mobile and wireless networks course instructor: dr. · pdf filemobile and wireless networks...
TRANSCRIPT
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Mobile and Wireless Networks
Course Instructor: Dr. Safdar Ali
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Satellite Communication
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INTRODUCTION
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INTRODUCTION
Communication satellite are off-course only one
means of telecommunication transmission.
The traditional means include copper wire and
microwave point- to-point links. Newer techniques
involves use of optics either point-to-point infrared
or fiber optics. Point-to-point radio system such as
short wave radio may also be used.
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Why Satellite ??
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INTRODUCTION
Inter- continent communication is difficult by using
the guided mediums.
The same feature enables satellites to provide
communications links to remote communities in
sparsely populated areas that are difficult to access
by other means.
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INTRODUCTION
Very large areas of the earth are visible from a
satellite, the satellite can form the star point of a
communications net, simultaneously linking many
users who may be widely separated geographically.
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INTRODUCTION
The information transferred most often correspondence
to voice (telephone), video (Television) and digital data.
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Satellite
Disadvantages??
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DISADVANTAGES OF SATELLITES
COMMUNICATION
Launching satellites into orbit is costly.
There is a larger propagation delay in satellite
communication than in terrestrial communication.
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What is Satellite ??
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INTRODUCTION
A communication satellite is basically an electronic communication package placed in orbit whose prime objective is to initiate or assist through space.
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INTRODUCTION
Satellite contains several transponders which
listens to some portion of spectrum, amplifies the
incoming signal and broadcasts it in another
frequency to avoid interference with incoming
signals.
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INTRODUCTION
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ELEMENTS OF SATELLITE COMMUNICATIONS
The basic elements of a communication satellite
service are divided between;
Space Segment
Ground Segment
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ELEMENTS OF SATELLITE COMMUNICATIONS
The space segment consist of the spacecraft &
launch mechanism.
Ground segment comprises the earth station and
network control center of entire satellite system.
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INTRODUCTION
TELSTARECHO 1
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INTRODUCTION
Asiasat 2SYNCOM 2
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EXAMPLES OF SATELLITE RADIO SERVICES
Fixed Satellite Service FSS
Mobile Satellite Service MSS
Broadcast Satellite Service BSS
Radio Navigation Sat. Serv. RNSS
Radio location Sat. Service RSS
Space Operation Service SOS
Earth observation Sat. Serv. ESS
. . .
In total more than 18 radio services
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SATELLITE OPERATING FREQUENCY BANDS
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APPLICATIONS
Communication (truncking call)
Teleconference
TV Broadcasting
Data communication
Weather telecast
Navigation
GPS
Security/Calamity monitoring
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Orbital Mechanics
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THE ORIGIN OF SATELLITE
The concept of using object in space to reflect
signals for communication was proved by Naval
Research Lab in Washington D.C. when it use the
Moon to establish a very low data rate link between
Washington and Hawaii in late 1940’s.
Russian started the Space age by successfully
launching SPUTNIK the first artificial spacecraft to
orbit the earth, which transmitted telemetry
information for 21 days in Oct. 1957.
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THE ORIGIN OF SATELLITE
The American followed by launching an
experimental satellite EXPLORER In 1958.
In 1960 two satellite were deployed “Echo” &
“Courier”
In 1963 first GEO “Syncom”.
The first commercial GEO (Intelsat & Molnya) in
1965 these provides video (Television) and voice
(Telephone) for their audience
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ORBITAL MECHANICS
Satellite technology has progressed tremendously ,since Arthur C. Clarke first proposed its idea in 1945 in his article in Wireless World.
When Clarke wrote, there were no satellite in orbit nor rockets were powerful enough to launch them.
After 20 years, his idea was proven, when early bird was launched.
It cost roughly $ 25,000 per kg to get a geostationary satellite in orbit.
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KU- BAND SATELLITE IN GEO
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MAJOR PROBLEMS FOR SATELLITE
Positioning in orbit in-term of Frequency & Orbit Selection
Stability
Power
Harsh environment
Interference Problem
High initial investment
New investment require in Ground Segment
Short life time
Spectrum crowding
Regulatory aspects (landing rights etc.)
Launch vehicle reliability
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ELECTROMAGNETIC SPECTRUM
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TYPES OF SATELLITE ORBITS
Based on the inclination, i, over the equatorial
plane:
Equatorial Orbits above Earth’s equator (i=0°)
Polar Orbits pass over both poles (i=90°)
Other orbits called inclined orbits (0°<i<90°)
Based on Eccentricity
Circular with centre at the earth’s centre
Elliptical with one foci at earth’s centre
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TYPE OF ORBITS
LEOs - Low Earth Orbit
MEOs - Medium Earth Orbit
GEO - Geostationary Earth Orbit
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GEOSTATIONARY EARTH ORBIT (GEO)
Originally proposed by Arthur C. Clarke
Circular orbits above the equator
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GEO
Orbital height above the earth about 23000
miles/35786.16km
Round trip time to satellite about 0.24 seconds
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GEO
GEO satellites require more power for communications
The signal to noise ratio for GEOs is worse because of the distances involved
A few GEOs can cover most of the surface of the earth
Note that polar regions cannot be “seen” by GEOs
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GEO
Since they appear stationary, GEOs do not require
tracking
GEOs are good for broadcasting to wide areas
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GEO
Angular separation about 2 degrees - allows 180
satellites
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THE VIEW FROM 36,000KM
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SOME GEO’S ABOVE US
Optus
AsiaSat
PAS
Intelsat
Inmarsat
Palapa
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LEO
Low Earth Orbit
200-3,000 km
High orbit speed
Many satellites
Predominately mobile
Iridium, Globalstar
(space shuttle orbit)
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MEO
Medium Earth Orbit
6,000 – 12,000km
About 12 satellites
Voice and mobile
ICO (Odyssey), Orbcomm,
Ellipso
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WHY SATELLITE REMAINS IN THE ORBIT
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SATELLITE ORBITS AND PERIODS
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SATELLITE ORBITS
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ASSIGNMENT # 2
Why Modulation??
Why do we use Decibel in Telecommunication??
Difference between the optical and radio horizon??
What is Multi-level signalling ???
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Kepler’s Laws
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KEPLER’S LAWS
LAW 1: The orbit of a planet about the Sun is
an ellipse with the Sun's center of mass at one
focus (1609)
LAW 2: A line joining a planet and the sun
sweeps out equal areas in equal intervals of
time (1609)
LAW 3: The squares of the periods of the
planets are proportional to the cubes of their
semi-major axes (1619)
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MAJOR AND MINOR AXIS
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KEPLER’S FIRST LAW
Kepler’s first law states that the path followed by a
satellite around the primary will be an ellipse.
An ellipse has two focal points shown as F1 and
F2.
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KEPLER’S FIRST LAW
The center of mass of the two-body system, termed
the barycenter, is always centered on one of the
foci.
Because of the enormous difference between the
masses of the earth and the satellite, the center of
mass coincides with the center of the earth, which
is therefore always at one of the foci.
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KEPLER’S FIRST LAW
The semi-major axis of the ellipse is denoted by a, and the semi-minor axis, by b. The eccentricity e is given by:
For an elliptical orbit, 0 < e < 1. When e= 0, the orbit becomes circular.
Earth’s orbit has an eccentricity of 0.017 (nearly circular)
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KEPLER’S SECOND LAW
Kepler’s second law states that, for equal time
intervals, a satellite will sweep out equal areas in its
orbital plane, focused at the barycenter.
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KEPLER’S SECOND LAW
Assuming the satellite travels distances S1 and S2
meters in 1 s, then the areas A1 and A2 will be
equal.
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KEPLER’S SECOND LAW
The average velocity in each case is S1 and S2
m/s, and because of the equal area law, it follows
that the velocity at S2 is less than that at S1.
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KEPLER’S SECOND LAW
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APOGEE AND PERIGEE
Satellites go faster at Perigee than at Apogee.
Apogee: A point for a satellite farthest from the
Earth.
Perigee: A point for a satellite closest from the
Earth.
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KEPLER’S THIRD LAW
Kepler’s third law states that the square of the
periodic time of revolution of the smaller body about
the larger body equals a constant multiplied by the
third power of the semi major axis of the orbital
ellipse.
Kepler's third law of orbital motion gives us a
precise relationship between the speed of the
satellite and its distance from the earth.
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KEPLER’S THIRD LAW
Satellites in circular orbits travel at a constant
speed. Simple. We just specify that speed, and
we're done.
Satellites in non-circular (i.e., eccentricity > 0) orbits
move faster when they are closer to the earth, and
slower when they are farther away.
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KEPLER’S THIRD LAW
The common practice is to average the speed. You
could call this number "average speed", but
astronomers call it the "Mean Motion“.
Mean Motion is usually given in units of revolutions
per day.
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NUMERICAL
The earth rotates once per sidereal day of 23 h 56
min 4.09 s. Show that the radius of the GEO is 42,
164.17 km.
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NUMERICAL
The Space Shuttle is an example of a low earth
orbit satellite. Sometimes, it orbits at an altitude of
250 km above the earth’s surface, where there is
still a finite number of molecules from the
atmosphere. The mean earth’s radius is
approximately 6378.14 km. Using these figures,
calculate the period of the shuttle orbit when the
altitude is 250km and the orbit is circular. Find also
the linear velocity of the shuttle along its orbit.
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WHAT IS A SATELLITE ?
A celestial body –In astronomical term, e.g. Moon
A space vehicle launched by humans and orbits the
earth or another celestial body –In aerospace terms
Communication Satellite –provides communication
and other services to variety of consumers
It is a microwave repeater in the sky
A satellite radio repeater is called a transponder
A satellite may have one to many transponders
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WHAT IS A SATELLITE SYSTEMS ?
A Satellite System may consist of :
one or more satellite space vehicles,
a ground based control earth station,
and a network of user earth stations that provides the
interface facilities for the transmission and reception of
terrestrial communications traffic through the satellite
systems.
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WHAT IS A SATELLITE SYSTEMS ?
Transmissions to and from the satellites are
categorized as either bus or payload.
The payload is the actual user information conveyed
through the system.
The bus includes control mechanisms that support the
payload operation.
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TYPES OF SATELLITES
Passive Satellites
Active Satellites
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PASSIVE SATELLITES
Simplest type of satellite is a passive reflector
It simply “bounces” signals from one place to another.
It reflects signals back to earth as there are no gain devices on board to amplify or modify the signals.
The passive satellites used in the early years of satellite communications were both artificial as well as natural.
Moon became the first passive satellite in 1954 when the U.S Navy successfully transmitted the first message over an Earth-to-moon-to-Earth communication system.
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PASSIVE SATELLITES
But moon proved to be unreliable communication
satellite as it is above the horizon only half of the
time and its position relative to earth is constantly
changing.
Radio beacon transmitters are required for tracking
and ranging purposes.
Beacon are used so that an earth station can lock
on to and use to determine the exact location of a
satellite so the earth station can align its antennas.
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ARTIFICIAL PASSIVE SATELLITE
The first artificial passive satellite
Echo-I of NASA was launched in
August 1960.
Echo-I was 100-ft. diameter
inflatable plastic balloon with
aluminum coating that reflected
radio signals transmitted from
huge earth station antennas.
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ARTIFICIAL PASSIVE SATELLITE
Echo-I had an orbital height of 1000 miles. Earth
Stations across US and Europe picked up the
signal and contributed a lot in motivating research
in communication satellite.
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DISADVANTAGE OF PASSIVE SATELLITE
Moon proved to be unreliable communication satellite
as it is above the horizon only half of the time and its
position relative to earth is constantly changing.
Earth Stations required high power (10 kW) to transmit
signals strong enough to produce an adequate return
echo.
Large Earth Stations with tracking facilities were
expensive.
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DISADVANTAGE OF PASSIVE SATELLITE
Control of satellites not possible from ground.
As little as 1 part in every 10 x 18 of an earth
station’s transmitted power is actually returned to
earth station receiving antennas.
A global system would have required a large
number of passive satellites accessed randomly by
different users.
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ACTIVE SATELLITES
It is capable of receiving, amplifying, reshaping,
regenerating and retransmitting the information.
Has sophisticated electronic equipment on-board.
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ACTIVE SATELLITES
Advantages of active satellites are:
Require lower power earth station
Directly controlled by operators from ground.
Disadvantages of active satellites are:
Disruption of service due to failure of electronics
components on-board the satellites
Requirement of on-board power supply
Requirement of larger and powerful rockets to launch
heavier satellites in orbit