final year project thesis.pdf

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RF Optimization of 2G/3G Networks

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Chapter # 1

Radio Network Optimization

1.1 Introduction

Radio network optimization is carried out in order to improve the network performance with

the existing resources. The main purpose is to increase the utilization of the network resources

solve the existing and potential problems on the network and identify the probable solutions

for future network planning.

Through Radio Network Optimization, the service quality and resources usage of the network

are greatly improved and the balance among coverage, capacity and quality is achieved. In

general, the following steps are followed during the Radio Network Optimization:

Data Collection and verification

Data analysis

Parameter and hardware adjustment

Optimization result confirmation and reporting.

Due to the mobility of subscribers and complexity of the radio wave propagation, most of

network problems are caused by increasing subscribers and the changing environment. Radio

Network Optimization is a continuous process that is required as the network evolves.

1.1.1 Causes that Inspire Carrying Out the RN Optimization:

New network or expansion on the existing network

The network quality decreased seriously and there are many complaints from subscribers.

An event occurs suddenly which affects the network performance seriously.

The number of subscribers increased and affects the network performance gradually.


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1.2 Optimization Process

Optimization process involves following major steps:

Data Collection

Analyzing collected data

Recommending Changes

Figure 1.1 Optimization Process


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Figure 1.2 Flow Chart of RN Optimization

1.2.1 Inputs for Optimization

Traffic statistics

Drive test

Customer complaints


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Figure 1.3 Data Collection

1.2.1.1 Traffic Statistics

Statistics are monitored on the NMS daily with the help of counters. The NMS usually measures

the functionalities such as call setup failures, dropped calls, and handovers (successes and

failures). It also gives data related to traffic and blocking in the radio network. An example of

KPI statistics is shown in figure below.

BSC Name Cell ID CSSR DCR HSR TCH

Blocking

Rate

HMLTBSC03 14480 98.49822 0.193237 98.80952 0

HMLTBSC03 24480 97.56784 0.284468 99.29078 0

HMLTBSC03 34480 97.04416 0.477834 97.51704 0

HMLTBSC03 14484 97.60172 0.17741 99.00596 0

HMLTBSC03 24484 97.69331 0.662252 98.29268 0

HMLTBSC03 34484 97.95067 0.15444 98.59873 0

HMLTBSC03 14919 98.16313 0.643696 94.2928 0

HMLTBSC03 24919 96.58074 2.777778 77.22772 0

HMLTBSC03 34919 91.47327 2.375566 77.57848 0

Table 1.1 Traffic Statistics


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1.2.1.2 Drive Testing

The quality of the network is ultimately determined by the satisfaction of the users of the

network, the subscribers. Drive tests give the feel of the designed network as it is experienced

in the field. The testing process starts with selection of the live region of the network where

the tests need to be performed, and the drive testing path.

Figure 1.4 Drive Testing

Before starting the tests the engineer should have the appropriate kits that include mobile

equipment (usually two mobiles), drive testing software (on a laptop), and a GPS (global

positioning system) unit. When drive testing starts, one mobile is used to generate calls with a

gap of few seconds (usually 1520 s). The second mobile is usually used for idle mode behavior.

The purpose of this testing is to collect enough samples at a reasonable speed and in a

reasonable time. If there are lots of dropped calls, the problem is analyzed to find a solution for

it and to propose changes.

An example of a drive test plan is shown in figure below


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Figure 1.5 Drive Test Route

1.2.1.3 Customer Complaints

Customer complaints are other big source of data for RNO teams. These complaints provide you

real network performance data. Customer may face Problems such as:

Call drop

Mute calls

Voice distortion

Network busy

Cross Talk

They report these problems to Service centers of the operator. These customer complaints are

then forwarded to the RNO teams. These complaints serve as source of network performance

data for RNO teams. They analyze these reports and identify issues. Then they make required

changes in the network to cater these problems.


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1.3 Analyzing Collected Data:

Data collected through above means is analyzed by RNO teams for problem identification. RNO

teams use special tools like TEMS and Mapinfo to analyze collected data. RNO teams use spread

sheet programs like Microsoft Excel for BSC statistics analysis.

1.4 Recommending Changes

After the problem has been identified RNO teams suggest the possible and best way out to

rectify the problem e.g.

faulty TRX replacement

frequency plan review for minimizing interference

addition of SDCCH channels to remove SD blocking etc.

A configuration mail sent to OMCR for addition of SD channels is displayed below.

Table 1.2 Configuration Mail of SD channels


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1.5 Benefits of Optimized Network

The GSM business model is changing. Competition for subscribers is fierce. Subscribers have

more choices than ever before about which wireless service to use. To attract, maintain and

move subscribers to high-value services such as data, network operators must provide

unprecedented quality of service. Higher quality will be achieved only through fast and accurate

network optimization.

Proper Network Optimization will benefit the operator in following ways.

Efficient spectrum utilization to meet capacity demands

Optimal frequency allocation to ensure good call quality

optimal use of network resources thanks to improved efficiency

Reduced dropped calls, resulting in less subscriber churn

Accurate neighbor topologies to ensure smooth handovers and call distribution

Improved customer loyalty, as high network quality is one of the most important factors for

customer retention

Increased revenue, due to the increased subscriber which is in turn due to higher quality

network

1.6 Network Optimization Tools

Network optimization tools are used for data collection, data analysis, and simulation analysis.

These are:

1. LAPTOP WITH TERMS INVESTIGATION 8.0 & MapInfo

2. CAR to carry out the Drive Test

3. FULL DRIVE TEST KIT

4. DIGITAL CAMERA

5. GPS

6. RECHARGEABLE BATTERIES and THE CHARGER

7. MAPS


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Chapter # 2

KPI ASSESSMENT & QoS ESTIMATION

In order to understand how the behavior of traffic channels (TCH) and control channels (SDCCH)

affects the networks performance; one has to analyze TCH and SDCCH blocking when

congestion in the network increases. Four major KPIs are frequently used in performance

judgment and QoS estimation of the network.

2.1 Call Set-Up Success Rate (CSSR)

Call set up success rate is one of the major KPI, which should be optimized to improve QoS

Where SD (usually called SDCCH stands for Stand-alone dedicated control channel) and TCH

stands for Traffic channel. A number of issues are related for its degradation as addressed

below.


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2.1.1 Issues Observed

CSSR might be affected and degraded due to following issues:

Due to radio interface congestion.

Due to lack of radio resources allocation (for instance: SDCCH).

Increase in radio traffic in inbound network.

Faulty BSS Hardware.

Access network Transmission limitations (For instance: abis expansion restrictions)

2.1.2 Analysis & Findings

Following methods are used to diagnose CSSR degradations as well as improvements:

Radio link Congestion statistics monitored using radio counter measurement.

Drive Test Reports.

Customer complaints related to block calls have been reviewed.

2.1.3 Improvement Methodologies

Following measures significantly improve the CSSR in live network:

Radio Resources enhancement (Parameter modification/changes in BSS/OMCR) such as half

rate, traffic load sharing and direct retry parameters implementation.

Transmission media Expansion to enhance hardware additions (such as TRX).

Faulty Hardware Replacement (such as TRX) in order to ensure the resources availability in

live network


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2.2 Call Drop Rate (CDR)

A number of issues are associated to its degradation as demonstrated below.


CDR might be affected due to following issues:

Interference (either external or internal) being observed over air interface. Internal

interference corresponds to in-band (900/1800 MHz) while external interference

corresponds to other wireless (usually military) networks.

Coverage limitation is also one of the factors, which increase CDR values.

Hardware faults (such as BTS transceiver) can also be incorporated in an increasing CDR,

which is a part of BSS failures.

Missing adjacencies (definition in BSS/OMCR) is also an important factor in CDR values

increment

2.2.2 Analysis & Findings:

Following methods are used to diagnose the rise in CDR values:

Radio uplink statistics monitored using radio counter measurement in order to confirm any

uplink interference.


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Path Balance stats which depict average of ERP-RX Power (where ERP stands for effective

radiated power over downlink and RX stands for receive power over uplink) also divert

attention towards faulty Transceivers hardware.

Customer complaints related to block calls would have been reviewed.

Interference band / Spectrum scanners are also useful in finding and tracing the

contaminated frequency carriers resulting in increasing CDR.

Drive Test Reports.


Following are some methods in order to improve the CDR value up to certain pre-Defined

baseline:

Faulty Hardware Replacement in order to ensure the resources availability in live network.

Frequency plans review and model tuning in order to ensure the clean band carriers for

serving cells. For instance; band conversion is done from 900 to 1800 MHZ in order to cater

uplink interference. Some times concentric cells (multi band cell having GSM & DCS

transceivers) solution is also devised.

New site integration is also suggested in order to improve indoor and outdoor coverage,

which is usually termed as Grid Enhancement.

Sometimes RF repeaters are also used in order to amplify the radio signal to extend

coverage area.

Existing coverage optimization might be done using physical optimization techniques.

Parameter tuning can also be done to improve call sustainability. This is done using OMCR

terminal. For Instance Power control parameters. Decrease emitted power when signal

receive level and quality (measured by peer entity) are better than a given value and vice

versa.

Frequency hopping technique is also incorporated to minimize the effect of interference.

Change of antenna orientation (azimuth/tilt) i.e., increase the down tilt of interferer cell

antenna.


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2.3 Handover Success Rate (HSR)

Handover Success rate is one of the major KPI that should be optimized to improve handover

quality:

A number of issues are related for its degradation as illustrated below:


HSR might be affected and degraded due to following issues:

Interference (either external or internal) being observed over air interface, which might

affect on going call switching in case of handover.

Missing adjacencies can also result in HSR degradation.

Hardware faults (such as BTS transceiver) can also be incorporated as a decreasing HSR,

which is a part of BSS failures.

Location area code (LAC) boundaries wrongly planned and/or defined (where Location area

represents a cluster of cells).

Coverage limitation is also one of the factors, which decrease HSR values.


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Following methods are used to diagnose HSR degradations as well as improvements:

Radio Congestion statistics monitored using radio counter measurement in order to confirm

congestion occurrence in a particular cell or area.

Neighboring plans reviewed and adjacencies audits being done.

Drive Test reports reviewed.


Following methods are employed in order to improve the HSR in live network:

Interference free band i.e., Spectrum analysis might be done to ensure it.

Adjacencies audits must be done in order to improve HSR.

Coverage improvement is also a vital factor of HSR enhancement.

BSS Resources addition (such as TRX) is also a factor for HSR improvement.

Parameter modification in OMCR such as Handover margin, traffic handover, power budget

parameters to assist better cell handovers.


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2.4 TCH Congestion Rate (TCHCR)

Traffic channel Congestion (TCH) rate is one of the major KPI, which should be optimized to

improve QoS

A number of issues are related for its degradation, which would be addressed here.


TCH (traffic channel) congestion might arise due to following issues:

TRX Hardware faults can also be incorporated as an increasing factor in TCH congestion.

Increasing number of subscribers and/or traffic in a certain area also causes congestion.

Lesser capacity sites (mainly due to the media issue or hardware resource unavailability)

also cause congestion problems.


Following methods are used to diagnose TCH congestion as well as improvements:

Radio Congestion statistics monitored using radio counter measurement in order to confirm

congestion occurrence in a particular cell or area.


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Customer complaints can also reveal the issue.

Drive Test reports reviewed.

WCR (Worst Cell Ratio) and CSSR (Call Set up Success Rate) KPIs also depict the TCH

congestion problem. Future subscriber density and growth is also a factor for the judgment of upcoming

congestion

2.4.3 Improvement Methodologies:

Following measures are used to minimize the TCH congestion in live network:

BSS Resources addition and expansion (including transceivers and transmission media) are

important factors for TCH congestion improvement.

Faulty hardware maintenance or replacement can also minimize TCH congestion.

Deployment of moving/portable BTS (commonly called COW BTS) can be used as a better

solution to improve congestion in case of foreseeable special events such as sports events,

important meetings, festivals and exhibitions etc.

Parameter modification in OMCR (such as half rate and traffic handover implementation) and

concentric cells additions are quite practical ways to improve congestion up to significant

extent.


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Chapter # 3

PHYSICAL PARAMETER OPTIMIZATION

Coverage may be optimized by careful design of the patterns of a broadcast antenna array.

Antenna tilt, antenna Elevation and azimuth patterns are generally independent of each other,

and may be altered to create improvements in coverage. Antenna azimuth and down tilt are

two important optimization parameters in Global System for Mobile Communication (GSM)

networks. Optimization of these two parameters can significantly improve system performance

as well as reduce interference with nearby sites. However, new networks sometimes use

inefficient optimization techniques and implement default values. Furthermore, inconsistencies

in setting these parameters during installation vary the network coverage and capacity.

Following physical parameters are of more importance:

Antenna Tilt

Antenna Azimuth

Antenna Height

Addition /Removal of TRXs

Antenna Patterns

3.1 Antenna Tilt Optimization

Antenna tilt is optimized for the following purposes:

To reduce coverage area

To reduce interference

To limit overshooting of a site

To improve coverage weakness between main lobe

To improve in building penetration

There are two methods to optimize antenna tilt:

Mechanical tilt

Electrical tilt


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3.1.1 Mechanical Tilt

Mechanical tilt of an antenna refers to physical alignment of antenna. Mechanical tilt means

physically leaning or giving slope to antenna.

Mechanical Down tilt is shown in fig:

Figure 3.1 mechanical down tilt

Figure 3.2 Adjusting Mechanical Tilt


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3.1.2 Electrical Tilt

Electrical Tilt of antenna is to tilt the beam by altering the signal Phasing, resulting decrease in

main, side and backward lobes. This has overcome the shortcomings of Mechanical Tilt Antenna

in which the whole antenna is physically tilted, which lifts the backward lobe in upward

direction and side lobes patterns are somewhat distorted.

Now a Days Remote Electrical Tilt Antennas are popular, In which Electrical Tilt can be done

from Remote Location for example NMS.

Electrical Down tilt is shown in fig:

Figure 3.3 Electrical down tilt

Electrical down tilt provides much better interference suppression.

Back lobes are also tilted with electrical tilt which is an added benefit whereas in mechanical tilt

whole antenna is physically tilted which raises back lobe in upward direction.


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Figure 3.4 Back lobes is tilted with Electrical down tilt

Figure 3.4a Back lobe is lifted upward in mechanical down tilt


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3.1.2.1 Benefits of Electrical Tilt

Improved Signal to Interference (C/I) ratio

Less dropped calls

o

Improved reputation

o Improved revenue

3.1.2.2 Effects of Electrical down tilt on Coverage

Figure 3.5 When tilt is smaller, larger area is covered. Here Electrical tilt is 2 degree and a larger area is covered.


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Figure 3.5a Here Electrical down tilt is increased to 5 degree and main lobe of antenna is focused. And coverage area is

reduced.


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Figure 3.5b Here Electrical down tilt is increased to 10 degree and beam of antenna is more focused. In this way, interference

with neighboring sites as well as coverage is reduced.

Altering antenna tilt must be done very carefully to really improve the situation. Typical down

tilts are between 0 and 10 degree. However, even higher values (up to 25 degree) can be used.

3.2 Antenna Azimuth

This chapter focuses on base stations with 3 sectors and a fixed spacing of 120 degree between

the three antennas. When adjusting the antenna azimuth, all three antennas are turned in the

same direction at the same time, so that the spacing between them will be kept constant at 120

degree.


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Figure 3.6: Adjustment of Antenna Azimuth

For finding the optimum azimuth settings in a network, the interference has to be taken into

account. The goal of the azimuth optimization is to reduce the intra- and inter-cell interference.

As a result the capacity of the network will be increased. In Figure 4.2, the horizontal pattern of

the used KATHREIN antenna. The pattern shows a difference in antenna gain of about 6 dB

between the main direction of the antenna (0) compared to an angle of 60 (at this angle the

adjacent sectors of this base station begin, and there the UEs will initiate a handover to the

neighboring cell). Due to that difference of 6 dB, the direction of the main beam of the antenna

is quite significant and thus it is important to adjust the azimuth of the antennas in order to

reach the highest antenna gain for the users in the own cell, as well as the lowest gain (or

highest attenuation) for the mobile stations located in neighboring cells. This way, less power is

needed for covering the area, and therefore less interference is generated.


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Figure 3.7: Horizontal Pattern of Antenna (in dB)

Altering antenna azimuth has following purposes:

To overcome coverage weakness between different sector

To reduce interference in certain directions

3.3 Antenna Height

The Aspects for Antenna heights considerations are depending upon the wave range and

economical reasons.


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Long wave/low frequency Antennas

At VLF, LF and MF the radio mast or tower is often used directly as an antenna. Its height

determines the vertical radiation pattern. Masts and towers with heights around a quarter

wave or shorter, radiate considerable power towards the sky. This allows only a small area of

fade-free reception at night, because the distance at which ground wave and sky wave are of

comparable strength and can interfere with each other is severely restricted (approximately 40

kilometers to 200 kilometers from the transmission site, depending on frequency and ground

conductivity).

For high power transmitters, masts with heights of about half the radiated wavelength are

preferred because they concentrate the radiated power toward the horizon. This enlarges the

distance at which selective fading occurs. However, masts with heights of around half a

wavelength are much more expensive than shorter ones and often too expensive for lower

power medium wave stations

Shortwave/high frequency antennas

For transmissions in the shortwave range, mast height has no influence on efficiency.

Masts are generally used to support the antenna. Most shortwave masts are less than 100

meters high.

Altering antenna height has following purposes:

To reduce or improve coverage

To reduce interference

However, antenna height is only changed only if it is really needed to improve situation.

3.4 Addition or Removal of TRXsDepending on real measured traffic load TRXs can either be removed (Switched off or blocked)

or added. Not really needed TRXs may interfere other cells.

The number of needed TRXs and configuration of different channels depend on offered traffic

and subscriber behavior.


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TRXs can be added for the following purposes:

To cater access network congestion problem

To remove blocking

TRXs can be removed for the following purpose:

To reduce interference ( Not really needed TRXs interfere with neighboring sites and may

Produce severe quality issues)

3.5Antenna Patterns (Radiation Patterns)

Antenna Pattern is a graphical representation of the antenna radiation properties as a function

of position (spherical coordinates).Or the antenna irradiation diagram is a graphical

representation of how the signal is spread through that antenna, in all directions. It is easier to

understand by seeing an example of a 3D diagram of an antenna (in this case, a directional

antenna with horizontal beam width of 65 degrees).

The representation shows, in a simplified form, the gain of the signal on each of these

directions. From the center point of the X, Y and Z axis, we have the gain in all directions. If you

look at the diagram of antenna 'from above', and also 'aside', we would see something like the

one shown below.


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These are the Horizontal (viewed from above) and Vertical (viewed from the side) diagrams of

the antenna.

But while this visualization is good to understand the subject, in practice do not work with the

3D diagrams, but with the 2D representation. So, the same antenna we have above may be

represented as follows.


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3.5.1

Common Types of Antenna Patterns

Power Pattern:Normalized power vs. spherical coordinate position.

Field Pattern: Normalized _E_ or _H_ vs. spherical coordinate position.

Antenna Field Types

Reactive field:The portion of the antenna field characterized by standing (stationary) waves

which represent stored energy.

Radiation field:The portion of the antenna field characterized by radiating (propagating)

waves which represent transmitted energy.

3.5.2

Antenna Field Regions

Reactive Near Field Region: The region immediately surrounding the antenna where the

reactive field (stored energystanding waves) is dominant.

Near-Field (Fresnel) Region: The region between the reactive near field and the far-field

where the radiation fields are dominant and the field distribution is dependent on the

distance from the antenna.

Far-Field (Fraunhofer) Region:The region farthest away from the antenna where the field

distribution is essentially independent of the distance from the antenna (propagating

waves).

3.5.3 Antenna Pattern Definitions

Isotropic Pattern: An antenna pattern defined by uniform radiation in all directions,

produced by an isotropic radiator (point source, a non-physical antenna which is the only

non-directional antenna).

Directional Pattern: A pattern characterized by more efficient radiation in one direction

than another (all physically realizable antennas are directional antennas).

Omni directional Pattern:A pattern which is uniform in a given plane.

Principal Plane Patterns:plane patterns of a linearly polarized antenna.


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E-plane: The plane containing the electric field vector and the direction of maximum

radiation.

H-plane: the plane containing the magnetic field vector and the direction of maximum

radiation.


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Chapter # 4

Drive Test

Drive testing is the most common and maybe the best way to analyze Cellular Network

performance by means of coverage evaluation, system availability, network capacity, network

retainibility and call quality. Although it gives idea only on downlink side of the process, it

provides huge perspective to the service provider about what's happening with a subscriber

point of view.

While statistics give an idea about the real behavior faced by all end users regardless of their

geographical location, drive testing or walk testing bring a simulation of end user perception of

the network on the field from one call perspective. Drive tests give the 'feel' of the designed

network as it is experienced in the field. The testing process starts with selection of the 'live'

region of the network where the tests need to be performed, and the drive testing path. Before

starting the tests the engineer should have the appropriate kits that include mobile equipment

(usually three mobiles), drive testing software (on a laptop), and a GPS (global positioning

system) unit.

4.1 Primary Motives behind Drive Test

Every alive Network needs to be under continues control to maintain/improve the

performance.

Optimization is basically the only way to keep track of the network by looking deep into

statistics and collecting/analyzing drive test data.

Drive test helps operation and maintenance for troubleshooting purposes.


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4.2 HW Requirements Involving Drive Test

Drive Test, as already mentioned, is the procedure to perform a test of Cellular Network

performance while driving. Following are the list of tools required while performing drive test.

1. A Laptop - or other similar device

2. Data Collecting Software installed

3. Security Key - Dongle - common to these types of software

4. At least one Mobile Phone

5. One GPS

Figure 4.1 Drive Test HW Components

Where,

GPS: collecting the data of latitude and longitude of each point / measurement data, time,

speed, etc.. It is also useful as a guide for following the correct routes.

MS:mobile data collection, such as signal strength, best server, etc.

Thus, the main goal is to collect test data, but they can be viewed / analyzed in real time (Live)

during the test, allowing a view of network performance on the field. Data from all units are

grouped by collection software and stored in one or more output files.


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4.3 Procedure Involved in Drive Testing

4.3.1 Drive Test Routes

Drive test routes are the first step to be set, and indicate where testing will occur. This area is

defined based on several factors, mainly related to the purpose of the test. The routes are

predefined in the office. A program of a lot of help in this area is Google earth. A good practice

is to trace the route on the same using the easy p paths or polygons.

Figure 4.2 Drive Test Routes using Google Earth

Some software allows the image to be loaded as the software background (geo-referenced).

This makes it much easier to direct routes to be followed.

It is advisable to check traffic conditions by tracing out the exact pathways through which the

driver must pass. It is clear that the movement of vehicles is always subject to unforeseen

events, such as congestion, interdicted roads, etc.. Therefore, one should always have on hand -

know alternate routes to be taken on these occasions.

Avoid running the same roads multiple times during a Drive Test (use the Pause if needed). A

route with several passages in the same way is more difficult to interpret.


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4.3.2 Drive Test Schedule

Again depending on the purpose, the test can be performed at different times - day or night.

A Drive Test during the day shows the actual condition of the network - especially in relation to

Traffic loading aspect of it. Moreover, a drive test conducted at night allows you to make, for

example, tests on transmitters without affecting most users.

Typically takes place nightly Drive Test in activities such System Design, for example with the

integration of new sites. And Daytime Drive Test applies to Performance Analysis and also

Maintenance.

Important: regardless of the time, always check with the responsible area which sites are with

alarms or even out of service. Otherwise, your job may be in vain.

4.3.3 Types of Calls

The Drive Test is performed according to the need, and the types of test calls are the same that

the network supports - calls can be voice, data, video, etc.. Everything depends on the

technology (GSM, CDMA, UMTS, etc. ...), and the purpose of the test, as always.

A typical Drive Test uses two phones. A mobile performing calls (CALL) for a specific number

from time to time, configured in the Collecting Software. And the other, in free or IDLE mode,

i.e. connected, but not on call. With this, we collect specific data in IDLE and CALL modes for the

network.


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The calls test (CALL) can be of two types: long or short duration.

Short calls should last the average of a user call - a good reference value is 180 seconds. Serve

to check whether the calls are being established and successfully completed (being a good way

to also check the network setup time).

Long calls serve to verify if the handovers (continuity between the cells) of the network are

working, i.e. calls must not drop.

4.4 Types of Drive Test

The main types of Drive Test are:

1. Performance Analysis

2. Integration of New Sites and change parameters of Existing Sites

3. Antenna Redesign

4. Benchmarking

1. Performance Analysis

Tests for Analysis Performance is the most common, and usually made into clusters (grouping

of cells), i.e., an area with some sites of interest. They can also be performed in specific

situations, as to answer a customer complaint.

2. Integration of New Sites and change in parameters of Existing Sites

In integration testing of new sites, it is recommended to perform two tests: one with the site

without handover permission - not being able to handover to another site - thus obtaining a

total visualization of the coverage area. The other, later, with normal handover, which is the

final state of the site.

Depending on the type of alteration of the site (if any change in EIRP) both tests are also

recommended. Otherwise, just perform the normal test.


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3. Antenna Redesign

This activity perform mainly for those cells/sites which are overshooting that means providing

coverage beyond there desired coverage limit. In this type of Drive Test Mobile Station first

camp on the desired cell and locked itself with the BCCH frequency of that particular cell and

than it start observing the coverage limit until its level completely diminished.

4. Benchmarking Tests

Benchmarking tests aims to compare the competing networks. If the result is better, can be

used as an argument for new sales. If worse, it shows the points where the network should be

improved.

4.5 Important Observing Indicators in Drive Test

Drive Test Indicators plays a vital role in analyzing, optimizing and troubleshooting the cell/site

radio end issues. Following are the list of indicators that needs to observe in Drive Test in order

to properly analyze and investigate the known issues emerging from radio end site.

1. Bit Error Rate (BER)

2. Rx-level

3. Rx-Quality

4. Frame Erasure Rate

5. Speech Quality Index (SQI)

1. Bit Error Rate (BER)

The BER is an estimated number of bit errors in a number of bursts to which corresponds a

value from 0 to 7 (best to worst) of the RX QUALITY. After the channel decoder has decoded a

456 bits block, it is coded again using the convolutional polynom in the channel coder and the

resulting 456 bits are compared with the 456 input bits. The number of bits that differs

between these two 456 bits blocks corresponds to the number of bit errors in the block. The

number of bit errors is accumulated in a BER sum for each SACCH multi frame and the result is

classified from 0 to 7 according to the BER-RX QUALITY conversion Table shown below.


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RX QUALITY is still considered as a basic measurement. It simply reflects the average BER over a

period of 0.5s. However, listener speech quality evaluation is a complex mechanism that is

influenced by several factors. Some of these factors that RX QUALITY does not consider are:

Time distribution of BER. For a given BER, if the rate fluctuates a lot, the perceived quality is

less than if the BER is constant over the time.

When entire frames are lost, speech quality is negatively impacted.

Handovers generate some frame losses. It is not evident in RX QUALITY measurements

since, during handovers, BER measurements are skipped.

Overall quality depends closely on the type of codec used.

In conclusion, RX QUALITY does not capture many phenomena t hat affect the listeners

perception of speech quality. That is why other metrics are defined.

BER to RX QUALITY Conversion table

2. Rx-Level

Rx-Level is defined as the power level corresponding to the average received signal level of the

downlink as measured by the mobile station. The range of Rx-level is between -55 to -110.It is

been further classified as Rx-Level Sub and Rx-level Full. Where Rx-Level sub is based on the


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mandatory frames on the SACCH multi frame. These frames must always be transmitted which

means that they carry intelligent signaling data. Whereas The FULL values are based upon all

frames on the SACCH multi frame, whether they have been transmitted from the base station

or not. This means that if DTX DL has been used, the FULL values will be invalid for that period

since they include bit-error measurements at periods when nothing has been sent resulting in

very high BER.

Figure 4.3 A Typical RX-Level Plot


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3. Rx-Qual

Rx-Qual is defined as the level corresponding to the mobile station's perceived quality of the

downlink signal. Rx Quality is a value between 0 and 7, where each value corresponds to an

estimated number of bit errors in a number of bursts. The Rx Quality value presented in TEMS is

calculated in the same way as values reported in the measurement report sent on the uplink

channel to the GSM network.

Each Rx Quality value corresponds to the estimated bit-error rate according to the following

table, which is taken from GSM technical specification shown in BER table.

Figure 4.4 A Typical RX-Qual Plot


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4. Frame Erasure Rate (FER)

A speech quality degrade factor that indicates fading and interference. Voice quality is judged

upon the Frame erasure rate. Even while experiencing bad Received Quality, the voice Quality

still could be maintained. FER (frame erasure rate) range goes from 0 (being the best

performance) to 100%. This represents the percentage of blocks with an incorrect CRC (cyclic

redundancy check). Since the BER is calculated before the decoding with no gain from

frequency hopping, the FER is then used in this case. Being even more stable than the BER, the

FER also depends on codec type. The smaller the speech codec bit rate, the more sensitive it

becomes to frame erasures. FER plays a major role in troubleshooting of Interference.

Formula for Calculating FER:

FER (%) = (no. of blocks with incorrect CRC / total no. of blocks)*100

Whereas,

Block represents 456 bits.

Figure 4.5 A Typical FER Plot


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5. Speech Quality Index (SQI)

SQI is an estimate of the perceived speech quality as experienced by the mobile user, is based

on handover events and on the bit error and frame erasure distributions. The need for speech

quality estimates in cellular networks have been recognized already in the GSM standard, and

the Rx Quality measure was designed to give an indication of the quality.

However, the Rx Quality measure is based on a simple transformation of the estimated average

bit error rate, and two calls having the same Rx Quality ratings can be perceived as having quite

different speech quality. One of the reasons for this is that there are other parameters than the

bit error rate that affects the perceived speech quality. Another reason is that knowing the

average bit error rate is not enough to make it possible to accurately estimate the speech

quality. A short, very deep fading dip has a different effect on the speech than a constant low

bit error level, even if the average rate is the same.

Generally Speech Quality Index, which is an estimate of the perceived speech quality as

experienced by the mobile user, is based on handover events and on the bit error and frame

erasure distributions. The quality of speech on the network is affected by several factors

including what type of mobile the subscriber is using, background noise, echo problems, and

radio channel disturbances. Extensive listening tests on real GSM networks have been made to

identify what type of error situations cause poor speech quality. By using the results from the

listening tests and the full information about the errors and their distributions, it is possible to

produce the Speech Quality Index. The Speech Quality Index is available every 0.5 second in and

predicts the instant speech quality in a phone call/radiolink in realtime.


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Figure 4.6 A Typical SQI Plot

4.6 Drive Testing Through TEMS

TEMS stands for Test Mobile System is considered as the most reliable tool across the globe

while dealing with drive test performing analysis, investigation and successful troubleshooting

in 2nd Generation based networks. Its primary task is to to read and control information sent

over the air Interface between the base station and the mobile station in GSM/Cellular system.

It can also used for radio coverage measurement.

4.6.1 A Quick look at TEMS

In this session we take a quick look of TEMS and its Interface. The information provided by

TEMS is displayed in status windows. This information includes cell identity, base station

identity code, BCCH carrier ARFCN, mobile country code, mobile network code and the location

area code of the serving cell.


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There is also information about RxLev, BSIC and ARFCN for up to six neighboring cells; channel

number(s), timeslot number, channel type and TDMA offset; channel mode, sub channel

number, hopping channel indication, mobile allocation index offset and hopping sequence

number of the dedicated channel; and RxLev, RxQual, FER, DTX down link, TEMS Speech Quality

Index (SQI), timing advance (TA), TX Power, radio link timeout counter and C/A parameters for

the radio environment.

The signal strength, Rx-Qual, C/A, TA, TX Power, TEMS SQI and FER of the serving cell and signal

strength for two of the neighboring cells can also be displayed graphically in a window.

Figure 4.7 A Typical TEMS Interface

4.7 MODES OF DRIVE TEST

1. Dedicated / Continuous / Long Call Mode

2. Idle Mode

3. Frequency Scan Mode


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1. Dedicated / Continuous / Long Call Mode

In this kind of Drive Test we Make a continuous call along drive test activity Before starting the

route, call the drive test number and only stop the call when the route (drive test) finish.

Figure 4.8 An Example of Continuous Long Mode Drive Test.

2. Idle Mode

A drive test activity in which, the MS is ON but no call occur. A powered on mobile station

(MS) that does not have a dedicated channel allocated is defined as being in idle mode (see

Figure 3). While in idle mode it is important that the mobile is both able to access and be

reached by the system. The idle mode behavior is managed by the MS. It can be controlled by

parameters which the MS receives from the base station on the Broadcast Control Channel

(BCCH). All of the main controlling parameters for idle mode behavior are transmitted on the

BCCH carrier in each cell. These parameters can be controlled on a per cell basis.


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Moreover, to be able to access the system from anywhere in the network, regardless of where

the MS was powered on/off, it has to be able to select a specific GSM base station, tune to its

frequency and listen to the system information messages transmitted in that cell. It must also

be able to register its current location to the network so that the network knows where to

route incoming calls.

Figure 4.9 An Example of Idle Mode Drive Test

3. SCAN Mode

One of TEMS feature

Scan all or selected frequencies on the selected spot or route

To find the clearest frequency

Its main application in frequency plan Application of frequency plan is to find the best

frequency to be use in the site and to identify interference adjacent channel and co channel.


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Figure 4.10 SCAN Mode Drive Test Example

The scanning can be performed by a TEMS mobile station or by a dedicated frequency scanner

mobile (TEMS Scanner). The mobiles supported in this version of TEMS investigation are

capable of all scanning tasks handled by TEMS Scanners, including CW scanning. As

measurement devices, Network Scanners are rigorously designed for the challenges in network

optimization and trouble shooting. They include a high-end RF front-end and sophisticated

algorithm to quickly and accurately scan the air interface and reliably detect all base stations

and their signal components. In contrast to mobile phones, they do not face the limitations of a

consumer product in precision, processing power and size.

In trouble shooting scenarios, Network Scanners come into play when a mobile phone for

example cannot register to the network, drops the call or faces degradations in its voice or data

quality. The Network Scanner can provide network information in situations which are beyond

the capabilities of a mobile phone.


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4.8 Testing WCDMA (3G) and GSM (2G) both i.e. in Dual mode

4.8.1 For GSM (2G)

You have to open 4 windows.

GSM radio parameter

GSM current channel

GSM serving + Neighbour

Events

First 3 windows are open from above toolbar.

Presentation-GSM

And last window is open from.

Presentation-Signaling-Event

Now connect the device by pressing the Green button (Connect all) or by pressing F2 button.

Figure 4.11 Window for GSM (2G)


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4.8.2 For WCDMA (3G)

Along with 4 window of GSM you have to open 3 window more and they are.

HSDPA analysis

WCDMA radio parameter

WCDMA Serving/Active+Neighbrs

The 1stwindow i.e. HSDPA analysis is used only for data call, And remaining 2 is used for WCDMA

analysis.

Figure 4.12 Window for WCDMA (3G)


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Figure 4.13 The above window shows that you are latch in 3G network. During 3G network all the parameters in the 2G window

are blank.

4.8.3 WCDMA (3G)

WCDMA Means Wideband Code Division Multiple Access.

Wideband Code Division Multiple Access is a CDMA channel that is four times wider

than the current channels that are typically used in 2G networks.

Wideband CDMA has a bandwidth of 5 MHz or more.

It is also called 3G system, allow for faster data transfer than GPRS and EDGE and also

let you talk while you transfer data.


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Figure 4.14 Window used in the WCDMA Drive Test


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4.8.4 WCDMA RADIO PARAMETERS

4.8.5 WCDMA Serving / Active Set + Neighbour

This windows shows the Serving cell & Neighbors

AS : Active Set

MN: Monitered Neighbors

DN: Dominant Neighbors


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4.9 HSDPA

HSDPA (High Speed Downlink Packet Access) is a technology based on the 3G network which

can support speeds of up to 7.2 Mbits per second. In reality you will most likely get a top speed

of around 3 Mbits but this is useful for mobile TV streaming and other high end data

transmissions. To use HSDPA your phone must be able to support the technology and of course

you will need to be located within range of a cell site that has been upgraded to offer the

service. HSUPA (High Speed Uplink Packet Access) is the other side of this coin, although for

mobile devices it is rarely mentioned as download speeds are considered more important.

Together the 2 technologies make HSPA (High Speed Packet Access).

HSDPA, short for High-Speed Downlink Packet Access, is a new protocol for mobile telephone

data transmission. It is known as a 3.5G (G stands for generation) technology. Essentially, the

standard will provide download speeds on a mobile phone equivalent to an ADSL (Asymmetric

Digital Subscriber Line) line in a home, removing any limitations placed on the use of your

phone by a slow connection. It is an evolution and improvement on W-CDMA, or Wideband

Code Division Multiple Access, a 3G protocol. HSDPA improves the data transfer rate by a factor

of at least five over W-CDMA. HSDPA can achieve theoretical data transmission speeds of 8-

10 Mbps (megabits per second). Though any data can be transmitted, applications with high

data demands such as video and streaming music are the focus of HSDPA.

HSDPA improves on W-CDMA by using different techniques for modulation and coding. It

creates a new channel within W-CDMA called HS-DSCH, or high-speed downlink shared

channel. That channel performs differently than other channels and allows for faster downlink

speeds. It is important to note that the channel is only used for downlink. That means that data

is sent from the source to the phone. It isn't possible to send data from the phone to a source

using HSDPA. The channel is shared between all users which lets the radio signals to be used

most effectively for the fastest downloads.

4.9.1 HSDPA TESTING

It shows the Speed of HSDPA

We consider Same window of HSUPA Analysis for the Speed Testing of HSUPA


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4.9.2 HSPA (Plus)

This is an evolution of the HSPA (HSDPA & HSUPA) standard and allows for faster speeds. The

maximum download speed allowed by the standard is 168 Mbit/s although in reality networks

that support HSPA (plus) will offer 21 Mbit/s download. This is because the existing 3G network

architecture operators would have deployed and made compatible was never designed to

handle such massive bandwidth.


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The idea of HSPA (plus) was to allow network carriers to move towards 4G speeds (defined as

100 Mbit/s download) without having to use new masts and radios. Networks which have been

upgraded to allow HSPA (plus) traffic are backwards compatible so phones with standard

HSDPA receivers will work on them but to take advantage of the higher speeds you must have a

device with an HSPA (plus) receiver. Many devices fitted with an LTE receiver are also capable

of HSPA (plus).

Figure 4.15 HSDPA / HSUPA Testing


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Chapter # 5

Problem Identification and Resolution

Using Drive Test

As mentioned earlier drive test plays a vital role in identifying, analyzing and Troubleshooting

issues arises specially at radio end. In this session we will analyze identify and resolve issues

that cause degradation in cellular networks. Following are the major issues faced by network in

day to day analysis of network performance

1. Coverage Problems

2. Lack of Dominant Server

3. Sudden Decrease on Signal Level

4. Cell Overshooting Problem

5. Cross Sector and Cross Feeder Problem

6. Missing Neighbor Relation

1. Coverage Problems

Low signal level is one of the biggest problems in a Network. The coverage that a network

operator can offer to customers mostly depends on efficiency of network design and

investment plans.

This problem usually pops up when building a new Network or as the number of subscribers

increases by the time resulting in new coverage demands.

Low signal level can result in unwanted situations that could directly lower the network

performance. Poor coverage problems are such problems that are really hard to solve, because

it is impossible to increase coverage by optimizing network parameters. Any hardware

configuration changes might improve the coverage a little. This is mainly effect mainly by

problems as shown in mentioned figure.


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Figure 5.1 Depicting causes of Coverage issues

2. Lack of Dominant Server

Signals of more than one cell can be reaching a spot with low level causing ping pong

handovers. This might happen because the MS is located on the cell borders and there is no any

best server to keep the call


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Figure 5.2 An Example of Lack of Dominant Server

3. Sudden Decrease on Signal Level

It may be notice sudden decrease on signal level when analyzing the log files emerges. This will

result in excessive number of handovers. Before suspecting anything else, check if the test was

performed on a highway and that particular area was a tunnel or not. Signal level on the chart

will make a curve rather than unstable changes. Tunnel effect will most likely result in ping

pong handovers. The other reason that It may happen for example that some peculiar

propagation conditions exist at one point in time that provide exceptional quality and level

although the serving BTS is far and another is closer and should be the one the mobile should

be connected to if the conditions were normal.


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It may then happen that these exceptional conditions suddenly drop and the link is lost, which

would not have happened if the mobile had been connected to the closest cell. So for these

reasons, this cause does not wait for the power control to react.

Figure 5.3 An Example of Sudden decrease in Signaling Level

4. Cell Overshooting Problem

When we get the signal from the site that not close to the current area drive test. Usually we

get bad RxQual and long/bigger TA.

We can suspect this as a overshoot case. This case happen when a site/cell is serving far away

from its area. This cause is used when a dominant cell provides a lot of scattered coverages

inside other cells, due to propagation conditions of the operational network. The consequence

of these spurious coverages is the probable production of a high level of co-channel

interference.


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It may happen for example that some peculiar propagation conditions exist at one point in time

that provide exceptional quality and level although the serving BTS is far and another is closer

and should be the one the mobile should be connected to if the conditions were normal.

It may then happen that these exceptional conditions suddenly drop and the link is lost, which

would not have happened if the mobile had been connected to the closest cell.

5. Cross Sector and Cross Feeder Problem

As the name suggests, this happens when the feeder cables of two different sectors are

completely crossed, which in turn leads to the fact that the coverage areas of the two adjacent

cells are swapped. Drive tester may observe a lot of HO failures and call drops.

A better understanding can be done while observing coverage level b/w Swap Sectors E and F

of Site NFZ0378 in below mention snap.

Figure 5.4 Drive Test results showing cross sector b/w two Cells


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As can be observed from above snap that on the coverage area of sector F of NFZ0378 its

adjacent cell NFZ0378 of Sector E is serving whereas same condition applies for Sector E where

NFZ0378F is serving instead of Sector E of NFZ0378. As mentioned earlier that because of sector

swap increase in call drop at radio end with sharp rise in Handover drops observed because the

cells are serving in opposite direction of there coverage.

6. Missing Neighbor Relation

Sometimes it is noticed that a good handover candidate in the neighbor list but handover will

not take place and call will drop. Although that neighboring cell with a very good signal level

appears to be a neighbor, It is because of missing adjacencies/Neighbors is considered as

common issue while monitoring network on day to day basis in which the problem arises of

which serving Site/Cell neighbors are not properly assigned. By assigning neighbors it means

that certain adjacencies should be defined at OMC in order to carry out successful handover.

Correct adjacency definitions are the basic requirement for mobility. Optimization of neighbour

cell lists saves BS and MS transmission powers, since MSs are connected to optimal cells. Also,

the number of dropped calls is reduced.

Figure 5.5 Drive Test results showing cross sector b/w two Cells


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As can be observed from above snap that on the coverage area of sector F of NFZ0378 its

adjacent cell NFZ0378 of Sector E is serving whereas same condition applies for Sector E where

NFZ0378F is serving instead of Sector E of NFZ0378. As mentioned earlier that because of sector

swap increase in call drop at radio end with sharp rise in Handover drops observed because the

cells are serving in opposite direction of there coverage.

5.1 Solutions for Problems Concerning Cell Coverage, Lack of Dominant Server

and Sudden Decrease on Signal Level

Possible solution ways can be listed as below:

1. New Site Proposal

2. Sector Addition

3. Repeater

4. Site Configuration Change (Antenna Type, height, azimuth, tilt changes)

5. Loss or Attenuation Check ( Feeders, Connectors, Jumpers, etc..)

The best thing to do in case of low signal strength could be recommending new site additions. A

prediction tool with correct and detailed height and clutter data supported with a reasonable

propagation model could be used to identify the best locations to put new sites. If client is not

eager to put new sites because of high costs to the budget or finds it unnecessary because of

low demand on traffic, then appropriate repeaters could be used to repeat signals and improve

the coverage. Adding repeaters always needs extra attention because they can bring extra

interference load to the network. The received level in the repeater should be above 80dBm

(or desired limits) so that it can be amplified and transmitted again. The mobile should not

receive both the original and the repeated signals at the same area, cause signal from the

repeater is always delayed and it will interfere with the original signal. A repeater should not

amplify frequencies outside the wanted band.


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If none of the above recommendations are accepted by the client, then cheaper and easier

ways should be followed. First things to be checked would be possible attenuation on the cells.

Faulty feedersjumpersconnectors or other faulty equipment, high combiner loss, reduced

EIRP, decreased output power, the orientations and types of antennas, unnecessary down tilts,

existence of diversity and height of the site should be deeply investigated. Putting higher gain

antennas, increasing output power, removing attenuations, changing antenna orientations

towards desired area, reducing down tilts, replacing faulty equipment or usage of diversity gain

could improve the coverage.

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