final year project thesis.pdf
TRANSCRIPT
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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.
2.2.1 Issues Observed
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.
2.2.3 Improvement Methodologies
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:
2.3.1 Issues Observed
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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2.3.2 Analysis & Findings
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.
2.3.3 Improvement Methodologies
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.
2.4.1 Issues Observed
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.
2.4.2 Analysis & Findings
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.