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MODULE A
LTE RADIO PLANNING
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Index
Telecoms Academy iii
CONTENTS
OBJECTIVES IX
SECTION 1 INTRODUCTION TO RADIO PLANNING XI
Lesson 1 Radio Planning Life Cycle 1
High Level Network Design Cycle 1
Phase 1 Detailed Procedure 2
Phase 1 Information 3
Phase 2 Detailed Procedures 4
Phase 3 Detailed Procedures 5
Phase 4 Detailed procedures 6
Phase 3 - 4 Information 6
Factors Affecting the LTE Planning Process 7
Allocated Spectrum and Channel Bandwidth 9
LTE Channel Parameters 10
Maximum Bit Rate per Channel 11
Equipment Performance 12
Coverage or Capacity 13
Service Area 14
Self Assessment Multiple Choice 17
Self Assessment Multiple Choice Answer Grid 21
Lesson 2 RF and Baseband Signal 23
The Electromagnetic Wave 23
Baseband Information 24
Self Assessment Multiple Choice 29
Self Assessment Multiple Choice Answer Grid 31
Lesson 3 Decibels (dB) and Noise in RF Theory 33
The Decibel and Applications for RF Practice 33
Calculating Noise in RF systems 36
Cascaded Noise 38
Self Assessment Multiple Choice 41Self Assessment Multiple Choice Answer Grid 43
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Lesson 4 Modulation Schemes for LTE 45
Modulation Systems 45
Binary Phase Shift Keying (BPSK) 46
High Level Modulation Schemes, QPSK, 8PSK 47
16QAM Modulation 48
64QAM Modulation 48
The Effect of Signal to Noise Ratio in Modulation 49
Adaptive Modulation Schemes 50
Self Assessment Multiple Choice 53
Self Assessment Multiple Choice Answer Grid 55
Lesson 5 Multiple access Schemes 57
Multiple Access in Radio Systems 57
Frequency Division Multiple Access 57
Time Division Multiple Access 58
TDMA and FDMA Hybrid 59
Code Division Multiple Access 59
OFDM (Orthogonal Frequency Division Multiplexing) 60
Orthogonal Frequency Division and Multiple Access 61
Duplex Schemes 62
Self Assessment Multiple Choice 63
Self Assessment Multiple Choice Answer Grid 65
End of Section 1 Questions 66
Self Assessment Multiple Choice Answer Grid 71
Section 1 Assignment Questions 72
SECTION 2 PROPAGATION PRINCIPLE, MODELLING AND ANTENNAS 75
Lesson 1 Propagation Basics 77
Refraction of the Radio Signal 77
Sub-Refraction 80
Super-Refraction 81
Extreme Cases, Ducting 82
Self Assessment Multiple Choice 85
Self Assessment Multiple Choice Answer Grid 89
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LTE Downlink Multiple Antenna Schemes 135
Open-loop Tx Diversity 136
Receive Diversity 136
Spatial Multiplexing MIMO 136
Spatial Multiplexing MIMO 137
Closed Loop Spatial Multiplexing 137
Open loop spatial Multiplexing 138
Reporting of UE Feedback 139
Self Assessment Multiple Choice 143
Self Assessment Multiple Choice Answer Grid 147
End of Section 2 Questions 148
Self Assessment Multiple Choice Answer Grid 155
Self Assessment Multiple Choice Answer Grid Cont....... 156
Section 2 Assignment 157
SECTION 3 LTE LINK BUDGETS 159
Lesson 1 Defning a Link Budget Statement 161
Intro to Basic Radio System 161
Typical Link Budget Requirements 162
LTE link Budget variables 163
Self Assessment Multiple Choice 165
Self Assessment Multiple Choice Answer Grid 167
Lesson 2 Transmitter Power in LTE Link Budgets 169
LTE Transmit Power Capability for the UE 169
Additional Factors Affecting UE Power Output 170
Maximum Power Reduction (MPR) 170
eNodeB Power Output Characteristics 171
Typical Losses in the eNB 172
Other Losses in the transmit/receive system 172
Self Assessment Multiple Choice 173
Lesson 3 eNB and UE Antenna Performance 175
Antenna Characteristics for the UE 175
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Introduction to Radio Planning
Telecoms Academy 1
LESSON 1 RADIO PLANNING LIFE CYCLE
High Level Network Design Cycle
Network design is a complex and time consuming affair with many steps and processes.
However from a high level it could be considered that there are 4 main steps in the
planning cycle.
The process begins with information gathering and objective setting. Information gathered
at this stage will include both marketing and technical data. The marketing information is
important so that realistic objectives can be set. Technical data will include information
about the technology to be used, spectrum related data and possibly equipment
performance data from a vendor.
Phase 1
Information Gathering+
Initial ObjectiveSetting
Phase 2
Site Selection+
Backhaul Planning
Phase 3
RF Predictions+
Confirm Assumptions
Phase 4
Build Plan+
Drive TestOptimisation
Figure 1 High Level Design Life Cycle
Information gather during this rst phase is used to test the objectives and determine the
viability of the business case. Since there are no major investments at this stage it is also
a good time to analyse the risks involved using known information. The assumptions and
objectives can be tested iteratively until some initial design is decided.
The second phase used the outputs of phase one to determine the best location for the
base sites and to determine the back haul requirements. Issues of co-location and new
site builds would be addressed at this stage.
Once all the site locations have been determined the initial assumptions regarding
coverage will need to be validated. This is possible through the use of software RF
planning tools. Some design optimisations can be determined during this stage. Choice
of software tools and models will have to be made, this is often a matter of scale and
budget.
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Phase 1
Equipment Performance
- Vendor Selection
- Antenna Type/Performance
- Costs
- UE Performance
- Additional Features
- MIMO/Beamforming
Coverage Objective
- Spectrum Frequency
- Capacity
- Area Type
- Service Level
Capacity Objective
- Mbps
- Mbps/Km2
- Rural Urban
- Quantity of Spectrum
Marketing
- Pop Density
- Demographics
- Market Penetration
- Number of Subs
- Revenues
- Services Offered
- Service level
- Service Quality
- Growth
Planning Process
- RF Model
- Capacity Models
- Spreadsheets
Figure 2 Information Required for Phase 1 Planning
Phase 1 Information
Phase 1 of planning is primarily about information gathering and initial system modelling,
the more information that can be gathered at his stage will allow for more detailed and
accurate modelling. More time spent at this at this stage understanding how the system
responds to changes in design inputs should result in more solid and reliable design in the
later stages. The basic premise of phase one design is to determine the optimum number
of base stations to meet the required objectives of coverage and capacity.
Some areas for investigation and fact nding are;
Marketing data
Vendor equipment data
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Which allows the setting of;
Coverage objectives
Capacity objectives
A planning process can also be considered at this time taking into account what tools
are available to the designer, RF planning tools, spreadsheets used to determine system
operating criteria etc.
Phase 2 Detailed Procedures
The output of phase 1 is, amongst others, is the number of base stations required to meet
the objectives, however the location of the base stations is yet to be determined. Phase
2 is about site selection and conrming the assumptions from the rst stage holds true
against the real location of sites.
Many operators will have existing sites on which they may co-locate the new LTE
equipment., however one of the implications of mobile broadband is the number of new
sites that may have to be deployed (depending on the spectrum used). This will involved
detailed site planning and acquisition to be carried out.
In addition the backhaul requirements for both the co-located sites and new sites will have
to be calculated and planned.
Introduce real site location including existing and new sites
Test system performance using real location against initial objectives
Begin site acquisition process
Determine the optimal build out plan
Investigate and plan backhaul requirements
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Phase 2
Site Acquisition
- Planning processes
- Site Availability
- Owned or Leased
- Cost
Backhaul
- Required Capacity
- Interconnects available
- Future Growth
- FO vs microwave
Site Selection
- Site Availability
- Collocated
- New Site
- Impact on Coverage
Figure 3 Phase 2 Information Required
Phase 3 Detailed Procedures
Once the site locations have been established, software tools can be used to conrm the
coverage and capacity assumptions made in the rst stage. Changes can be made to
the initial design at this stage as well the selection of ideal locations for new sites. It is
important at this stage to develop a build out plan that will quickly establish the required
coverage and capacity in the least amount of time with the least amount of cost, there are
software tools available that can develop this plan.
Use software tools to conrm initial assumptions for coverage and capacity
Make changes to site planning
Optimise the build plan
Begin the build
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Phase 4 Detailed procedures
Before a major build is undertaken the accuracy of the software tools must be determined,
therefore it is not uncommon to run drive test against a test site, this can be used to
conrm the coverage predicted by the RF tools and if the site is fully functional some
estimate of cell capacity can also be determined. Any major discrepancy between the
RF prediction and the actual measurements can be used to tune the prediction models.
Tuning of the software models is important in order to reduce the amount of retro planning/
site building further in to the build process.
Drive test to conrm the software planning models used
Optimise radio plan if necessary
Phase 3 - 4 Information
Phase 3 and 4 are primarily about site selection and building, where the use of RF
software planning, capacity planning tools and optimisation tools are heavily used. The
selection of tools is based on the type of system that is being planned and the budget
given to the planning department. There are many different stand-alone tools that ca
be used in the process and an increasing number of integrated tools that will allow the
planner to manage the design process from start to nish.
Typical tools required during the third and fourth stages are:
RF Planning
Capacity Planning
Drive Test
Roll out and Optimisation Planning
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Phase
3/4
Drive Test
- Tool Type
- Features
- Integration with Planning tool
- Interpreting Results
- Optimisation
Capacity Planning
- Tool Type
- Accuracy
- Capacity Models
Optimisation
- Tool Type
- Features
- Integration with Planning tool
Planning Tools
- Tool Type/Capability
- Cost
- Terrain/Clutter Database
- Building Database
- Planning Models
Figure 4 Phase 3 4 Information Required
Factors Affecting the LTE Planning Process
Whilst LTE technology is new and complex some of the basic rules of system planning
do not change. Much of the complexity of LTE is designed to make the best use of the
available spectrum, better spectral efciency, in other words. Achieving better efciency
means that higher data rates can be achieved in systems that are spectrum limited.
Indeed LTE is design to support a single channel reuse pattern with out resorting to tricks
like spread spectrum.
When considering capacity planning, or general system planning, these are some of the
factors that should be taken in to account.
Frequency Band
Amount of Allocated Spectrum
Channel Bandwidth
Equipment Performance
Service Area
Population Density
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Population Demographic
Population Penetration
Expected level of service
Each of the factors mentioned above will have some impact on the overall system design
and the ultimate capacity in each cell and across the system as a whole.
Frequency Band
There are many frequency bands potentially available for the deployment of LTE, the
bands listed opposite have been identied through work done by the ITU and the WRCs.
The bands are part of the IMT spectrum and many are in use already with cellular
technologies like GSM, UMTS and WiMAX.
It is not expected for a UE to support all of the bands shown here, but is highly likely that
UE will support a sunset of the bands depending on the intended are of deployment,
allowing national and international roaming as cost effectively as possible.
Figure 5 FDD IMT Frequency Bands
The chosen spectrum will have a very large impact on the planning process since
the nominal radius of the LTE radio cell is dependant on the frequency of operation.
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Generally speaking the lower the frequency the larger the radio cell, the better the building
penetration, the less sensitive to atmospheric issues the system becomes. This is of great
interest to operators since the cost of deploying LTE networks is likely to be very high,
lower frequency allocations can save many millions of dollars in CAPEX, i.e. there will be
less eNBs to buy.
e.g. The US operator Verizon is deploying its LTE network in the 700MHz band (band 13)
whilst DoCoMo in Japan have won spectrum in the 1500MHz band. A band of interest for
many European operators is the 2.6GHz band.
Figure 6 TDD IMT Frequency Bands
Allocated Spectrum and Channel Bandwidth
The bands are regulated in terms of the allowed operating bandwidth. This is driven
largely by the amount of available spectrum in each of the bands. Some of the bands
do not allow the use of the narrow channels, whilst others prohibit the use of the larger
bandwidths.
The amount of allocated spectrum will impact the overall network capacity and the
individual sector capacity. As with many aspects of system planning more is better.
Planning a system with 1 or 2 channels is very challenging, even when the technology
provides some complex mechanisms to allow for reuse factors of 1, there will still be a
negative impact on capacity.
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In some cases the operator may have the exibility to choose the channel bandwidth
depending on the total amount of spectrum they have. Some analysis may have to be
done on the advantages and disadvantages of a few large bandwidth channels (e.g.
2x10MHz) versus more, lower bandwidth channels (e.g. 4x5MHz)
Figure 7 Available Capacity and Channel Bandwidths for LTE
LTE Channel Parameters
Once the individual channel bandwidths are know, it is possible to work out what the likely
capacity of the channel will be. This is less straight forward in LTE for many reasons, not
least of which is the nature of the OFDM technique employed on the radio interface.
The table opposite shows the main attributes of the various channel bandwidths. It can be
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seen that the entire channel is not occupied due to the FFT sampling of the channel, this
will yield a lower than expected capacity using the Nyquist and Shannon assumptions
Figure 8 LTE Channel Parameters
Maximum Bit Rate per Channel
Based on a simple Nyquist calculation and an assumption of the overall efciency (80%)
of the radio, the table opposite shows the maximum data rates that could be expected
from the various channel bandwidths.
Figure 9 Maximum Downlink Capacity per Radio Channel
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However the actual cell capacity in LTE may vary due to considerations of serving cell
load and adjacent cell load and also the interference coordination feature of LTE.
Figure 10 Maximum Uplink Capacity per Radio Channel
Equipment Performance
System performance will be affected by many factor related to the equipment used in the
network. The fundamental aspects of the link budget rely entirely on the performance
of the equipment. In many case the vendor spec sheet will provide the majority of the
information required to perform basic ink budgets. This may be enough during the
initial phase of planning to establish a baseline for capacity and performance. Once the
basic performance parameters have been worked out and certain levels of performance
have been determined, it is then possible to include the more complex features of the
equipment to determine the additional gains possible. For example MIMO, beamforming
antennas, vendor specic algorithms for interference management.
BS/UE Power Output BS/UE Antenna Gains
Receiver sensitivity
Link Budget Gains and Losses
MIMO Gains
Vendor Specic Requirements
Figure 11 Equipment Parameters Considered for Capacity
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Coverage or Capacity
Coverage limited design
Coverage limited systems are those whose performance is limited by the coverage
possible from a given set of performance attributes. The system design for coverage will
maximise the range from the base station at the expense of capacity. Coverage limited
systems will likely have a few widely spaced base stations.
Capacity Limited Design
A system that is limited by its capacity will deliver maximum capacity for a given set of
conditions. Capacity will be delivered at the expense of coverage. Systems designed for
capacity will have many closely spaced base stations.
Figure 12 Capacity Limited Design
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Figure 12 Coverage Limited Design
Service Area
Having established the performance capabilities of LTE and the vendor specic equipment
the job of planning must then determine the capacity or coverage objectives. The
objectives will of course vary from area to area depending on the planning criteria.
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Private
Residential
Council
Residential
Heavy Industrial
Light Industrial
Figure 13 Area to be served
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Self Assessment Multiple Choice
Radio Planning Life Cycle
Q1
which phase of the planning cycle would include site selection and backhaul planning?
a) Phase 1
b) Phase 2
c) Phase 3
d) Phase 4
Q2
which of the following processes are most likely to occur in phase 1 of the planning life
cycle?
a) network build plan
b) drive test and optimisation
c) initial objective setting
d) RF predictions
Q3
when setting coverage objectives, which if the following information is most useful?
a) Vendor selection
b) Market penetration
c) Allocated spectrum
d) Number of subscribers
Q4
completion of phase 1 planning yields what kind of information ?
a) The nal location of the base stations.
b) The approximate number of base stations required.
c) Detailed description of subscriber services.
d) The radio channel frequency plan.
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Q5
in which phase of the planning cycle would real site locations be determined ?
a) Phase 1
b) Phase 2
c) Phase 3
d) Phase 4
Q6
drive test tools and optimisation processes are most like to occur in which phase of the
planning cycle ?
a) Phase 1
b) Phase 2
c) Phase 3
d) Phase 4
Q8
which of the following may cause potential problems for LTE deployment when
considering handset complexity and roaming ?
a) No interworking with existing 3G systems
b) The radio interface is not standardised for LTE
c) LTE can be deployed in many frequency bands
d) LTE antennas will be very large
Q9
how many FFT points will be used to decode an LTE radio channel of 10MHz bandwidth?
a) 512
b) 1000
c) 1024
d) 2048
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Q10
which of the following statements are true regarding the relationship between capacity and
coverage ?
a) Cell capacity increases with coverage
b) Cell capacity is independent of coverage
c) Increased cell coverage results in smaller cells
d) Cell coverage reduces as capacity is increased
Q11
radio systems which are designed with many radio cell with close spacing can be said to
be
a) Capacity limited
b) Capacity reduced
c) Coverage limited
d) Coverage reduced
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Self Assessment Multiple Choice Answer Grid
Transfer your answers onto the grid for easy assessment and future reference
Name...
Question set
Question a b c d
1
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The radio frequency signal has that property that it will propagate away from the
transmitting element making it suitable to act as a carrier of information.
Baseband Information
Early systems of radio transmission made used of very simple information systems, simply
switching the transmitter on and off the send information, Morse code maybe the best
know example of this kind of transmission system.
However today we have much more complex signals that we wish to transmit, voice,
video, high speed broadband information, the information that represents the data that
we wish to transmit is known as the baseband information.
The diagram below shows an analogue representation of the speech band, human
speech happens to be very wide, up to 20KHz, however we choose not to transmit all
of the information since our brains are able to understand what is being said with much
less information in the signal. This is also convenient for transmission systems since
the amount of information they can typically carry is limited. In voice based transmission
systems, wired or wireless the amount of speech information that is transmitted is
normally limited to only 3.1KHz of the total amount of information.
Figure 16 A Comparison of Audio Signal Bandwidths
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The diagram below represents the speech information in the time domain, showing how
the amplitude of the information varies with time.
Figure 17 An Analogue Signal Shown in the Time Domain
This diagram shows the same information but now the amplitude is shown against the
frequency domain, it is possible to see from this kind of spectral analysis the bandwidth of
the voice signal and the nature of the individual frequency components.
Figure 18 An Analogue Signal Shown in the Frequency Domain
In todays communication systems it is more common to convert the analogue information
(shown above) into digital signals. The diagram below shows the time domain
representation of a digital signal. This signal is simply an ON/OFF wave form, real digital
systems would have much more complex waves, however it is a good starting point to
describe the way in which digital system attributes can be described.
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Figure 19 A Time Domain Representation of a Square Wave
The same information from above can be shown in the frequency domain. From the
signals shown below it is possible to see that the simple square waveform has signal
components at the fundamental frequency of the wave form and then odd harmonic
components. This is a simplied description of a much more complex theory in
communication known as the Fourier Transform.
Figure 20 A Frequency Domain Representation of a Square Wave
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In fact Fourier stated that any complex wave form can be described by the sum of a series
of sinusoidal components. The diagram below again illustrates the principle of the simple
square wave built from sinusoidal wave forms.
Figure 21 Showing the Addition of Fundamental and Harmonic Components
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Self Assessment Multiple Choice
RF and Baseband Signals
Q1
the plane of polarisation of an electromagnetic (EM) wave is determined from the angle of
which EM component ?
a) The magnetic eld
b) The static eld
c) The electric eld
d) The magnestatic eld
Q2
analogue and digital data that represents information before coding and modulation is
referred to as
a) Broadband
b) Wideband
c) In-band
d) Baseband
Q3
Fourier state that any complex wave can be represented by..
a) The sum of a series of sinusoidal signals
b) The inverse of a series of sinusoidal signals
c) The sum of all its fundamental sinusoidal components
d) The sum of a series of square waves
Q4
a spectrum analyser displays information from which of the following domains ?
a) Time and space
b) Amplitude and time
c) Amplitude and frequency
d) Frequency and time
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Q5
a square wave consists of what sinusoidal components ?
a) A fundamental component only
b) A fundamental component and odd harmonics
c) A fundamental component and even harmonics
d) Harmonic components only
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Self Assessment Multiple Choice Answer Grid
Transfer your answers onto the grid for easy assessment and future reference
Name...
Question set
Question a b c d
1
2
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LESSON 3 DECIBELS (dB) AND NOISE IN RF THEORY
The Decibel and Applications for RF Practice
In general it can be said that the Decibel (dB) is another way of representing factors or
absolute values, it turns out to be a very convenient way to represent very small or very
large numbers, and consider them on a reasonable scale.
To dene the decibel we should rst look at the way in we represent the numbers
associated with the logarithms
Figure 22 Dening the Base and Index of a Number
When considering the product of two number that are raised to the power of some index,
m and n in this case, the indexes can be added or subtracted as shown below.
Figure 23 Showing the Addition and Subtraction of Number Indexes
From the statement below it can be understood that the value 10 raised to the indexxwill
yield the value N, and that the logarithm to the base 10 of N will yield the valuex.
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This can be seen in the following numerical example.
The vaules above however are simply logarithms, the decibel refers much more
specicaly to factors and absolute values.
The example below shows the ratio of two values P1 and P2. If P1 = 10 and P2 = 5 then
the linear value would be 5 , the logartihm i.e. log10
(P1/P2) would be 0.7.
However the answer in dB requires a mutilication by 10 there for the ratio of 10 and 2 is
7dB. The answer in this case is a simple factor, and can be used to describe the gain or
loss of ampliers, components, pathloss etc.
Figure 24 Finding the Total Gain of a System
In some case it is necessary to describe absolute values in dB therefore the value in
question must be referenced against some know value. For measurements of power the
reference value of 1mW is often used. The following expression can be used to convert
from linear Watts to dBm.
Figure 25 Converting Power in W to dBm
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In the example system shown below, each component has a value of performance
expressed as a gure of gain in dB, to establish the total performance of the combined
components we can simply add the gures together.
Figure 26 Gain and Loss Expressed as dB can be Added and Subtracted
It should be noted that dB values that expressed absolute level of power or ratios cannot
be added in this way, the gures must converted back in to linear values before the
addition is made.
The table below shows some commonly used dB values and their linear conversions.
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Figure 27 Table of Typical Values and their Conversions
Calculating Noise in RF systems
Thermal noise is the wideband electromagnetic radiation that is emitted from all objects,
the cosmos, the stars, the earth and the conducting components that comprise a radio
system. Noise is something that is inevitable in radio systems and cannot be completely
eliminated. However its possible quantify the noise and to design system that will still
work satisfactorily despite the noise.
The expression below determines the amount of noise present in a radio channel of a
dened bandwidth. The constant k and temperature T are often taken together to be a
constant value of -174dBm/Hz, this amounts to -174dBm of noise power present in one
hertz of radio bandwidth, it follows therefore that the total amount of noise present will be
proportional to the to actual bandwidth of the channel. (This is covered in more detail in
Section 3)
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System components congured in series or cascade will contribute to the overall noise
present in any radio system. The diagram below illustrates the principle. If we could
measure the signal to noise ratio (SNR) at the input and output of a system, represented
by the box in the middle, then the total noise contribution is the difference of the SNR dB
at the input and output. This gure is often expressed as the Noise Figure (NFdB
) of the
system.
Figure 28 Calculating Noise Figure from Signal to Noise Ratios
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Cascaded Noise
Where there are multiple components in the receiver system, such as feeders, lters,
ampliers, each component will contribute noise to the total NF of the system. However
the noise gure of the total system cannot be better than the noise gure of the rst
component. Also the gain of the rst stage will impact the noise seen in the subsequent
stages of the system, thus a cascade calculation must be carried out to determine the
total noise in the system, this concept is outlined in the diagram below and is covered in
more detail in section 3.
Figure 29 Noise in Cascaded Systems
Noise in radio systems will also be affected by the ambient noise level generated from
man made sources, such as street lighting, car ignition systems, electricity distribution. It
follows that urban areas will exhibit more noise than rural areas given the greater density
of electrical systems. This noise may need to be considered as a margin when planning
mobile radio systems, however radio systems operating above 1GHz or so are less
affected by this source of noise.
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Figure 30 Noise from Man Made Sources
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Self Assessment Multiple Choice
Decibels (dB) and Noise in RF Theory
Q1
in the following expression X is referred to as the
Xn
a) Base
b) Index
c) Logarithm
d) Power
Q2
Convert the following from linear units of Watt to dBm
a) 10mW..dBm
b) 30W.dBm
c) 1WdBm
d) 121pW.dBm
e) 99nWdBm
Q3
Convert the following from dBm to linear units of power, Watts
a) 14dBm..W
b) 60dBm..W
c) -87dBm....W
d) -100dBm..W
e) 0dBm...W
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Q4
a system consists of a x1000 gain amplier and an cable which loses half the power, what
id the total gain of the system in dB?
a) 30dB
b) 500dB
c) 15dB
d) 27dB
Q5
thermal background noise in radio systems is proportional to
a) Boltzmans constant
b) Radio frequency
c) Channel bandwidth
d) Transmitter power
Q6
in a cascaded system of three components the noise contributed by the second stage to
the overall noise gure is primarily determined by
a) The gain of the third stage
b) The noise of the rst stage
c) The gain of the rst stage
d) The noise in the third stage
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LESSON 4 MODULATION SCHEMES FOR LTE
Modulation Systems
As suggested earlier in this section there are two types of signal in radio systems, the
carrier and the baseband information. The process of modifying the radio frequency
carrier to represent or carry the baseband data is known as modulation.
The diagram below shows the 3 principle methods used by digital modulation schemes.
Amplitude Shift Keying (ASK) the amplitude or power of the radio carrier is varied to
represent the baseband information, in this example a low power represents a digital 0
and a high power represents a digital 1. Such systems are simple in there concept but
rather more difcult to implement with good performance in practice, since any variation
in the radio signal during propagation will also distort the baseband information leading
errors in the receiver.
Frequency Shift Keying (FSK) systems keep the power constant and vary the transmitted
frequency to represent the baseband information. In this example a higher frequency
represents the 0 whilst a lower frequency represents the 1. This is a more practical system
and is used in mobile technologies such as GSM, it also has the advantage of being rather
power efcient since the constant envelope of the modulated signal can be amplied
easily. It could be said that FSK systems are not as spectrally efcient since they occupy
a wide radio channel compared to the amount of data that can be sent over the channel.
Phase Shift Keying (PSK) are arguable the most spectrally efcient of modulation
schemes allowing a large amount of data to be sent relative to the amount of radio
spectrum occupied. However these systems tend to be rather complex and less power
efcient than FSK systems. The baseband information is no encoded in to the angle or
phase of the transmitted radio carrier. PSK system can be absolute, in that the angle of
the carrier directly represents the baseband information, or they can be differential where
the information is encoded in to the direction and magnitude of the phase change.
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Figure 31
Binary Phase Shift Keying (BPSK)
BPSK modulation is the simplest of the PSK family, the transmitted radio signal has only
two possible angle, typically 0oand 180o. the angles can represent the 1 or the 0 of the
baseband data. The diagram below shows the phase change occurring during the change
of the baseband data from a 0 to 1 or 1 to 0.
Figure 32
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The time domain representation of the BPSK modulated signal is sometimes a little
complex to study there fore the diagram below is a vector representation of the same
signal. In fact most PSK based modulation schemes are shown using this representation.
Figure 33
High Level Modulation Schemes, QPSK, 8PSK
Using this vector based approach it is easier to show the high order modulation schemes.
Below is the QPSK (used in LTE) modulation constellation where each point or angle can
represent 2 bits of information and 8 PSK where each angle represents 3 bit of information
(EDGE uses 8PSK)
Figure 34
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16QAM Modulation
When the number of angle is more than 8 the receivers become more sensitive to noise
and interference and it becomes more efcient to use the angle domain and the amplitude
domain together, these system are known as Quadrature Amplitude Modulation (QAM)
schemes. The constellation shown below is 16QAM and each point on the constellation
now represents 4 bits of information. Such systems are highly spectrally efcient,
however there is a requirement for low noise in the radio link in order that the receiver can
correctly determine the point on the constellation. LTE also uses the 16QAM scheme.
Figure 35
64QAM Modulation
Below is the 64QAM modulation scheme, each point on the constellation now represents
6 bits of information. This is a very efcient scheme however it can only be used
successfully in the best signal areas. 64QAM is used by LTE.
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Figure 36
The Effect of Signal to Noise Ratio in Modulation
In the diagram below we can see the impact of noise and interference on the 16QAM
modulation system. Instead of the information being perfectly aligned with each target
point, the noise in the radio channel causes the information to arrive in a less than perfect
location, thus the information appears spread out over the angle and amplitude domains.
Some distortion is allowed in the channel however the more complex the scheme the less
distortion can be tolerated before the receiver begins to make errors.
There is more detail about the maximum distortion allowed in section 3 where we discuss
more the required SNR for each of the modulation schemes of LTE
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Figure 37
Adaptive Modulation Schemes
In todays advanced mobile radio systems multiple modulation and error coding schemes
are used and the link can dynamically adapt to the current radio conditions. This will
ensure that the link can trade throughput or capacity for reliability for any given UE across
the cell. What this means in practice is that many users in the radio cell will be using
different modulation and coding schemes depending on their location. The diagram below
shows the probable situation where 4 modulation and coding schemes are available.
This also means that is becomes very difcult to dimension the raio cell for capacity since
a user communicating using the QPSK modulation scheme will use 3 times more cell
resources than a user that is situated closer to the base station using 64QAM.
In cases like this a base station function known generally as the scheduler is highly
important to the efcient use of system resources.
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Figure 38
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Self Assessment Multiple Choice
Modulation Schemes for LTE
Q1
which of the following modulation schemes could be said to be more spectrally efcient
than power efcient ?
a) ASK
b) FSK
c) PSK
d) GMSK
Q2
QAM based modulation schemes use which of the following to represent the modulated
data ?
a) Time and Frequency
b) Angle and Phase
c) Amplitude and Frequency
d) Phase and Amplitude
Q3
in 16 QAM how many bit of information are represented by each symbol ?
a) 16
b) 2
c) 4
d) 6
Q4
higher order modulation schemes such as 16QAM and 64QAM generally require
a) A lower SNR
b) A higher SNR
c) Higher noise in the channel
d) Lower signal in the channel
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LESSON 5 MULTIPLE ACCESS SCHEMES
Multiple Access in Radio Systems
Given the limited resources of the radio spectrum it is important that these communication
systems off the highest possible capacity i.e. large number if users able to communicate
apparently simultaneously. These systems are known as Multiple Access systems. in
radio systems there is generally only two domains that can be shared to achieve multiple
access, the frequency and time domains, other systems such as those based on spread
spectrum techniques exploit information theory to allow user to communicate at the same
time.
.
Figure 39 The Multiple Access Concept
Frequency Division Multiple Access
FDMA (Frequency Division Multiple Access) schemes divide a spectrum allocation into
smaller frequency segments, allocating each signal a different frequency. Simple 1st
Generation systems used this method.
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Figure 40 Separate Radio Channels in FDMA Multiple Access
Time Division Multiple Access
TDMA (Time Division Multiple Access) allows signals to be transmitted on the same
frequencies, but not at the same time each signal is given its own time slot within
this frequency band. Note that GSM uses a combination of both of these schemes.
Network Operators are allocated a portion of spectrum which is divided into radio carrier
frequencies spaced 200kHz apart (FDMA). Each carrier frequency band is then divided
into eight separate timeslots (TDMA).
Figure 41 Individual User Time Slots in TDMA Multiple Access
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TDMA and FDMA Hybrid
Systems like GSM use both the time and frequency domains to create multiple sperate
radio channels each divided in the time domain into timeslots. Thus a channel allocation
will include both a frequency domain and time domain description.
Figure 42 Radio Channels and Time Slots in Hybrid TDMA/FDMA
Code Division Multiple Access
The third type of access scheme, CDMA (Code Division Multiple Access), allows all
signals to share the same frequency and time domains. In order to distinguish signals at
the receiver, unique codes are attached to each signal. A common analogy which is made
between the TDMA and CDMA schemes which are the basis of 2G cellular systems is as
follows
Figure 43 User Information Spread in the Time and Frequency Domains
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Imagine a crowded room. In a TDMA system, everyone in the room is speaking the same
language. Therefore in order to hear someone speaking on the other side of the room, it is
necessary for everyone else to stop speaking. Each person could therefore be allocated
a recurring timeslot during which they could speak, with multiple conversations supported
by allocating a different timeslot to each. In CDMA, everyone in the room is speaking a
different language. Therefore even when other people in the room are speaking at the
same time, it is still possible to pick out what the person on the other side of the room is
saying, so long as they are speaking the language that you understand.
Multi Carrier Transmission (OFDM)
Multi-carrier systems split the high speed stream of serial baseband data in to lower speed
parallel streams. The lower bit rate on each sub-carrier results in a narrower radio channel
that is resistant to the frequency selective fade.
Figure 44 Single Carrier and Multiple Carrier Comparison
OFDM (Orthogonal Frequency Division Multiplexing)
However, these multi-carrier systems need to exhibit good spectral efciency, each sub
carrier must be placed close to its adjacent carrier without causing interference. The
channel spacing is 1/Ts where Ts is the symbol time of information modulated onto the
carrier. Spacing the channels in this manner ensures that the centre of each carrier
corresponds with a zero crossing point for each of the neighbouring sub-carriers. This
means that the centre of the sub-carriers can be sampled, free from interference of the
adjacent sub-carriers.
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Figure 45 Data is Sent in Parallel Radio Channels
Orthogonal Frequency Division and Multiple Access
Whilst the concept of multi-channel systems have many performance benets in the multi-
path environment, there is still a requirement to allow multiple access, that is allow many
people at one time to access the services of the system.
LTE uses Orthogonal Frequncy Division Multiple Access (OFDMA) to organsise and
schedule data transmission to the users in the cell. Simple OFDM systems on exploit
the time domain to allow multiple access however OFDMA also allows multiple access
to extend to the frequency domain. This yeilds a system that is very exible and efceint
but at the same time fairly complicated to manage, hence the importance, again, of the
scheduler funciton within the base station.
Figure 46 Time and Frequency Sharing in OFDMA
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Duplex Schemes
LTE supports both Time Division and Frequency Division Duplex (TDD, FDD).
In FDD the uplink and downlink communications are separated from each other in the
frequency domain, the base station and mobile device will transmit and receive on
different frequencies/
Figure 47 Frequency Division Duplexing
TDD on the other hand uses the same frequency uplink and downlink so the uplink data
and downlink data is transmitted at different times.
Figure 48 Time Division Duplexing
Most LTE deployments will make use of the FDD mode, requiring paired spectrum
allocations
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Self Assessment Multiple Choice
Multiple access Schemes
Q1
which of the following multiple access schemes use the frequency domain as the primary
means of sharing the radio channel resources?
a) FDMA
b) CDMA
c) TDMA
d) OFDM
Q2
in TDMA systems the time allocated to the users for transmission and reception is
generally know as a
a) Slot
b) Burst
c) Time slot
d) Sub channel
Q3
which of the following multiple access schemes is generally thought to be more a more
efcient use of the radio spectrum?
a) TDMA
b) FDMA
c) TDMA/FDMA Hybrid
d) CDMA
Q4
which of the following modulation schemes will perform better in a multipath environment
for mobile broadband systems ?
a) TDMA
b) WCDMA
c) OFDMAd) FDMA
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Q5
in OFDMA systems the data is..
a) Modulated on to a single wideband carrier
b) Spilt in to parallel narrow radio channels
c) Spread with a wideband code before transmission
d) Transmitted in parallel on multiple wide band channels
Q6
in OFDMA the user information uses which of the following to enable a multiple access
scheme
a) Only the time domain
b) Only the frequency domain
c) Both frequency and time domains
d) The code domain
Q7
the individual radio channels that form the overall OFDMA radio channels are know as?
a) Radio channels
b) Sub-channels
c) Sub-carriers
d) Tones
Q8
which of the following statements is true regarding the LTE radio channel?
a) LTE is a TDD only system
b) LTE is an FDD only system
c) LTE supports both FDD and TDD but will be deployed using TDD
d) LTE supports both FDD and TDD but will be deployed using FDD
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End of Section 1 Questions
Q1
which phase of the planning cycle would include site selection and backhaul planning?
a) Phase 1
b) Phase 2
c) Phase 3
d) Phase 4
Q2
when setting coverage objectives, which if the following information is most useful?
a) Vendor selection
b) Market penetration
c) Allocated spectrum
d) Number of subscribers
Q3
how many FFT points will be used to decode an LTE radio channel of 10MHz bandwidth?
a) 512
b) 1000
c) 1024
d) 2048
Q4
which of the following statements are true regarding the relationship between capacity and
coverage ?
a) Cell capacity increases with coverage
b) Cell capacity is independent of coverage
c) Increased cell coverage results in smaller cells
d) Cell coverage reduces as capacity is increased
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Q5
the plane of polarisation of an electromagnetic (EM) wave is determined from the angle of
which EM component ?
a) The magnetic eld
b) The static eld
c) The electric eld
d) The magnestatic eld
Q6
analogue and digital data that represents information before coding and modulation is
referred to as
a) Broadband
b) Wideband
c) In-band
d) Baseband
Q7
Fourier states that any complex wave can be represented by..
a) The sum of a series of sinusoidal signals
b) The inverse of a series of sinusoidal signals
c) The sum of all its fundamental sinusoidal components
d) The sum of a series of square waves
Q8
Convert the following from linear units of Watt to dBm
a) 20mW..dBm
b) 25W.dBm
c) 0.11W..dBm
d) 140pW.dBm
e) 0.004nW dBm
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Q9
Convert the following from dBm to linear units of power, Watts
a) 24dBm..W
b) -60dBm..W
c) -90dBm....W
d) -103dBm..W
e) 0dBm...W
Q10
a system consists of a x1000 gain amplier and an cable which loses half the power, what
id the total gain of the system in dB?
a) 30dB
b) 500dB
c) 15dB
d) 27dB
Q11
QAM based modulation schemes use which of the following to represent the modulated
data ?
a) Time and Frequency
b) Angle and Phase
c) Amplitude and Frequency
d) Phase and Amplitude
Q12
in 64QAM how many bit of information are represented by each symbol ?
a) 16
b) 2
c) 4
d) 6
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Q13
lower order modulation schemes such as 16QAM and 64QAM generally require
a) A lower SNR
b) A higher SNR
c) Higher noise in the channel
d) Lower signal in the channel
Q14
the effect of decreasing noise in the transmission channel will..
a) Reduce the signal level
b) Decrease the BER
c) Increase the throughput
d) Increase the BER
Q15
in adaptive modulation systems users close to the cell centre are more likely to use which
of the following modulation schemes?
a) QPSK
b) 16QAM
c) 64QAM
d) 8PSK
Q16
in systems that support adaptive modulation schemes the capacity of the radio cell will be
reduced when
a) Most of the users are close to the base station
b) The radio cell has fewer users
c) Most of the users are closer to the cell edge
d) The radio cell has many users
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Q17
in TDMA systems the time allocated to the users for transmission and reception is
generally know as a
Slota)
Burstb)
Time slotc)
Sub channeld)
Q18
which of the following modulation schemes will perform better in a multipath environment
for mobile broadband systems ?
a) TDMA
b) WCDMA
c) OFDMA
d) FDMA
Q19
in OFDMA systems the data is..
a) Modulated on to a single wideband carrier
b) Spilt in to parallel narrow radio channels
c) Spread with a wideband code before transmission
d) Transmitted in parallel on multiple wide band channels
Q20
which of the following statements is true regarding the LTE radio channel?
a) LTE is a TDD only systemb) LTE is an FDD only system
c) LTE supports both FDD and TDD but will be deployed using TDD
d) LTE supports both FDD and TDD but will be deployed using FDD
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Self Assessment Multiple Choice Answer Grid
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Section 1 Assignment Questions
Q1
For your own company discover what the radio planning practices are, and comment on
the differences between your own practice and those described in lesson 1.
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Q2
Look at a typical link budget for your own system and comment on where the main
differences would be when considering an LTE link budget. Where possible include
details of the vendors you may choose for the LTE network.
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SECTION 2
PROPAGATION PRINCIPLE,
MODELLING AND ANTENNAS
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LESSON 1 PROPAGATION BASICS
Refraction of the Radio Signal
It is generally assumed that the radio wave will travel in straight lines, however this is not
the case. The radio wave will follow a curved trajectory determined by the properties of
the medium though which it travels. This means that the radio horizon is further that the
optical or geometric horizon, the diagram below illustrates this.
Figure 49 the Geometric and Radio Horizon
The radio wave can be assumed to have a vertical dimension which increases as the
wave front travels further from the transmission source, this means that the top and
bottom of the wave front will be travelling through a transmission medium which as
different properties. The air in this case is the transmission medium, and the air has a
certain refractive index which is determined by the air pressure, temperature, and water
vapour pressure. It can be generally stated that the refractive index is less as height
increases.
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Figure 50 Calculation the Refractive Index
The variation in refractive index will vary the speed at which the radio wave travels,
effectively moving faster at the top of the wave front, thus causing the entire wave front to
follow the curved path.
Figure 51 Refractive Index Reduces with Altitude
The gure below shows an alternative view where the radio wave is shown as a straight
line and the geometric line is drawn as a curved line. This is referred to as the 4/3 model,
where the relative size of the earths radius would have to be increase to 4/3s of it actual
radius to cause the radio wave to be drawn as a straight line. The 4/3 rule applies to
normal refractive and propagation conditions, however there are extreme conditions
where the 4/3 does not apply
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Figure 52 the 4/3s Earth
The 4/3 earth radius scan be calculated based on the following expression
Figure 53 Calculating Earth Radius
The refractive index is given the value N, which is a unitless value. Under normal
refractive conditions this value can be seen to change by 40 units for every 1000m gained
in altitude. It is normally shown in a graphical format as seen below.
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Figure 54 N Decreasing Under Normal Condition
There are non standard conditions that can occur where the value of N changes by more
than 40 units/1000m or less than 40 units/1000m.
Sub-Refraction
When the refractive index falls more slowly as height is increased, this is referred to as
sub-refractive condition and is illustrated in the graph below.
Figure 55 Sub-Refraction
The impact of this condition on the radio signal is that it will tend to follow a less curved
trajectory and in extreme case can lift off and fail to reach the target receiver.
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Figure 56 The Effect of Sub Conditions On the Radio Path
Super-Refraction
When the refractive index falls more rapidly than standard it is referred to as super-
refractive conditions and is illustrated below.
Figure 57 Super-Refraction
When this condition occurs the radio wave will follow a more curved trajectory causing it
to be bent more toward the earth than under standard conditions. The impact in this case
maybe reduced radio range.
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Figure 58 The Effect of Super Conditions On the Radio Path
Extreme Cases, Ducting
Where there are extreme variations in temperature, air pressure or water vapour pressure
a phenomenon known as ducting can occur.
In the case below the refractive index falls with altitude but then reverses and begins to
increase, this sharp change in refractive index will cause the radio wave to be reected
from the boundary and become trapped in the duct. The duct can exhibit a very low
propagation loss and the signal may travel for many miles before becoming very weak.
Ducts like this may be the cause of de-coupled point to point link an interference.
Areas around the Middle East and other regions where there is extreme temperature and
humidity, particularly in coastal areas, would tend to suffer from the ducting effects.
Figure 59 Surface Duct
The two diagrams below illustrate other forms of ducting that may occur, areas where cool
thermal layers sit over warm surface air (or vice versa) will cause these elevated ducts.
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Self Assessment Multiple Choice
Propagation Basics
Q1
Generally speaking the radio horizon will be ____________ than the optical horizon?
a) Greater
b) Smaller
c) The same
d) Wider
Q2
which one of the following parameters will inuence the refractive behaviour of the radio
wave ?
a) Radio frequency
b) Antenna height
c) Air pressure
d) Distance
Q3
under normal refractive conditions the radio wave can be drawn as a straight line when
the earth radius is considered to be
a) 3/4
b) 4/4
c) 4/3
d) 2/3
Q4
the refractive index N will decrease by ______ units for every 1000 metres gained in
altitude.
a) 40
b) 80
c) 20
d) 1000
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Q5
sub-refractive conditions may be observed when the refractive index _______________
with altitude?
a) Decreases more rapidly
b) Increases more rapidly
c) Remains constant
d) Decreases more slowly
Q6
sub-refractive conditions may cause the radio wave to be..
a) Bent upward away from the earth
b) Bent downwards toward the earth
c) Follow a straight line
d) Be attenuated more rapidly
Q7
super-refractive conditions may be observed when the refractive index N ____________
with altitude?
a) Decreases more rapidly
b) Increases more rapidly
c) Remains constant
d) Decreases more slowly
Q8
super-refractive conditions may cause the radio wave to be..
a) Bent upward away from the earth
b) Bent downwards toward the earth
c) Follow a straight line
d) Be attenuated more rapidly
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Q9
where two layers of extreme temperature differences are observed the effect on the radio
wave is known as
a) Propagation
b) Pathloss
c) Ducting
d) Super-refractive
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Self Assessment Multiple Choice Answer Grid
Transfer your answers onto the grid for easy assessment and future reference
Name...
Question set
Question a b c d
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LESSON 2 MECHANISMS OF PROPAGATION
There are many mechanisms by which radio energy propagates around the environment,
the actual effect of these mechanisms depend largely on the wavelength of the radio
signal
Reection
Radio energy which arrives at a surface will be reected or scattered. The amount energy
reected depends on the wavelength and the nature of the material itself and the angle of
incidence. Smooth, conducting surfaces such as metal or sea water will tend to reect the
signal. A reected signal will carry most of the energy of the incident wave, some of the
energy will be absorbed or transmitted through the surface.
Figure 62 Radio Wave Refection
Scattering
Scattering of the radio wave would tend to occur when the height of the surface features
is large relative to the wave length of the signal. The incident wave would be dispersed in
multiple directions each of the new signal components having a low energy compared to
the incidence wave.
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Figure 63 Radio Wave Scattering
Diffraction
When planning macro or micro level cells diffraction of radio energy around objects
in the radio path is one of the main mechanisms that is analysed when making signal
predictions. A radio wave that strikes an object would tend to be bent around the object
yielding a soft shadow behind the object.
Figure 64 Radio Wave Diffraction
The amount of energy diffracted is dependant on the wave length an shape of the object,
basic mathematical analysis of diffraction would model spherical and knife edge objects.
The path between transmitter and receiver may of course have multiple objects therefore
more advance analysis will calculate multiple edge diffraction in order to predict the signal
strength. Software planning tools do this as a matter of course and use both terrain and
building features in their predictions.
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Figure 65 Modelling Radio Wave Diffraction
Through this analysis it is possible to determine curves such as the one shown below for
the amount of signal energy behind the object, the shape of the curve being dened by
the wave length, the shape of the object and the percentage of obstruction of the radio
signal.
Figure 66 The Effect of Diffraction on the Radio Wave
Attenuation through Penetration
Another major mechanism of interest when making propagation predictions is the amount
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of energy that will pass though objects, this is of particular importance when predicting
in-building coverage in macro and micro cellular systems. The radio frequency, building
material, thickness and the angle of incidence will all determine the amount of energy
transmitted thought the object. These penetration loss values are often built empirically
from tests on different types of building using different frequency bands. There is no
single reference table that can be consulted during the planning stages since local
variations play a large part in the nal attenuation value.
Figure 67 Loss of Energy Through Penetration
Fresnel Clearance
In point to point or LOS systems it is expected the radio path can be designed largely free
from mid path objects however the denition of path clearance must be determined with
respect to the 1stFresnel zone.
Fresnel zones are described by path lengths that are , 1, 1 wavelengths longer
than a direct bore sight path between the transmitter and receiver antennas. When
determining clearance it is only the 1stFresnel zone that is of interest. The 1stFresnel
zone is all paths between the transmitter and receiver that are wavelength longer than
the bore sight path
The radius of this zone can be calculated using the expression shown below
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Figure 68 Fresnel Clearance for Radio Links
The 1stFresnel zone is shown in cross section below, in point to point links about 9%
of the transmitted power is delivered in this zone, therefore clearance of the zone is
important.
Figure 69 The Fresnel Zones in Cross Section
The zone however does not need to be 100% clear. It is sufcient to have 60% of the 1st
Fresnel zone clear for maximum power over the link. Engineers who plan these links will
establish a path prole and determine the height of the transmitting and receiving antenna
based on a 60% clearance.
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Figure 71 Calculating Fresnel Clearance
Rician Environment
Multipath can exist where one of the signal paths has a much higher energy than the other
paths, fading will still occur however the magnitude of the fading is much less than that
experienced in the Rayleigh case, fades of up to 10-20 dB less than the expected mean
can be seen.
Figure 72 The Rician Channel
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Self Assessment Multiple Choice
Mechanisms of Propagation
Q1
A ____________ radio wave will carry most of the energy of the incident wave ?
a) Refracted
b) Diffracted
c) Reected
d) Scattered
Q2
when a radio wave encounters a surface where the surface features are large relative to
the signal wave length the signal is more likely to be
a) Refracted
b) Diffracted
c) Reected
d) Scattered
Q3
when considering path clearance which one of the following Fresnel zones are normally
taken in to account?
a) 1st
b) 2nd
c) 3rd
d) 4th
Q4
for point to point links at least _________ of the 1st Fresnel zone must be clear from
obstruction ?
a) 100%
b) 90%
c) 60%
d) 40%
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Q5
in a Rayleigh multipath environment the radio signal would tend exhibit which of the
following properties?
a) Many radio paths each of low signal strength
b) Many radio paths each of high signal strength
c) A dominant signal path with other weaker signal paths
d) A single radio path
Q6
in a Rician multipath environment the radio signal would tend exhibit which of the following
properties?
a) Many radio paths each of low signal strength
b) Many radio paths each of high signal strength
c) A dominant signal path with other weaker signal paths
d) A single radio path
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LESSON 3 INTERFERENCE AND FREQUENCY REUSE
Frequency Reuse Concepts
Radio systems that use large radio cells (traditional PMR) may not use very many base
stations but they are unable to offer very high capacity (number of simultaneous call,
Mbps). Since the 1940s it has been known that using smaller radio cells and reusing the
same bock of frequencies over and over again will yield much higher network capacities.
However the regulatory regime and the technology were unavailable at that time to allow
such systems to be built.
The diagram below illustrates the main concept of frequency reuse, where cell A though
G will use the same radio channel or set of radio channels. The trick in these types of
systems is to manage the amount of co-channel interference across the system. The
more capacity require the greater the number of time the same radio channel will be used
over the same area, unfortunately this also means that the level of interference will also
be higher. It is a ne balance in designing high capacity networks.
Figure 73 Typical View of Frequency Reuse
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The parameter that affects the amount of interference is the distance between the cell
centres of the reuse cells, this is illustrated in the diagram below. Whilst the reuse
distance is of some importance, the ratio of cell radius to reuse distance has more of an
impact on the amount of interference.
Figure 74 Calculating the Re-Use Distance
The expression above for the reuse distance can be transposed to ;
D/R = 3N
Where N is the number of cells in the reuse pattern. A value of N = 7 will yield a
particular capacity and interference value, where N=4 the capacity will be higher and the
interference will also be higher.
The diagram below describes the interference concept. At the cell edge the mobile device
will receive a wanted signal C but will also receive unwanted power from the interferer
I. The amount interference is expressed as a ratio of these two values, C/I. C/I is also
a factor when calculating the total SNR experience by the device and will determine the
capacity available to the user in that location. This is particularly important in systems like
LTE since the selection of modulation and coding scheme is driven largely by the SNR.
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Figure 75 The Co-Channel to Interference Ratio
Frequency Reuse in LTE
For LTE networks the challenge of frequency reuse is very high since it is very unlikely
that operators will have more than 3-6 channels. Verizon in the USA, for example has
deployed the rst phase of its LTE system using only a single 10MHz radio channel. This
means that every radio cell will be using the same radio channel, potentially leading to
very high co-channel interference.
LTE uses a mechanism called interference coordination where each base station is
network to its neighbour cells and will negotiate the use of time and frequency resources.
In some cases where there will be very high use of the radio channel a base station can
announce what amounts to an interference warning to all the adjacent sites, thus allowing
them to avoid resource collisions and therefore high interference. This coordination
mechanism is crucial to the successful operation of LTE networks.
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Cell Size and Capacity
Practically, a network will not have cells of only one size, the cell sizes will depend
on factors such as the type of area to be covered and the capacity required in those
locations. The diagram below show the progressive splitting of cells to meet the local
capacity requirements of an urban area.
Figure 76 Cell Splitting of Capacity Increase
Cell Deployment in LTE
LTE is designed to work using radio cells from just a few meters wide to 100Km.
Depending on the frequency band used it seems that the initial deployments will be micro-
cells and smaller, certainly in Europe and the Middle East where the most likely frequency
band to become available will be the 2.6GHz band. The gure below shows the basic
concept and names given to radio cells of different sizes.
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Figure 77 Typical Cell Sizes for Cellular Systems
Systems that support mobility often have multiple layers of cells to increase network
reliability and capacity. It is possible in these systems to services mobiles with different
levels of mobility i.e. speed, with the different layers of radio cell. Smaller radio cells
can be overlaid on the larger macro cells and will have antenna heights of lower altitude.
Of course these days it is very common to have base stations inside public buildings to
increase the reliability of the network.
Large buildings such as shopping centres and airports may use distributed antenna
systems and remote radio heads to provide coverage in a cost effective manner.
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Figure 78 Layering of Different Cell Sizes
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Self Assessment Multiple Choice
Interference and Frequency Reuse
Q1
a radio systems that employs a high radio frequency reuse factor will tend to exhibit which
of the following characteristics.
a) Low capacity and low interference
b) Low capacity and high interference
c) High capacity and high interference
d) High capacity and low interference
Q2
in a simplied frequency reuse system the amount of interference is governed primarily
by..
a) The reuse distance
b) The cell radius
c) the number of cells in the reuse pattern
d) the frequency of the radio channels
Q3
if the frequency reuse factor N is reduced from 7 to 4 the capacity of the systems will.
a) Stay the same
b) Be increased
c) Be reduced
d) Be reduced but interference will lower
Q4
as well as increasing the frequency reuse factor, operators may also increase network
capacity in specic areas by..
a) Cell splitting
b) Introducing more large radio cells
c) Increasing the transmitted power of base stationsd) Using a lower reuse factor
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Q5
radio frequencies in the 2.6GHz range are more suitable for which type of radio cell
deployment?
a) Macro cell
b) Micro cell
c) Nano cell
d) Overlay cells
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Self Assessment Multiple Choice Answer Grid
Transfer your answers onto the grid for easy assessment and future reference
Name...
Question set
Question a b c d
1
2
3
4
5
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LESSON 4 ANTENNA BASIC THEORY
All antenna theory stems from the basic concept of the isotropic radiator. The isotropic
radiator is a theoretical point source of energy that radiates equally in all directions. The
concept is show in the diagram below. From this concept the gain of real antennas can be
dened as well as the basics of radio propagation and pathloss.
Figure 79 The Isotropic Radiator
The Dipole Antenna
The simplest antenna that can be practically constructed is the wave dipole. A have
wave dipole is a self resonating antenna which is normally fed from the centre. The
vertical dimension of the antenna is determined from the wavelength of the radio signal
that is being transmitted, maximum power transfer is achieved if the antenna is the
wavelength and the feeder in impedance match to the antenna.
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Figure 80 A Simple Self Resonations Wave Diploe
The radiation pattern of the half wave dipole is shown below. Since there is no radiation
from the ends of the dipole the radiation is more concentrated perpendicular to the
antenna orientation.
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Figure 81 The Radiation Patterns for a Dipole Antenna
When compared to the radiation from the isotopic antenna the dipole effectively focus the
energy in a more specic direction. In the diagram below the edges of the radiation elds
are effectively equal power contours, therefore the dipole appears to push the energy eld
further from the point of radiation. This can be described as the gain of the antenna.
Figure 82 Dipole Radiation Compared to the Isotropic
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