Satellite Systems

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IT351 Mobile Wireless Computing
Satellite Systems

• Objective
• To introduce satellite communications and
  provide details of the particulars of satellite
  systems design

2
Outline

• 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 chapters
Chapter 10 Support for Mobility
Chapter 9 Mobile Transport Layer
Chapter 8 Mobile Network Layer
Chapter 4 Telecommunication Systems
Chapter 5 Satellite Systems
Chapter 6 Broadcast Systems
Chapter 7 Wireless LAN
Chapter 3 Medium Access Control
Chapter 2 Wireless Transmission
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Introduction

• 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

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History of satellite communication

• Satellite 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
  phones

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Applications

• 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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Applications

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

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Satellite Systems
Inter Satellite Link (ISL)
Mobile User Link (MUL)
MUL
Gateway Link (GWL)
GWL
base station or gateway
footprint
GSM
PSTN
ISDN
User data
PSTN Public Switched Telephone Network
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Basics

• 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
  regeneration

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Inclination
plane of satellite orbit
satellite orbit
d
inclination d
equatorial plane
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Elevation
Elevation angle e between center of satellite
beam and surface
minimal elevation elevation needed at least to
communicate with the satellite
e
footprint
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Link 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

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Satellite 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 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)

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Satellite 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
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Orbits

• 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

GEO (Inmarsat)
MEO (ICO)
LEO (Globalstar,Irdium)
inner and outer Van Allen belts
earth
1000
10000
35768
km
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Satellite 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)

• 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

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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 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)

• 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 MEO

• Medium Earth Orbit (MEO)
• Altitude 6000 to 20000km
• 6 hour orbits
• Coverage diameter 10000 to 15000km
• Signal delay lt80ms
• Visible for a few hours
• Proposed for data communication services

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MEO 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 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 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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Routing

• 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 networks

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Localization 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
  satellites 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

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Summary

• 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