Satellite SystemsIT351: Mobile & Wireless ComputingObjective: To introduce satellite communications and provide details of the particulars of satellite systems design

OutlineIntroductionHistoryBasicsCategorization of satellite systemsGeostationary earth orbit (GEO)Medium earth orbit (MEO)Low earth orbit (LEO)RoutingLocalization

Overview of the main chaptersChapter 2: Wireless TransmissionChapter 3: Medium Access ControlChapter 4: Telecommunication SystemsChapter 5: Satellite SystemsChapter 6: Broadcast SystemsChapter 7: Wireless LANChapter 8: Mobile Network LayerChapter 9: Mobile Transport LayerChapter 10: Support for Mobility

IntroductionSatellite is a system that supports mobile communicationsIt offers global coverage without wiring costs for base stations and is almost independent of varying population densitiesTwo or more stations on EarthCalled Earth StationsOne or more stations in Earth OrbitCalled SatellitesUplink = transmission to satelliteDownlink = transmission to earth stationThe satellite converts uplink transmissions into downlink transmission via a transponder

History 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 Relays1957 first satellite SPUTNIK by Soviet Union during the cold war1960first reflecting communication satellite ECHO by US1963 first geostationary satellite SYNCOM for news broadcasting1965 first commercial geostationary satellite Early Bird (INTELSAT I): 240 duplex telephone channels or 1 TV channel, 1.5 years lifetime 1976three MARISAT satellites for maritime communication1982 first mobile satellite telephone system INMARSAT-A1988 first satellite system for mobile phones and data communication INMARSAT-C (data-rates about 600 bits/s)1993first digital satellite telephone system 1998 global satellite systems for small mobile phones

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

ApplicationsTelecommunicationGlobal 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 areasGlobal 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)

BasicsElliptical or circular orbitsComplete rotation time depends on distance satellite-earthInclination: angle between orbit and equatorElevation: angle between satellite and horizonLOS (Line of Sight) to the satellite necessary for connectionhigh elevation needed, less absorption due to e.g. buildingsFootprint: area on earth that is covered by satellite (where signals of satellite can be received)typically separated frequencies for uplink and downlinktransponder used for sending/receiving and shifting of frequenciestransparent transponder: only shift of frequenciesregenerative transponder: additionally signal regeneration

Inclinationinclination ddsatellite orbitplane of satellite orbitequatorial plane

ElevationElevation:angle e between center of satellite beam and surfaceeminimal elevation:elevation needed at leastto communicate with the satellitefootprint

Evolving of Satellite SystemsAt 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 beginningThe use of digital signals allows for signal regenerationThe satellite decodes the signal into a bit stream and codes it again into a signal higher quality of the received signalTodays communication satellites provides many functions of higher communication layers, e.g., inter-satellite routing and error correction.

base stationor gatewaySatellite SystemsInter Satellite Link (ISL)Mobile User Link (MUL)Gateway Link (GWL)footprintUser dataPSTNISDNGSMGWLMULPSTN: Public Switched Telephone Network

Link Problems of SatellitesPropagation delayPropagation loss of signals depends on distance, angle and atmospheric conditionParameters like attenuation or received power determined by four parameters:sending powergain of sending antennadistance between sender and receivergain of receiving antennavarying strength of received signal due to multipath propagationinterruptions due to shadowing of signal (no LOS)Possible solutionssatellite diversity (usage of several visible satellites at the same time) helps to use less sending power

Satellite CommunicationsCategorisationCoverage area: global, regional or national. Larger systems require more satellitesService type: fixed satellite service (FSS), broadcast satellite service (BSS), or mobile satellite service (MSS)

Satellite CommunicationsDesign considerationsArea/coverage; some satellites can cover almost 33% of earths surface, transmission cost becomes invariant of distanceBandwidth; is a very limited resource.Transmission quality; is usually very high, though delay can be up to secondFrequency bands:C-band (4 and 6 GHz)Ku-band (11 and 14 GHzKa-band (19 and 29 GHZ)

Satellite CommunicationsOrbitCan be circular or elliptical around the center of earthCan be in different (e.g. polar or equatorial) or same planesCan be Geostationary (GEO), Medium (MEO) or Low (LEO)Coverage is affected by objects such as buildings, by atmospheric attenuation, and electrical noise from earthGEOMEOLEO

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 kmMEO (Medium Earth Orbit) or ICO (Intermediate Circular Orbit): 6000 - 20000 km

Orbits earthkm35768100001000LEO (Globalstar, Irdium)inner and outer VanAllen beltsMEO (ICO)GEO (Inmarsat)

Satellite Communications: GEOGeostationary Earth Orbit (GEO)Proposed by Arthur C Clarke in 1945 and have been operational since 1960sSame speed as EarthAppears to stay still35,863km above the Earth above EquatorCommon for early applications like Weather and military

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 rotationfix antenna positions, no adjusting necessarysatellites 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 equatorhigh transmit power neededhigh 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

Geostationary Satellites (cont)GEOAdvantagesRelative stationary property means frequency changes are not a problemTracking by Earth stations is simpleCan see huge areas, so less satellites needed

Disadvantages35,000km is a long way for signals to travelPolar regions not well servedLong delay (2 * 35,863)/300000 = 0.24s

Satellite Communications: LEOLow Earth Orbit (LEO)Circular or Elliptical orbit, under 2000kmOften in polar orbit at 500 to 1500 km altitudeAppear to move, usually 1.5 to 2 hours to orbit onceCoverage diameter about 8000kmDelay low, about 20msOnly visible to Earth stations for about 20 minutesFrequencies change with movement (Doppler shifts)

Low Earth Orbit (cont)Requires many satellites in many planes for global coverageSmall foot-print, better frequency reuseSatellites must communicate with each other to hand- over signalsMore complex systemCheaper kit with better signal strength, and bandwidth efficiencyUsed in mobile communications systems, with increased use in 3G systems

Satellite Communications: MEOMedium Earth Orbit (MEO)Altitude 6000 to 20000km6 hour orbitsCoverage diameter 10000 to 15000kmSignal delay

MEO systemscomparison with LEO systems:slower moving satellitesless satellites neededsimpler system designfor many connections no hand-over neededhigher latency, ca. 70 - 80 mshigher sending power neededspecial antennas for small footprints neededExample: ICO (Intermediate Circular Orbit, Inmarsat) start 2000

Satellite CommunicationsSatellite Network ConfigurationsPoint to PointTwo earth stations and one satellite

Broadcast LinkOne earth transmitter, one satellite, many receivers

Satellite CommunicationsVSAT (Very Small Aperture Terminal)Two-way communications via ground hubSubscribers have low cost antennasSubscribers communicate via hub

RoutingOne solution: inter satellite links (ISL)reduced number of gateways needed forward connections or data packets within the satellite network as long as possibleonly one uplink and one downlink per direction needed for the connection of two mobile phones Problems:more complex focusing of antennas between satelliteshigh system complexity due to moving routershigher fuel consumptionthus shorter lifetimeIridium and Teledesic planned with ISLOther systems use gateways and additionally terrestrial networks

Localization of mobile stationsMechanisms similar to GSMGateways maintain registers with user dataHLR (Home Location Register): static user dataVLR (Visitor Location Register): (last known) location of the mobile stationSUMR (Satellite User Mapping Register): satellite assigned to a mobile stationpositions of all satellitesRegistration of mobile stationsLocalization of the mobile station via the satellites positionrequesting user data from HLRupdating VLR and SUMRCalling a mobile stationlocalization using HLR/VLR similar to GSMconnection setup using the appropriate satellite

SummaryThe 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 customersA new application for satellite is the satellite digital multi-media broadcasting

*Universitt KarlsruheInstitut fr TelematikMobilkommunikationSS 1998Prof. Dr. Dr. h.c. G. KrgerE. Dorner / Dr. J. Schiller**********