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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)

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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
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 7:

Wireless

LAN

Chapter 3:

Medium Access Control

Chapter 2:

Wireless Transmission


Introduction
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
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
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.


Applications1
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
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
Inclination

plane of satellite orbit

satellite orbit

d

inclination d

equatorial plane


Elevation
Elevation

Elevation:

angle e between center of satellite beam

and surface

minimal elevation:

elevation needed at least

to communicate with the satellite

e

footprint


Evolving of satellite systems
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., inter-satellite routing and error correction.


Satellite systems
Satellite Systems

Inter Satellite Link (ISL)

Mobile User Link (MUL)

MUL

Gateway Link (GWL)

GWL

base station

or gateway

footprint

GSM

ISDN

PSTN

User data

PSTN: Public Switched

Telephone Network


Link problems of satellites
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
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


Satellite communications1
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 communications2
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


Orbits
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


Satellite communications geo
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
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 cont1
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
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
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
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
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 communications3
Satellite Communications

  • Satellite Network Configurations

    • Point to Point

      • Two earth stations and one satellite

    • Broadcast Link

      • One earth transmitter, one satellite, many receivers


Satellite communications4
Satellite Communications

  • VSAT (Very Small Aperture Terminal)

    • Two-way communications via ground hub

    • Subscribers have low cost antennas

    • Subscribers communicate via hub


Routing
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
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
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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