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
Outline
•
•
•
•
Introduction
History
Basics
Categorization of satellite systems
– Geostationary earth orbit (GEO)
– Medium earth orbit (MEO)
– Low earth orbit (LEO)
• Routing
• Localization
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 3:
Medium Access Control
Chapter 2:
Wireless Transmission
Chapter 7:
Wireless
LAN
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’
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
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.
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)
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
Inclination
plane of satellite orbit
satellite orbit
d
inclination d
equatorial plane
Elevation
Elevation:
angle e between center of satellite beam
and surface
minimal elevation:
elevation needed at least
to communicate with the satellite
e
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
• Today’s communication satellites provides many
functions of higher communication layers, e.g., intersatellite routing and error correction.
Satellite Systems
Inter Satellite Link
(ISL)
Mobile User
Link (MUL)
Gateway Link
(GWL)
GWL
base station
or gateway
footprint
ISDN
PSTN: Public Switched
Telephone Network
MUL
PSTN
User data
GSM
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
Satellite Communications
• Categorisation
– Coverage area: global,
regional or national. Larger
systems require more
satellites
– Service type: fixed satellite
service (FSS), broadcast
satellite service (BSS), or
mobile satellite service
(MSS)
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)
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
MEO
LEO
GEO
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
GEO (Inmarsat)
Orbit): 6000 - 20000 km
MEO (ICO)
LEO
(Globalstar,
Irdium)
inner and outer Van
Allen belts
earth
1000
10000
35768
km
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
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
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
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)
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
Satellite 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
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
Satellite Communications
• Satellite Network
Configurations
– Point to Point
• Two earth stations
and one satellite
– Broadcast Link
• One earth transmitter,
one satellite, many
receivers
Satellite Communications
– VSAT (Very Small Aperture
Terminal)
• Two-way communications via
ground hub
• Subscribers have low cost antennas
• Subscribers communicate via hub
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
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 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
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
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