Procedures and Guidelines for Pre-Launch Optimisation_DT

7/28/2019 Procedures and Guidelines for Pre-Launch Optimisation_DT

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CONFIDENTIAL

Procedures andGuidelines for Pre-

launch Optimisation

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Commercial in Confidence Page 2 of 46

CONTENTS

1 REVISION HISTORY.............................................................................................. 3

2 OBJECTIVES ......................................................................................................... 4

3 BACKGROUND...................................................................................................... 4

4 MONITORING AND BENCHMARKING ................................................................. 4

5 PRE-LAUNCH OPTIMISATION PROCESS ........................................................... 6

5.1 DRIVE TESTS......................................................................................................................7 5.1.1 Field Test Survey Strategy ....................................................................... ........................7

5.1.2 Field Test Routes Definition ....................................................................... .....................75.1.3 Field Test Measurement Collection.................................................................................7

5.2 DATA ANALYSIS AND INTERPRETATION ...........................................................................7 5.2.1 Undefined or Missing Neighbour ................................................................. ...................85.2.2 Low Coverage Problem ..................................................................... ............................115.2.3 High Interference and Poor Quality Problem...............................................................14

5.3 SITE CONFIGURATION CHANGE REQUESTS .....................................................................19 5.4 IMPLEMENTATION............................................................................................................19


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1 REVISION HISTORY

Revision Date Name Comments

1.0 04 August Paul Teixeira First Release


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2 OBJECTIVES

This document aims to explain the procedures and guidelines used whileoptimising the network before commercial launch.

After reading this document, the reader should be familiar with the activities thatare performed in this pre-launch optimisation phase and be able to address some

of the common problems found during this phase.

3 BACKGROUND

Up to the Pre-launch Optimisation phase almost all the previous Rollout activities

have been based on:

Theoretical knowledge of propagation characteristics, and GSM System design,

Computer modelling, relying on digital topographic and morphologic data, and

Previous experience of the Planning Engineer

It is therefore only logical that problems will be encountered in the real worldthat were not foresee in this theoretical approach. These problems can degrade

the performance of the network and in some cased degrade the performance

below the initial design criteria or targets.

The aim of the pre-launch optimisation phase is to detect the problems arising

from the real world or practical limitations and to improve and where possible

solve these problems. It must be remember that not all problems are solvable buthopeful they can be controlled to a point where they are acceptable.

4 MONITORING AND BENCHMARKING

There are two sources available for an engineer to use to monitor the network.

The table below presents advantages and drawbacks of the two complementary

sources.

Features Cost Efficiency Geographical Scope

Field Test

Measurements

Represent a

reproducible andobjective customer

view of network quality suitable for

competitors analysis

Very resources and

time consuming

Restricted to specific

areas good togeographically locate

problems pinpointcoverage holes

NMS/OMC

Allows centralised data

collection permanent

flow of information useful to monitor trends

Cost effective way to

monitor network

quality

Limited geographical

location of problems

can locate problems on aper cell (TRX,TSL) level


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Before the commercial launch of the network, the subscriber numbers are low

which results in an almost zero-count for most statistics collected by the NMS.The engineer is therefore unable to use this source during the pre-launch

optimisation stage to diagnose most problems, hence almost all pre-launchoptimisation are based from the detection of problems by monitoring Field TestMeasurements and the improvement/degradation of the results from these

measurements.

However other NMS Reports such as Equipment Alarms are useful to detect

possible problems and can also provide answer to the poor performance found in

a Drivetest.

Recent developments in vendor support systems have provided one more source

of NMS performance data. The source of data is derived from the Measurement

Reports sent from the mobile, and uplink measurements made by the BTS onactive connections.

Alcatels RMS and Ericssons MRR are two such systems and can provide more

insight to a problem by collecting data such as Timing Advance Distributions,

Distribution of Signal Level on the Uplink and Downlink, Uplink and DownlinkRXQUAL Distribution and Power Reduction/Control. The data collected from

each of these indicators can be displayed individually or combined to help

diagnose problems. For example the RXQUAL versus the Signal level

distribution could indicate if the interference problem is due to low signalstrength or poor frequency allocation plan.

As mentioned before, this data is collected from active connections and hence if

the number of subscribers or network traffic is low, the sample size may be toosmall for valid statistical analysis. However this document will indicate where

such data may be of use.


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5 PRE-LAUNCH OPTIMISATION PROCESS

Pre-LaunchOptimisationProcess

Pre-launch Optimisation

Completed

Problems Found?

Implementation

Field Tests Survey

Strategy

Field Test RoutesDefinition

Data Analysis andInterpretation

Site ConfigurationChange Request

NMS ReportsField TestMeasurement

Collection

Achieved Targets?

YES

NO

NO

YES

Physical SiteOptimisation

NeighbourOptimisation

FrequencyOptimisation

ParameterOptimisation

CustomerRequirements

Drive Test


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5.1 Drive Tests

5.1.1 Field Test Survey Strategy

The RNP group will organise teams and equipment to carry out field test

measurement surveys. Since this activity can be very time consuming, extra

personnel can be involved. In this case the RNP group will instruct the survey

teams to be able to concentrate more on the interpretation of the measurementsresults.

5.1.2 Field Test Routes Definition

The RNP group will define the routes to be followed by the survey teams. These

routes should be adhered to and reviewed only ever few months or where an

areas dynamics have changed.

Examples of such changes include but are not limited to: new site integrations,new building developments and new major roads.

In the case where extra personnel carries out the measurements, it is

recommended that the RNP engineers issue a field test measurement request formwith a route map attached.

5.1.3 Field Test Measurement Collection

Drive Test Teams should conduct the field tests in keeping with the guidelines,strategies and routes defined by the RNP engineers.

Teams should take notes of any equipment problems and other abnormal events

(site outages, swapped sectors etc.) and inform the appropriate parties. If theproblem can be rectified on-the-spot, the teams should continue the drive test,

else discontinue the Drivetest until a further date.

After carrying out the requested measurements, the survey team will produce a

report of the key performance indicators of the network. This report will then beprovided to the customer.

5.2 Data Analysis and Interpretation

The analysis and interpretation of the drive tests will allow the RNP engineers to

assess the performance of the network, detect network problems and recommend

changes to improve the network.

The combined use of all sources of data is strongly recommended for a complete

investigation of a later stage network. This section will contain some example ofcommon problems found on the network and how they are interpreted.


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5.2.1 Undefined or Missing Neighbour

This problem is listed first because the symptoms can mimic those problems

described later. An example of misdiagnosing of a missing neighbour is when theDrivetest shows that the signal level is dropping below the coverage target, whichis then misinterpreted as a low coverage problem. Therefore it is better to analyse

the Drivetest data for a missing handover before proceeding to detect the other

possible problems.

Before going through examples of the diagnosis of a missing handover, one must

be aware if Downlink Power Control is active on the current connection or not.Power Control has the effect that the signal level measured may not made with

the BTS at full power and hence can not be compared with the reported

neighbour signal levels which are always made on the BTS transmissions at fullpower.

This can be done by checking the parameters of the cell in the OMC, theDrivetest and using the following table. This will help in deciding the likelihoodthat the signal level of the current connection has been reduced by the effect of

power control.

Current connection only usingBCCH carrier

No Power Control Used

Power Control disabled on cell No Power Control Used

Serving Cell BCCH in BA-Active

list and is reported to have a RXLEV

consistently close to the RXLEV ofthe connection (by 1-2dB)

No Power Control Used

RXLEV below the desired DL SignalStrength value or outside the DL

Power Control Window (by 2-3dB)

Most likely No PowerControl Used

RXLEV near DL Desired Signal

Strength Value or within the DL

Power Control Window

Most likely Power Control

Used

Serving Cell BCCH in BA-Active

list and is reported to have a RXLEVconsistently greater than the RXLEV

of the connection (by 2-3dB)

Most likely Power Control

Used

As it can be seen in the above table there are cases where one can not definitelysay that power control is or is not being used mainly due to the fact that the

BTS does not inform the mobile of the power control used. Therefore in caseswhere there is doubt, further Drivetests can be made to verify the problem.


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In cases where the serving cell RXLEV is also reported as one of the neighbours

then by using this RXLEV value the effect of power control can be ignored. Ifthis is not the case and power control has most likely been used for the

connection then the Drivetest may be showing normal operation.

If the missing neighbour uses a BCCH frequency that is already defined as part of

the serving cell Active BCCH Allocation list then detecting the missing handoveris fairly easy. An example of this scenario can be found in the next figure.

From the figure we can see that the connection is on the BCCH, so from the

previous table Power control is not in use. We can further see that the mobile is

reporting a signal (blue line) that is stronger than the serving signal (brown area)

but that no handover has been attempted to this stronger cell.

This example suggests that the blue cell is a strong candidate for a missing

neighbour, but the checks below need to be performed to confirm the diagnosis:

Check if reported BCCH/BSIC combination match to a known cell in thenetwork. If no BSIC is decoded, use planning tool to find a probable cell.If no matching or probable cell can be found as a candidate then

investigate if other operators are using that frequency

If a neighbour relationship is already defined to the candidate cell:

o In the case that the BSIC was not decoded or only availableintermittently then check for possible inference on the candidate. Ifa BSIC can not be reliably decoded, a handover will not occur.

o Check if the cells are controlled within the same BSC, LAC and/orMSC. If any of these are different, then check the relevant

databases to ensure that cell identities and handover information aredefined correctly.

o Check for congestion on the candidate cell. If it is congested, nohandover will occur.


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o Check parameter Disable Incoming Handover for the candidatecell. If incoming handover is disabled, mobiles will not hand to thecell.

o Check HCS layer settings and other vendor specific handoveralgorithm parameters for possible causes.

o Check the transmission of the candidate cell. Some vendors havedesigned their equipment to continue transmitting the BCCH carrier

even if the BTS is not connected to the rest of the network.

However if you see soon after in the Drivetest that the candidate

cell was in fact used, transmission failure is unlikely the cause.

A change request should be issued only for the missing neighbour in the case the

all the checks show up no other problem. Problems that are detected during thechecks should also be corrected by issuing a change request

If the missing neighbour uses a BCCH frequency that is not defined in the

serving cell Active BCCH Allocation list then detecting the missing handover isharder but not always impossible. An example of this case can be found in the

next figure.

In this example we can see that a handover occurred to a cell (the blue line before

the first handover) that was detected as stronger than the server, then immediately

after the handover a new cell (light blue line) is measured which is stronger thanboth the new and old serving cells. A second handover is then made to this new

strongest server.

In this case there is a strong indication that the original serving cell and finalserving cell should be defined as neighbours. If a neighbour relationship is

already defined to the candidate cell following checks should be made:

Check the Measurement Frequency List from either the System TypeInformation 5 or from the OMC. If the neighbour frequency does not

appear in the list then no handover will occur.


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Check if the cells are controlled within the same BSC, LAC and/or MSC.If any of these are different, then check the relevant databases to ensure

that cell identities and handover information are defined correctly.

If no relationship is defined then a change request should be issued to add themissing neighbour.

In some cases it may be advisable to delete and recreate the neighbour relation in

case there has been a corruption of the OMC/BSC configuration database.

5.2.2 Low Coverage Problem

This is generally the simplest problem to detect and analyse. However thesolution to this problem is normally not the easiest to implement.

The reason that the low signal level condition is evaluated for is because ageneral side effect of a low signal is that the C/I also reduces. This lowering C/I

causes then quality of the call to be degraded, firstly the BER increases to a point

where voice quality is degraded and then later to when the FER increases wherevoice samples are lost all together and finally to a dropped call.

If the interference condition was evaluated before the signal level, the optimisizer

might be tempted to diagnose the problem as poor quality and retune the

frequencies only to find that he or she has wasted their time because the real

cause of the interference is thermal or background noise which cannot bemitigated by a frequency change.

The solutions for correcting low coverage problems are:

1) Increase radiated power by increasing BTS output power or reducing BTSloses2) Redirect, tilt or increase the height of antennas.3) Use a repeater to extend coverage area of existing cell4) Build new base station to expand network coverage.

Depending on vendor equipment, it may be possible to increase the output power

to the maximum rated power via a software command or replace the TRX with ahigher power TRX.

Another possibility may be to reduce combiner and/or feeder system loses by

either implementing the concept of air-combining or using lower-loss feeders.Air-combining is a concept that instead of using a filter/hybrid combiner to

combine 2 TRX outputs to one antenna a separate antenna is used for each TRX

and thereby removing the 3dB to 3.5dB loss introduced by the combiner.

It is important to remember that the downlink and uplink coverage must be

balanced, so by increasing the downlink coverage it may be necessary to match

the uplink coverage by adding a TMA.

If the low coverage problem exists in an area that is close to an existing site and

is caused by obstructions, then it may be possible to solve the problem by


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increasing the height of the serving cell antennas sufficiently to overcome the

obstruction.

If the low coverage problem existing in an area near an existing cell but issituated in a direction that is between the sectors of the site (the null) then it

maybe be possible to redirect the sectors slightly to obtain some improvement.

Redirection is a give and take technique, it will give some signal for the area

that is now in the main beam but will take some signal away from the area that

was in the old main beam. However if in the direction of the old main beam therewas excess signal strength or there is another site that could cover the area within

the design targets then the redirection can be advantageous and increase the totalarea meeting the design target.

Uptilting of sector antennas may provide some gains in cases where the bore-line

angle falls short of the problem area. However the negative side of doing so is the

increased spillage from the cell which may cause interference to other cells. Inmost cases placing the top -3dB angle/point above the horizon will not provide

any signal level gain.

In rural areas and where the capacity requirements are low, it may be possible to

use a repeater to cover the low coverage area. There are many repeater products

available on the market so selecting the best repeater must be done on a case-by-case basis.

When analysing low coverage areas it is important to check if the low signal is

due to power control in which case if the quality is acceptable there is no problemto be fixed.

The figure above and the next map show an example of a low coverage problem.The possibility of a missing neighbour relationship has been previously ruled out.

In the next signal level graph you can see the signal level is decreasing steadily

from when the call is started until about midway of the call at which stage it

begins to increase. As mentioned previously as the signal level decreases so doesthe C/I and quality, this can be seen from the graphs; the calculated C/I decreases,


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the RXQUAL (BER representation) increases, the Speech Quality (SQI)

decreases, and the Frame Erasure Rate increases.

In the map the route has been shown and the midway point is has been selected.

The signal level decreases while the mobile is moving away from the base station

up until the midpoint, where at the mobile changes direction and starts to moveback towards the base station. In the list of the serving and neighbour cells it can

also be seen that the signal level of the server and the neighbours are all below -100dBm and the designed threshold.

In this example the distance measured to the low coverage area fairly far and

from the current antenna directions the low coverage is in the null of sectors 1

and 2, but it would be unlikely that a major improvement could be obtained

(without degrading the other areas below acceptable limits) by redirecting thesectors to better cover the area.

Further analysis shows the signal level is below the outdoor coverage level so the

use of high powered TRXs or low loss feeders will not significantly help. There

are no more sites to the South-East so the serving cell is on the edge of thenetwork, but considering that the low coverage area is still within the town limits

the best solution would be to build a new base station.

The next map view on the following page shows another example of a lowcoverage problem, however in this example the area is surrounded by sites and

not on the edge of network. Analysis of the area shows that there is no significant

obstruction near the serving site and the distance to the problem area is on the

edge of the designed coverage radius. Further inspection of the low coverage arearesults in the conclusion that the area is significantly more dense (roads are very

narrow 4-5m and buildings are contiguous) with little chance of line of sight or

reflected and refracted signals from the surrounding cells penetrating enough toreach the mobile.

The best solution again is to build another site in or near the area because area isdense and hence will have high traffic (repeater does not increase capacity) and

only a over-the-rooftop signal will be able to penetrate enough to reach the


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mobile (reducing the feeder losses or increasing the output power will not

improve penetration into the area)..

5.2.3 High Interference and Poor Quality Problem

The degradation to the network caused by high interference or poor quality caninclude the following:

An increase in Bad Speech Quality

A decrease in GPRS transfer rates with high number of retransmissions

An increase in dropped calls (from radio link timeout)

An increase in handover failures and handover drops

An increase in call setup failures

The major source of interference is the cells within the operators own network

and therefore is normally controllable or sometimes can be completely eliminated

by the operator.

Other same technology and band operators followed by other differenttechnology and same band operators are more sources of interference to a lesserdegree. A point to bear in mind is that sometimes the interference from these

sources can not be detected from Drivetests because they are either within theuplink band of GSM or use a difference technology. In theses cases the idle

channel measurement statistics available from most vendor systems and

wideband receiver (spectrum analyser) tests can help to detect these external

sources.


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Background noise or thermal noise is the last source of interference but it is

usually only a problem at very low (


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Single Interferer

In cases where interference is between two specific cells the first method afrequency change on either the server or the interferer can correct the

interference problem.

The detection of whether or not the interference is from a single source or many

can vary in complexity. The first step is to look at the frequency plan for the area

and find the most likely interferers. Sometimes a very dominant cell or a cell that

is very high (a boomer) can be quickly identified with this method.

Multiple InterferersWhen the area with high interference receives many signals from different cells

at similar levels the problem is compounded exponentially for every extrainterferer. In these cases local area frequencies retune maybe necessary to reduce

the interference to within acceptable limits. The extreme case of this is wherethere are so many signals that effectively you have a raised noise floor, and

frequency retuning will rarely obtain any gain.

In networks which have tight reuse of frequencies or that use fixed groups, or inlow cell density areas bordering high cell density areas, this raised noise floor can

become a major problem. Implementation of Frequency Hopping may help byaveraging this interference out but it cannot eliminate it.

When faced with such an increased noise floor, the remaining methods are better

suited to improving the quality in these high interference areas. Their main

objective is to decrease the interference without decreasing the carrier power suchthat the C/I ratio improves sufficiently to achieve good quality.


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Down Tilting of Antennas

The goal of downtiliting is to match the coverage area (footprint) of a cell withthe serving area of the cell. If the coverage area of the cell is larger than the

serving area then the cell is effectively spilling RF interference energy into thesurrounding area.

To correctly tilt the cell, the planner must determine the border of the cell and

then tilt the antenna such that the cell edge is within the top half of the main lobe.The exact tilt depends on the vertical beamwidth and electrical tilt of the antenna

and what the target range of the cell is.

As a first guideline, placing the top -3dB point of the antenna above the horizonis not recommended normally since this would result in a large portion of the

transmitted energy being lost into space or worse being trapped in atmospheric

thermal ducts to only return to the ground at some far off distance and interfering

with the cells in that area. The following paragraph may suggest a tilt that does

not follow this initial recommendation and if so, the pros and cons of doing soneed to be assessed and a compromise reached.

If the cell is there to provide coverage in areas where there is low site density

(rural or open areas), then the centreline of the antenna main lobe may be pointed

towards the cell edge. If the cell is to provide coverage in an area where there is ahigh site density (urban areas), then the top -3dB point of the antenna main lobe

should be pointed towards the cell edge.

In areas with uneven terrain, the decision process for tilt angle needs to consider

the relative difference in height between the BTS antenna and the MS. For

example, if the antenna is pointed up towards a hill, then the top -3dB point

should be just above the top of the hill. This will reduce possible spillage further

on (also reduce uplink interference to the cell) and concentrate the transmittedand received power within the target area.

In order to gain a clear definition of the cell edge/border many Drivetests need to

be performed and analysed. Look for the area where the cells are received at

equal powers, if both the cells are down tilted to this edge then the final powerlevels at the edge will not change by much but the spillage will be reduced

significantly. However if the signal levels at the cell edge are still very high

(+3dB above the level considered to be sufficient for good indoor coverage) then

tilt the antenna further until the signal level at the cell edge is acceptable.

Cell edges should roughly be equidistant between 2 cells which will in turnbalance the traffic between them. Sometimes a large barrier or obstruction which

one of the cells can not overcome defines the cell edge. Examples of suchobstructions include top of hills or large built up areas.

Once the cell edge or range is known, some simple trigonometry constructions

can be used to calculate the correct tilt angle.


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If Timing Advance distribution data is available from the OMC/NMS then these

may also help in defining the cell edge or range. Using this data, an engineer cantilt the antenna such that the top -3dB point of the antenna main lobe to the cover

the majority of the traffic/users.

Below is a Timing Advance Distribution Graph of cell that has areas of spillage.

In this example it would be good to downtilt the antenna such that the top -3dB

point falls about 6km (TA = 12) from the site. This should shed the unwantedtraffic around 9 and 15km away which is degrading the cells performance

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Power Adjustments to Server or Interferer

In areas with good coverage but poor interference it may be possible to reducethe signal level of either the interferer. This increases the C/I ratio of the server

and may lead to an improvement in performance.

Increasing power on a server that is well contained (a low site or coverage area islimited by barriers or obstructions) and is interfered with by another cell can

usually lead to a better C/I and improved performance as well.

Network FeaturesImplementing conservative power control settings (those that have quality

thresholds well within good RXQUAL values and high RXLEV) can still offerimprovements since the major gains are obtained with the first 2-3dB of power

reduction.

DTX and Frequency Hopping can both reduce the interference that any single

connected MS receives and hence obtain better network results.


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5.3 Site Configuration Change Requests

All problems detected in the previous section should result in some requirement

for a configuration change. The change could be one that just requires a software

command to be implemented or one that requires some physical change to be

implemented.

Whatever the case may be it is important to document the changes by completing

a Change Request. This prevents someone in the future removing a much needed

neighbour or changing an antenna configuration on the notion that the

configuration does not seem logical.

Change requests should document the reason and the change itself, identify thepossible effects to the network such that the performance counters can be

evaluated to see if the change has improved the performance or not.

5.4 Implementation

The optimisation cycle should start again once the change requests have beenimplemented by either change in:

Allocated Frequencies on Server or Interferer

Antenna Configurations

Neighbour Definitions

Parameters controlling Network Features

Because not all changes will result in performance improvements it is important

to re-evaluate the network after the change has been made and fall back whennecessary. Such fall backs should also be documented as an annex to the original

change request.

The optimisation process is repeated until the performance targets are achieved oruntil all possible solutions have been tried. Those problems that were not solved

should be documented and revisited every few months in case a new solution canbe found (for example after a new site has been integrated or a new feature

available).

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