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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



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



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



End of Section 1 Questions 66


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




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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



End of Section 2 Questions 148


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



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


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


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


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

2

3

4

5

6

7

8

9

10

11


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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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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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Name...

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Question a b c d

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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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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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Name...

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Question a b c d e

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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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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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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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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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Telecoms Academy 75

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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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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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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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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Telecoms Academy 103

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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LTE Radio Planning

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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LTE Radio Planning

Figure 78 Layering of Different Cell Sizes


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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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LTE Radio Planning

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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Name...

Question set

Question a b c d

1

2

3

4

5


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LTE Radio Planning


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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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LTE Radio Planning

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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