SATELLITE COMMUNICATIONS
BISHOY MOFIED
OMAR SHAABAN
VARTAN SHOUSHANIAN
1
Outline
Introduction
How satellite works
History of satellites
Satellite Frequency Bands
Digital modulation schemes
Satellite components
Orbital altitudes
GPS satellites
Antenna Gain efficiency
Antenna Examples
2
Satellites Theory
3
History of Satellites(Read Only)
Early in October 1957 communications stations started picking up a regular
beeping noise coming from space.
The signals were coming from Sputnik 1, the world's first man-made satellite by
the soviet union.
At 1958, NASA successfully launched Explorer 1, the first American satellite.
4
History of Satellites(Read Only)
The first satellite picture of Earth came from NASA's Explorer 6 in 1959
In 1963 , the world's first geosynchronous communications satellite NASA
Syncom 2 was launched as Its earlier sister, Syncom 1, had been blown up
on launch earlier that year, but the second version was a huge success.
Now a days 3,600 satellites are orbiting the earth out of 6,600 satellite
launched the rest are part of the space debris
5
Advantages of Satellites
 High channel capacity (>100 Mb/s)
 Low error rates (Pe ~ 10-6)
 Bit Rate (Bits/Sec) = BW * log2(1 + SNR)
 Stable cost environment (no long-distance cables or national boundaries)
 Wide area coverage (whole North America, for instance)
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Disadvantages of Satellites
 Expensive to launch
 Expensive ground stations required
 Cannot be maintained
 Limited orbital space (geosynchronous)
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How satellite works
A Earth Station sends message modulated in GHz range. (Uplink)
Satellite Receive signal and retransmit it back to the earth.
(Downlink)
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Satellite communication
bands(read only)
• Microwave frequency range 300MHz to 300GHz
• Satellite frequency range 1GHz to 170GHz divided
into bands with letters
• Wide bandwidth
• Line of sight required
9
Satellite communication bands
Frequency bands
300
250
Frequency (GHz)
200
170
140
150
110
90
100
75
60
50
50
110
40
26.5
0
2
1
L band
4
2
S band
8
4
12
8
C band
X band
18
26.5
30
12
18
Ku band
K band
Ka band
Q band
40
U band
50
V band
60
E band
75
W band
90
F band
D band
Band
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Satellite communication bands
L band
0.950 - 1.450 GHz
GPS at 1.57542 GHz
C band
3.7 - 4.2 GHz (Downlink)
5.925 - 6.425 GHz (Uplink)
Ka band
18.3 - 18.8, 19.7 - 20.2 GHz
(Downlink)
30 GHz (Uplink)
V band
40 - 75 GHz
Ku band
60 GHz allocated for unlicensed
(WiFi) use
11.7 - 12.2 GHz (Downlink)
70, 80, and 90 GHz for other wireless
14 - 14.5 GHz (Uplink)
11
Satellite communication
bands(read only)
• Uplink and downlink
12
Digital Modulation schemes
13
Phase Shift Key Modulation
Binary phase-shift keying:
Sinusoidal wave with 0 and 180 represent binary
zero and one .
Offset quadrature phase-shift keying :
uses phase shift of 45,135,225,315 deg to
represent 11,01,00,10
This doubles the channel bit rate
14
Offset quadrature phase-shift
keying(read only)
The I/Q modulator mixes the I signal with the carrier cosine wave, and it
mixes the Q signal with the same carrier sin wave
15
Satellite system components
There are many ways to build a satellite, but the key parts are
always the same and these include:
 Communication and processing system Antenna, Transceivers
and On-board processor.
 Propulsion system: Rocket motors and Thrusters.
 Power System : Fuel tanks,Solar panels,Batteries.
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Satellite system components
• Communication and processing
-Transponder can either operate on bent pipe principle or regenerative principle
Bent pipe:sending back to earth of what goes in with only amplification and a shift
from uplink to downlink frequency.
Regenerative:use on-board processing, to demodulate, decode, re-encode and
modulated aboard the satellite.
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Bent Pipe Principle
Regenerative Principle
Satellite system components
Bent Pipe
Advantages
• Reduced power and weight
(simpler)
Regenerative
Advantages
• Reduce error rate (error correction)
• Noise doesn't carry over
Disadvantages
• Increased system flexibility
• Inter modulation between
Disadvantages
carriers to be avoided
• No error correction
• Increased power consumption
• Heavier payload
• Difficult to change signal format
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(change modulation scheme)
Satellite system components
-Transponder example (Bent pipe)
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Satellite system components
• Example Base station to user flow
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Travelling wave tube amplifier
Low noise block downconverter (LNB) diagram
Read only:
Dual-band LNBs
These will typically have two alternative local oscillator frequencies, for example 9.75 GHz and 10.6
GHz with the higher frequency option selected using a 22 kHz tone injected into the cable. Such an
LNB may be used to receive 10.7 - 11.7 GHz using the lower 9.75 GHz LO frequency or the higher
band 11.7 - 12.75 GHz using the higher 10.6 GHz LO frequency.
LNB Frequency stability
All LNBs used for satellite TV reception use dielectric resonator stabilised local oscillators. The DRO is just a
pellet of material which resonates at the required frequency. Compared with quartz crystal a DRO is
relatively unstable with temperature and frequency accuracies may be +/- 250 kHz to as much as +/- 2
MHz at Ku band. This variation includes both the initial value plus variations of temperature over the full
extremes of the operating range. Fortunately most TV carriers are quite wide bandwidth (like 27 MHz) so
even with 2 MHz error the indoor receiver will successfully tune the carrier and capture it .
LNB supply voltages
The DC voltage power supply is fed up the cable to the LNB. Often by altering this voltage it is possible to
change the polarisation or, less commonly, the frequency band. Voltages are normally 13 volts or 19 volts.
Satellite system components
• The propulsion system
- large rocket motor and a smaller
thruster rockets that keep the satellite
at that location.
-forces like the pressure of the solar
wind, the effects of the Earth's and
moon's gravity,..
-Attitude control through spinning
gyroscopes.
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Satellite system components
Power System
-Solar panel with battery
• Power degrades over long time
-Radioactive isotopes
• Low power over long time
-Fuel Cells
• High power but needs resupply
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TYPES OF SATELLITES (BASED ON ORBITS)
• Geostationary or geosynchronous earth orbit (GEO)
• Low Earth Orbit (LEO)
• Medium Earth Orbit (MEO)
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~35000 Km
~8000-16000 Km
~1600 Km
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GEOSYNCHRONOUS EARTH ORBIT (GEO)
• GEO satellites are synchronous with respect to earth. Looking from a fixed
point from Earth, these satellites appear to be stationary.
• Covers around 1/3 (120 degrees latitude) of the earth’s surface.
• Three conditions which lead to a Geosynchronous satellites :
• The satellite should be placed 37,786 kms (approximated to 36,000 kms) above the surface
of the earth.
• These satellites must travel in the rotational speed of earth, and in the direction of motion of
earth, that is eastward.
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• The inclination of satellite with respect to earth must be 0.
GEOSYNCHRONOUS EARTH ORBIT (GEO)
•
Disadvantages of GEO: Northern or southern regions of the Earth (poles) have more
problems receiving these satellites due to the low elevation above a latitude of 60°,
i.e., larger antennas are needed in this case.
•
The transmit power needed is relatively high which causes problems for battery
powered devices. These satellites cannot be used for small mobile phones.
•
The biggest problem for voice and also data communication is the high latency as
without having any handovers, the signal has to at least travel 72,000 kms.
•
Transferring a GEO into orbit is very expensive
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LOW EARTH ORBIT (LEO) SATELLITES
• These satellites are placed 500-1500 kms above the surface of the earth.
• As LEOs circulate on a lower orbit, hence they exhibit a much shorter period
that is 95 to 120 minutes.
• Each LEO satellite will only be visible from the earth for around ten minutes.
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LOW EARTH ORBIT (LEO) SATELLITES
•
Disadvantage: The biggest problem of the LEO concept is the need for many
satellites if global coverage is to be reached. (50–200 satellites)
•
One general problem of LEOs is the short lifetime of about five to eight years due to
atmospheric drag and radiation from the inner Van Allen belt1.
•
The short time of visibility with a high elevation requires additional mechanisms for
connection handover between different satellites.
•
the need for routing of data packets from satellite to if a user wants to communicate
around the world.
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MEDIUM EARTH ORBIT (MEO) SATELLITES
• MEOs can be positioned somewhere between LEOs and GEOs,both in terms of
their orbit and due to their advantages and disadvantages. Using orbits
around 10,000 km
• The system only requires a dozen satellites which is more than a GEO system,
but much less than a LEO system.
• These satellites move more slowly relative to the earth's rotation allowing a
simpler system design (satellite periods are about six hours).
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MEDIUM EARTH ORBIT (MEO) SATELLITES
• Disadvantage : Again, due to the larger distance to the earth, delay increases
to about 70–80 ms. the satellites need higher transmit power and special
antennas for smaller footprints.
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THE GLOBAL POSITIONING SYSTEM (GPS)
•
The Global Positioning System (GPS) is a
worldwide radio-navigation system formed
from a constellation of 24 satellites and
their ground stations.
•
They are constantly moving, making two
complete orbits in less than 24 hours.
•
These satellites are traveling at speeds of
roughly ~11000 Km an hour.
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THE GLOBAL POSITIONING SYSTEM (GPS)
• The whole idea behind GPS is to use satellites in space as reference points for
locations here on earth.
• GPS satellites use a "triangulate," system where the GPS receiver measures
distance using the travel time of radio signals.
• By using triangulation, we can accurately measure our distance and find out
position from three satellites position anywhere on earth
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THE GLOBAL POSITIONING SYSTEM (GPS)
•
Even though the satellites positions are constantly monitored, they can't be watched
every second.
•
The atomic clocks they use are very, very precise but they're not perfect. Minute
discrepancies can occur, and these translate into travel time measurement errors.
•
The signal may not actually get to the ground station receivers first. It may bounce
off various objects before it gets to the receivers.
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Antenna Design
•
To begin the derivation of the Friis Equation, consider two antennas in
free space(no obstruction nearby) separated by a distance R, 𝑃𝑡 is
power transmitted to transmitter antenna with Gain 𝐺𝑡 , while Pr is
received power to receiving antenna with Gain 𝐺𝑟 .
•
The Term Antenna Gain describes how much power is transmitted in the
direction of peak radiation to that of an isotropic source.
•
Power density 𝑃 =
𝑃𝑡
4𝜋𝑅2
𝐺𝑡
• Flux density 𝐹 = 10 log(𝑃)
• 𝐹 = 10 log 𝑃𝑡 + 𝐺𝑡 − 10 log(4𝜋) − 10 log(38,500 ×
103 ) dBW
POWER LOSS
• Pr =P x AE
• 𝐺𝑟 = 4 𝜋 𝐴𝑒𝑓𝑓 / 𝝀2
• 𝐴𝑒𝑓𝑓 =
𝝀2
4𝜋
𝐺𝑟
𝐴𝑒𝑓𝑓 = 𝐺𝑟 + 20 log 𝝀 - 11.0 dB
• 𝐴𝑒𝑓𝑓 is the efficiency of an antenna relates the power delivered to the antenna and
the power radiated or dissipated within the antenna.
• 𝐿𝑝 (path loss): 20 log(
4𝜋R
𝝀
)
• 𝐹 = 10 log 𝑃𝑡 + 𝐺𝑡 − 10 log(4𝜋) − 10 log(𝑅 ) dBW
• Pr =
P𝑡
4𝜋
𝑅2
𝐺𝑡 𝐴𝑒𝑓𝑓 = 𝑃 x
𝐴𝑒𝑓𝑓
• Pr = Pt + Gt + Gr - Lp dBW = 𝐹 +𝐴𝑒𝑓𝑓 dB
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Question #1.
A C-band earth station has an antenna with a transmit gain of 54 dB. The transmitter output
power is set to 100 W at a frequency of 6.100 GHz. The signal is received by a satellite at a
distance of 37,500 km by an antenna with a gain of 26 dB.
a. Calculate the path loss at 6.1 GHz.
Wavelength is 0.04918 m.
4𝜋R
Answer: Path loss = 20 log(
𝝀
103
)
= 20 log ( 4 𝜋 x 37,500 x
/ 0.04918) dB
Lp = 199.6 dB
b. Calculate the power at the output port (sometimes called the output waveguide flange) of the
satellite antenna, in dBW.
Answer: Uplink power budget gives
Pr = Pt + Gt + Gr - Lp dBW
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= 20 + 54 + 26 – 199.6 = -99.6 dBW
2. A geostationary satellite carries a transponder with a 20 watt transmitter at 4 GHz. The transmitter is operated at an output power of 10
watts and drives an antenna with a gain of 30 dB. An earth station is at the center of the coverage zone of the satellite, at a range of 38,500
km.Using decibels for all calculations, find:
a. The flux density at the earth station in dBW/m2
Answer: Flux density is given by F = 20 log [ Pt Gt / (4 𝜋 𝑹𝟐 ) ] dBW/m2
Hence for R = 38,500 km, f = 4 GHz, 𝝀 = 0.075 m
F = 10 log Pt + 𝑮𝒕 - 10 log (4 𝜋) - 20 log (38,500 x 103 ) dBW / m2
= 10.0 +30.0 - 11.0 - 151.7 = -122.7 dBW / m2
b. The power received by an antenna with a gain of 39 dB, in dBW.
Answer: Received power can be calculated from the effective area of the antenna aperture and the incident flux density, but since the
antenna gain is given in dB, it is better to use path loss and the link budget.
Path loss Lp = 20 log (4 𝜋 R / 𝝀) = 10 log (4 𝜋 x 38,500 x 𝟏𝟎𝟑 / 0.075) = 196.2 dB
Downlink power budget gives
Pr = Pt + Gt + Gr - Lp dBW
= 10.0 + 30.0 + 39.0 – 196.2 = -117.2 dBW
Alternatively, the received power can be found from
Pr = F + 𝑨𝒆𝒇𝒇 where 𝑨𝒆𝒇𝒇 is the effective aperture area of the antenna.
Given G = 4 𝜋 𝐴𝑒𝑓𝑓 / 𝝀2 = 39 dB, we can find 𝑨𝒆𝒇𝒇 from
𝑨𝒆𝒇𝒇 = G + 20 log 𝝀 - 11.0 dB = 39.0 – 22.5 –11.0 = 5.5 dB m2
Pr = -122.7 + 5.5 = - 117.2 dBW / m2
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