Satellite SystemsSatellite Systems

IT351: Mobile & Wireless ComputingIT351: Mobile & Wireless Computing

Objective:– To introduce satellite communications and provide details of the particulars of

satellite systems design

OutlineOutline

• Introduction• History• Basics• Categorization of satellite systems

– Geostationary earth orbit (GEO)

– Medium earth orbit (MEO)

– Low earth orbit (LEO)

• Routing• Localization

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Overview of the main chaptersOverview of the main chapters

Chapter 2: Wireless Transmission

Chapter 3: Medium Access Control

Chapter 4: Telecommunication Systems

Chapter 5: Satellite Systems

Chapter 6: Broadcast Systems

Chapter 7: Wireless LAN

Chapter 8: Mobile Network Layer

Chapter 9: Mobile Transport Layer

Chapter 10: Support for Mobility

IntroductionIntroduction• Satellite is a system that supports mobile communications• It offers global coverage without wiring costs for base

stations and is almost independent of varying population densities

• Two or more stations on Earth

– Called ‘Earth Stations’

• One or more stations in Earth Orbit

– Called ‘Satellites’

• Uplink = transmission to satellite

• Downlink = transmission to earth station

• The satellite converts uplink transmissions into downlink

transmission via a ‘transponder’ 4

History of satellite communicationHistory of satellite communicationSatellite communication began after the Second World War when

scientists knew that it was possible to build rockets that would carry radio transmitters into space.

1945 Arthur C. Clarke publishes an essay about “Extra Terrestrial Relays”

1957 first satellite SPUTNIK by Soviet Union during the cold war

1960 first reflecting communication satellite ECHO by US

1963 first geostationary satellite SYNCOM for news broadcasting

1965 first commercial geostationary satellite “Early Bird“ (INTELSAT I): 240 duplex telephone channels or 1 TV channel, 1.5 years lifetime

1976 three MARISAT satellites for maritime communication

1982 first mobile satellite telephone system INMARSAT-A

1988 first satellite system for mobile phones and data communication INMARSAT-C (data-rates about 600 bits/s)

1993 first digital satellite telephone system

1998 global satellite systems for small mobile phones5

ApplicationsApplications

• Traditionally – Weather forecasting: several satellites deliver pictures

of the earth.

– Radio and TV broadcast satellites: hundreds of radio and TV programs are available via satellite. This technology competes with cable in many places as it is cheap

– Military satellites

– Satellites for navigation and localization (e.g., GPS). Almost all ships and aircraft rely on GPS in addition to traditional navigation systems.

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ApplicationsApplications

• Telecommunication– Global telephone backbones: one of the first applications was

the establishment of international telephone backbones. However, these satellites are increasingly being replaced by fiber optical cables crossing the oceans.

– Connections for communication in remote places or underdeveloped areas

– Global mobile communication: the latest trend is the support of global mobile data communication. Due to high latency, GEO satellites are not ideal for this task, but satellite in lower orbits are used. The purpose is not to replace the existing mobile phone network but to extend the area of coverage.

• Satellite systems to extend cellular phone systems (e.g., GSM or AMPS)

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Evolving of Satellite SystemsEvolving of Satellite Systems

• At the beginning satellite systems were simple transponders. – Transponders receive a signal on one frequency, amplify

it and transmit on another frequency.

– Only analog amplification was possible at the beginning

• The use of digital signals allows for signal regeneration– The satellite decodes the signal into a bit stream and

codes it again into a signal – higher quality of the received signal

• Today’s communication satellites provides many functions of higher communication layers, e.g., inter-satellite routing and error correction.

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base stationor gateway

Satellite SystemsSatellite Systems

Inter Satellite Link (ISL)

Mobile User Link (MUL) Gateway Link

(GWL)

footprint

User data

PSTNISDN GSM

GWL

MUL

PSTN: Public Switched Telephone Network

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BasicsBasics

• Elliptical or circular orbits• Complete rotation time depends on distance satellite-earth• Inclination: angle between orbit and equator• Elevation: angle between satellite and horizon• LOS (Line of Sight) to the satellite necessary for connection

- high elevation needed, less absorption due to e.g. buildings

- Footprint: area on earth that is covered by satellite (where signals of satellite can be received)

• typically separated frequencies for uplink and downlink– transponder used for sending/receiving and shifting of

frequencies

– transparent transponder: only shift of frequencies

– regenerative transponder: additionally signal regeneration10

InclinationInclination

inclination

satellite orbit

plane of satellite orbit

equatorial plane

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ElevationElevation

Elevation:angle between center of satellite beam and surface

minimal elevation:elevation needed at leastto communicate with the satellite

footprint

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Link Problems of SatellitesLink Problems of Satellites

• Propagation delay

• Propagation loss of signals depends on distance, angle and atmospheric condition– Parameters like attenuation or received power determined by four

parameters:• sending power

• gain of sending antenna

• distance between sender and receiver

• gain of receiving antenna

• varying strength of received signal due to multipath propagation

• interruptions due to shadowing of signal (no LOS)• Possible solutions

– satellite diversity (usage of several visible satellites at the same time) helps to use less sending power 13

Satellite CommunicationsSatellite Communications

• Categorisation

– Coverage area: global,

regional or national. Larger

systems require more

satellites

– Service type: fixed satellite

service or mobile satellite

service (MSS), point to point

or broadcast satellite

service (BSS) or

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Satellite CommunicationsSatellite Communications

• Design considerations

– Area/coverage; some satellites can cover almost 33% of earths surface, transmission cost becomes invariant of distance

– Bandwidth; is a very limited resource.

– Transmission quality; is usually very high, though delay can be up to ¼ second

• Frequency bands:

– C-band (4 and 6 GHz)

– Ku-band (11 and 14 GHz)

– Ka-band (19 and 29 GHZ)15

Satellite CommunicationsSatellite Communications• Orbit

– Can be circular or elliptical around the center of earth

– Can be in different (e.g. polar or equatorial) or same planes

– Can be Geostationary (GEO), Medium (MEO) or Low (LEO)

– Coverage is affected by objects such as buildings, by atmospheric attenuation, and electrical noise from earth

GEO

MEO LEO 16

Three different types of satellite orbits can be identified depending on diameter of the orbit:

• GEO (Geostationary Earth Orbit), 36000 km above earth surface

• LEO (Low Earth Orbit): 500 - 1500 km• MEO (Medium Earth Orbit) or ICO (Intermediate Circular

Orbit): 6000 - 20000 km

Orbits Orbits

earth

km35768

10000

1000

LEO (Globalstar,

Irdium)

inner and outer VanAllen belts

MEO (ICO)

GEO (Inmarsat)

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Satellite Communications: GEOSatellite Communications: GEO

• Geostationary Earth Orbit (GEO)– Proposed by Arthur C

Clarke in 1945 and have been operational since 1960s

– Same speed as Earth• Appears to stay still• 35,863km above the

Earth above Equator

– Common for early applications like Weather and military

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Geostationary Satellites (cont)Geostationary Satellites (cont)

• Orbit 35,786 km distance to earth surface, orbit in equatorial plane (inclination 0°)– complete rotation exactly one day, satellite is

synchronous to earth rotation

• fix antenna positions, no adjusting necessary• satellites typically have a large footprint (up to 34% of

earth surface!), therefore difficult to reuse frequencies • bad elevations in areas with latitude above 60° due to

fixed position above the equator• high transmit power needed• high latency due to long distance (0.24 sec)

– not useful for global coverage for small mobile phones and data transmission, typically used for radio and TV transmission 19

Geostationary Satellites (cont)Geostationary Satellites (cont)

• GEO– Advantages

• Relative stationary property means frequency changes are not a problem

• Tracking by Earth stations is simple• Can ‘see’ huge areas, so less satellites needed

– Disadvantages• 35,000km is a long way for signals to travel• Polar regions not well served• Long delay… (2 * 35,863)/300000 = 0.24s

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Satellite Communications: LEOSatellite Communications: LEO

• Low Earth Orbit (LEO)– Circular or Elliptical orbit, under

2000km

– Often in polar orbit at 500 to

1500 km altitude

– Appear to move, usually 1.5 to

2 hours to orbit once

– Coverage diameter about 8000km

– Delay low, about 20ms

– Only visible to Earth stations for about 20 minutes

– Frequencies change with movement (Doppler shifts)

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Low Earth Orbit (cont)Low Earth Orbit (cont)

– Requires many satellites in many planes for global coverage

– Small foot-print, better frequency reuse

– Satellites must communicate with each other to hand- over signals

– More complex system

– Cheaper kit with better signal strength, and bandwidth efficiency

– Used in mobile communications systems, with increased use in 3G systems

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Satellite Communications: MEOSatellite Communications: MEO

• Medium Earth Orbit (MEO)– Altitude 6000 to 20000km

– 6 hour orbits

– Coverage diameter 10000 to 15000km

– Signal delay <80ms

– Visible for a ‘few’ hours

– Proposed for data communication services

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MEO systemsMEO systems

• comparison with LEO systems:– slower moving satellites

– less satellites needed

– simpler system design

– for many connections no hand-over needed

– higher latency, ca. 70 - 80 ms

– higher sending power needed

– special antennas for small footprints needed

• Example: ICO (Intermediate Circular Orbit, Inmarsat) start 2000

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Satellite CommunicationsSatellite Communications

• Satellite Network Configurations– Point to Point

• Two earth stations and one satellite

– Broadcast Link

• One earth transmitter, one satellite, many receivers

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Satellite CommunicationsSatellite Communications

– VSAT (Very Small Aperture Terminal)

• Two-way communications via ground hub

• Subscribers have low cost antennas

• Subscribers communicate via hub

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RoutingRouting

• One solution: inter satellite links (ISL)– reduced number of gateways needed – forward connections or data packets within the satellite

network as long as possible– only one uplink and one downlink per direction needed for

the connection of two mobile phones

• Problems:– more complex focusing of antennas between satellites– high system complexity due to moving routers– higher fuel consumption– thus shorter lifetime

• Iridium and Teledesic planned with ISL• Other systems use gateways and additionally terrestrial

networks27

Localization of mobile stationsLocalization of mobile stations• Mechanisms similar to GSM

• Gateways maintain registers with user data– HLR (Home Location Register): static user data– VLR (Visitor Location Register): (last known) location of the mobile

station– SUMR (Satellite User Mapping Register):

• satellite assigned to a mobile station

• positions of all satellites

• Registration of mobile stations– Localization of the mobile station via the satellite’s position– requesting user data from HLR– updating VLR and SUMR

• Calling a mobile station– localization using HLR/VLR similar to GSM– connection setup using the appropriate satellite 28

SummarySummary

• The trend for communication satellite is moving away from big GEOs, towards the smaller MEOs and LEOs for the reason of lower delay.

• Special problems of LEOs is the high system complexity and the relatively short lifetime

• Most LEO satellites fly over non or sparsely populated areas- too few customers

• A new application for satellite is the satellite digital multi-media broadcasting

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