintenance Center(OMC).

The BSS parameters given in this lesson are applicable for GSM softwareRelease 6.5

LESSON OBJECTIVES 3

At the end of this lesson, participants will be able to:

s Explain the BSS parameters which can be adjusted to improve the qualityof the air-interface.

s Explain the effects of adjusting BSS parameters in relation to optimizingthe air-interface.


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BSS PARAMETERS AND FUNCTIONS 9W535

GROUPS OF PARAMETERS:

s Cell (re)selection

s Power Control

s Handover Control

s Radio link

s Signaling timers

AREAS OF IMPROVEMENT:

s Minimization of interference

s Handover behavior improvement

s Trafc distribution

s Increase call completion


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BSS PARAMETERS AND FUNCTIONS



9W535

GROUPS OF PARAMETERS 3

The BSS parameters which are related to the improvement of the air interface,can be divided into ve groups. Each group consists of parameters which arerelated to:

s Cell (re)selection

s Power Control

s Handover Control

s Radio link

s Signaling timers

In this lesson these ve groups will be discussed in ve separate sections.

By adjusting the BSS parameters improvement in the following areas can beachieved:

s Minimizing interference

s Handover behavior improvement (avoiding unnecessary handovers).

s Trafc distribution (a maximum amount of trafc can be handled by eachBTS)

s Increase call completion.


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Figure 3-1. CELL SELECTION

.

Table 3-1. CELL PRIORITY LIST

CELL_BARQUALIFY

CELL_BARACCESS

Priority for CellSelection

Priority for CellReselection

False False Normal Normal

False True Barred Barred

True False Low Normal

True True Low Normal

?


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CELL (RE)SELECTION 3

WHAT IS CELL (RE)SELECTION 3

To ensure that an MS is camped on to a cell with the best transmission quality, thecell selection criteria is introduced. The cell selection is not only for selection of aproper cell but can also be used to nd a better cell. In that case it is called cellreselection .

The mobile station attempts to nd a suitable cell by passing through the list indescending order of received signal strength ; the rst BCCH channel whichsatises a set of requirements, is selected. The requirements that a cell mustsatisfy before a mobile station can provide service from it, are:

1. It should be a cell of the selected PLMN The mobile station checks whether the cell is part of the selected PLMN.For this purpose the MCC and MNC are used.

2. It should not be barred The PLMN operator may decide not to allow mobile stations to accesscertain cells. These cells may, for example, only be used for handovertrafc. Barred cell information is broadcast on the BCCH to instruct mobilestations not to access these cells.

3. The radio path loss between the mobile station and the selected BTS must be below a threshold set by the PLMN operator

There are two radio path loss cell (re)selection criteria dened by the GSMspecications. The rst one, C1, is used for cell selection and reselection. The

second one, C2, is used in a hierarchical cell structure for reselection only.

CELL PRIORITY 3

Cells can have two priority levels, low and normal priority. Suitable cells which areof low priority are only camped on if there are no other suitable cells of normalpriority available. Operators may prefer a certain type of cell not to be selectedunless it is the only suitable type.

The operator may also decide not to allow MSs to camp on certain cells (e.g. cellsonly used handover trafc). Barred cell information is broadcast on the BCCH toinstruct MSs not to camp on these cells.

The barred status of a cell depends on the parameters CELL_BAR ACCESS andon the cells priority indicated by CELL_BAR QUALIFY . Table 3-1 shows therelationship between the two parameters for cell selection and cell reselection


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Satised if C1 > 0

Figure 3-2. C1 CRITERION

C 1 A( Max B 0( , ) )=


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9W535

C1 CRITERION 3

The MS uses a path loss criterion C1 to determine whether a cell is suitable tocamp on. The following parameters are used to calculate the C1 criterion:

s The received signal level at the MS side.

s The parameter RXLEV_ACCESS_MIN , which is broadcast on the BCCH,is related to the minimum received level at the MS required for access tothe network.

s The parameter MS_TXPWR_MAX_CCH , which is also broadcast on theBCCH, is the maximum power that a MS may use when initially accessingthe network

s The maximum power of the MS

Calculation of C1 3The pathloss criterion C1 can be calculated with the following formula:

Where:

A = Received Level Average - RXLEV_ACCESS_MIN

B = MS_TXPWR_MAX_CCH - max. output power of the MS

except for class 3 GSM1800 MS where:

B = MS_TXPWR_MAX_CCH + POWER OFFSET - max. power of MS

The path loss criterion is satised if C1 > 0 . This means that a cell can be selectedif the C1 value is positive.

Operational Aspects 3

It is up to the operator to choose the values for RXLEV_ACCESS_MIN andMS_TXPWR_MAX_CCH to obtain the correct compromise between cellboundaries, trafc, and quality of transmission for different classes of mobiles, aswell as consistency with the handover algorithms and parameters.A default value for RXLEV_ACCESS_MIN is -102 dBm. A default value forMS_TXPWR_MAX_CCH is 35 dBm.

C 1 A( Max B 0( , ) )=


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Figure 3-3. CELL SELECTION EXERCISE

BTS1

BTS2

BTS3


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EXERCISE CELL SELECTION 3

Two mobile stations at nearly the same location, are trying to access a GSMnetwork (refer to the gure on the opposite page). The maximum output power of

the handset is 0.8 W (29 dBm). The maximum output power of the car set is 8 W(39 dBm). The table bellow gives the values of the cell selection parameters.

Questions:

1. To which BTS will each mobile camp on?

2. What would happen if the operator sets the CELL_BAR ACCESS parameter to False for BTS 2?

BTS 1 BTS 2 BTS 3Received level (at MS) -75 dBm -80 dBm -85 dBm

RXLEV_ACCESS_MIN -90 dBm -85 dBm -90 dBm

MS_TXPWR_MAX_CCH 39 dBm 35 dBm 35 dBm

CELL_BAR QUALIFY True False False

CELL_BAR ACCESS False True False


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C2 = C1 + CELL_RESELECT_OFFSET - TEMPORARY_OFFSET H(PENALTY_TIME - T)

(for PENALTY_TIME innity)

C2 = C1 - CELL_RESELECT_OFFSET (for PENALTY_TIME = innity)

Where for non serving cells: For serving cells:H(x) = 0 for x < 0

= 1 for x 0H(x) = 0

C2 Criterion Calculation:


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Figure 3-4. CELL RESELECTION FOR FAST MOVING MS

A

B

C

D

M1

M2

M3

U1

36 km/h

20 40 60 80 100 120A B C D

C2

U1

Temporary Offset M3

Penalty Time M3 T

Distance:A - B = 100mB - C = 500mC - D = 500m

0

Penalty Time M2

(sec)

Temporary Offset M2


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NOTE:When the previous serving cell is placed by the MS on the list of strongestcarriers at cell reselection, the timer T shall be set to the value ofPENALTY_TIME.

Cell Reselection For Fast Moving MS 3

Figure 3-4 shows an example of a fast moving subscriber in a hierarchical cellstructure. The subscriber remains camped on to the umbrella cell due to thetemporary offset and the penalty time for the two microcells. To travel through themicro cell it takes 500m: 10 m/s = 50 s. In this case the Penalty Time should be atleast 50 s. For this period of time the temporary offset should be high enough toreduce the C2 value of the micro cell.

NOTE:ETSI specied steps of 20 s for the Penalty Time, in this case in means thatthe Penalty Time should be set to 60 s.

s At point A the MS camps on to the umbrella cell because this is the onlysuitable cell.

s At point B the MS also receives signals from micro cell M3. To avoid thereselection to M3 a Temporary Offset is set to reduce the C2 value of theM3.

s It takes the MS 50 s to travel through the M3. The Penalty Time for M3 isset to 60 s, so the MS remains camped on to U1.

s

At point C the MS receives signals from M2, but again due to theTemporary Offset and the Penalty Time the MS remains camped on to U1.

s At point D the MS leaves the hierarchical cell structure and is still campedon to U1.


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Figure 3-5. CELL RESELECTION FOR SLOW MOVING MS

A

B

C

D

M1

M2

M3

U1

3.6 km/h

100 200 600 700 800A B C

M3

C2

U1

Temporary Offset

Penalty Time M3T (sec)

M2

Distance:A - B = 100mB - C = 500mC - D = 500m

0

U1


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Cell Reselection For Slow Moving MS 3Figure 3-5 shows an example of a slow moving subscriber. Every time thesubscriber moves to an other cell reselection takes place. In this case it takes500m: 1m/s = 500 s to travel trough the micro cell. After the Penalty Time isexpired (50 s) the MS will camp on to the micro cell.

NOTE:Keep in mind that this is a theoretical example with strict boundaries. In anormal network these boundaries are not strict.

s At point A the MS camps on to the umbrella cell because this is the onlysuitable cell.

s At point B the MS also receives signals from micro cell M3. Due to theTemporary Offset of M3 the MS is still camped on to U1.

s It takes the MS 500 s to travel through micro cell M3. The Penalty Time forM3 is set to 60 s, so after 60 s reselection takes place to M3 (the C2 valueof M3 is higher than the C2 value of U1.)

s At point C the MS receives signals from M2, but again due to theTemporary Offset and the Penalty Time the MS performs a reselection toU1.

s After 60 s in M2 again a reselection occurs from U1 to M2.

s At point D the MS leaves the hierarchical cell structure and the MS campson to U1 immediately (there is no Temporary Offset dened for U1).


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Figure 3-6. CELL RESELECTION FLOW DIAGRAM

MEASURE C2 AND

C2 EVERY 5 SEC.

YES

NO

NO

NO

YES

YES

SELECT C2

C2 > C2

PREV.15 SECONDS

RESELECTION?

C2 > C2 + 5dBFOR

5 SECONDS

FOR5 SECONDS


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TIMING FOR CELL RESELECTION 3

At least every 5 seconds the MS calculates the value C1 and C2 for the servingcell and recalculate the C1 and C2 values for non serving cells. The MS checks

whether:s The path loss criterion C1 for the current serving cell falls bellow zero for a

period of 5 seconds. This indicates that the path loss to the cell hasbecome too high.

s The calculated value of C2 for a non serving suitable cell exceeds the valueof C2 for the serving cell for a period of 5 seconds.

Figure 3-6 gives the ow diagram of how the MS selects the proper cell using theC2 algorithm. C2 is the C2 value of the current serving cell. C2 is the C2 value ofthe new cell.


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Figure 3-7. C2 EXERCISE

M1

U1

M23.6 km/h

36 km/h

0.8 W

8 W

TXPWR_MAX_CCH U1=37 dBmTXPWR_MAX_CCH M1=36 dBmTXPWR_MAX_CCH M2 =36 dBm

RXLEV_ACCESS_MIN U1=-104 dBmRXLEV_ACCESS_MIN M1= -102 dBmRXLEV_ACCESS_MIN M2 = -102 dBm


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C2 EXERCISE 3

Two MSs are traveling through a hierarchical cell structure of an umbrella cell andtwo micro cells. The radius of the micro cells is 100m. The signal level received

from U1 is -89 dBm. The signal level received is -76 dBm from M1 and, -79 dBm.from M2 These values

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