Department of: Technical programs Satellite communication1 Propagation of radio waves pages (1-34)...

Department of: Technical programs

Satellite communication

1 Propagation of radio waves pages (1-34)

Sub – Sections 2 Transmission principals pages (1-22)

3 Single Sideband working pages (1-26)

4 Microwave communication system pages (1-37)

5 Satellite communication systems pages (1-43)

6 multiplexing pages (1-12)

Satellite communication 7 VSAT pages(1-6)

This document consists of 180 pages

Chapter 1: Propagation of Radio Waves

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Chapter 1 : Propagation of Radio Waves

Aim of study

This Chapter introduces the Definition of radio spectrum Classification of Polarization, fading.

Contents Pages

1-1 Radio Frequency Spectrum 2

1-2 Radiation and induction field 5

1-3 Classification of Polarization 12

1-4 What is propagation? 15

1-5 Ground Wave Signal Propagation 16


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Chapter 1

Propagation of Radio Waves

1.1 Radio Frequency Spectrum

The radio waves' part of the spectrum ranges from 104 to 10

8 Hz. This range is

divided into frequency bands called Very Low Frequency (VLF), Low

Frequency (LF), Medium Frequency (MF), High Frequency (HF), and Very

High Frequency (VHF). Radio waves are easy to generate and can travel long

distances. When radio waves are generated, they spread out from the

transmitting antenna in all directions. Because of this property, radio waves are

said to be omnidirectional.

Radio waves are used for AM and FM broadcasting, television, low frequency

and high frequency radio communication, mobile radio, and amateur radio. [1]

The properties of radio waves are frequency dependent. At low frequencies,

radio waves pass through obstacles well, but the power falls off sharply with

distance from the source. At high frequencies, radio waves travel in straight

lines and bounce off obstacles. [1,5]

In VLF, LF, and MF bands, waves follow the curvature of the Earth. To send

them, one of the wires that supply oscillations for radiation is connected to the

antenna and the other is grounded. This produces a ground wave that is a double

of the antenna wave. The limit ground waves can travel is from 50 to 200 km,

depending upon the design and power of the transmitter, the frequency, and the


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transmitting characteristics of the soil. Salt water is 5,000 times better than dry

earth for transmitting ground waves. Hence they can travel great distances at

sea. [4,5]

In HF and VHF bands, the ground waves will no longer propagate well; instead,

they are absorbed by Earth near the transmitting station. Fortunately, a portion

of the radiation known as the sky wave radiates outward and upward to the

ionosphere in the upper atmosphere. As a result of the sun's radiation, the

ionosphere contains many ionized (electrified) particles. These react to waves by

reflecting them back to the Earth. The sky wave from a very powerful

transmitter can be reflected several times between the ionosphere and the Earth.

[4,5]

At all frequencies, radio waves, being made of electric and magnetic forces, are

subject to interference from various electrical equipment. There is also another

problem: at some frequencies, waves far travel by both ground and sky, so the

receiver actually gets multiple copies of the same wave. What's worse, even in

the case where all waves receivable by receiver travel by sky, it can still get

multiple copies because of the different paths waves take to arrive (multipath

fading). [4]

This is the table of contents to a list showing how the radio frequency spectrum

is allocated to different users


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Table of Contents:

Frequency Band

10 kHz to 30 kHz Very Low Frequency (VLF)

30 kHz to 300 kHz Low Frequency (LF)

300 kHz to 3 MHz Medium Frequency (MF)

3 MHz to 30 MHz High Frequency (HF)

30 MHz to 144 MHz

144 MHz to 174 MHz

174 MHz to 328.6 MHz

Very High Frequency (VHF)

328.6 MHz to 450 MHz

450 MHz to 470 MHz

470 MHz to 806 MHz

806 MHz to 960 MHz

960 MHz to 2.3 GHz

2.3 GHz to 2.9 GHz

Ultra High Frequency (UHF)

2.9 GHz to 30 GHz Super High Frequency (SHF)

30 GHz and above Extremely High Frequency (EHF)

Other charts of the radio spectrum

Cable TV channel frequencies

Letter designations of microwave bands

Satellite to L-band conversion

Frequency coordination

Other communications resources on the net


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1. 2 Radiation and induction field

Fields are really just mathematical descriptions of forces between charges. We

have three simple conditions that create physical forces between charges. They

are electric, magnetic, and electromagnetic (radiation) forces. They are all

created by distinctly different physical actions in a system. We can't mix the

various names for effects resulting from a certain physical condition and create a

new cause! Fields describe the effects of certain causes, they don't create the

causes of those effects!!!

Electric Field

Electric field describes a force created by uneven charge distribution. Nature

wants charges to be evenly distributed, she can only take so much piling up of

charges in one spot! The force between charges, caused by nature trying to

balance or even the distribution of charges, is called an electric field.

Uneven charge distribution goes hand-in-hand with a voltage difference between

two physical points. We can obviously can have a difference in charge

distribution in insulators as well as conductors. A comb, "charged" by running

through our hair (if we have any left), can have an electric field. The force of

this field can pick up tiny bits of paper as nature tries to equalize the charge

distribution. The terminals of a battery have an electric field between them and

when we place a conductor in that field the charges try to equalize. Another

example would be an antenna, where a voltage difference (uneven charge

distribution) between two points creates an electric field.

Any difference in charge distribution, whether the charges are "moving" or

standing, causes a physical force. We name this effect an electric field, and


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describe it mathematically in volts over a certain distance (like millivolts per

meter). This field (or force) decreases rapidly with distance.

Magnetic Field

Magnetic field describes a force created by moving charges. When charges are

moving, they exert a force on all other charges around them. We call this effect

the magnetic field.

One example of a magnetic field is a conductor carrying current. Perhaps it is a

wire connected between two terminals of a battery. Another example would be a

RF-current carrying conductor in an antenna.

The movement of charges causes a magnetic field, and somewhere rooted in the

creation of that magnetic field is an uneven distribution of charges causing an

electric field! Once again, this field (or force) decreases rapidly with distance.

Electromagnetic Field

An electromagnetic field is created whenever charges are accelerated.

Acceleration occurs whenever a charge changes direction or velocity. When a

charge accelerates, all the other charges in the universe feel a force trying to

make them move. The only thing that stops that force from going on forever is

when another charge (or combination of charges) accelerate to create an

opposing force. We call the velocity at which this force or effect ripples through

the universe the speed of light.

One example of electromagnetic fields is in an AC current carrying conductor,

like a power line. The time-varying voltage causes charges to move back and

forth, and the change in velocity and direction causes the effect called

electromagnetic radiation.


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It is easy to see why our antennas have all three fields, and why we can

communicate so well over large distances with low power. While the very

strong electric and magnetic induction fields drop off rapidly with distance, the

initially much weaker electromagnetic radiation field goes on until something

cancels it. The radiation field allows us to communicate, not the electric or

magnetic induction fields!

(Electric and magnetic induction fields store and return energy to the system,

the forces from that effect are very strong, but they decay very rapidly with

distance.)

Near the Antenna

Near any antenna fields are a complex mixture or " soup" of various effects from

charges. When viewed from distances very close to the antenna, charges are

almost always moving in multiple directions and distributed over vary distances

from our viewpoint. It isn't always easy to picture or get a feel for what actually

happens, especially when the area of the antenna is very large compared to the

distance from which we observe the effects of charges. Near the antenna, pattern

and field impedance is generally nothing like we might intuitively imagine!

It is the response in this area, generally within 01/λ distance from the antenna,

that small "magnetic loop" and "electric dipole" antennas get their names.

Very close to the high-current area of a small loop antenna (but not near the

capacitor end, because that is where the electric field dominates), the magnetic

field dominates. Magnetic fields are mathematical descriptions of forces derived

from moving charges, or current flow. This effect, when large compared to the

electric field, is sometimes described by saying the "field impedance" is "low".


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Conversely, near a small dipole or monopole with high voltage and little current,

the electric field dominates. The largest force is from the very high open-end

voltages, and very uneven charge distribution. We might say such an antenna

has a " high field impedance" in the area where the electric field dominates any

forces cause by moving charges.

In all of these cases, if the antenna is electrically small, the dominant fields

apply only within approximately 01/λ distance from the antenna!

As we move out further the weaker radiation field, because it is attenuated less

with distance, starts to have a noticeable contribution to the charge forces.

Because the phase of the fields (fields are a way of describing effects) is

different at the antenna, the sum of the effects is different with distance. At some

distance the low field impedance of a small loop becomes high, and the high

field impedance of a small dipole becomes low!


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Since the distance of a wavelength in the above graph (thanks W7EL) is 100

meters, we can also considered the bottom scale as a percentage of a

wavelength. We can see at about 11 percent of a wavelength (which would be

about 50 feet on 160 meters), there is no field impedance difference between a

small "magnetic" loop and a small "electric" dipole. At distances beyond 50 feet

on 160 meters, the loop actually has a higher field impedance than a dipole.

Losses in the area around the antenna

The field impedance close to the antenna, when other than 377 ohms, has

nothing to do with high losses near the antenna. Losses are directly related to the

field density, and when we are close to any antenna the fields are very intense.

Losses are not a field ratio problem, they are field intensity related. In very small

Fig. (1-1)


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antennas, virtually ALL of the losses are related to reactance canceling and

resistive losses in the antenna and any lossy media around the antenna!

We also must be mindful of the painful truth that we can not take either electric

or magnetic fields to zero or all radiation stops. By definition, radiation is an

electromagnetic wave. We can't modify the field impedance of an antenna

without changing the voltage and current distribution of the antenna.

Nearfield

The nearfield area is an area where the ultimate pattern is not fully formed, and

where induction fields (from charge distribution and charge movement) have a

noticeable effect on the forces we measure or observe.

It is possible, with large arrays of small elements, to be out of the induction field

region but still be in the area called the "nearfield" area or zone. Let's consider

individual groups of elements as "cells", and the array a combination of small

directional cells occupying a very large physical area. Each cell has formed a

radiation field. Depending on the size and type of radiator in each cell, induction

fields that charge distribution plays a role in may be attenuated so much as to be

negligible....yet the radiation pattern of the entire array may not be totally

formed. The radiation pattern might not be fully formed even though the

induction effects are no longer observable. We are in the nearfield, but not in an

area where the energy storage fields have a noticeable effect.

This is the case with my phased Beverages and phased verticals. The individual

antennas making up the array are so distant that the effects of charge distribution

(electric induction field, sometimes called the electrostatic field) or steady

movement (considered at one infinitely brief instant of time, or magnetic

induction field) have no effect. For example, at about 1 wavelength distance the


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electric and magnetic induction fields are negligible from either my circle of

eight verticals or 780-foot Beverages, yet the pattern of the overall array

established by the phasing of multiple cells is not fully formed. The pattern

would only be fully formed several wavelengths from each array, where the

distance between cells or elements is only a small fraction of the distance we are

looking back from.

The total pattern of two 780-foot long Beverages spaced 350 feet apart is not

fully formed even at distances of several thousand feet, yet nearfield induction

effects are totally gone at much shorter distances. The field impedance is

established, yet the antenna pattern is not.

The nearfield generally refers to or includes the area where "static" or induction

fields still have a noticeable influence.

Fresnel Zone

The Fresnel (fre-nel, no "S" sound) zone is the area where pattern is still being

formed. It may or may not include induction field areas.

Physically large arrays almost always have a physically large Fresnel zone. Even

simple omni-verticals have a Fresnel zone extending out a few wavelengths. The

field impedance may or may not have already been established in the Fresnel

zone.

You may have heard about Fresnel zones during discussions of vertical antenna

loss at low wave angles, or Fresnel lenses for lighthouses or other beacon lights.


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Farfield

The farfield is the area where any changes in distance result in no noticeable

change in pattern or field impedance. Losses are lower in the farfield area

because field density is lower, not because we call it farfield.

1. 3 Classification of Polarization

Light in the form of a plane wave in space is said to be linearly polarized. Light

is a transverse electromagnetic wave, but natural light is generally unpolarized,

all planes of propagation being equally probable. If light is composed of two

plane waves of equal amplitude by differing in phase by 90°, then the light is

said to be circularly polarized. If two plane waves of differing amplitude are

related in phase by 90°, or if the relative phase is other than 90° then the light is

said to be elliptically polarized.

• Linear Polarization

A plane electromagnetic wave is said to be linearly polarized. The

transverse electric field wave is accompanied by a magnetic field wave as

illustrated.

Fig. (1-2)


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Circular Polarization

Circularly polarized light consists of two perpendicular electromagnetic plane

waves of equal amplitude and 90° difference in phase. The light illustrated is

right- circularly polarized.

Fig. (1-3)

Fig. (1-4)


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If light is composed of two plane waves of equal amplitude but differing in

phase by 90°, then the light is said to be circularly polarized. If you could

see the tip of the electric field vector, it would appear to be moving in a

circle as it approached you. If while looking at the source, the electric

vector of the light coming toward you appears to be rotating

counterclockwise, the light is said to be right-circularly polarized. If

clockwise, then left-circularly polarized light. The electric field vector

makes one complete revolution as the light advances one wavelength

toward you. Another way of saying it is that if the thumb of your right hand

were pointing in the direction of propagation of the light, the electric vector

would be rotating in the direction of your fingers.

Circularly polarized light may be produced by passing linearly polarized

light through a quarter-wave plate at an angle of 45° to the optic axis of the

plate.

• Elliptical Polarization

Elliptically polarized light consists of two perpendicular waves of unequal

amplitude which differ in phase by 90°. The illustration shows right-

elliptically polarized light.


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If the thumb of your right hand were pointing in the direction of propagation of

the light, the electric vector would be rotating in the direction of your fingers.

1. 4 What is propagation?

How radio waves travel between two points. They generally do this in four

ways:

• Directly from one point to another

• Following the curvature of the earth

• Becoming trapped in the atmosphere and traveling longer distances

• Refracting off the ionosphere back to earth.

Fig. (1-5)


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1. 5 Ground Wave Signal Propagation

The ground wave used for radio communications signal propagation on the long,

and medium wave bands for local radio communications

Ground wave propagation is particularly important on the LF and MF portion of

the radio spectrum. Ground wave radio propagation is used to provide relatively

local radio communications coverage, especially by radio broadcast stations that

require to cover a particular locality.

Ground wave radio signal propagation is ideal for relatively short

distance propagation on these frequencies during the daytime. Sky-wave

ionospheric propagation is not possible during the day because of the attenuation

of the signals on these frequencies caused by the D region in the ionosphere. In

view of this, radio communications stations need to rely on the ground-wave

propagation to achieve their coverage.

A ground wave radio signal is made up from a number of constituents. If the

antennas are in the line of sight then there will be a direct wave as well as a

reflected signal. As the names suggest the direct signal is one that travels

directly between the two antenna and is not affected by the locality. There will

also be a reflected signal as the transmission will be reflected by a number of

objects including the earth's surface and any hills, or large buildings. That may

be present.

In addition to this there is surface wave. This tends to follow the

curvature of the Earth and enables coverage to be achieved beyond the horizon.

It is the sum of all these components that is known as the ground wave.

Beyond the horizon the direct and reflected waves are blocked by the curvature

of the Earth, and the signal is purely made up from the diffracted surface wave.


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It is for this reason that surface wave is commonly called ground wave

propagation.

Surface wave

The radio signal spreads out from the transmitter along the surface of the Earth.

Instead of just traveling in a straight line the radio signals tend to follow the

curvature of the Earth. This is because currents are induced in the surface of the

earth and this action slows down the wave-front in this region, causing the

wave-front of the radio communications signal to tilt downwards towards the

Earth. With the wave-front tilted in this direction it is able to curve around the

Earth and be received well beyond the horizon.

Ground wave radio propagation

Fig. (1-6)


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Effect of frequency

As the wave front of the ground wave travels along the Earth's surface it is

attenuated. The degree of attenuation is dependent upon a variety of factors.

Frequency of the radio signal is one of the major determining factor as losses

rise with increasing frequency. As a result it makes this form of propagation

impracticable above the bottom end of the HF portion of the spectrum (3 MHz).

Typically a signal at 3.0 MHz will suffer an attenuation that may be in the

region of 20 to 60 dB more than one at 0.5 MHz dependent upon a variety of

factors in the signal path including the distance. In view of this it can be seen

why even high power HF radio broadcast stations may only be audible for a few

miles from the transmitting site via the ground wave.

Effect of the ground

The surface wave is also very dependent upon the nature of the ground over

which the signal travels. Ground conductivity, terrain roughness and the

dielectric constant all affect the signal attenuation. In addition to this the ground

penetration varies, becoming greater at lower frequencies, and this means that it

is not just the surface conductivity that is of interest. At the higher frequencies

this is not of great importance, but at lower frequencies penetration means that

ground strata down to100 meters may have an effect.

Despite all these variables, it is found that terrain with good conductivity gives

the best result. Thus soil type and the moisture content are of importance. Salty

sea water is the best, and rich agricultural, or marshy land is also good. Dry

sandy terrain and city centres are by far the worst. This means sea paths are

optimum, although even these are subject to variations due to the roughness of

the sea, resulting on path losses being slightly dependent upon the weather! It


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should also be noted that in view of the fact that signal penetration has an effect,

the water table may have an effect dependent upon the frequency in use.

Effect of polarisation

The type of antenna has a major effect. Vertical polarisation is subject to

considerably less attenuation than horizontally polarised signals. In some cases

the difference can amount to several tens of decibels.

It is for this reason that medium wave broadcast stations use vertical antennas,

even if they have to be made physically short by adding inductive loading.

Ships making use of the MF marine bands often use inverted L antennas as

these are able to radiate a significant proportion of the signal that is vertically

polarised.

At distances that are typically towards the edge of the ground wave coverage

area, some sky-wave signal may also be present, especially at night when the D

layer attenuation is reduced. This may serve to reinforce or cancel the overall

signal resulting in figures that will differ from those that may be expected.

Line-of-Sight

• Signals travel in a straight line from transmitting to receiving antenna

• Useful in VHF and UHF ranges

• Television, AM/FM broadcast

• Signals are easily reflected, causing problems with mobile operation


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Tropospheric Ducting

• Troposphere: region of the atmosphere close to the earth’s surface

• Causes bending of radio waves

• Radio waves travel further than usual

• Radio-path horizon about 15% farther away than the true horizon

• Radio waves can also be trapped in the troposphere, extra distance!

Ground-Wave

• Radio waves follow the Earth’s surface

• AM broadcasts during the day

• Works best at lower frequencies (40, 80, and 160 meters)

• Relatively short-range communications

• Amateur priv’s are higher than broadcast frequencies, thus less ground-

wave range

The Ionosphere

• Region that stops UV rays from the sun

• Contains various layers, or regions

• Keeps radio waves “in” (most of the time)

• Allows long-distance radio propagation


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Ground Waves

• Radio wave that travels along the earth’s surface (surface wave)

• Vertically polarized

• Changes in terrain have strong effect

• Attenuation directly related to surface impedances

• More conductive the more attenuated

• Better over water

Attenuation related to frequency

• Loses increase with increase in frequency

• Not very effective at frequencies above 2Mhz

• Very reliable communication link

Fig. (1-7)


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• Reception is not affected by daily or seasonal weather changes

• Used to communicate with submarines

• ELF (30 to 300 Hz) propagation is utilized

Sky Waves

Fig. (1-8)

Fig. (1-9)


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• Radio waves radiated from the transmitting antenna in a direction toward

the ionosphere

• Long distance transmissions

• Sky wave strike the ionosphere, is refracted back to ground, strike the

ground, reflected back toward the ionosphere, etc until it reaches the

receiving antenna

• Skipping is he refraction and reflection of sky waves

Atmospheric Phenomenon

Three layers

• Troposphere: earth’s surface to about 6.5 mi

• Stratosphere: extends from the troposphere upwards for about 23 mi

• Ionosphere: extends from the stratosphere upwards for about 250mi

• Beyond this layer is free space

Fig. (1-10)


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• Temperature in the stratosphere is believed to be fairly constant and is not

to temperature changes or inversions and will not cause significant

refractions

• This is called an isothermal region

• The ionic density in the ionosphere varies from very dense at the border

between the ionosphere and stratosphere to very low density as it

approaches free space

• The ions in the far reaches of the ionosphere are easily susceptible to the

sun’s radiation with the susceptibility reducing as one approaches the

Stratosphere

Ionosphere

Three layers

• D: low frequencies can be refracted but the high frequencies tend to pass

on through

• E: signals as high as 20MHz can be refracted while higher ones pass

through

• F: during the day light hours there are two layers:

F1 and F2

• F: during the night hours the ionization layer is relatively constant and the

higher frequencies can be refracted

• During the night hours, the D and E layers virtually disappear and signals

that would be refracted at lower levels now are refracted at higher levels.

• This results in greater skip distances and better reception at greater

distances than in the daytime hours.


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• The layers that form the ionosphere vary greatly in altitude, density, and

thickness with the varying degrees of solar activity.

• The upper portion of the F layer is most affected by sunspots or solar

disturbances

• There is a greater concentration of solar radiation during peak sunspot

activity.

• The greater radiation activity the more dense the F layer and the higher the

F layer becomes and the greater the skip distance

Critical Frequency

The highest frequency that will be returned to the earth when transmitted

vertically under given ionospheric conditions

Fig. (1-11)


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Critical Angle

The highest angle with respect to a vertical line at which a radio wave of a

specified frequency can be propagated and still be returned to the earth from the

ionosphere

Maximum usable frequency (MUF)

The highest frequency that is returned to the earth from the ionosphere between

two specific points on earth.

Optimum Working frequency

The frequency that provides for the most consistent communication path via sky

waves

Fig. (1-12)


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Quiet Zone or Skip Zone

The space between the point where the ground wave is completely dissipated

and the point where the first sky wave is received

Fading

Variations in signal strength that may occur at the receiver over a period of time.

Tropospheric Scattering

Signals are aimed at the troposphere rather than the ionosphere

350 Mhz to 10GHz for paths up to 400 mi

Received signal = 10-6 th of the transmitted power

Fading a problem

Skip Distance/Skip Zone

In figure 2-19, note the relationship between the sky wave skip distance, the skip

zone, and the ground wave coverage.

The Skip Distance is the distance from the transmitter to the point where the

sky wave is first returned to Earth. The size of the skip distance depends on the

frequency of the wave, the angle of incidence, and the degree of ionization

present.

Relationship between skip zone, skip distance, and ground wave.


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The Skip Zone is a zone of silence between the point where the ground wave

becomes too weak for reception and the point where the sky wave is first

returned to Earth. The size of the skip zone depends on the extent of the ground

wave coverage and the skip distance. When the ground wave coverage is great

enough or the skip distance is short enough that no zone of silence occurs, there

is no skip zone.

Occasionally, the first sky wave will return to Earth within the range of the

ground wave. If the sky wave and ground wave are nearly of equal intensity, the

sky wave alternately reinforces and cancels the ground wave, causing severe

fading. This is caused by the phase difference between the two waves, a result of

the longer path traveled by the sky wave.

Fig. (1-13)


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Propagation Paths

The path that a refracted wave follows to the receiver depends on the angle at

which the wave strikes the ionosphere. You should remember, however, that the

energy radiated by a transmitting antenna spreads out with distance. The energy

therefore strikes the ionosphere at many different angles rather than a single

angle.

After the rf energy of a given frequency enters an ionospheric region, the paths

that this energy might follow are many. It may reach the receiving antenna via

two or more paths through a single layer. It may also, reach the receiving

antenna over a path involving more than one layer, by multiple hops between the

ionosphere and Earth, or by any combination of these paths.

The figure shows how radio waves may reach a receiver via several paths

through one layer. The various angles at which rf energy strikes the layer are

represented by dark lines and designated as rays 1 through 6.

Ray paths for a fixed frequency with varying angles of incidence.

Fig. (1-14)


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When the angle is relatively low with respect to the horizon (ray 1), there is only

slight penetration of the layer and the propagation path is long . When the angle

of incidence is increased (rays 2 and 3), the rays penetrate deeper into the layer

but the range of these rays decreases . When a certain angle is reached (ray 3),

the penetration of the layer and rate of refraction are such that the ray is first

returned to Earth at a minimal distance from the transmitter. Notice, however,

that ray 3 still manages to reach the receiving site on its second refraction (called

a hop) from the ionospheric layer.

As the angle is increased still more (rays 4 and 5), the rf energy penetrates the

central area of maximum ionization of the layer. These rays are refracted rather

slowly and are eventually returned to Earth at great distances. As the angle

approaches vertical incidence (ray 6), the ray is not returned at all, but passes on

through the layer.

Absorption in the Ionosphere

Many factors affect a radio wave in its path between the transmitting and

receiving sites. The factor that has the greatest adverse effect on radio waves is

Absorption. Absorption results in the loss of energy of a radio wave and has a

pronounced effect on both the strength of received signals and the ability to

communicate over long distances.

You learned earlier in the section on ground waves that surface waves suffer

most of their absorption losses because of ground-induced voltage. Sky waves,

on the other hand, suffer most of their absorption losses because of conditions in

the ionosphere. Note that some absorption of sky waves may also occur at lower

atmospheric levels because of the presence of water and water vapor. However,

this becomes important only at frequencies above 10,000 megahertz.


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Most ionospheric absorption occurs in the lower regions of the ionosphere

where ionization density is greatest. As a radio wave passes into the ionosphere,

it loses some of its energy to the free electrons and ions. If these high-energy

free electrons and ions do not collide with gas molecules of low energy, most of

the energy lost by the radio wave is reconverted into electromagnetic energy,

and the wave continues to be propagated with little change in intensity.

However, if the high-energy free electrons and ions do collide with other

particles, much of this energy is lost, resulting in absorption of the energy from

the wave. Since absorption of energy depends on collision of the particles, the

greater the density of the ionized layer, the greater the probability of collisions;

therefore, the greater the absorption. The highly dense D and E layers provide

the greatest absorption of radio waves.

Because the amount of absorption of the sky wave depends on the density of the

ionosphere, which varies with seasonal and daily conditions, it is impossible to

express a fixed relationship between distance and signal strength for ionospheric

propagation. Under certain conditions, the absorption of energy is so great that

communicating over any distance beyond the line of sight is difficult.

Fading

The most troublesome and frustrating problem in receiving radio signals is

variations in signal strength, most commonly known as FADING. There are

several conditions that can produce fading. When a radio wave is refracted by

the ionosphere or reflected from the Earth's surface, random changes in the

polarization of the wave may occur. Vertically and horizontally mounted

receiving antennas are designed to receive vertically and horizontally polarized

waves, respectively. Therefore, changes in polarization cause changes in the


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received signal level because of the inability of the antenna to receive

polarization changes.

Fading also results from absorption of the rf energy in the ionosphere.

Absorption fading occurs for a longer period than other types of fading, since

absorption takes place slowly.

Usually, however, fading on ionospheric circuits is mainly a result of multipath

propagation.

Multipath Fading

Multipath is simply a term used to describe the multiple paths a radio wave

may follow between transmitter and receiver. Such propagation paths include

the ground wave, ionospheric refraction, radiation by the ionospheric layers,

reflection from the Earth's surface or from more than one ionospheric layer, etc.

the figure shows a few of the paths that a signal can travel between two sites in

a typical circuit. One path, XYZ, is the basic ground wave. Another path, XEA,

refracts the wave at the E layer and passes it on to the receiver at A. Still another

path, XFZFA, results from a greater angle of incidence and two refractions from

the F layer. At point Z, the received signal is a combination of the ground wave

and the sky wave. These two signals having traveled different paths arrive at

point Z at different times. Thus, the arriving waves may or may not be in phase

with each other. Radio waves that are received in phase reinforce each other and

produce a stronger signal at the receiving site. Conversely, those that are

received out of phase produce a weak or fading signal. Small alternations in the

transmission path may change the phase relationship of the two signals, causing

periodic fading. This condition occurs at point A. At this point, the double-hop F

layer signal may be in or out of phase with the signal arriving from the E layer.


33

TS02TX11ST

Multipath transmission

Multipath fading may be minimized by practices called SPACE DIVERSITY

and Frequency Diversity. In space diversity, two or more receiving antennas are

spaced some distance apart. Fading does not occur simultaneously at both

antennas; therefore, enough output is almost always available from one of the

antennas to provide a useful signal. In frequency diversity, two transmitters and

two receivers are used, each pair tuned to a different frequency, with the same

information being transmitted simultaneously over both frequencies. One of the

two receivers will almost always provide a useful signal.

Selective Fading

Fading resulting from multipath propagation is variable with frequency since

each frequency arrives at the receiving point via a different radio path. When a

wide band of frequencies is transmitted simultaneously, each frequency will

vary in the amount of fading. This variation is called Selective Fading. When

selective fading occurs, all frequencies of the transmitted signal do not retain

Fig. (1-15)


34

TS02TX11ST

their original phases and relative amplitudes. This fading causes severe

distortion of the signal and limits the total signal transmitted.

Chapter 2: Transmission Principals

TS02TX11ST 1

Chapter 2 Transmission Principals

Aim of study :

This Chapter introduces the Definition of carrier wavesTypes of modulation ,

types of transmitter & Receiver

Contents Pages

2-1 Carrier wave 2

2-2 Modulation 3


TS02TX11ST 2

Chapter 2

Transmission Principals

2.1 Carrier wave

Definition of carrier waves

In telecommunication, the term carrier (cxr) or carrier wave has the following

meanings:

1. A waveform suitable for modulation by an information-bearing signal for

the purpose of conveying information.

2. An unmodulated emission. Note: The carrier is usually a sinusoidal wave

or a uniform or predictable series of pulses.

3. Sometimes employed as a synonym for a carrier system, or a synonym for

a telecommunications provider company (operator), such as a common

carrier.

This carrier wave is usually of much higher frequency than the input signal.

Frequency modulation (FM) and amplitude modulation (AM) are commonly

used methods to modulate the carrier. In the case of single-sideband modulation

(SSB) the carrier is suppressed (and in some forms of SSB eliminated). The

carrier must be reintroduced at the receiver by a beat frequency oscillator (BFO).

The frequency for a given radio or television station is actually the carrier wave's

center frequency.


TS02TX11ST 3

sin[2 ]c cV f tπ

sin[2 ]m mV f tπ

[ ]amV t

Modern modulation systems & the carrier wave

Newer forms of radio communication, such as spread spectrum and ultra wide

band, do not transmit a conventional carrier wave, nor does COFDM, which is

used in DSL and in the European standard for HDTV.

• COFDM should be thought of as an array of symmetrical carrier waves.

The rules governing carrier wave propagation affect COFDM differently

than 8VSB.

• Some forms of spread spectrum transmission and most forms of ultra wide

band transmission are mathematically defined as being devoid of carrier

waves.

2.2 Modulation

2.2.1 Amplitude modulation

• a process of changing the amplitude of relatively high frequency carrier

signal in proportion with instantaneous value of the modulating signal

• inexpensive, low quality of modulation

• used for commercial broadcasting (audio & video), 2 way mobile radio

communication

AM Envelope

• AM double-sideband full carrier (DSBFC)

• most commonly used

• conventional AM


TS02TX11ST 4

• the carrier

• the modulating signal

• the modulated wave

Fig. (2-1)


TS02TX11ST 5

AM Frequency Spectrum and Bandwidth

AM – nonlinear device

• non linear mixing occurs

• the output envelope is complex wave, made of:

• dc voltage, carrier frequency, sum and difference frequencies

• AM signal contains – frequency components spaced fm Hz on either side

of the carrier

• modulated wave does not contain a frequency component that is equal to

the modulating signal

• the effect of modulation – to translate the modulating signal in the

frequency domain so that it is reflected symmetrically about the carrier

frequency

Fig. (2-2)


TS02TX11ST 6

AM spectrum

• fc – fm(max) to fc + fm(max)

• fc – fm(max) – lower sideband (LSB) – lower side frequency

• fc + fm(max) – upper sideband (USB) – upper side frequency

• Bandwidth (B) – the difference between the highest upper side freq. and the

lowest side freq. B = 2 fm(max)

AM DSBFC modulator

• carrier frequency, fc = 100 kHz

• maximum modulating signal frequency, fm(max) = 5 kHz

• determine:

• Frequency limits for the upper and lower sidebands

• Bandwidth

• Upper and lower side frequencies produced when the modulating signal is

3 kHz tone


TS02TX11ST 7

AM Transmitter

Low level Transmitter

• Used for low power, low-power, low capacity systems

• Wireless intercoms, remote control units, pager, short range walkie-talkie

Source of modulating signal:

Fig. (2-3)

Fig. (2-4)


TS02TX11ST 8

• Microphone, magnetic tape, CD, phonograph record

Preamplifier – sensitive, high impedance

• To raise the amplitude of the source to a usable level

• Producing minimum nonlinear distortion

• Adding as little thermal noise as possible

Modulating signal driver

• Linear amplifier

• Amplifies the information signal to an adequate level to sufficiently drive

the modulator

RF oscillator

• Any oscillator configurations

• Crystal-controlled oscillator

Buffer amplifier

• Low-gain, high input impedance linear amplifier

• Function: to isolate the oscillator from high power amplifiers

Modulator

• To combine the modulating signal with the carrier frequency

High-level Transmitter

• Modulating signal process – as same as low level


TS02TX11ST 9

• Power amplifier Because the carrier is at full power at the modulation

occurrence.

• Additional carrier power amplifier

• High level modulator

Provides the necessary circuits for modulation to occur

o Final power amplifier

o Frequency up-converter

o Translate the low frequency signal to radio frequency signals

o To be efficiently radiated from an antenna


TS02TX11ST 10

AM Demodulation

• Reverse process of Am modulation

• The receiver simply converts a received AM wave back to the original source

information

• The receiver must capable of band-limiting the total radio freq. spectrum –

tuning the receiver

• RF section – detecting, band-limiting and amplifying the received signal

• The mixer/converter – concerts the the received RF freq. to intermediate freq

(IF)

• IF section – amplify the intermediate freq.

• AM detector – demodulates the AM wave

• Audio section – amplify the recovered information

Fig. (2-5)


TS02TX11ST 11

2.2.2 Superheterodyne Receivers

• Superheterodyne receivers convert all incoming signals to a lower

frequency, known as the intermediate frequency (IF), at which a single set

of amplifiers is used to provide a fixed level of sensitivity and selectivity.

• Gain and selectivity are obtained in the IF amplifiers.

• The key circuit is the mixer, which acts like a simple amplitude

• Modulator to produce sum and difference frequencies.

• The incoming signal is mixed with a local oscillator signal.

Fig. (2-6)


TS02TX11ST 12

Superheterodyne Receiver Block Diagram

RF Amplifier

• The antenna picks up the weak radio signal and feeds it to the RF amplifier

• provide some initial gain and selectivity and are sometimes called

preselectors.

• Pick up desired station by tuning filter to right frequency band

Fig. (2-7)


TS02TX11ST 13

Mixing Principles

• Mixers accept two inputs: The signal to be translated to another frequency is

applied to one input, and the sine wave from a local oscillator is applied to

the other input.

• Like an amplitude modulator, a mixer essentially performs a mathematical

multiplication of its two input signals.

• The oscillator is the carrier, and the signal to be translated is the modulating

signal.

• The output contains not only the carrier signal but also sidebands formed

when the local oscillator and input signal are mixed.

From RF

output

Fig. (2-8)


TS02TX11ST 14

IF Amplifiers

• The primary objective in the design of an IF stage is to obtain good

selectivity.

• Narrow-band selectivity is best obtained at lower frequencies.

• At low frequencies, circuits are more stable with high gain.

Demodulators

• The highly amplified IF signal is finally applied to the demodulator, which

recovers the original modulating information.

• The demodulator may be a diode detector (for AM), a quadrature detector

(for FM), or a product detector (for SSB).

• The output of the demodulator is then usually fed to an audio amplifier.

Advantage Superhetrodyne

• Overcome equipment : cannot operate at high frequency

• Component operate at fixed frequency

• Optimize utilization

• Reduce cost


TS02TX11ST 15

AM Vs FM

Radio AM Radio FM

Carrier range RF 0.535 – 1.605

MHz

88 – 108 MHz

IF 0.455 kHz 10.7 MHz

Bandwidth IF 10 kHz 200 kHz

2.2.3 Frequency Modulation (FM)

Unmodulated carrier, full power

at all times

Waveform of modulating signal

Modulated carrier with frequency

deviation and constant amplitude


TS02TX11ST 16

FM Transmitter

• In an FM transmitter, Block 1 represents a Reactance Modulator.

• The Reactance Modulator changes the phase or frequency of the oscillator in

response to the audio input.

If the Audio Amplifier in this FM transmitter failed the output would be an

unmodulated carrier.

CLIPPER

FILTER

AUDIO

AMP

MULTIPLIER POWER

AMP MULTIPLIER

MULTIPLIER

OSCILLATOR

BLOCK 1

MIC

Fig. (2-9)


TS02TX11ST 17

Modulating

signal

source

FM wave

( )cos 2c c

V f tπ

Frequency

modulatorFM wave

( )cos 2c c

V f tπ

Direct

Indirect

Crosby

Phase Locked Loop

Armstrong

Types of FM Transmitter

Frequency Modulation

• FM transmitters operate at full power at all times, even with no audio

input.

• When an FM transmitter over-modulates, the transmitted signal becomes

so wide (bandwidth) it may cause out-of-channel emissions and interfere

with adjacent channels. This is called over-deviation.

• If you are told you are over deviating you can talk farther away from the

microphone.

• FM is effective for local VHF/UHF communications because the audio is

less affected by static-type electrical noise.

Fig. (2-10)


TS02TX11ST 18

Simple Direct FM Modulator

• The carrier is generated by LC or crystal oscillator circuits.

• In LC oscillators, the carrier frequency can be changed by varying either

the inductance or capacitance.

• The idea is to find a circuit or component that converts a modulating

voltage to a corresponding change in capacitance or inductance.

• In crystal oscillators, the frequency is fixed by the crystal.

• This modulatior is impractical

Demodulation of FM signal

• Demodulation process is done in order to recover/get back the information

signal transmitted.

Fig. (2-11)


TS02TX11ST 19

• Basic concepts of demodulation circuit is to detect the frequency

variation.

Two techniques can be used:

Five most commonly used demodulator are:

Convention Circuit FM to AM (Discriminator)

• This technique is required to convert FM signal to AM signal and then by

using AM demodulation circuit is to get back the information signal.

• This technique is called (slope detection) or discriminator.

• Block diagram of the detection circuit is as shown below:

FM Demodulation

Indirect Direct

• Discriminator Phase Lock Loop(PLL)/

Quadrature detector

Slope detector

Foster-Seeley discriminator

Ratio detector

PLL demodulator

Quadrature detector

Tuned-circuit

frequency

discriminator


TS02TX11ST 20

FM Receiver

• The IF Amplifier and Limiter remove unwanted amplitude variation.

t

t

t

y(t)

Envelope

detector

dt

d

vFM(t) y(t) ( )tv

FM&

( )tvFM&

vFM(t)

Mixer

Intermediate

Frequency

Amplifier

1 Audio

Amplifier

Oscillator

Limiter

Wide

Filter

Radio

Frequency

Amplifier

Discriminator

Fig. (2-12)

Fig. (2-13)


TS02TX11ST 21

• The Discriminator recovers the modulation signal from the Frequency

Modulated IF signal.

• If a receiver has a DISCRIMINATOR it is an FM receiver.

• If the discriminator FAILS there is no audio output.

This receiver could receive signals on 147.0 MHz or 168.4 MHz.

157.7 MHz Oscillator minus 10.7 Mhz IF = 147.0 Mhz

or 157.7 MHz plus 10.7 MHz = 168.4 MHz

• This is a single-conversion superhetrodyne receiver because it has only

one IF stage.

• This is an FM receiver because it has a DISCRIMINATOR.

• FM receivers have a SQUELCH which should be set at the point that it

just silences background noise.

Mixer

10.7 MHz

IF Amplifier

De-emphasis

Audio Amp 157.7 MHz Oscillator

Limiter -

Discriminator

Broadband RF

Amplifier

Fig. (2-14)


TS02TX11ST 22

2.2.4 PM (Phase Modulation)

Direct

Indirect

Phase

modulator PM wave

( )co s 2c c

V f tπ

Modulating

signal

source

PM wave

( )c o s 2c cV f tπ

Chapter 3: Single Sideband working

TS02TX11ST 1

Chapter 3 Single Sideband working

Aim of study

This Chapter introduces the methods of Generating SSB Signals

Types of sideband communication systems., Types of sampling

& Quantization

Contents Pages

3-1 SSB Circuits 2

3-2 Sampling 9


TS02TX11ST 2

Chapter 3

Single Sideband working

3.1 SSB Circuits

Generating SSB Signals: The Filter Method

• The filter method is the simplest and most widely used method of

generating SSB signals.

• The modulating signal is applied to the audio amplifier.

• The amplifier’s output is fed to one input of a balanced modulator.

• A crystal oscillator provides the carrier signal which is also applied to

the balanced modulator.

• The output of the balanced modulator is a double-sideband (DSB) signal.

• An SSB signal is produced by passing the DSB signal through a highly

selective bandpass filter.

• With the filter method, it is necessary to select either the upper or the

lower sideband.


TS02TX11ST 3

An SSB transmitter using the filter method

Generating SSB Signals: Phasing Method

• The phasing method of SSB generation uses a phase-shift technique that

causes one of the sidebands to be canceled out.

• The phasing method uses two balanced modulators which eliminate the

carrier.

Fig. (3-1)


TS02TX11ST 4

• The carrier oscillator is applied to the upper balanced modulator along

with the modulating signal.

• The carrier and modulating signals are both shifted in phase by 90 degrees

and applied to another balanced modulator.

• Phase-shifting causes one sideband to be canceled out when the two

modulator outputs are added together.

An SSB generator using the phasing method

Fig. (3-2)


TS02TX11ST 5

DSB and SSB Demodulation

• To recover the intelligence in a DSB or SSB signal, the carrier that was

suppressed at the receiver must be reinserted.

• A product detector is a balanced modulator used in a receiver to recover

the modulating signal.

• Any balanced modulator can be used as a product detector to demodulate

SSB signals.

A balanced modulator used as a product detector to demodulate an SSB signal

Fig. (3-3)


TS02TX11ST 6

Types of Single Sideband

There are several types of sideband communication systems. Some of them

conserve bandwidth, some conserve power, and some conserve both.

1. Full carrier single sideband

Transmit at full power but only one sideband is transmitted.

• Thus require only half AM DSBFC bandwidth.

• Power carrier constitutes 4/5 of total transmission power and signal

1/5.

• In DSBFC power of the carrier is 2/3 and signal 1/3.

2. Suppressed carrier single sideband

• Carrier is completely suppressed and so signal power is equal to

transmission power.

• One sideband transmitted and so require half bandwidth of DSBFC.

3. Reduced carrier single sideband

• Carrier amplitude reduce to approximately 10% of its unmodulated

value. Consequently, as much as 96% of total power transmitted is in

the sideband.

• Only one sideband transmitted so only half the bandwidth of the

DSBFC needed.


TS02TX11ST 7

4. Independent sideband also known as twin sideband suppressed

carrier

• Carrier modulated with two independent intelligence signal and so

bandwidth is conserved.

• The Transmitter Consists of two SSB-SC Modulators (DSB signal with

two independent SSBs). finally carrier is reinserted as in SSBRC.

• Carrier is reduced and so intelligent signal as a higher transmission

power.

5. Vestigial sideband

• The carrier and one complete sideband are transmitted, but only part of

the second sideband is transmitted.

• The lower modulating-signal frequencies are transmitted double

sideband and the higher modulating-signal frequencies single sideband.

Consequently, the lower frequencies and appreciate the benefit of 100%

modulation, where as the higher frequencies cannot achieve more than

50% modulation.


TS02TX11ST 8

Comparisons of frequency Spectra & Power Distribution

Fig. (3-4)


TS02TX11ST 9

X

Digital signal

s(t)

ms(t)m(t)

m(t)

t

ms(t)

t

s(t)

t

X

Digital signal

s(t)

ms(t)m(t) X

Digital signal

s(t)

ms(t)m(t)

m(t)

t

m(t)

t

ms(t)

t

ms(t)

t

s(t)

t

s(t)

t

ISB Wave

3.2 Sampling

A process of periodically sampling the continually changing analog input

voltage and convert it to a series of constant amplitude pulses

WAVE IS SIMILAR TO A

DSBSC WAVE BUT WITH

A REPETITION RATE

TWICE THAT OF THE

MODULATING SIGNAL

FREQUENCY

Fig. (3-5)

Fig. (3-6)


TS02TX11ST 10

Block diagram for digital transmission system

Nyquist Sampling Theorem

Nyquist Sampling Theorem states that an analogue signal is completely

described by its samples, taken at equal time Intervals, the sampling frequency

fs is greater than, or equal to, twice the maximum frequency component of the

analogue signal

Nyquist theorem states that:

fs= 2 x (bandwidth of analogue signal) = 2B Hz

The choice of sampling frequency, fs must follow the sampling theorem to

overcome the problem of aliasing and loss of information.

Analog ADC Line coding Digital

transmissio

Sampling

Quantization

Coding

RZ,

NRZ,

AMI

ASK,

FSK,

PSK

Block diagram for digital transmission system

ms ff 2≥

Fig. (3-7)


TS02TX11ST 11

Shannon sampling theorem=> fs ≥≥≥≥ 2fm

Nyquist frequency

⇒ fs = 2fm= fN

(a) Sampling frequency=> fs1 < 2fm (max)

f

2fs1

3fs1

fs1 fm

Aliasing mmmmssss(f)(f)(f)(f)

(b) Sampling frequency=> fs2 > 2fm (max)

f

2fs2

3fs2

fs2 fm

mmmmssss(f)(f)(f)(f)

Fig. (3-8)


TS02TX11ST 12

Nyquist theorem

Ideal

Aliasing

2s af f=

2s af f<

ffffssss > > > > 2222ffffmmmm

ffffssss = = = = 2222ffffmmmm ffffssss < < < < 2222ffffmmmm

Therefore, the maximum Therefore, the maximum Therefore, the maximum Therefore, the maximum

frequefrequefrequefrequency that can be ncy that can be ncy that can be ncy that can be

processed by the sampled data processed by the sampled data processed by the sampled data processed by the sampled data

using sampling frequency, using sampling frequency, using sampling frequency, using sampling frequency, fs fs fs fs

(without aliasing) (without aliasing) (without aliasing) (without aliasing) is:is:is:is: ⇒ffffmmmm = f= f= f= fssss / / / /

2222 = = = = 1111 / / / / 2222TTTTssss

Fig. (3-9)

Fig. (3-10)


TS02TX11ST 13

Types of sampling

1. Natural Sampling

Tops of the sample pulses retain their natural shape, making it difficult for

ADC to convert to PCM codes.

2. Flat-top Sampling

Input voltage is sampled with narrow pulses and then held relatively

constant until next sampling.

Natural SamplingNatural SamplingNatural SamplingNatural Sampling FlatFlatFlatFlat----top Samplingtop Samplingtop Samplingtop Sampling

Information signalInformation signalInformation signalInformation signal

Pulse signalPulse signalPulse signalPulse signal

Sampled signal (PAM)Sampled signal (PAM)Sampled signal (PAM)Sampled signal (PAM)

t

m(t

t

s(t

T

τ

t

ms(t

T

τ

t

ms(t

)

T

τ( )sT<<τ

Fig. (3-11)


TS02TX11ST 14

Quantization

• Quantization level, L = 2N

Quantization level depends on the number of binary bits, N used to

represent each sample.

For example:For N= 3; Quantization level, L = 23 = 8 level.

In this example, first level (level 0) is represented by 000, whereas bit

111 represents the eigth level.

• Quantization Interval (∆V)

Represent the voltage value for each quantized level

For example: For a sampled signal that has 5V amplitude, Vpp = 10 V

divide by the quantized level, L = 8 level,

Therefore, quantized interval ,

V 25.18

V 102===∆

L

mV

p


TS02TX11ST 15

Folded binary code

• The binary codes used for PCM are n-bit codes (sign-magnitude code)

where the MSB bit is the sign bit.

• A code for a sample voltage value can be found from:

Folded PCM code = sample voltage

quantization interval

+mp

-mp

0

1 11

1 10

1 01

1 00

0 00

0 01

0 10

0 11

t

∆∆∆∆VVVV

Fig. (3-12)


TS02TX11ST 16

• If PCM is 3-bit codes, then the sign and magnitude are shown below:

Sign Magnitude Decimal value Quantization range

(V)

1 1 1 +3 +2.5 to +3.5

1 10 +2 +1.5 to +2.5

1 01 +1 +0.5 to 1.5

1 00 +0 0 to +0.5

0 00 -0 0 to -0.5

0 01 -1 -0.5 to -1.5

0 10 -2 -1.5 to -2.5

0 11 -3 -2.5 to -3.5

In terms of Voltage, the maximum signal voltages are 3 V or -3 V and the

minimum signal voltages are 1 V or -1 V.

Quantization Error

• Folded PCM code = sample voltage

resolution

• For input at 2.6 V, the PCM code is therefore:

2.6/1 = 2.6

But since there is no code for +2.6, the magnitude is rounded off to the nearest

valid code, which is 111 (+3V)

• Thus there is difference of 0.4

• Quantization Error (Qe) or also known as quantization noise (Qn)

Qe =sample voltage - original analog signal


TS02TX11ST 17

• Maximum magnitude Qe is equal to one-half a quantum

• Resolution , more accurate the quantized signal will resemble the

original analog sample

Linear input-output transfer curve

Linear

Quantization

Error

re s o lu t io n

2e

Q =

Fig. (3-13)


TS02TX11ST 18

Non uniform quantization

nonuniform: to improve SNR (SQR)

⇒ More levels is available for low level amplitudes compared to high

amplitude.

⇒ Increase SNR for low level amplitude and decrease SNR for higher

amplitudes.

analog compression is done to the input signal before sampling and

quantization at the transmitter, Expansion is done at the receiver

COMPANDING (compression and expanding)

Companding => Compress - Expanding

A method used to produce a uniform SNR for all input signal range is

compression-expansion (Companding).

Input signal is compressed at the transmitter and expanded at the receiver.

• Why use compandor?

– Inputs of smaller values suffer higher percentage of distortion under

uniform quantizer.

– Nonlinearity in perceived luminance small difference in low luminance

is more visible.

– Overall companding system may approximate Lloyd-Max quantizer.

• Analog – Compression process is done on the input signal before

sampling and coding.


TS02TX11ST 19

• Digital – compression process is done after the signal is sampled.

Popular companding system

• EUROPE => A - Law

• USA/NORTH AMERICA => µµµµ - Law

analog

compressor

ADC

DAC Analog

expander

Analog signal

(output)

PCM with analog compress-expand

To digital channel

Axfor

xA

for

A

VAVinV

A

VAVin

V

MAX

OUT 10

11

ln1

max/ln1

max/ln(1

⟨⟨

⟨⟨

+

+

+

=

AAAA ---- compressor paramater. Usually compressor paramater. Usually compressor paramater. Usually compressor paramater. Usually the value of the value of the value of the value of AAAA is is is is 87.687.687.687.6. . . .

analog signal

(input)

Fig. (3-14)

Fig. (3-15)


TS02TX11ST 20

Digital-To-Analog Conversion

Digital-to-analog conversion is the process of changing one of the

characteristics of an analog signal based on the information in digital data.

• Digital data needs to be carried on an analog signal.

• A carrier signal (frequency fc) performs the function of transporting the

digital data in an analog waveform.

• The analog carrier signal is manipulated to uniquely identify the digital

data being carried.

Digital-to-analog conversion

Fig. (3-16)


TS02TX11ST 21

Types of digital-to-analog conversion

Note

Bit rate, N, is the number of bits per second (bps). Baud rate is the number of

signal elements per second (bauds).

In the analog transmission of digital data, the signal or baud rate is less than

or equal to the bit rate.

S=Nx1/r bauds

Where r is the number of data bits per signal element.

Amplitude Shift Keying (ASK)

• ASK is implemented by changing the amplitude of a carrier signal to

reflect amplitude levels in the digital signal.

• For example: a digital “1” could not affect the signal, whereas a digital

“0” would, by making it zero.

• The line encoding will determine the values of the analog waveform to

reflect the digital data being carried.


TS02TX11ST 22

Bandwidth of ASK

Binary amplitude shift keying

Implementation of binary ASK

Fig. (3-18)

Fig. (3-19)


TS02TX11ST 23

Frequency Shift Keying

• The digital data stream changes the frequency of the carrier signal, fc.

• For example, a “1” could be represented by f1=fc +∆f, and a “0” could be

represented by f2=fc-∆f.

Binary frequency shift keying

Bandwidth of FSK

• If the difference between the two frequencies (f1 and f2) is 2∆f, then the

required BW B will be:

B = (1+d)xS +2∆f

Multi level FSK

• Similarly to ASK, FSK can use multiple bits per signal element.

• That means we need to provision for multiple frequencies, each one to

represent a group of data bits.

• The bandwidth for FSK can be higher

B = (1+d)xS + (L-1)/2∆f = LxS

Fig. (3-20)


TS02TX11ST 24

Phase Shift Keyeing

• We vary the phase shift of the carrier signal to represent digital data.

• The bandwidth requirement, B is:

B = (1+d)xS

• PSK is much more robust than ASK as it is not that vulnerable to noise,

which changes amplitude of the signal.

Binary phase shift keying

Fig. (3-21)


TS02TX11ST 25

Implementation of BASK

Quadrature PSK

• To increase the bit rate, we can code 2 or more bits onto one signal

element.

• In QPSK, we parallelize the bit stream so that every two incoming bits are

split up and PSK a carrier frequency. One carrier frequency is phase

shifted 90o from the other - in quadrature.

• The two PSK signals are then added to produce one of 4 signal elements.

L = 4 here.

Fig. (3-22)


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QPSK and its implementation

Fig. (3-23)

Chapter 4: Microwave communication system

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Chapter 4 : Microwave communication system

Aim of study:

This Chapter introduces the definition of microwave spectrum Advantages and

disadvantages of microwave, Common Fixed Microwave Link Frequency, Radio signal

path loss.

Contents Pages

4-1 Microwaves 2

4-2 The microwave spectrum 7

4-3 Advantages and disadvantages of microwave 8

4-4 Available of frequency bands 9

4-5 Microwave repeaters 10

4-6 Path loss 12

4-7 Microwave Link Design 18

4-8 Components of Microwave Systems 24


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Chapter 4

Microwave communication system

4-1 Microwaves

Microwaves are short , high-frequency radio waves. It varies from .03937 inch

to 1 foot (1 millimeter to 30 centimeters) in length . Like light waves,

microwaves may be reflected and concentrated. But they pass easily through

rain, smoke, and fog, which block light waves. They can also pass through the

ionosphere, which surrounds the earth and blocks or reflects longer radio waves.

Thus, microwaves are well suited for long-distance, satellite, and space

communications and for control of navigation.

Microwaves are generated in special electron tubes, such as the klystron and the

magnetron, with built-in resonators to control the frequency or by special

oscillators or solid-state devices.

Microwaves first came to public notice through the use of radar in World War II

(1939-1945). Today, many satellite communications systems use them. In TV,

microwave transmission sends programs from pickup cameras in the field to the

TV transmitter. These programs can then be sent via satellites to locations

around the world. Microwaves have also many applications in meteorology,

distance measuring, research into the properties of matter, and cooking food in

microwave ovens.

Microwave ovens operate by agitating the water molecules in the food, causing

them to vibrate, which produces heat. The microwaves enter through openings

in the top of the cooking cavity, where a stirrer scatters them evenly throughout


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the oven. They are unable to enter a metal container to heat food, but they can

pass through nonmetal containers.

Microwaves can be detected by an instrument consisting of a silicon-diode

rectifier connected to an amplifier, and a recording or display device.

Exposure to microwaves is dangerous mainly when high densities of microwave

radiation are involved, as with masers. They can cause burns, cataracts, damage

to the nervous system, and sterility. The possible danger of long-term exposure

to low-level microwaves is not yet well known. Nevertheless, the U.S.

government limits the exposure level, in general, to 10 milliwatts per square

centimeter. Stricter limits are placed on microwave ovens.

Fig. (4-1)


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High-frequency microwave transmissions are beamed from point to point

using tall antennas. The antennas must be within sight of each other, since the

microwave signals travel in straight, narrow paths.

In the case of point-to-point radio links, antennas are placed on a tower or other

tall structure at sufficient height to provide a direct, unobstructed line-of-sight

(LOS) path between the transmitter and receiver sites. In the case of mobile

radio systems, a single tower provides point-to-multipoint coverage, which may

include both LOS and non-LOS paths. LOS microwave is used for both short-

and long-haul telecommunications to complement wired media such as optical

transmission systems. Applications include local loop, cellular back haul,

remote and rugged areas, utility companies, and private carriers. Early

applications of LOS microwave were based on analog modulation techniques,

but today’s microwave systems used digital modulation for increased capacity

and performance.

Fig. (4-2)


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Television antennas are built on tall towers so that high-frequency signals (which

only travel in a straight line) can reach viewers without being blocked by nearby

hills or buildings. Small dishes on this tower send and receive microwave signals

from other stations or from reporters broadcasting live from nearby locations

4-1-1 Standards

In the United States, radio channel assignments are controlled by the Federal

Communications Commission (FCC) for commercial carriers and by the

National Telecommunications and Information Administration (NTIA) for

government systems.

The FCC's regulations for use of spectrum establish eligibility rules, permissible

use rules, and technical specifications. FCC regulatory specifications are

intended to protect against interference and to promote spectral efficiency.

Equipment type acceptance regulations include transmitter power limits,

frequency stability, out-of-channel emission limits, and antenna directivity.

The International Telecommunications Union Radio Committee (ITU-R) issues

recommendations on radio channel assignments for use by national frequency

allocation agencies. Although the ITU-R itself has no regulatory power, it is

important to realize that ITU-R recommendations are usually adopted on a

worldwide basis.


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4-1-2 Historical Milestones

1950s Analog Microwave Radio

• Used FDM/FM in 4, 6, and 11 GHz bands for long-haul

• Introduced into telephone networks by Bell System

1970s Digital Microwave Radio

• Replaced analog microwaves

• Became bandwidth efficient with introduction of advanced modulation

techniques (QAM and TCM)

• Adaptive equalization and diversity became necessary for high data rates

1990s and 2000s

• Digital microwave used for cellular back-haul

• Change in MMDS and ITFS spectrum to allow wireless cable and point-

to-multipoint broadcasting

• IEEE 802.16 standard or WiMax introduces new application for

microwave radio

• Wireless local and metro area networks capitalize on benefits of

microwave radio.


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4-2 The microwave spectrum

Fig. (4-3)The electromagnetic spectrum


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Common uses of the Microwave spectrum

Fig.(4-4)

4-3 Advantages and disadvantages of microwave

Advantages

• Adapts to difficult terrain

• Loss versus distance (D) = Log D (not linear)

• Flexible channelization

• Relatively short installation time

• Can be transportable

• Cost usually less than cable

• No “back-hoe” fading


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Disadvantages

• Paths could be blocked by buildings

• Spectral congestion

• Interception possible

• Possible regulatory delays

• Sites could be difficult to maintain

• Towers need periodic maintenance

• Atmospheric fading

4-4 Available of frequency bands

The International Telecommunication Union (ITU) ITU-R organization defines a

number of specific frequency bands that are allocated to fixed services - i.e. for

microwave point-to-point links. Table (4-1) shows the ITU-R bands covered the

Codan 8800 series, and outlines the usage for digital telecommunications

purposes.

Table (4-1) Common Fixed Microwave Link Frequency Bands


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Different frequency bands are subject to differing propagation criteria, which

results in attenuation in the link received signal. As a general rule, the higher the

frequency band, the shorter the usable distance of the link.

In the extreme case, use of frequency bands above 20 GHz in tropical areas will

limit path length to just a few kilometers.

Frequency management organizations will also make most effective use of

frequency spectrum by imposing a link length policy; i.e. shorter links will be

licensed only in the higher frequency bands and vice-versa.

Fig.(4-5)

4-5 Microwave repeaters

There are two types of repeaters

- Active repeaters

- Passive repeaters


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Passive repeaters are used to change the direction of a microwave path to

overcome obstructions, reduce the number of active repeaters installed and

allow more convenient locations for active

repeaters near existing roads and power lines. A

passive repeater requires no access or power

supply and virtually no maintenance.

Microflect brand passive repeaters have been

manufactured since 1956 with thousands of units operating successfully

throughout the world.

Fig. (4-6)


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4-6 Path losses

Radio signal path loss is a particularly important element in the design of any

radio communications system or wireless system. The radio signal path loss will

determine many elements of the radio communications system in particular the

transmitter power, and the antennas, especially their gain, height and general

location. The radio path loss will also affect other elements such as the required

receiver sensitivity, the form of transmission used and several other factors.

As a result, it is necessary to understand the reasons for radio path loss, and to

be able to determine the levels of the signal loss for a give radio path.

The signal path loss can often be determined mathematically and these

calculations are often undertaken when preparing coverage or system design

activities. These depend on a knowledge of the signal propagation properties.

Accordingly, path loss calculations are used in many radio and wireless survey

tools for determining signal strength at various locations. These wireless survey

tools are being increasingly used to help determine what radio signal strengths

will be, before installing the equipment. For cellular operators radio coverage

surveys are important because the investment in a macrocell base station is high.

Also, wireless survey tools provide a very valuable service for applications

such as installing wireless LAN systems in large offices and other centres

because they enable problems to be solved before installation, enabling costs to

be considerably reduced. Accordingly there is an increasing importance being

placed onto wireless survey tools and software.

Signal path loss basics

The signal path loss is essentially the reduction in power density of an

electromagnetic wave or signal as it propagates through the environment in

which it is travelling.


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There are many reasons for the radio path loss that may occur:

Free space loss: The free space loss occurs as the signal travels through space

without any other effects attenuating the signal it will still diminish as it spreads

out. This can be thought of as the radio communications signal spreading out as

an ever increasing sphere. As the signal has to cover a wider area, conservation

of energy tells us that the energy in any given area will reduce as the area

covered becomes larger.

Absorption losses: Absorption losses occur if the radio signal passes into a

medium which is not totally transparent to radio signals. This can be likened to a

light signal passing through transparent glass.

Diffraction: Diffraction losses occur when an object appears in the path. The

signal can diffract around the object, but losses occur. The loss is higher the

more rounded the object. Radio signals tend to diffract better around sharp

edges.

Multipath: In a real terrestrial environment, signals will be reflected and they

will reach the receiver via a number of different paths. These signals may add or

subtract from each other depending upon the relative phases of the signals. If the

receiver is moved the scenario will change and the overall received signal will

be found vary with position. Mobile receivers (e.g. cellular telecommunications

phones) will be subject to this effect which is known as Rayleigh fading.

Terrain: The terrain over which signals travel will have a significant effect on

the signal. Obviously hills which obstruct the path will considerably attenuate

the signal, often making reception impossible.

Additionally at low frequencies the composition of the earth will have a marked

effect. For example on the Long Wave band, it is found that signals travel best


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over more conductive terrain, e.g. sea paths or over areas that are marshy or

damp. Dry sandy terrain gives higher levels of attenuation.

Buildings and vegetation: For mobile applications, buildings and other

obstructions including vegetation have a marked effect. Not only will buildings

reflect radio signals, they will also absorb them. Cellular communications are

often significantly impaired within buildings. Trees and foliage can attenuate

radio signals, particularly when wet.

Atmosphere: The atmosphere can affect radio signal paths. At lower

frequencies, especially below 30 - 50MHz, the ionosphere has a significant

effect, reflecting (or more correctly refracting) them back to Earth. At

frequencies above 50 MHz and more the troposphere has a major effect,

refracting the signals back to earth as a result of changing refractive index. For

UHF broadcast this can extend coverage to approximately a third beyond the

horizon.

These reasons represent some of the major elements causing signal path loss for

any radio system.

Predicting path loss

One of the key reasons for understanding the various elements affecting radio

signal path loss is to be able to predict the loss for a given path, or to predict the

coverage that may be achieved for a particular base station, broadcast station,

etc.

Although prediction or assessment can be fairly accurate for the free space

scenarios, for real life terrestrial applications it is not easy as there are many

factors to take into consideration, and it is not always possible to gain accurate

assessments of the effects they will have.


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Despite this there are wireless survey tools and radio coverage prediction

software programmes that are available to predict radio path loss and estimate

coverage. A variety of methods are used to undertake this.

Free space path loss varies in strength as an inverse square law, i.e.

1/(range)^2, or 20 dB per decade increase in range. This calculation is very

simple to implement, but real life terrestrial calculations of signal path loss are

far more involved.

Most path loss predictions are made using techniques outlined below:

Statistical methods: Statistical methods of predicting signal path loss rely on

measured and averaged losses for typical types of radio links. These figures are

entered into the prediction model which is able to calculate the figures based

around the data. A variety of models can be used dependent upon the

application. This type of approach is normally used for planning cellular

networks, estimating the coverage of PMR (Private Mobile Radio) links and for

broadcast coverage planning.

Deterministic approach: This approach to radio signal path loss and coverage

prediction utilises the basic physical laws as the basis for the calculations. These

methods need to take into consideration all the elements within a given area and

although they tend to give more accurate results, they require much additional

data and computational power. In view of their complexity, they tend to be used

for short range links where the amount of required data falls within acceptable

limits.


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These wireless survey tools and radio coverage software packages are growing

in their capabilities. However it is still necessary to have a good understanding

of radio propagation so that the correct figures can be entered and the results

interpreted satisfactorily.

Line-of-Sight Considerations

Fig.(4-7) Line of sight (LOS)

• Microwave radio communication requires a clear line-of-sight (LOS)

condition.

• Under normal atmospheric conditions, the radio horizon is around 30

percent beyond the optical horizon.

• Radio LOS takes into account the concept of Fresnel ellipsoids and their

clearance criteria.

• Fresnel Zone - Areas of constructive and destructive interference created

when electromagnetic wave propagation in free space is reflected

(multipath) or diffracted as the wave intersects obstacles. Fresnel zones

are specified employing ordinal numbers that correspond to the number of

half wavelength multiples that represent the difference in radio wave

propagation path from the direct path.

• The Fresnel Zone must be clear of all obstructions.


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• Radius of the first Fresnel zone

R=17.32(x(d-x)/fd)1/2

where d = distance between antennas (in Km)

R= first Fresnel zone radius in meters

f= frequency in GHz

• Typically the first Fresnel zone (N=1) is used to determine obstruction

loss.

• The direct path between the transmitter and the receiver needs a clearance

above ground of at least 60% of the radius of the first Fresnel zone to

achieve free space propagation conditions.

• Earth-radius factor k compensates the refraction in the atmosphere

• Clearance is described as any criterion to ensure sufficient antenna

heights so that, in the worst case of refraction (for which k is minimum)

the receiver antenna is not placed in the diffraction region

Effective Earth’s Radius = k * True Earth’s Radius

True Earth’s radius= 6371 Km

k=4/3=1.33, standard atmosphere with normally refracted path (this value

should be used whenever local value is not provided)

Clearance criteria to be satisfied under normal propagation conditions

- Clearance of 60% or greater at the minimum k suggested for the certain path

- Clearance of 100% or greater at k=4/3

- In case of space diversity, the antenna can have a 60% clearance at k=4/3

plus allowance for tree growth, buildings (usually 3 meter)


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4-7 Microwave Link Design

Microwave Link Design is a methodical, systematic and sometimes lengthy

process that includes

• Loss/attenuation Calculations

• Fading and fade margins calculations

• Frequency planning and interference calculations

• Quality and availability calculations

The whole process is iterative and may go through many redesign phases before

the required quality and availability are achieved

Loss / Attenuation Calculations

The loss/attenuation calculations are composed of three main contributions

- Propagation losses (Due to Earth’s atmosphere and terrain)

- Branching losses (comes from the hardware used to deliver the

transmitter/receiver output to/from the antenna)

- Miscellaneous (other) losses

(unpredictable and sporadic in character like fog, moving objects crossing

the path, poor equipment installation and less than perfect antenna alignment

etc).

This contribution is not calculated but is considered in the planning process as

an additional loss


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Propagation Losses

• Free-space loss - when the transmitter and receiver have a clear,

unobstructed

line-of-sight

Lfsl=92.45+20log(f)+20log(d) [dB] where f = frequency (GHz)

d = LOS range between antennas (km)

• Vegetation attenuation (provision should be taken for 5 years of

vegetation growth)

L=0.2f 0.3R0.6(dB)

f=frequency (MHz) R=depth of vegetation in meter’s (for R<400m)

• Obstacle Loss –also called Diffraction Loss or Diffraction

Attenuation.

One method of calculation is based on knife edge approximation.

Having an obstacle free 60% of the Fresnel zone gives 0 dB loss

• Gas absorption

- Primarily due to the water vapor and oxygen in the atmosphere in the

radio relay region.The absorption peaks are located around 23GHz for

water molecules and 50 to 70 GHz for oxygen molecules.The specific

attenuation (dB/Km)is strongly dependent on frequency, temperature and

the absolute or relative humidity of the atmosphere.

• Attenuation due to precipitation

– Rain attenuation is the main contributor in the frequency range used by

commercial radio links

– Rain attenuation increases exponentially with rain intensity


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– The percentage of time for which a given rain intensity is attained or

exceeded is available for 15 different rain zones covering the entire

earth’s surface .

– The specific attenuation of rain is dependent on many parameters such as

the form and size of distribution of the raindrops, polarization, rain

intensity and frequency

– Horizontal polarization gives more rain attenuation than vertical

polarization

– Rain attenuation increases with frequency and becomes a major

contributor in the frequency bands above 10 GHz

– The contribution due to rain attenuation is not included in the link budget

and is used only in the calculation of rain fading

Ground Reflection

• Reflection on the Earth’s surface may give rise to multipath propagation

• The direct ray at the receiver may interfered with by the ground-reflected

ray and the reflection loss can be significant

• Since the refraction properties of the atmosphere are constantly changing

the reflection loss varies.

• The loss due to reflection on the ground is dependent on the total

reflection coefficient of the ground and the phase shift

• The highest value of signal strength is obtained for a phase angle of 0o

and the lowest value is for a phase angle of 180o

• The reflection coefficient is dependent on the frequency, grazing angle

(angle between the ray beam and the horizontal plane), polarization and

ground properties

• The grazing angle of radio-relay paths is very small – usually less than 1o


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• It is recommended to avoid ground reflection by shielding the path against

the indirect ray

• The contribution resulting from reflection loss is not automatically

included in the link budget.When reflection cannot be avoided, the fade

margin may be adjusted by including this contribution as “additional loss”

in the link budget

Fading and Fade margins

Fading is defined as the variation of the strength of a received radio carrier

signal due to atmospheric changes and/or ground and water reflections in the

propagation path.Four fading types are considered while planning links.They are

all dependent on path length and are estimated as the probability of exceeding a

given (calculated) fade margin

• Multipath fading

- Flat fading

- Frequency-selective fading

• Rain fading

• Refraction-diffraction fading (k-type fading)

• Multipath Fading is the dominant fading mechanism for frequencies lower

than 10GHz. A reflected wave causes a multipath, i.e.when a reflected

wave reaches the receiver as the direct wave that travels in a straight line

from the transmitter

• If the two signals reach in phase then the signal amplifies. This is called

upfade

Upfademax=10 log d – 0.03d (dB)

Where d is path length in Km


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• If the two waves reach the receiver out of phase they weaken the overall

signal.A location where a signal is canceled out by multipath is called null

or downfade

• As a thumb rule, multipath fading, for radio links having bandwidths less

than 40MHz and path lengths less than 30Km is described as flat instead

of frequency selective.

Flat fading

• A fade where all frequencies in the channel are equally affected.There is

barely noticeable variation of the amplitude of the signal across the

channel bandwidth

• If necessary flat fade margin of a link can be improved by using larger

antennas, a higher-power microwave transmitter, lower –loss feed line

and splitting a longer path into two shorter hops

• On water paths at frequencies above 3 GHz, it is advantageous to choose

vertical polarization.

Frequency-selective fading

• There are amplitude and group delay distortions across the channel

bandwidth

• It affects medium and high capacity radio links (>32 Mbps)

• The sensitivity of digital radio equipment to frequency-selective fading

can be described by the signature curve of the equipment

• This curve can be used to calculate the Dispersive Fade Margin (DFM)

DFM = 17.6 – 10log[2(∆∆∆∆f)e-B/3.8/158.4] dB

∆f = signature width of the equipment

B = notch depth of the equipment


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• Modern digital radios are very robust and immune to spectrum- distorting

fade activity. Only a major error in path engineering (wrong antenna or

misalignment) over the high-clearance path could cause dispersive fading

problems.

• Rain Fading

– Rain attenuates the signal caused by the scattering and absorption

of electromagnetic waves by rain drops

– It is significant for long paths (>10Km)

– It starts increasing at about 10GHz and for frequencies above 15

GHz, rain fading is the dominant fading mechanism

– Rain outage increases dramatically with frequency and then with

path length

– Microwave path lengths must be reduced in areas where rain

outages are severe

– The available rainfall data is usually in the form of a statistical

description of the amount of rain that falls at a given measurement

point over a period of time.The total annual rainfall in an area has

little relation to the rain attenuation for the area

– Hence a margin is included to compensate for the effects of rain at

a given level of availability. Increased fade margin (margins as high

as 45 to 60dB) is of some help in rainfall attenuation fading.

• Reducing the Effects of Rain

– Multipath fading is at its minimum during periods of heavy rainfall

with well aligned dishes, so entire path fade margin is available to

combat the rain attenuation (wet-radome loss effects are minimum

with shrouded antennas)


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– When permitted, crossband diversity is very effective

– Route diversity with paths separated by more than about 8 Km can

be used successfully

– Radios with Automatic Transmitter Power Control have been used

in some highly vulnerable links

– Vertical polarization is far less susceptible to rainfall attenuation

(40 to 60%) than are horizontal polarisation frequencies.

Refraction – Diffraction Fading

– Also known as k-type fading

– For low k values, the Earth’s surface becomes curved and terrain

irregularities, man-made structures and other objects may intercept

the Fresnel Zone.

– For high k values, the Earth’s surface gets close to a plane surface

and better LOS(lower antenna height) is obtained

– The probability of refraction-diffraction fading is therefore

indirectly connected to obstruction attenuation for a given value of

Earth –radius factor

– Since the Earth-radius factor is not constant, the probability of

refraction-diffraction fading is calculated based on cumulative

distributions of the Earth-radius factor.

4-8 Components of Microwave Systems

Microwave Transceivers consist of :

1. Antenna

2. Transmitter

3. Receiver


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4. Control unit

5. Power supply

6. Amplification

4-8-1 Antenna

Antenna, also referred to as an aerial, device used to radiate and receive radio

waves through the air or through space. Antennas are used to send radio waves

to distant sites and to receive radio waves from distant sources. Many wireless

communications devices, such as radios, broadcast television sets, radar, and

cellular radio telephones, use antennas.

Fig. (4-8)

4-8-1-1 Antenna types

- Win antennas i.e. dipole and circular

- Aperture antennas i.e. pyramidal horn


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- Micro strip antennas i.e. circular patch

- Array antennas i.e. Yagi Uda array

- Lens antennas i.e. convex lens and concave lens

4-8-1-2 How Antennas Work ?

A transmitting antenna takes waves that are generated by electrical signals

inside a device such as a radio and converts them to waves that travel in an open

space. The waves that are generated by the electrical signals inside radios and

other devices are known as guided waves, since they travel through transmission

lines such as wires or cables. The waves that travel in an open space are usually

referred to as free-space waves, since they travel through the air or outer space

without the need for a transmission line. A receiving antenna takes free-space

waves and converts them to guided waves.

Radio waves are a type of electromagnetic radiation, a form of rapidly changing,

or oscillating, energy. Radio waves have two related properties known as

frequency and wavelength. Frequency refers to the number of times per second

that a wave oscillates, or varies in strength. The wavelength is equal to the speed

of a wave (the speed of light, or 300 million m/sec) divided by the frequency.

Low-frequency radio waves have long wavelengths (measured in hundreds of

meters), whereas high-frequency radio waves have short wavelengths (measured

in centimeters).

An antenna can radiate radio waves into free space from a transmitter, or it can

receive radio waves and guide them to a receiver, where they are reconstructed

into the original message. For example, in sending an AM radio transmission,

the radio first generates a carrier wave of energy at a particular frequency. The

carrier wave is modified to carry a message, such as music or a person’s voice.


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The modified radio waves then travel along a transmission line within the radio,

such as a wire or cable, to the antenna. The transmission line is often known as a

feed element. When the waves reach the antenna, they oscillate along the length

of the antenna and back. Each oscillation pushes electromagnetic energy from

the antenna, emitting the energy through free space as radio waves.

The antenna on a radio receiver behaves in much the same way. As radio waves

traveling through free space reach the receiver’s antenna, they set up, or induce,

a weak electric current within the antenna. The current pushes the oscillating

energy of the radio waves along the antenna, which is connected to the radio

receiver by a transmission line. The radio receiver amplifies the radio waves and

sends them to a loudspeaker, reproducing the original message.

4-8-1-3 Properties Of Antennas

An antenna’s size and shape depend on the intended frequency or wavelength of

the radio waves being sent or received. The design of a transmitting antenna is

usually not different from that of a receiving antenna. Some devices use the

same antenna for both purposes.

4-8-1-4 Antenna Sizes

An antenna works best when its physical size corresponds to a quantity known

as the antenna’s electrical size. The electrical size of an antenna depends on the

wavelength of the radio waves being sent or received. An antenna radiates

energy most efficiently when its length is a particular fraction of the intended

wavelength. When the length of an antenna is a major fraction of the

corresponding wavelength (a quarter-wavelength or half-wavelength is often

used), the radio waves oscillating back and forth along the antenna will


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encounter each other in such a way that the wave crests do not interfere with one

another. The waves will resonate, or be in harmony, and will then radiate from

the antenna with the greatest efficiency.

If an antenna is not long enough or is too long for the intended radio frequency,

the wave crests will encounter and interfere with one another as they travel back

and forth along the antenna, thus reducing the efficiency. The antenna then acts

like a capacitor or an inductor (depending on the shape of the antenna) and

stores, rather than radiates, energy. The electrical length of an antenna can be

altered by adding a metal loop of wire known as a loading coil to one end of the

antenna, thus increasing the amount of wire in the antenna. Loading coils are

used when the practical length of an antenna would be too long. Adding a coil to

a short antenna increases the antenna’s electrical length, improves its resonance

at the desired frequency, and increases the antenna’s efficiency.

The radio waves used by AM radio have wavelengths of about 300 m (about

1,000 ft). Most AM transmitter antennas are built to a height of about 75 m

(about 250 ft), which, in this case, is the length of a quarter-wavelength. With a

tower of this height, an AM radio antenna will radiate radio waves most

efficiently. Since an antenna that is 75 meters tall would be impractical for a

portable AM radio receiver, AM radios use a special coil of wire inside the radio

for an antenna. The coil of wire is wrapped around an iron-like magnetic

material called a ferrite. When radio waves come into contact with the coil of

wire, they induce an electric charge within the coil. The magnetic ferrite helps

confine and concentrate the electrical energy in the coil and aids in reception.

Television and FM radio use tall broadcast towers as well but use much shorter

wavelengths, corresponding to much higher frequencies, than AM radio.

Therefore, television and FM radio waves have wavelengths of only about 3 m

(about 10 ft). As a result, the corresponding antennas are much shorter.


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Buildings and other obstructions close to the ground can block these high-

frequency radio waves. Thus the towers are used to raise the antennas above

these obstructions in order to provide a greater broadcasting range. Receiving

antennas for television sets and FM radios are small enough to be installed on

these devices themselves, but the antennas are often mounted high on rooftops

for better reception.

4-8-1-5 Antenna Shapes

Antennas come in a wide variety of shapes. One of the simplest types of

antennas is called a dipole. A dipole is made of two lengths of metal, each of

which is attached to one of two wires leading to a radio or other

communications device. The two lengths of metal are usually arranged end to

end, with the cable from the transmitter or receiver feeding each length of the

dipole in the middle. The dipoles can be adjusted to form a straight line or a V-

shape to enhance reception. Each length of metal in the dipole is usually a

quarter-wavelength long, so that the combined length of the dipole from end to

end is a half-wavelength.

The familiar “rabbit-ear” antenna on top of a television set is a dipole antenna.

Another common antenna shape is the half-dipole or monopole antenna, which

uses a single quarter-wavelength piece of metal connected to one of the twin

wires from the transmitter or receiver. The other wire is connected to a ground,

or a point that is not connected to the rest of the circuit. The casing of a radio or

cellular telephone is often used as a ground. The telescoping antenna in a

portable FM radio is a monopole. This arrangement is not as efficient as using

both ends of a dipole, but a monopole is usually sufficient to pick up nearby FM

signals.


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Satellites and radar telescopes use microwave signals. Microwaves have

extremely high frequencies and, thus, very short wavelengths (less than 30 cm).

Microwaves travel in straight lines, much like light waves do. Dish antennas are

often used to collect and focus microwave signals. The dish focuses the

microwaves and aims them at a receiver antenna in the middle of the dish. Horn

antennas are also used to focus microwaves for transmission and reception.

Fig. (4-9)

Receiving antennas come in many different shapes, depending on the frequency

and wavelength of the intended signal. A portable FM radio uses a half-dipole

antenna to receive radio signals. The other half of the dipole is attached to the

radio casing and acts as a ground. VHF television antennas use multiple

elements to receive a broader range of broadcast signals. Many TV antennas

include directors and reflectors, which are extra pieces of metal that reflect and

focus TV waves into the dipole elements. TV satellite dishes are also reflectors.

They focus high-frequency microwaves from satellites into the receiving

element mounted in front of the dish.


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4-8-1-6 Antennas Directivity

Directivity is an important quality of an antenna. It describes how well an

antenna concentrates, or bunches, radio waves in a given direction. A dipole

transmits or receives most of its energy at right angles to the lengths of metal,

while little energy is transferred along them. If the dipole is mounted vertically,

as is common, it will radiate waves away from the center of the antenna in all

directions. However, for a commercial radio or television station, a transmitting

antenna is often designed to concentrate the radiated energy in certain directions

and suppress it in others. For instance, several dipoles can be used together if

placed close to one another. Such an arrangement is called a multiple-element

antenna, which is also known as an array. By properly arranging the separate

elements and by properly feeding signals to the elements, the broadcast waves

can be more efficiently concentrated toward an intended audience, without, for

example, wasting broadcast signals over uninhabited areas.

The elements used in an array are usually all of the same type. Some

arrays have the ability to move, or scan, the main beam in different directions.

Such arrays are usually referred to as scanning arrays.

Arrays are usually electrically large and have better directivity than single

element antennas. Since their directivity is large, arrays can capture and deliver

to the receiver a larger amount of power. Two common arrays used for rooftop

television reception are the Yagi-Uda array and the log-periodic array.

A Yagi-Uda consists of one or more dipoles mounted on a crossbar. The

dipoles are of different lengths, corresponding to the different frequencies used

in broadcast television transmission. Additional pieces of metal, which are

called directors and reflectors, are placed on the crossbar in front of and behind

the dipoles. Directors and reflectors are not wired into the feed element of the


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antenna at all but merely reflect and concentrate radio waves toward the the

directors.

Yagi-Uda antennas are highly directive, and receiving antennas of this type are

often mounted on rotating towers or bases, so that these antennas can be turned

toward the source of the desired transmission. Log-periodic arrays look similar

to Yagi-Uda arrays, but all of the elements in a log-periodic array are active

dipole elements of different lengths. The dipoles are carefully spaced to provide

signal reception over a wide range of frequencies.

While the dipole, monopole, microwave dish, horn, Yagi-Uda, and log-

periodic are among the most common types of antennas, many other designs

also exist for communicating at different frequencies. Submarines traveling

underwater can receive coded radio commands from shore by using extremely

low frequency (ELF) radio waves. In order to receive these signals, a submarine

unravels a very long wire antenna behind as it travels underwater. Television

camera crews broadcasting from locations outside the studio use powerful

microwave transmitter antennas, which can send signals to satellites or directly

to the television station. Amateur, or “ham,” radio enthusiasts, who generally

use frequencies between those of AM and FM radio, often construct their own

antennas, customizing them for sending and receiving signals at desired

frequencies.

4-8-2 Transmitter

A transmitter is an important subsystem in a wireless system. In any active

wireless system, a signal will be generated and transmitted through an antenna.

The signal’s generating system is called a transmitter. The specifications for a

transmitter depend on the applications. For long-distance transmission, high

power and low noise are important. For space or battery operating systems, high


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efficiency is essential. For communication systems, low noise and good stability

are required. A transmitter can be combined with a receiver to form a

transceiver. In this case, a duplexer is used to separate the transmitting and

receiving signals. The duplexer could be a switch, a circulator, or a diplexer.

A transmitter generally consists of an oscillator, a modulator, an upconverter,

filters, and power amplifiers. A simple transmitter could have only an oscillator,

and a complicated one would include a phase-locked oscillator or synthesizer

and the above components. Figure (4-10) shows a typical transmitter block

diagram. The information will modulate the oscillator through AM, FM, phase

modulation (PM), or digital modulation. The output signal could be upconverted

to a higher frequency.

The power amplifiers are used to increase the output power before it is

transmitted by an antenna. To have a low phase noise, the oscillator or local

oscillator can be phase locked to a low-frequency crystal oscillator. The

oscillator could also be replaced by a frequency synthesizer that derives its

frequencies from an accurate high-stability crystal oscillator source.

The following transmitter characteristics are of interest:

1. Power output and operating frequency: the output RF power level

generated by a transmitter at a certain frequency or frequency range.

2. Efficiency: the DC-to-RF conversion efficiency of the transmitter.

3. Power output variation: the output power level variation over the

frequency range of operation.

4. Frequency tuning range: the frequency tuning range due to mechanical

or electronic tuning.

5. Stability: the ability of an oscillator=transmitter to return to the original

operating point after experiencing a slight thermal, electrical, or

mechanical disturbance.


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6. Circuit quality (Q) factor: the loaded and unloaded Q-factor of the

oscillator’s resonant circuit.

7. Noise: the AM, FM, and phase noise. Amplitude-modulated noise is the

unwanted amplitude variation of the output signal, frequency-modulated

noise is the unwanted frequency variations, and phase noise is the

unwanted phase variations.

8. Spurious signals: output signals at frequencies other than the desired

carrier.

9. Frequency variations: frequency jumping, pulling, and pushing.

Frequency jumping is a discontinuous change in oscillator frequency due

to nonlinearities in the device impedance. Frequency pulling is the

change in oscillator frequency versus a specified load mismatch over

360_ of phase variation.

Frequency pushing is the change in oscillator frequency versus DC bias

point variation.

10. Post-tuning drift: frequency and power drift of a steady-state oscillator

due to heating of a solid-state device.

Fig. (4-10) Transmitter system

4-8-3 Receiver

A receiver picks up the modulated carrier signal from its antenna. The carrier

signal is downconverted, and the modulating signal (information) is recovered.

Figure (4-11) shows a diagram of typical radio receivers using a double-

conversion scheme. The receiver consists of a monopole antenna, an RF

amplifier, a synthesizer for LO signals, an audio amplifier, and various mixers,


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IF amplifiers, and filters. The input signal to the receiver is in the frequency

range of 20–470 MHz; the output signal is an audio signal from 0 to 8 kHz. A

detector and a variable attenuator are used for automatic gain control (AGC).

The received signal is first downconverted to the first IF frequency of 515MHz.

After amplification, the first IF frequency is further downconverted to 10.7

MHz, which is the second IF frequency. The frequency synthesizer generates a

tunable and stable LO signal in the frequency range of 535– 985MHz to the first

mixer. It also provides the LO signal of 525.7 MHz to the second mixer.

Other receiver examples are shown in Fig. (4-12). shows a simplified transceiver

block diagram for wireless communications. A T=R switch is used to separate

the transmitting and receiving signals. A synthesizer is employed as the LO to

the upconverter and downconverter.

Fig. (4-11) Radio receiver

The receiver is used to process the incoming signal into useful information,

adding minimal distortion. The performance of the receiver depends on the

system design, circuit design, and working environment. The acceptable level of

distortion or noise varies with the application. Noise and interference, which are


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unwanted signals that appear at the output of a radio system, set a lower limit on

the usable signal level at the output. For the output signal to be useful, the signal

power must be larger than the noise power by an amount specified by the

required minimum signal-to-noise ratio. The minimum signal-to-noise ratio

depends on the application, for example, 30 dB for a telephone line, 40 dB for a

TV system, and 60 dB for a good music system.

The receiver system considerations are listed below:

1. Sensitivity. Receiver sensitivity quantifies the ability to respond to a weak

signal. The requirement is the specified signal-noise ratio (SNR) for an

analog receiver and bit error rate (BER) for a digital receiver.

2. Selectivity. Receiver selectivity is the ability to reject unwanted signals on

adjacent channel frequencies. This specification, ranging from 70 to 90

dB, is difficult to achieve. Most systems do not allow for simultaneously

active adjacent channels in the same cable system or the same

geographical area.

3. Spurious Response Rejection. The ability to reject undesirable channel

responses is important in reducing interference. This can be accomplished

by properly choosing the IF and using various filters. Rejection of 70 to

100 dB is possible.

4. Intermodulation Rejection. The receiver has the tendency to generate its

own on-channel interference from one or more RF signals. These

interference signals are called intermodulation (IM) products. Greater than

70 dB rejection is normally desirable.

5. Frequency Stability. The stability of the LO source is important for low

FM and phase noise. Stabilized sources using dielectric resonators, phase-

locked techniques, or synthesizers are commonly used.


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6. Radiation Emission. The LO signal could leak through the mixer to the

antenna and radiate into free space. This radiation causes interference and

needs to be less than a certain level specified by the FCC.

Fig. (4-12) Simplified transceiver block diagram for wireless communications.

Chapter 5 : Satellite communication system

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Aim of study

This Chapter introduces the History of Satellite, Satellite frequency bands of

induction fields , definition of wave propagation ,fading.

Contents Pages

5-1 Introduction 2

5-2 History 5

5-3 Satellite frequency bands 4

5-4 Satellite orbits 5

5-5 Satellite's earth station and transponder 10

5-6 Types of artificial satellites 17

5-7 Satellite Configuration 21

5-8 Getting into space and back 24

5-9 Remaining in orbit 27

5-10 Protection against the dangers of space 28

5-11 Some of the advantages of using a Satellite 28

5-12

5-13

INTELSAT

INMARSAT

29

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Chapter 5

Satellite communication system

5.1 Introduction

Satellite, in astronomy, secondary object that revolves in a closed orbit about a

planet or star, referred to as the primary of the satellite. The best-known satellite

is the earth's moon—just as the earth itself is a satellite of the sun—although the

moon and earth are close enough in size to be considered sometimes as a

double-planet system. The motion of most of the solar system's known satellites

about their planets is direct—that is, from west to east—and in the same

direction as the rotation of their planets. Only a few satellites of the large outer

planets revolve in the retrograde direction—that is, from east to west—and

opposite the direction of rotation of their planets; they probably were captured

by the planets' gravitational fields some time after the formation of the solar

system. Many astronomers believe that Pluto, which moves in an independent

orbit about the sun, is an escaped satellite of Neptune; Pluto itself was recently

discovered to have a satellite. Information about the individual satellites is given

in the articles on the planets that they orbit.

Satellites have revolutionized communication by making worldwide telephone

links and live broadcasts common occurrences. A satellite receives a microwave

signal from a ground station on the earth (the uplink), then amplifies and

retransmits the signal back to a receiving station or stations on earth at a

different frequency (the downlink). A communication satellite is in

geosynchronous orbit, which means that it is orbiting at the same speed as the

earth is revolving. The satellite stays in the same position relative to the surface


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of the earth, so that the broadcasting station will never lose contact with the

receiver.

Fig. (5.1)

5.2 History

In 1955, the United States and the Soviet Union announced plans to launch

artificial satellites. On Oct. 4, 1957, the Soviet Union launched Sputnik 1, the

first artificial satellite. It circled the earth once every 96 minutes and

transmitted radio signals that could be received on the earth. On Nov. 3, 1957,

the Soviets launched a second satellite, Sputnik 2. It carried a dog named Laika,

the first animal to soar in space. The United States launched its first satellite,

Explorer 1, on Jan. 31, 1958, and its second, Vanguard 1, on March 17, 1958.

In August 1960, the United States launched the first communications satellite,

Echo I. This satellite reflected radio signals back to the earth. In April 1960, the

first weather satellite, Tiros I, sent pictures of clouds to the earth. The U.S.

Navy developed the first navigation satellites. The Transit 1B navigation

satellite first orbited in April 1960. By 1965, more than 100 satellites were

being placed in orbit each year.


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Since the 1970's, scientists have created new and more effective satellite

instruments and have made use of computers and miniature electronic

technology in satellite design and construction. In addition, more nations and

some private businesses have begun to purchase and operate satellites. By the

early 1990's, more than 20 countries owned satellites. About 2,000 satellites

were operating in orbit.

5.3 Satellite frequency bands

The three most commonly used satellite frequency bands are the C band, Ku-

band, and Ka-band. C-band and Ku-band are the two most common frequency

spectrums used by today's satellites. To help understand the relationship

between antenna diameter and transmission frequency, it is important to note

that there is an inverse relationship between frequency and wavelength--when

frequency 11 increases, wavelength decreases. As wavelength increases, larger

antennas (satellite dishes) are necessary to gather the signal.

C-band satellite transmissions occupy the 4 to 8 GHz frequency range.

These relatively low frequencies translate to larger wavelengths than Ku-band or

Ka-band. These larger wavelengths of the C-band mean that a larger satellite

antenna is required to gather the minimum signal strength, and therefore the

minimum size of an average C-band antenna is approximately 2-3 meters in

diameter.

Ku-band satellite transmissions occupy the 11 to 17 GHz frequency range.

These relatively high frequency transmissions correspond to shorter wavelengths

and therefore a smaller antenna can be used to receive the minimum signal

strength. Ku-band antennas can be as small as 18 inches in diameter.

Ka-band satellite transmissions occupy the 20 to 30 GHz frequency range. These

very high frequency transmissions mean very small wavelengths and very small

diameter receiving antennas.


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Table (5.1) Commercial Satellite Communication Frequencies

5.4 Satellite orbits

The defining characteristics of an orbit are its shape, its altitude, and the angle it

makes with Earth’s equator. A satellite’s controllers choose an orbit with a

particular combination of shape, altitude, and angle that will best serve the

satellite’s mission. Most orbits are circular, but some satellites use elliptical

orbits—that is, orbits in which the satellite’s distance from Earth is not constant.

The altitude of an orbit determines how long the satellite takes to circle Earth

and how much of the planet is visible to the satellite at one time. Satellites pass

over different ranges of Earth’s latitude depending on the angle of their orbits

with respect to the equator. Some satellites orbit along the equator. Satellites

that pass over high northern and southern latitudes have orbits that form a large

angle to the equator. Some satellites move clockwise around Earth as seen from

the North Pole, but most satellites move counterclockwise around Earth.


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5.4.1 GEO Orbit

AGeostationary Equatorial Orbit Satellites in geostationary equatorial orbit

(GEO) orbit Earth around the equator at a very specific altitude that allows them

to complete one orbit in the same amount of time that it takes Earth to rotate

once. As a result, these satellites stay above one point on Earth’s equator at all

times. The altitude of GEO is about 5.6 times the radius of Earth, or about

35,800 km (about 22,200 mi).

Direct-broadcast television satellites are in GEO. A few satellites in GEO can

provide coverage for the entire Earth, and antennas do not need to track the

satellite to receive a signal. Earth-surveillance missions, including military

surveillance and weather tracking missions, also use GEO.

5.4.2 LEO Orbit

Low Earth Orbit A satellite in low Earth orbit (LEO) orbits at an altitude of

2,000 km (1,200 mi) or less. Almost every satellite enters a LEO after it is

launched. If a satellite’s mission requires an orbit other than LEO, it uses

rockets to move into its final orbit.

A low Earth orbit minimizes the amount of fuel needed. In addition, a satellite

in LEO can obtain clearer surveillance images and can avoid the Van Allen

radiation belts, which contain harmful high-energy particles. It needs less

powerful signals to communicate with Earth than satellites with higher orbits. A

signal to or from a low Earth orbit also reaches its destination more quickly,

making LEO satellites especially good for transmitting data.


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5.4.3 MEO Orbit

Medium Earth orbit (MEO) satellites orbit at an altitude about 10,000 km (about

6,000 mi) and balance the benefits and problems between LEO and GEO. The

most common uses of MEO are by navigation and communication satellites.

The U.S. navigation system NAVSTAR Global Positioning System (GPS), the

Russian Global Navigation Satellite System (GLONASS), and Odyssey, a

private U.S. communications satellite program, all use MEO.

Fig.(5.2)

5.4.4 Polar Orbit

Satellites in polar orbits orbit around Earth at right angles to the equator over

both the North and South poles. Polar orbits can occur at any altitude, but most

satellites in polar orbits use LEOs. Two polar satellites belonging to the U.S.

National Oceanic and Atmospheric Administration provide weather information

for all areas of the world every six hours. The satellites also map ozone levels


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(see Ozone Layer) in the atmosphere, including the level over the poles. Landsat

is a U.S. government remote-sensing satellite system that operates in polar orbit.

Scientists often use Landsat to view agricultural phenomena such as

deforestation and crop blight. Transit, the first satellite-based navigation system,

used polar orbits in order to support navigation around the world, especially for

submarines in the polar regions.

Fig. (5.3) shows the satellite Nimbus circles the earth in an orbit that passes over

the North and South Poles several times a day, taking photos on each pass.

Because the earth rotates, each pass produces a new set of images, and the entire

earth can be photographed every day. Pictorial information about the earth’s

atmosphere and oceans is relayed back to the surface, where it is used to

monitor changes in the environment

Fig. (5.3)

5.4.5 Sun-Synchronous Orbit

A satellite in a Sun-synchronous orbit always passes over a certain point of

Earth when the Sun is at the same position in Earth’s sky. A Sun-synchronous

satellite has a retrograde orbit (it moves clockwise around Earth), orbits in a

low Earth orbit, and orbits at a specific angle with respect to Earth’s equator


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(about 98°). The satellite crosses each latitude about 1° east of where it crossed

the latitude the previous day. Thus, the satellite stays synchronized with the

location of the Sun relative to Earth. Sun-synchronous orbits are useful for

satellites photographing Earth, because the Sun will be at the same angle each

time the satellite passes over a point on Earth.

A sun-synchronous, polar orbit passes almost directly over the North and South

poles. A slow drift of the orbit's position is coordinated with the earth's

movement around the sun in such a way that the satellite always crosses the

equator at the same local time on the earth. Because the satellite flies over all

latitudes, its instruments can gather information on almost the entire surface of

the earth. One example of this type of orbit is that of the NOAA-H satellite,

which monitors the weather. The altitude of the orbit is 540 miles (870

kilometers), and the orbital period is 102 minutes. When the satellite crosses the

equator, the local time is always either 1:40 a.m. or 1:40 p.m.

Fig. (5.4)


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5.5 Satellite's earth station and transponder

A simple satellite communication link is shown in Fig. (5.5). The earth station A

transmits an uplink signal to the satellite at frequency fU . The satellite receives,

amplifies, and converts this signal to a frequency fD . The signal at fD is then

transmitted to earth station B. The system on the satellite that provides signal

receiving, amplification, frequency conversion, and transmitting is called a

repeater or transponder. Normally, the uplink is operating at higher frequencies

because higher frequency corresponds to lower power amplifier efficiency. The

efficiency is less important on the ground than on the satellite. The reason for

using two different uplink and downlink frequencies is to avoid the interference,

and it allows simultaneous reception and transmission by the satellite repeaters.

The repeater enables a flow of traffic to take place between several pairs of

stations provided a multiple-access technique is used. Frequency division

multiple access (FDMA) will distribute links established at the same time

among different frequencies. Time division multiple access (TDMA) will

distribute links using the same frequency band over different times. The repeater

can distribute thousands of telephone lines, many TV channels, and data links.

For example, the INTELSAT repeater has a capacity of 1000 telephone lines for

a 36-MHz bandwidth.


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Fig.(5.5) Simple satellite communication link

Fig.(5.6) Earth station architecture


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Fig.(5.7) Up & down links between satellite and earth station

The earth stations and satellite transponders consist of many RF and microwave

components. As an example, Fig. (5.8) shows a simplified block diagram

operating at the Ku-band with the uplink at 14–14.5 GHz and downlink at 11.7–

12.2 GHz. The earth terminal has a block diagram shown in Fig(5.9). It consists

of two upconverters converting the baseband frequency of 70MHz to the uplink

frequency.

A power amplifier (PA) is used to boost the output power before transmitting.

The received signal is amplified by a low-noise RF amplifier (LNA) before it is

downconverted to the baseband signal. The block diagram for the transponder

on the satellite is shown in Fig. (5.10). The transponder receives the uplink

signal (14–14.5GHz). It amplifies the signal and converts the amplified signal to

the downlink frequencies (11.7–12.2 GHz). The downlink signal is amplified by

a power amplifier before transmitting. A redundant channel is ready to be used if

any component in the regular channel is malfunctional. The redundant channel

consists of the same components as the regular channel and can be turned on by

a switch.


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Fig. (5.8)


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Fig. (5.9)A block diagram of the earth terminal

Fig. (5.10) A block diagram for satellite transponder

5.5.1 Low noise block (LNB)

The abbreviation LNB stands for Low Noise Block. It is the device on the front

of a satellite dish that receives the very low level microwave signal from the

satellite, amplifies it, changes the signals to a lower frequency band and sends

them down the cable to the indoor receiver.


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The expression low noise refers the the quality of the first stage input amplifier

transistor. The quality is measured in units called Noise Temperature, Noise

Figure or Noise Factor. Both Noise Figure and Noise Factor may be converted

into Noise Temperature. The lower the Noise Temperature the better. So an

LNB with Noise Temperature = 100K is twice as good as one with 200K.

The expression Block refers to the conversion of a block of microwave

frequencies as received from the satellite being down-converted to a lower

(block) range of frequencies in the cable to the receiver. Satellites broadcast

mainly in the range 4 to 12 to 21 GHz.

Fig. (5.11) Low noise block downconverter (LNB) diagram

The diagram shows the input waveguide on the left which is connected to the

collecting feed or horn. As shown there is a vertical pin through the broad side

of the waveguide that extracts the vertical polarisation signals as an electrical

current. The satellite signals first go through a band pass filter which only

allows the intended band of microwave frequencies to pass through. The signals

are then amplified by a Low Noise Amplifier and thence to the Mixer. At the

Mixer all that has come through the band pass filter and amplifier stage is

severely scrambled up by a powerful local oscillator signal to generate a wide


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range of distorted output signals. These include additions, subtractions and

multiples of the wanted input signals and the local oscillator frequency.

Amongst the mixer output products are the difference frequencies between the

wanted input signal and the local oscillator frequencies. These are the ones of

interest. The second band pass filter selects these and feeds them to the output

L band amplifier and into the cable. Typically the output frequency = input

frequency - local oscillator frequency. In some cases it is the other way round

so that the output frequency = local oscillator frequency - input frequency. In

this case the output spectrum is inverted.

Examples of input frequency band, LNB local oscillator frequency and output

frequency band are shown in table (5.2).

Input frequency

band from

satellite

waveguide

Input band

GHz

Local

Oscillator

(LO)

frequency

Output L band

into cable. Comments

C band 3.4-4.2 5.15 950-1750 inverted output

spectrum

3.625.4.2 5.15 950-1525 "

4.5.4.8 5.75 950-1250 "

4.5.4.8 5.95 1150-1450 "

Ku band 10.7-11.7 9.75 950-1950

10.95.11.7 10 950-1700

10.95 - 12.15 10 950-2150 Invacom SPV-

50SM

11.45.11.95 10.5 950-1450


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11.2-11.7 10.25 950-1450

11.7-12.75 10.75 950-2000 Invacom SPV-

60SM

12.25.12.75 11.3 950-1450 Invacom SPV-

70SM

11.7-12.75 10.6 1100-2150

Ka band 19.2-19.7 18.25 950-1450

19.7-20.2 18.75 950-1450

20.2-20.7 19.25 950-1450

20.7-21.2 19.75 950-1450

Table (5.2)

All the above illustrate a simple LNB, with one LNA and one LO frequency.

More complex LNBs exist, particularly for satellite TV reception where people

wish to receive signals from multiple bands, alternative polarisations, and

possibly simultaneously.

5.6 Types of artificial satellites

Engineers have developed many kinds of satellites, each designed to serve a

specific purpose or mission. For instance the telecommunications and

broadcasting industries use communications satellites to carry radio, television,

and telephone signals over long distances without the need for cables or

microwave relays. Navigational satellites pinpoint the location of objects on

Earth, while weather satellites help meteorologists forecast the weather. The

United States government uses surveillance satellites to monitor military

activities. Scientific satellites serve as space-based platforms for observation of


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Earth, the other planets, the Sun, comets, and galaxies, and are useful in a wide

variety of other applications.

5.6.1 Scientific Research Satellites

Scientific research satellites gather data for scientific analysis. These satellites

are usually designed to perform one of three kinds of missions. (1) Some gather

information about the composition and effects of the space near the earth. They

are placed in a variety of orbits. (2) Other satellites record changes in the earth

and its atmosphere. Many of them travel in sun-synchronous, polar orbits. (3)

Still others observe planets, stars, and other distant objects. Most of these

satellites operate in low altitude orbits. Scientific research satellites also orbit

other planets, the moon, and the sun.

5.6.2 Weather Satellites

Weather satellites carry cameras and other instruments pointed toward Earth’s

atmosphere. They can provide advance warning of severe weather and are a

great aid to weather forecasting. NASA launched the first weather satellite,

Television Infrared Observation Satellite (TIROS) 1, in 1960. TIROS 1

transmitted almost 23,000 photographs of Earth and its atmosphere. NASA

operates the Geostationary Operational Environmental Satellite (GOES) series,

which are in geostationary orbit. GOES provides information for weather

forecasting, including the tracking of storms. GOES is augmented by Meteosat

3, a European weather satellite also in geostationary orbit. The National Oceanic

and Atmospheric Administration (NOAA) operates three satellites that collect

data for long-term weather forecasting. These three satellites are not in

geostationary orbit; rather, their orbits carry them across the poles at a relatively

low altitude.


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Broadcasters use data from meteorological satellites to predict weather and

broadcast storm warnings when necessary. Satellites like GOES (Geostationary

Operational Environmental Satellite) collect meteorological and infrared

information about the atmosphere and the ocean. A camera on the GOES is

continuously pointed at the earth, broadcasting satellite images of cloud patterns

both day and night. Here, the GOES-C satellite is being encapsulated inside its

payload fairing aboard a Delta rocket.

5.6.3 Communications Satellites

Communications satellites serve as relay stations, receiving radio signal

messages from one location and transmitting them to another. A

communications satellite can relay several television programs or many

thousands of telephone calls at once. Communications satellites are usually put

in a high altitude, geosynchronous orbit over a ground station. A ground station

has a large dish antenna for transmitting and receiving radio signals. Countries

and commercial organizations such as television broadcasters and telephone

companies use these satellites continuously.

5.6.4 Navigation Satellites

Navigation satellites can help in locating the position of ships, aircraft, and even

automobiles that are equipped with special radio receivers. A navigation satellite

sends continuous radio signals to Earth. These signals contain data that a special

radio receiver on Earth translates into information about the satellite’s position.

The receiver further analyzes the signal to find out how fast and in what

direction the satellite is moving and how long the signal took to reach the

receiver. From this data, the receiver can calculate its own location. Some


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navigation satellite systems use signals from several satellites at once to provide

even more exact location information. ( GPS service )

The U.S. Navy launched the first navigation satellite, Transit 1B, in 1960. The

United States ended its support of the Transit system in 1996.

The U.S. Air Force operates a system, called the NAVSTAR Global Positioning

System (GPS), that consists of 24 satellites. Depending on the type of receiver

and the method used, GPS can provide position information with an accuracy

from 100 m (about 300 ft) to less than 1 cm (less than about 0.4 in). The Global

Orbiting Navigation Satellite System (GLONASS) of the Russian Federation

consists of 24 satellites and provides accuracy similar to GPS.

5.6.5 Earth Observation Satellites

Earth observation satellites are used to map and monitor our planet's resources.

They follow sun-synchronous, polar orbits. Under constant illumination from

the sun, they take pictures in different colors of visible light and in infrared

radiation. Computers on the earth combine and analyze the pictures. Scientists

use earth observation satellites to locate mineral deposits, to determine the

location and size of freshwater supplies, to identify sources of pollution and

study its effects, and to detect the spread of disease in crops and forests.

5.6.6 Military Satellites

Many military satellites are similar to commercial ones, but they send encrypted

data that only a special receiver can decipher. Military surveillance satellites

take pictures just as other earth-imaging satellites do, but cameras on military

satellites usually have a higher resolution.

The U.S. military operates a variety of satellite systems. The Defense Satellite

Communications System (DSCS) consists of five spacecraft in geostationary


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orbit that transmit voice, data, and television signals between military sites. The

Defense Support Program (DSP) uses satellites that are intended to give early

warning of missile launches. DSP was used during the Persian Gulf War (1991)

to warn of Iraqi Scud missile launches.

Some military satellites provide data that is available to the public. For instance,

the satellites of the Defense Meteorological Satellite Program (DMSP) collect

and disseminate global weather information. The military also maintains the

Global Positioning System (GPS), described earlier, which provides navigation

information that anyone with a GPS receiver can use.

5.7 Satellite Configuration

Fig. (5.12)


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5.7.1 What's Inside a Satellite ?

Satellites have a great deal of equipment packed inside them. Most satellites

have seven subsystems, and each one has special work to do.

Fig. (5.13)

1. The propulsion subsystem includes the rocket motor that brings the

spacecraft to its permanent position, as well as small thrusters (motors) that

help to keep the satellite in its assigned place in orbit. Satellites drift out of

position because of solar wind or gravitational or magnetic forces. When

that happens, the thrusters are fired to move the satellite back into the right

position in its orbit.

2. The power subsystem

A satellite provides its own power for the duration of its mission, which

can extend to ten years or more. The most common source of power for

Earth-orbiting satellites is a combination of solar cells with a battery

backup. Solar cells need to be large enough to provide the power that the

satellite requires. For example, the solar array of the complex Hubble


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Space Telescope is about 290 sq m (about 3,120 sq ft) in area and generates

about 5,500 watts of electricity, while the solar array of a smaller Global

Positioning System satellite is about 4.6 sq m (about 50 sq ft) in area and

generates about 700 watts of electricity. Solar cells are often mounted on

wing like panels that unfold from the body of the satellite after it reaches its

final orbit. Batteries provide power before the solar panels are deployed

and when sunlight does not reach the solar panels.

3. The communications subsystem handles all the transmit and receive

functions. It receives signals from the Earth, amplifies them, and transmits

(sends) them to another satellite or to a ground station.

4. The structures subsystem helps provide a stable framework so that the

satellite can be kept pointed at the right place on the Earth's surface.

Satellites can't be allowed to jiggle or wander, because if a satellite is not

exactly where it belongs, pointed at exactly the right place on the Earth, the

television program or the telephone call it transmits to you will be

interrupted.

5. The thermal control subsystem keeps the active parts of the satellite cool

enough to work properly. It does this by directing the heat that is generated

by satellite operations out into space, where it won't interfere with the

satellite.

6. The attitude control subsystem points the spacecraft precisely to maintain

the communications "footprints" in the correct location. When the satellite

gets out of position, the attitude control system tells the propulsion system

to fire a thruster that will move the satellite back where it belongs.

7. Operators at the ground station need to be able to transmit commands to the

satellite and to monitor its health. The telemetry and command system

provides a way for people at the ground stations to communicate with the

satellite.


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5.8 Getting into space and back

Fig. (5.14)

Overcoming gravity is the biggest problem for a space mission.

A spacecraft must be launched at a particular velocity (speed and direction).

Gravity gives everything on the earth its weight and accelerates free-falling

objects downward. At the surface of the earth, acceleration due to gravity,

called g, is about 32 feet (10 meters) per second each second.


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A powerful rocket called a launch vehicle or booster helps a spacecraft

overcome gravity. All launch vehicles have two or more rocket sections known

as stages. The first stage must provide enough thrust (pushing force) to leave

the earth's surface. To do so, this stage's thrust must exceed the weight of the

entire launch vehicle and the spacecraft. The booster generates thrust by

burning fuel and then expelling gases. Rocket engines run on a special mixture

called propellant. Propellant consists of solid or liquid fuel and an oxidizer, a

substance that supplies the oxygen needed to make the fuel burn in the

airlessness of outer space. Lox, or liquid oxygen, is a frequently used oxidizer.

The minimum velocity required to overcome gravity and stay in orbit is called

orbital velocity. At a rate of acceleration of 3 g's, or three times the acceleration

due to gravity, a vehicle reaches orbital velocity in about nine minutes. At an

altitude of 120 miles (190 kilometers), the speed needed for a spacecraft to

maintain orbital velocity and thus stay in orbit is about 5 miles (8 kilometers)

per second.

In many rocket launches, a truck or tractor moves the rocket and its payload

(cargo) to the launch pad. At the launch pad, the rocket is moved into position

over a flame pit, and workers load propellants into the rocket through special

pipes.

At launch time, the rocket's first-stage engines ignite until their combined thrust

exceeds the rocket's weight. The thrust causes the vehicle to lift off the launch

pad. If the rocket is a multistage model, the first stage falls away a few minutes

later, after its propellant has been used up. The second stage then begins to fire.

A few minutes later, it, too, runs out of propellant and falls away. If needed, a

small upper stage rocket then fires until orbital velocity is achieved.

The launch of a space shuttle is slightly different. The shuttle has solid-

propellant boosters in addition to its main rocket engines, which burn liquid

propellant. The boosters combined with the main engines provide the thrust to


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lift the vehicle off the launch pad. After slightly more than two minutes of

flight, the boosters separate from the shuttle and return to the earth by parachute.

The main engines continue to fire until the shuttle has almost reached orbital

velocity. Small engines on the shuttle push it the remainder of the way to

orbital velocity.

To reach a higher altitude, a spacecraft must make another rocket firing to

increase its speed. When the spacecraft reaches a speed about 40 per cent faster

than orbital velocity, it achieves escape velocity, the speed necessary to break

free of the earth's gravity.

Returning to the earth involves the problem of decreasing the spacecraft's great

speed. To do this, an orbiting spacecraft uses small rockets to redirect its flight

path into the upper atmosphere. This action is called de-orbit. A spacecraft

returning to the earth from the moon or from another planet also aims its path to

skim the upper atmosphere. Air resistance then provides the rest of the

necessary deceleration (speed reduction).

At the high speeds associated with reentering the atmosphere from space, air

cannot flow out of the way of the onrushing spacecraft fast enough. Instead,

molecules of air pile up in front of it and become tightly compressed. This

squeezing heats the air to a temperature of more than 10,000 °F (5,500 °C),

hotter than the surface of the sun. The resulting heat that bathes the spacecraft

would burn up an unprotected vehicle in seconds. Insulating plates of quartz

fiber glued to the skin of some spacecraft create a heat shield that protects

against the fierce heat. Refrigeration may also be used. Early spacecraft had

ablative shields that absorbed heat by burning off, layer by layer, and

vaporizing.


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Many people mistakenly believe that the spacecraft skin is heated through

friction with the air. Technically, this belief is not accurate. The air is too thin

and its speed across the spacecraft's surface is too low to cause much friction.

For unmanned space probes, deceleration forces can be as great as 60 to 90 g's,

or 60 to 90 times the acceleration due to gravity, lasting about 10 to 20 seconds.

Space shuttles use their wings to skim the atmosphere and stretch the slowdown

period to more than 15 minutes, thereby reducing the deceleration force to about

11/2 g's.

When the spacecraft has lost much of its speed, it falls freely through the air.

Parachutes slow it further, and a small rocket may be fired in the final seconds

of descent to soften the impact of landing. Some spacecraft, including the space

shuttle, use their wings to glide to a runway and land like an airplane. The early

U.S. space capsules used the cushioning of water and "splashed down" into the

ocean.

5.9 Remaining in orbit

A satellite remains in orbit because of a balance between two factors: (1) the

satellite's velocity (speed at which it would travel in a straight line), and (2) the

gravitational force between the satellite and the earth. Were it not for the pull of

gravity, a satellite's velocity would send it flying away from the earth in a

straight line. But were it not for velocity, gravity would pull a satellite back to

the earth.

To help understand the balance between gravity and velocity, consider what

happens when a small weight is attached to a string and swung in a circle. If the

string were to break, the weight would fly off in a straight line. However, the

string acts like gravity, keeping the weight in its orbit. The weight and string

can also show the relationship between a satellite's altitude and its orbital

period. A long string is like a high altitude. The weight takes a relatively long


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time to complete one circle. A short string is like a low altitude. The weight

has a relatively short orbital period.

5.10 Protection against the dangers of space

Engineers working with specialists in space medicine have eliminated or greatly

reduced most of the known hazards of living in space. Space vehicles usually

have double hulls for protection against impacts. A particle striking the outer

hull disintegrates and thus does not damage the inner hull.

Astronauts are protected from radiation in a number of ways. Missions in earth

orbit remain in naturally protected regions, such as the earth's magnetic field.

Filters installed on spacecraft windows protect the astronauts from blinding

ultraviolet rays.

The crew must also be protected from the intense heat and other physical effects

of launch and landing. Space vehicles require a heat shield to resist high

temperatures and sturdy construction to endure crushing acceleration forces. In

addition, the astronauts must be seated in such a way that the blood supply will

not be pulled from their head to their lower body, causing dizziness or

unconsciousness.

Aboard a spacecraft, temperatures climb because of the heat given off by

electrical devices and by the crew's bodies. A set of equipment called a thermal

control system regulates the temperature. The system pumps fluids warmed by

the cabin environment into radiator panels, which discharge the excess heat into

space. The cooled fluids are pumped back into coils in the cabin.

5.11 Some of the advantages of using a Satellite

• As little as three satellites can cover almost the whole of the earth's

surface, with the exclusion of the sparsely populated polar regions. To


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achieve the same coverage by terrestrial means would require a very

large and expensive network of ground-based transmitters.

• Services can be quickly introduced, since coverage is available for

everyone from day one. There is no need for a phased introduction of

ground-based transmissions with a simple antenna , those located even in

the most remote locations can still enjoy the same level of service as

someone living in the centre of a major city.

• Satellites naturally span national boundaries, providing numerous

possibilities for truly international services.

5.12 INTELSAT

5.12.1 Commercial Satellites

Commercial satellites provide a wide range of communications services.

Television programs are relayed internationally, giving rise to the phenomenon

known as the “global village.” Satellites also relay programs to cable television

systems as well as to homes equipped with dish antennas.

5.12.2 What is the INTELSAT?

Deployment and operation of communications satellites on a commercial basis

began with the founding of the Communications Satellite Corporation

(COMSAT) in 1963. When the International Telecommunications Satellite

Organization (INTELSAT) was formed in 1964, COMSAT became the U.S.

member. Based in Washington, D.C., INTELSAT is owned by more than 120

nations.

INTELSAT is an association that manages international satellite

communications.


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INTELSAT maintains the world's largest network of communications satellites.

The network enables telephone messages, television signals, and other forms of

communication to be sent worldwide almost instantly. Because of INTELSAT,

TV viewers can see the Olympic Games and other special events as they

happen.

Intelsat 1, known as Early Bird, launched in 1965, provided either 240 voice

circuits or one two-way television channel between the United States and

Europe. During the 1960s and 1970s, message capacity and transmission power

of the Intelsat 2, 3, and 4 generations were progressively increased. The first of

the Intelsat 4s, launched in 1971, provided 4,000 voice circuits. With the Intelsat

5 series (1980), innovations in signal focusing resulted in additional increases in

capacity. A satellite's power could now be concentrated on small regions of the

earth, making possible smaller-aperture, lower-cost ground stations. An Intelsat

5 satellite can typically carry 12,000 voice circits.

The Intelsat 6 satellites, which entered service in 1989, can carry 24,000 circuits

and feature dynamic on-board switching of telephone capacity among six

beams, using a technique called SS-TDMA (satellite-switched time division

multiple access). In the late 1990s, INTELSAT had 19 satellites in orbit,

providing the world's most extensive telecommunications system. Other systems

also provide international service in competition with INTELSAT. The growth

of international systems has been paralleled by domestic and regional systems,

such as the U.S. Telstar, Galaxy, and Spacenet programs and Europe's Eutelsat

and Telecom.

Intelsat satellites now carry over 100,000 telephone circuits, with growing use

of digital transmission. Digital source coding methods (see

Telecommunications) have resulted in a ten-fold reduction in the transmission


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rate needed to carry a voice channel, thus enhancing the capacity of existing

facilities and reducing the size of ground stations that provide telephone service.

5.12.3 INTELSAT Satellites Positions

Fig. (5.15)


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5.12.4 Satellites Shapes

Fig. (5.16)

5.12.5 INTELSAT Generations

Fig. (5.17)


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Fig. (5.18)

Fig. (5.19)


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Fig. (5.20)

Fig. (5.21)


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Fig. (5.22)

Fig. (5.23)


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Fig. (5.24)

Fig. (5.25)


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Fig. (5.26)

Fig. (5.27)


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Fig. (5.28)

5.13 INMARSAT

5.13.1 What is the INMARSAT ?

The International Mobile Satellite Organization (INMARSAT), founded in 1979

as the International Maritime Satellite Organization, is a mobile

telecommunications network, providing Global satellite coverage. The satellites

are in geo-synchronous orbits (GEO) above the equator, which means they are

always in the same place in the sky.

Inmarsat is a 62 member-country, internationally owned cooperative, which

operates and maintains the network, satellites and all the earth stations.

It is providing digital data links, telephone, and facsimile transmission, or fax,

service between ships, offshore facilities, and shore-based stations throughout


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the world. It is also now extending satellite links for voice and fax transmission

to aircraft on international routes.

Starting with a user base of 900 ships in the early 1980s, it now supports links

for phone, fax and data communications at up to 64kbit/s to more than 210,000

ship, vehicle, aircraft and portable terminals. That number is growing at several

thousands a month.

The Inmarsat business strategy is to pursue a range of new opportunities at the

convergence of information technology, telecoms and mobility while continuing

to serve traditional maritime, aeronautical, land-mobile and remote-area

markets.


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Fig. (5.29)

Fig. (5.30)


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Fig. (5.31)

5.13.2 Coast Station architecture

Fig. (5.32)


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5.13.3 INMARSAT Network

Fig. (5.33)


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5.13.4 INMARSAT Equipments

Fig. (5.34)

5.13.5 INMARSAT Satellite Image

Fig. (5.35)

Chapter 6 : Multiplexing

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Aim of study

This Chapter introduces the definition of Frequency Division Multiplexing

(FDM), Time Division Multiplexing (TDM),ways of Frame synchronization,

TDM Hierarchy.

Contents Pages

6-1 Introduction to multiplexing 2

6-2 Frequency Division Multiplexing (FDM) 3

6-3 Time Division Multiplexing (TDM) 9


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Chapter 6

Multiplexing

6-1 Introduction to multiplexing

Multiplexing is a technique for sending more than one information signal at a

time down a single communication path(e.g. medium, circuit or channel).

Multiplexing is sometimes loosely referred to as MANY into ONE. This is

true as shown in Figure (6-1).

Fig. (6-1) Multiplexer

Multiplexing can be achieved in a number of ways. The following three will

be covered in this class:

1. Space Division Multiplexing

2. Frequency Division Multiplexing

3. Time Division Multiplexing

Let us now take a look at basic examples of Multiplexing. People who share

an office in their workplaces also share a communication medium (air inside

the room) to converse at the same time.

If there are six people in the office and they all want to talk at the same time,

there obviously will be some interference between the conversations taking

place. To reduce the interference they may divide themselves into three


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groups of two, such that the conversation is between each pair of people. If

the pairs continue talking whilst sitting next to each other, the interference

would still be present.

The best way for each pair to converse with minimal interference would be to

sit a few feet away from the other pairs (within the same room) and converse.

They would still be sharing the same medium for their conversations but the

physical space in the room would be divided for each conversation. This is an

example of SPACE DIVISION MULTIPLEXING. The pairs could also try

conversing using different pitch tones (i.e., same medium but different

frequencies). This will require filters such that each pair hears its own

conversation but not that of others. This is an example of FREQUENCY

DIVISION MULTIPLEXING.

Another option would be for the pairs to converse in turns i.e. sharing the

same medium and have time in which to say something and give others a

chance to converse, too. This will continue until the message/conversation is

over for all pairs. This would be an example of TIME DIVISION

MULTIPLEXING.

6-2 Frequency Division Multiplexing (FDM)

Frequency division multiplexing is the position of signal spectra in

frequency such that each signal spectrum can be separated out from all the

others by filtering. FDM does not preclude the use of other modulating

methods.

There are N signals in frequency, each is band limited to fm Hz. In order to

separate N signals in frequency, each is modulated with a carrier frequency

fC1 , fC2 , …, fCN . Using DSB-LC, the spectral density of every

modulated signal has a bandwidth of 2fm and each is centered at various


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carrier frequencies fC1 , fC2 ,…, fCN. These carrier frequencies are chosen

far enough apart such that each signal spectral density is separated from all

the others.

Fig. (6-2)

Fig. (6-3)

In Telephony, the most widely used method of modulation in FDM is single

sideband modulation, which, in the case of voice signals, requires a


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bandwidth that is approximately equal to that of the original voice signal.

Each voice input is usually assigned a bandwidth of 4 KHz. The band pass

filters following the modulators are used to restrict the band of each

modulated signal to its prescribed range. The resulting band pass filter

outputs are combined in parallel to form the input to the common channel.

Why 4 kHz?

In practical telecommunication systems, a nominal channel allocation of 0 – 4

kHz is assumed to allow for finite cut-off filters. Hence the 4kHz channel

spacing in F.D.M. assemblies. The frequency shifting process (also called

frequency translation) of the information signal produces two new frequency

bands. One is called the upper sideband (USB) and the other lower sideband

(LSB). Both are referred to as side frequencies.

At the receiving terminal, a bank of band pass filters, with their inputs

connected in parallel, is used to separate the message signals on a frequency-

occupancy basis. The original message signals are recovered by individual

demodulators.

Fig. (6-4)Block diagram of FDM system


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Fig. (6-5)Illustration of the modulation steps in an FDM system

The first multiplexing step combines 12 voice inputs into a basic group,

which is formed by having the nth input modulate a carrier at frequency fc =

60 + 4n KHz, where n = 1,2, …, 12. The lower sidebands are then selected

by band pass filtering and combined to form a group of 12 lower sidebands

(one for each voice channel). Thus the basic group occupies the frequency

band 60 ~ 108 KHz.

The next step in the FDM hierarchy involves the combination of five basic

groups into a supergroup. This is accomplished by using the nth group to

modulate a carrier of frequency fc = 372 + 48n KHz, where n = 1, 2, …, 5.

Note: fC1 = (372 + 48 ) = 420 KHz ; 420 KHz − 108 KHz = 312 KHz

fC2 = (372 + 48×2 ) = 468 KHz ; 468 KHz − 108 KHz = 360 KHz

fC3 = (372 + 48×3 ) = 516 KHz ; 516 KHz − 108 KHz = 408 KHz

The supergroup occupies the frequency band 312 ~ 552 KHz. A supergroup

is designed to accommodate 60 independent voice inputs.

In practical systems, complex multiplexing formats are encountered. For

example, the Table (6-1) shows an F.D.M. structure for telephony which is

recommended by the Consultative Committee of the International


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Telecommunications Union (CCITT). CCITT is an international advisory

body. The structure allows telephone calls to be combined in blocks of

appropriate sizes for transmission through a national or international

telephone network.

Channel 1 channel (4kHz)

Group 12 channels (48 kHz)

Supergroup (5 groups) 60 channels (240 kHz)

Mastergruop (5 groups) 300 channels (1.2 MHz)

Supermastergroup (3 groups) 900 channels (3.6 MHz)

Table (6-1) CCITT F.D.M. Hierarchy

The frequency assignments set forth by CCITT, which is based in Geneva,

Switzerland, makes sure that systems using these in various countries are

compatible.

We can now take a look at the basic group (known as the standard telephone

group-12 channel) and see how the information signals are combined into a

composite signal for transmission over a single communication channel. It is

also worth mentioning at this point that the CCITT recommends that all

multichannel systems should be based on two alternative 12 channel group

(i.e. one group should utilise the USB and the other should use the LSB).


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Remember!

Fig. (6-6) LSB and USB

We will take a look at an example of a 12 channel multiplex system utilizing

the lower sideband.

Fig. (6-7) A 12 Channel Multiplex System

LSB BPF is a Band Pass Filter which will allow only the Lower Sidebands to

pass through to the transmission line.


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6-3 Time Division Multiplexing (TDM)

TIME DIVISION MULTIPLEXING (T.D.M.) allows multiple conversations to

take place by the sharing of medium or channel in time. A channel is

allocated the whole of the line bandwidth for a specific period of time. This

means that each subscriber is allocated a time slot. AS in Pulse Code

Modulation (PCM), a signal is sampled in time. This is also done in T.D.M. If

we have a number of analog signals, each signal is sampled first. Then, the

samples from each are combined and the composite signal is transmitted.

Sampling is an essential component in T.D.M. Individual channels are

sampled at higher rates [normally 8 kHz (i.e. 8 samples per cycle of 1 kHz)].

The samples are converted into digital signals and a series of zeros and ones is

transmitted on the line.

Fig.(6-8) Basic T.D.M. System

TDM (Time-division multiplexing) is the time interleaving of samples from

several sources so that the information form these sources can be transmitted

serially over a single communication channel.


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Figure (6-9) illustrates TDM applied to three analog sources that are

multiplexed over a PCM system.

Fig. (6-9)

At the receiver the decommutator (sampler) has to be synchronized with the

incoming waveform so that the PAM samples corresponding to source-1 will

appear on the channel-1 output.

6-3-1 Frame synchronization

multiplexed data can be sorted and directed to appropriate output channel.

Frame sync is provided to the receiver in two different ways:

- provided to the de-multiplexer circuit by sending a frame sync signal from

the transmitter over a separate channel.

- derive the frame sync from the TDM signal itself Figure (6-10) show that

frame sync can be multiplexed along with the information words in an N

channel TDM system.


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Frame synchronization word

A segmented bits data stream which obeys some rules. Usually, it should be

unique in the data stream, or at least, the appear probability is very small.

main

Fig. (6-10)

Synchronous and Asynchronous Lines

For bit sync, data transmission systems are designed to operate with either

synchronous or asynchronous serial data lines.

- In a synchronous system, each device is designed so that its internal clock is

relatively stable for a long period of time and it is synchronized to the

master clock

- In an asynchronous system, the timing is precise only for the bits within

each character (word). Also called start-stop signaling.

advantage of TDM:

It can easily accommodate both analog and digital sources.

Unfortunately, when analog signals are converted to digital signals without

redundancy reduction, they consume a great deal of digital system capacity.

6-3-2 TDM Hierarchy

In practice TDM(s) may be grouped into two:

1. multiplexers used in conjunction with digital computer systems to merge

digital signals from several sources.

2. used by common carriers to combine different sources into high speed

digital TDM signal for transmission.


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Standards adopted by North America and Japan are different from those

that have been adopted in other parts of the world. North America/Japan

standards were first adopted by AT&T.

Later an other set of standards were adopted by CCITT under the

auspices of ITU.

North America TDM hierarchy (T1) is as shown in Fig. (6-11).

Fig. (6-11) North American TDM hierarchy

The corresponding CCITT TDM standard that is used elsewhere in the world

is as shown in Fig. (6-12).

Fig. (6-12) CCITT Digital TDM hierarchy

Chapter 7 : VSAT

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Chapter 7 : VSAT

Aim of study

This Chapter introduces the definition of vsat, advantages & Operations of

vsat, Shapes Of VSAT Remotes,vsat types ,equipments

Contents Pages

What is the VSAT 2 7-1

Advantages & Operations 2 7-2

Shapes Of VSAT Remotes 3 7-3

VSAT Network 4 7-4

VSAT Communications Types 4 7-5

VSAT Equipments 5 7-6

Chapter 7 : VSAT

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Chapter 7

VSAT

7.1 What is the VSAT ?

VSAT stands for Very Small Aperture Terminal and refers to combined

send/receive terminals with a typical antenna diameter of 1 to 3.7m. VSAT

networks are well suited for business applications, offering solutions for large

networks with low or medium traffic. They provide very efficient point-to-

multipoint communications, are easy to install, and can be expanded at very

low extra cost. Any business operation with long-distance telecommunications

needs will find VSAT an attractive solution.

With VSAT networks, there is no response delays, no interruptions. They

offer immediate accessibility and continuous high-quality transmissions. And

as they are adapted for any kind of transmission, from data to voice, fax, and

video, VSAT networks offer the operational flexibility needed for all

information transfers, with very simple installation.

7.2 Advantages & Operations

The great advantage of VSAT is its flexibility. It permits any kind and size of

network based around a central hub and remote locations. This makes them

particularly pertinent for corporate networks or, for example, communications

among educational, government or health-care institutions. Through a VSAT

network, a corporation can communicate freely and constantly with branch

offices.

• Voice and fax transmissions

• Local Area Network interconnection

Chapter 7 : VSAT

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• Data broadcasting

• Videoconferencing

• In-house training

• Business TV or radio

7.3 Shapes Of VSAT Remotes

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7.4 VSAT Network

7.5 VSAT Communications Types

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7.6 VSAT Equipments

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Department of: Technical programs Satellite communication1 Propagation of radio waves pages (1-34)...

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Transcript of Department of: Technical programs Satellite communication1 Propagation of radio waves pages (1-34)...