Satellite-Related TermsEarth Stations – antenna systems on or near earthUplink – transmission from an earth station to a satellite

(a transmission from Earth to a spacecraft or the path of such a transmission).

Downlink – transmission from a satellite to an earth station

Transponder – electronics in the satellite that convert uplink signals to downlink signals

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earth stationA ground-based receiving or

transmitting/receiving station in a satellite communications system. The counterpart to the earth station is the satellite in orbit, which is the "space station." Earth stations use dish-shaped antennas, the diameters of which can be under two feet for satellite TV to as large as fifty feet for satellite operators. Antennas for space exploration have diameters reaching a hundred feet.

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What are uplink and downlink?

The communication going from a satellite to ground is called downlink, and when it is going from ground to a satellite it is called uplink. When an uplink is being received by the spacecraft at the same time a downlink is being received by Earth, the communication is called two-way. If there is only an uplink happening, this communication is called upload. If there is only a downlink happening, the communication is called one-way.

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What is a Satellite? (Contd.)Communications Satellite: It is a

microwave repeater in the sky that consists of a diverse combination of one or more components including transmitter, receiver, amplifier, regenerator, filter onboard computer, multiplexer, demultiplexer, antenna, waveguide etc.

A satellite radio repeater is also called transponder. This is usually a combination of transmitter and receiver.

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Ground based satellite

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Ways to CategorizeCommunications Satellites

Coverage areaGlobal, regional, national

Service typeFixed service satellite (FSS)Broadcast service satellite (BSS)Mobile service satellite (MSS)

General usageCommercial, military, amateur, experimental

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Basics: How do Satellites WorkTwo Stations on Earth want to communicate

through radio broadcast but are too far away to use conventional means.

The two stations can use a satellite as a relay station for their communication

One Earth Station sends a transmission to the satellite. This is called a Uplink.

The satellite Transponder converts the signal and sends it down to the second earth station. This is called a Downlink.

Basics: Advantages of SatellitesThe advantages of satellite communication

over terrestrial communication are: The coverage area of a satellite greatly exceeds

that of a terrestrial system. Transmission cost of a satellite is independent

of the distance from the center of the coverage area.

Satellite to Satellite communication is very precise.

Higher Bandwidths are available for use.

Basics: Disadvantages of SatellitesThe disadvantages of satellite

communication: Launching satellites into orbit is costly. Satellite bandwidth is gradually becoming used

up. There is a larger propagation delay in satellite

communication than in terrestrial communication.

Basics: Factors in satellite communicationElevation Angle: The angle of the horizontal of

the earth surface to the center line of the satellite transmission beam. This effects the satellites coverage area. Ideally, you

want a elevation angle of 0 degrees, so the transmission beam reaches the horizon visible to the satellite in all directions.

However, because of environmental factors like objects blocking the transmission, atmospheric attenuation, and the earth electrical background noise, there is a minimum elevation angle of earth stations.

Basics: Factors in satellite communication (cont.)Coverage Angle: A measure of the portion of

the earth surface visible to a satellite taking the minimum elevation angle into account.

R/(R+h) = sin(π/2 - β - θ)/sin(θ + π/2) = cos(β + θ)/cos(θ)R = 6370 km (earth’s radius)h = satellite orbit heightβ = coverage angleθ = minimum elevation angle

Basics: Factors in satellite communication (cont.)Other impairments to satellite communication:

The distance between an earth station and a satellite (free space loss).

Satellite Footprint: The satellite transmission’s strength is strongest in the center of the transmission, and decreases farther from the center as free space loss increases.

Atmospheric Attenuation caused by air and water can impair the transmission. It is particularly bad during rain and fog.

Basics: How Satellites are usedService Types

Fixed Service Satellites (FSS)• Example: Point to Point Communication

Broadcast Service Satellites (BSS)• Example: Satellite Television/Radio• Also called Direct Broadcast Service (DBS).

Mobile Service Satellites (MSS)• Example: Satellite Phones

Types of SatellitesSatellite Orbits

GEO LEO MEO Molniya Orbit HAPs

Frequency Bands

Classification of Satellite OrbitsCircular or elliptical orbitCircular with center at earth’s center Elliptical with one foci at earth’s center

Orbit around earth in different planesEquatorial orbit above earth’s equatorPolar orbit passes over both polesOther orbits referred to as inclined orbits

Altitude of satellitesGeostationary orbit (GEO)Medium earth orbit (MEO)Low earth orbit (LEO)

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Geometry TermsElevation angle - the angle from the horizontal to the

point on the center of the main beam of the antenna when the antenna is pointed directly at the satellite

Minimum elevation angleCoverage angle - the measure of the portion of the earth's

surface visible to the satellite

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Minimum Elevation AngleReasons affecting minimum elevation angle of earth

station’s antenna (>0o)Buildings, trees, and other terrestrial objects block the line

of sightAtmospheric attenuation is greater at low elevation anglesElectrical noise generated by the earth's heat near its surface

adversely affects reception

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GEO OrbitAdvantages of the the GEO orbit

No problem with frequency changesTracking of the satellite is simplifiedHigh coverage area

Disadvantages of the GEO orbitWeak signal after traveling over 35,000 kmPolar regions are poorly servedSignal sending delay is substantial

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GEO Orbit

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Low Earth Orbit (LEO)LEO satellites are much closer to the earth

than GEO satellites, ranging from 500 to 1,500 km above the surface.

LEO satellites don’t stay in fixed position relative to the surface, and are only visible for 15 to 20 minutes each pass.

A network of LEO satellites is necessary for LEO satellites to be useful

LEO (cont.)Advantages

A LEO satellite’s proximity to earth compared to a GEO satellite gives it a better signal strength and less of a time delay, which makes it better for point to point communication.

A LEO satellite’s smaller area of coverage is less of a waste of bandwidth.

LEO (cont.)Disadvantages

A network of LEO satellites is needed, which can be costly

LEO satellites have to compensate for Doppler shifts cause by their relative movement.

Atmospheric drag effects LEO satellites, causing gradual orbital deterioration.

LEO Satellite CharacteristicsCircular/slightly elliptical orbit under 2000 kmOrbit period ranges from 1.5 to 2 hoursDiameter of coverage is about 8000 kmRound-trip signal propagation delay less than 20 msMaximum satellite visible time up to 20 minSystem must cope with large Doppler shiftsAtmospheric drag results in orbital deterioration

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LEO Satellite Characteristics

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LEO CategoriesLittle LEOs

Frequencies below 1 GHz 5MHz of bandwidth Data rates up to 10 kbpsAimed at paging, tracking, and low-rate messaging

Big LEOsFrequencies above 1 GHz Support data rates up to a few megabits per secOffer same services as little LEOs in addition to voice and

positioning services

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MEO Satellite CharacteristicsCircular orbit at an altitude in the range of 5000 to

12,000 kmOrbit period of 6 hoursDiameter of coverage is 10,000 to 15,000 kmRound trip signal propagation delay less than 50 msMaximum satellite visible time is a few hours

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Geostationary Earth Orbit (GEO)These satellites are in orbit 35,863 km above

the earth’s surface along the equator.Objects in Geostationary orbit revolve around

the earth at the same speed as the earth rotates. This means GEO satellites remain in the same position relative to the surface of earth.

GEO (cont.)Advantages

A GEO satellite’s distance from earth gives it a large coverage area, almost a fourth of the earth’s surface.

GEO satellites have a 24 hour view of a particular area.

These factors make it ideal for satellite broadcast and other multipoint applications.

GEO (cont.)Disadvantages

A GEO satellite’s distance also cause it to have both a comparatively weak signal and a time delay in the signal, which is bad for point to point communication.

GEO satellites, centered above the equator, have difficulty broadcasting signals to near polar regions

Low Earth Orbit (LEO)LEO satellites are much closer to the earth

than GEO satellites, ranging from 500 to 1,500 km above the surface.

LEO satellites don’t stay in fixed position relative to the surface, and are only visible for 15 to 20 minutes each pass.

A network of LEO satellites is necessary for LEO satellites to be useful

LEO (cont.)Advantages

A LEO satellite’s proximity to earth compared to a GEO satellite gives it a better signal strength and less of a time delay, which makes it better for point to point communication.

A LEO satellite’s smaller area of coverage is less of a waste of bandwidth.

LEO (cont.)Disadvantages

A network of LEO satellites is needed, which can be costly

LEO satellites have to compensate for Doppler shifts cause by their relative movement.

Atmospheric drag effects LEO satellites, causing gradual orbital deterioration.

Medium Earth Orbit (MEO)A MEO satellite is in orbit somewhere between

8,000 km and 18,000 km above the earth’s surface.

MEO satellites are similar to LEO satellites in functionality.

MEO satellites are visible for much longer periods of time than LEO satellites, usually between 2 to 8 hours.

MEO satellites have a larger coverage area than LEO satellites.

MEO (cont.)Advantage

A MEO satellite’s longer duration of visibility and wider footprint means fewer satellites are needed in a MEO network than a LEO network.

Disadvantage A MEO satellite’s distance gives it a longer

time delay and weaker signal than a LEO satellite, though not as bad as a GEO satellite.

Other OrbitsMolniya Orbit Satellites

Used by Russia for decades. Molniya Orbit is an elliptical orbit. The

satellite remains in a nearly fixed position relative to earth for eight hours.

A series of three Molniya satellites can act like a GEO satellite.

Useful in near polar regions.

MEO Satellite Characteristics

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Frequency Bands Available for Satellite Communications

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Satellite Link Performance Factors

Distance between earth station antenna and satellite antenna

For downlink, terrestrial distance between earth station antenna and “aim point” of satelliteDisplayed as a satellite footprint (Figure 9.6)

Atmospheric attenuationAffected by oxygen, water, angle of elevation, and

higher frequencies

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Satellite Footprint

Satellite Network Configurations

Satellite Network Configurations

Very Small Aperture Terminal

Capacity Allocation StrategiesFrequency division multiple access (FDMA)Time division multiple access (TDMA)Code division multiple access (CDMA)

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Capacity AllocationFDMA

FAMA-FDMA DAMA-FDMA

TDMA Advantages over FDMA

FDMASatellite frequency is already broken into

bands, and is broken in to smaller channels in Frequency Division Multiple Access (FDMA).

Overall bandwidth within a frequency band is increased due to frequency reuse (a frequency is used by two carriers with orthogonal polarization).

FDMA (cont.)The number of sub-channels is limited by

three factors: Thermal noise (too weak a signal will be

effected by background noise). Intermodulation noise (too strong a signal will

cause noise). Crosstalk (cause by excessive frequency

reusing).

FDMA (cont.)FDMA can be performed in two ways:

Fixed-assignment multiple access (FAMA): The sub-channel assignments are of a fixed allotment. Ideal for broadcast satellite communication.

Demand-assignment multiple access (DAMA): The sub-channel allotment changes based on demand. Ideal for point to point communication.

TDMATDMA (Time Division Multiple Access) breaks

a transmission into multiple time slots, each one dedicated to a different transmitter.

TDMA is increasingly becoming more widespread in satellite communication.

TDMA uses the same techniques (FAMA and DAMA) as FDMA does.

TDMA (cont.)Advantages of TDMA over FDMA.

Digital equipment used in time division multiplexing is increasingly becoming cheaper.

There are advantages in digital transmission techniques. Ex: error correction.

Lack of intermodulation noise means increased efficiency.

Frequency-Division Multiplexing

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Frequency-Division MultiplexingAlternative uses of channels in point-to-point

configuration1200 voice-frequency (VF) voice channelsOne 50-Mbps data stream16 channels of 1.544 Mbps each400 channels of 64 kbps each600 channels of 40 kbps eachOne analog video signalSix to nine digital video signals

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Frequency-Division Multiple Access

Factors which limit the number of subchannels provided within a satellite channel via FDMAThermal noiseIntermodulation noiseCrosstalk

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Forms of FDMAFixed-assignment multiple access (FAMA)

The assignment of capacity is distributed in a fixed manner among multiple stations

Demand may fluctuateResults in the significant underuse of capacity

Demand-assignment multiple access (DAMA)Capacity assignment is changed as needed to respond

optimally to demand changes among the multiple stations

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FAMA-FDMAFAMA – logical links between stations are preassignedFAMA – multiple stations access the satellite by using

different frequency bandsUses considerable bandwidth

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FAMA-FDMA

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DAMA-FDMASingle channel per carrier (SCPC) – bandwidth divided

into individual VF channelsAttractive for remote areas with few user stations near each

siteSuffers from inefficiency of fixed assignment

DAMA – set of subchannels in a channel is treated as a pool of available links For full-duplex between two earth stations, a pair of

subchannels is dynamically assigned on demandDemand assignment performed in a distributed fashion by

earth station using CSC

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Time Division Multiple Access

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Reasons for Increasing Use of TDM Techniques

Cost of digital components continues to dropAdvantages of digital components

Use of error correctionIncreased efficiency of TDM

Lack of intermodulation noise

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FAMA-TDMA OperationTransmission in the form of repetitive sequence of

framesEach frame is divided into a number of time slotsEach slot is dedicated to a particular transmitter

Earth stations take turns using uplink channelSends data in assigned time slot

Satellite repeats incoming transmissionsBroadcast to all stations

Stations must know which slot to use for transmission and which to use for reception

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FAMA-TDMA Uplink

FAMA-TDMA

Downlink

Efficiency

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