01 Introduction to Communications

8/10/2019 01 Introduction to Communications

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The radio spectrum


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1. Introduction

Radio waves are the principal means by which

wireless communications can be achieved

what they are, how they behave, what they

are used for and what their limitations are

Need to keep information up to date since the

technologies that are the focus of this course

are all changing rapidly


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2. Radio waves

Most forms of wireless communication involve radiowaves and the words wireless and radio are oftensynonymous.

Radio waves are a particular form of electromagneticradiation; other forms include light, X-rays and gammarays.

The term electromagnetic is used because all thesewaves involve the physics of electricity and magnetism.


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2.1 Frequency and wavelength

The time between successive oscillations is called theperiod of the wave, and is measured in seconds.

The number of complete oscillations (or cycles) whichtake place in a second called the frequency, and ismeasured in hertz (1 Hz = 1 cycle per second).

There is a simple relationship between the frequencyfand period T:

f=1/T.


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Wavelength and speed of light

relation:

Wavelength is the distance between cycles, and isusually measured in meters or fractions of ameter

Where c is speed of light (3x108m/s).


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Wavelength and speed of light

relation cont.


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Activity 2 (self-assessment/revision)

Calculate the period of a radio wave whose

frequency is 2 GHz.

Calculate the frequency of a radio wave whose

period is 4 ms.

Solution:


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Assignments

1. The mains electricity supply is sinusoidal,

with a frequency of 50 Hz. What is its period?

2. The period of the mains supply in the USA is

approximately 0.0167 s. Is the frequency of

the mains in the USA therefore higher or

lower than in the KSA?


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The electromagnetic spectrum:

Spectrumrepresents a spread (or range) of frequencies.

Complete spectrum of electromagnetic waves is shownin Figure 3.

Radio waves are electromagnetic waves withwavelengths above 1 mm.

In this range, Waves are suitable for communications.

At shorter wavelengths are infrared radiation, visible

light, ultraviolet, X-rays and gamma rays


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The electromagnetic spectrum Cont.


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3. The radio spectrum in Figure 5

All electromagnetic waves at frequencies less than 300 GHz.

It is a part (from VLF to EHF) of the bigger electromagneticspectrum.

It only deals with the RF(radio frequency) portion of thespectrum.

Used by radio transmitters etc.

Each decade in Figure 5 is a defined frequency band, often referredto by name, from very low frequency (VLF) at the bottom up toextremely high frequency (EHF) at the top.

The region above 1 GHz,which includes the EHF, SHF and part ofthe UHF bands, is also commonly called the microwavespectrum.

Terminology is not completely consistent, however, and someauthors regard microwaves as beginning at 3 GHz.

These wavelengths range from 100 km for 3 kHz, down to 1 mm for300 GHz.


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Usage of bands:

Each frequency band in Figure 5 includes some typicalwireless application areas, from military and navigationapplications at the lowest frequencies up to satellitecommunications and radar at the top but these are justa few examples of use.

In the UK the spectrum is coordinated by the NationalFrequency Planning Group, who issues a FrequencyAllocation Table (FAT) from time to time.


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Interference Concept:

If two radio stations are transmitting on the

same frequency oron two frequencies that

are very closethen their signals get mixed up-

- there is interference between them whichcan be heard as whistles, distortion or mixed-

up sounds.

Hence Spectrum management is required.


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Log Scale:

Commonly used radio frequencies range from 3 kHz up to 300 GHz,which is a largefactor of higher.

Because of this large range it is useful to represent frequencies

graphically on a logarithmic (or log) scale.

The main characteristicof a log scale is that equal distances along itcorrespond to a multiplication by a constant factor.

The intervals which represent a factor-of-10 change, such asbetween 3 and 30, and between 30 and 300, are called decades.


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4 Bandwidth and channels


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4.1 Bandwidth

Bandwidth (BW):

Spread over a range of radio frequencies is called the bandwidth.

In Figure 6(a) the box drawn between f1 and f2 on the frequencyaxis represents BW.

Generally speaking, the more data that is conveyed by a signal, thelarger is its bandwidth.

Video signals generally require a much greater bandwidth thanspeech signals.


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4.2. Channels

Interference is avoided by ensuring that the bandwidth ofa signal does not overlap with an adjacent one.

Therefore each frequency band can only cope with acertain number of signals.

To provide an adequate bandwidth for each signal and to

avoid overlap, many application bands are structured intospecified channels, as shown in Figure 7.


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Channels Cont.

A specific example is broadcast TV, which in

the UK takes place in the frequency range of

470--854 MHz, part of the UHF band. This

frequency range is shared out equally into 48channels. These are numbered from channel

21 at the lowest frequency up to channel 68 at

the highest.


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Activity 4 (self-assessment/revision)

If each UHF TV channel occupies the same bandwidth inthe 470854 MHz frequency range, what is the bandwidthavailable to each channel? Which frequencies are occupiedby channel 21? Which frequencies are occupied by channel68?

Solution:


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5. Some common radio applications


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Some common radio applications

The early days of radio broadcasting and

communications were centered on the LF and

MF bands.

Most of the current applications relevant to

this course are in the VHF, UHF and SHF

bands.

This trend to higher frequencies is expected to

continue.


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Licensed and unlicensed bands:

There is also a distinction between licensed and unlicensed bands.

Unlicensed bandsare a bit of a free-for-all, in which finding an

unused channel cannot be guaranteed.

However, there are regulations which specify maximum transmitted

power, so that any interference effects are local.

This is appropriate for short range equipment such as Wi-Fi

networks, cordless headphones and DECT digital phones.

The use of licensed bandsinvolves paying a fee for the legal right to

use certain Frequencies and these are controlled much more closely.

Mobile phone systems and broadcasting fall within this category


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6. The propagation of radio signals


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The propagation of radio signals

We shall briefly describe some of the main

factors which contribute to the characteristics

of radio propagation and some of the

differences between the various frequencybands


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6.1 The inverse square law

As you move further away from a transmitter, the power you receive becomessmaller.

We will discuss factors which cause the radio signal to decrease.

How the received power varies with distancein what is perhaps the simplest

situation -- in free spacewhen there is no other matter nearby to affect

propagation between the transmitter and receiver.

Also assume that the transmitting and receiving antennas transmit or receive

equally in all directions. These are called isotropicantennas.

In free space, a radio signal spreads out in three dimensions, so rather than a

circle, the ripples spread out as a spherical surface. This is illustrated in Fig. 8

The inverse square law of radio propagation: the received power decreases with

1/d2. So in free spacewith an isotropictransmitting antenna, power receivedby a fixed

size of antenna varies according to the inverse square law.


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The inverse square law


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Non-free space

i.e. on the surface of the earth:

Communications on the surface of the earth arenot always as simple as above.

Although the inverse square law must operate, inaddition there is the ground to consider, theatmosphere, the weather, mountains, valleys,buildings, furniture, people, vehicles and trees.

These can all alter the propagation of radiowaves, sometimes dramatically, and the effectsare described in the following sections.


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Activity 6 (self-assessment)

If an antenna in free space receives 16MW of

power at a distance of 2 km from an isotropic

transmitter, how much will it receive at 4 km?

How much at 8 km?


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6.2. Absorption

Absorption causes attenuation.

Gases of the atmosphere or the walls can

cause wave energy to reduce even more

quickly than under the inverse square law

alone.


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Attenuation Coefficient of radio waves in

the atmosphere


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Remarks on Fig. 9

The horizontal scale is frequency, from 10 GHz to 400 GHz on a logarithmic

scale.

The vertical scale represents attenuationof a signal, and runs from zero to

50 dB/km on a linear scale.

An attenuation value per unit distanceis often called an attenuation

coefficient.

To find the total loss of a path in dB, you simply multiply this attenuation

coefficient in dB/km by the path length (in km).

Form the fig. 9 it is clearas the frequency increases towards 400 GHz, the

amount of absorption risessteadily, with a number of peaks.


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Remarks on Fig. 9

Absorption and other losses by building materials such as brick or wood

are also significant, and increase as the frequency goes into the gigahertz

range.

The overall picture of these materials is very complicated.


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Attenuation Coefficient of radio waves

in different materials


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Example

A 1.8 GHz radio wave propagates 18 kmthrough theatmosphere, and then through twobrick walls, each of100mm(0.1m)thickness. Rain leads to an atmosphericloss of 1.5dB/km, and brick attenuates at 30 dB/matthis frequency. Calculate the total power loss in dBover this path that is due to attenuation by theatmosphere and by the wall. (Ignore the inverse squarelaw.) Type only the final total power loss number.

18 km * 1.5 db / km = 27 db 2 * (100*10^-3m) * 30 db/m = 6 db

total power = 27 + 6 = 33


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Positive side of signal loss:

The positive side of signal loss through the inverse square law, absorptionand other effects.

Because radio waves decay with distance of propagation, then frequenciescan be reusedin different places without interfering with each other.

This is the principle behind cellularphone systems, which allow a farhigher number of simultaneous users than non-cellular systems.

It makes feasible all the short range devicessuch as Bluetooth headsetsand remote car locking.

And it allows reuse of broadcast frequencies on a national or regionallevel, thus using the limited spectrum much more efficiently.


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Decibels:

A decibel is a way to express a ratio of powers, such as:

Figure 10plots the decibel values for a number of power ratios.

For example, 3 dB is equivalent to a power ratio of 2.

10 dB represents a power ratio of 10. 20 dB is a ratio of 100, and so on.

Similarly, a ratio of 1/2 is -3 dB, 1/10 is -10 dB and 1/100 is -20 dB.

An amplifier which increases signal power by 1000 times is said to have again of 30 dB.

If the signal power is reduced by a factor of 1000, then you can say thatthis is a loss of 30 dB or alternatively a gain of -30 dB.

However, you may also come across sources which refer to a loss of -30dB.The bottom line is you should always know whether the signal is gettingbigger or smaller and interpret the plus or minus signs.


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Power Conversion

Power in db

Power Ratio


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6.3 Line of sight (LoS)


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Line of sight Cont.

LoS require an uninterrupted path between transmitter (Tx) and receiver(Rx).

But the line of sight is often limited because of two main factors.

Curvature of the earth:The first is the curvature of the earth, illustrated inFigure 11(a).

With a transmitter and receiver placed at certain heights above thesurface, the radio horizon means that waves can only propagate directlybetween them up to a maximum distance.

Obstacles:The second factor limiting line of sight is the presence ofobstacles in the path such as mountains, forests, buildings and vehicles.

If these attenuate radio waves then they will affect reception. It is nolonger a line of sight when, say, a building which absorbs most of theradiation at a certain frequency is in the way.

Again taking an optical analogy, the building is casting a radio shadowinwhich the signal will not be received.


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Antennas


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Basics:

Transmitting Antenna:Radio waves are producedby

an oscillating electric current in the transmitting

antenna.

ReceivingAntenna:Radio waves then go on togeneratea small electric current in the receiving

antenna.

Same antenna can be used for both transmission and

reception.

A properly designed antenna will generate a much

larger signal than will an arbitrary piece of wire.

H lf l th ( ) Di l


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Half-wavelength (or ) Dipole

Antenna: A fundamental form of antenna is called a dipole. A key property of the dipole is its length: if the length is halfthe

wavelengthof the radio signal, then the antenna will resonateatthis frequency.

It effectively becomes tunedto this resonant frequency, and the

efficiency of converting radio waves to electrical signals and viceversa is veryhigh.

The efficiency falls away at higher or lower frequencies, although itresonates as well at frequencies which are 3x, 5x, 7x... higher.(Theseare called the oddharmonicsof the fundamental resonantfrequency).

The evenharmonics are 2x, 4x, etc., which do notresonate.) If the dipole is half the wavelength, then it is known as a half-

wavelength (or ) dipole, as shown in Figure 19(a).


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Vertical rod or whip antenna:

Used for FMreception in cars and for many othercommon wirelessapplications.

It turns out that a length of (or 4) works best in thisconfiguration, as illustrated in Figure 19(b).

One side of the connectionis referred to as theground plane, which often means the surface thattheantenna is mounted on (such as a car body).

If the length is less than l/4 then, although the received

signal strength is reduced, theperformance is oftenadequate in many practical situations.


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Bandwidth of an antenna:

The bandwidth of an antenna can be made wider

or narrowerthan that of a simpledipole by using

more complicateddesigns and by connecting it to

suitable electronic circuitry. While many applications, such as a single-

frequencytransmitter, work better with a sharply

resonant antenna with a narrowbandwidth,

other applications, such as a widebandreceiver,

require a wide bandwidth antenna.


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Omnidirectional Antennas:

An important property of antennas is the way in whichtheir sensitivityvaries with direction.

A vertical rodwill radiate (or receive) equally as well inallhorizontal directions around it. This is termed an

omni-directional radiation pattern. Note that such an antenna is not isotropic(i.e. it does

not radiate equally in all three dimensions) becausethere is very little signal radiated in the up and downdirections, i.e. along the length of the rod.

But such an omnidirectional pattern is very useful, forexample, for a transmitter broadcastingto the regionaround it.


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Directional Antennas:

Example: The dish antenna used for microwave orsatellite communications.

For transmission, a dipole at the focus of the dishradiates energy, and the dish reflectslike a

curved mirror to directmost of this energy alonga narrow beam, rather like a spotlight forming anarrow beam of light.

For receptionthe dish reflectsthe incoming

radiation to a focusat the dipole. This is anexample of a highly directional antenna that isvery efficient within its narrow beam.


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Activity 11 (self-assessment)

Estimate the physical length of a l/4 rod to be

used for a 100 MHz FM radio station. How

long would it be for a 2.4 GHz Wi-Fi link?

(Hint: in each case, assume a free spacewavelength.)


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Solution


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9.1. Polarisation

EM Wave: A slightly more detailedpicture of an electromagnetic (EM) wave is

shown in Figure 20.

This represents the wave propagating from left to right across thepage, with an electricfield oscillating in a direction up and down

the page, and a magneticfield oscillating in a direction that is intothe page.

The magnetic field, electric field and direction of propagation are allperpendicularto one another, like three edges of a cube whichmeet at a corner.

The electromagnetic wave is the groupingof these electric and

magnetic fields which are oscillating in time with each other, butwhich are perpendicular to each other and to the direction ofpropagation.


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Polarization:

Although the electric and magnetic fields must be perpendicular toeach other, this pair together can be oriented at any angle aroundthe axis of propagation of the wave.

The directionalong which the electric fieldis oriented is called thepolarization of the wave.

For radio waves propagating parallel to the ground, the case whenthe electric field is also parallel to the groundis called horizontalpolarization.

If the electric field is perpendicularto the ground then it is verticalpolarization.

Although any angle of polarization is possible, usually radio waves

are deliberately polarized to be eitherhorizontal or vertical.

Documents

01 Introduction to Communications