Mobile Communications Chapter 5: Satellite Systems History LEO, - - PowerPoint PPT Presentation

Mobile Communications Satellite Systems 1

Mobile Communications Chapter 5: Satellite Systems

 History  Basics  Orbits  LEO, MEO, GEO  Examples  Handover, Routing

History of satellite communication

1945 Arthur C. Clarke publishes an essay about „Extra Terrestrial Relays“ 1957 first satellite SPUTNIK 1960 first reflecting communication satellite ECHO 1963 first geostationary satellite SYNCOM 1965 first commercial geostationary satellite Satellit „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 1993 first digital satellite telephone system 1998 global satellite systems for small mobile phones

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Applications

 Traditionally

 weather satellites  radio and TV broadcast satellites  military satellites  satellites for navigation and localization (e.g., GPS)

 Telecommunication

 global telephone connections  backbone for global networks  connections for communication in remote places or underdeveloped areas  global mobile communication

 satellite systems to extend cellular phone systems

replaced by fiber optics

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base station

• r gateway

Classical satellite systems

Inter Satellite Link (ISL) Mobile User Link (MUL) Gateway Link (GWL) footprint small cells (spotbeams) User data PSTN ISDN GSM GWL MUL PSTN: Public Switched Telephone Network Mobile Communications Satellite Systems 4

Basics

Satellites in circular orbits

 attractive force Fg = m g (R/r)²  centrifugal force Fc = m r ω²  m: mass of the satellite  R: radius of the earth (R = 6370 km)  r: distance to the center of the earth  g: acceleration of gravity (g = 9.81 m/s²)  ω: angular velocity (ω = 2 π f, f: rotation frequency)

Stable orbit

 Fg = Fc

3 2 2

) 2 ( f gR r π =

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Satellite period and orbits

10 20 30 40 x106 m 24 20 16 12 8 4 radius satellite period [h] velocity [ x1000 km/h] synchronous distance 35,786 km

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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

 Uplink: connection base station - satellite  Downlink: connection satellite - base station  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 Mobile Communications Satellite Systems 7

Inclination

inclination δ δ satellite orbit perigee plane of satellite orbit equatorial plane

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Elevation

Elevation: angle ε between center of satellite beam and surface

ε

minimal elevation: elevation needed at least to communicate with the satellite

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Link budget of satellites

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

Problems

 varying strength of received signal due to multipath propagation  interruptions due to shadowing of signal (no LOS)

Possible solutions

 Link Margin to eliminate variations in signal strength  satellite diversity (usage of several visible satellites at the same time)

helps to use less sending power

2

4       = c f r L π

L: Loss f: carrier frequency r: distance c: speed of light Mobile Communications Satellite Systems 10

Atmospheric attenuation

Example: satellite systems at 4-6 GHz elevation of the satellite 5° 10° 20° 30° 40° 50° Attenuation of the signal in % 10 20 30 40 50 rain absorption fog absorption atmospheric absorption ε Mobile Communications Satellite Systems 11

Four different types of satellite orbits can be identified depending

• n the shape and diameter of the orbit:

 GEO: geostationary orbit, ca. 36000 km above earth surface  LEO (Low Earth Orbit): ca. 500 - 1500 km  MEO (Medium Earth Orbit) or ICO (Intermediate Circular Orbit):

• ca. 6000 - 20000 km

 HEO (Highly Elliptical Orbit) elliptical orbits

Orbits I

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Orbits II

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

Van-Allen-Belts: ionized particles 2000 - 6000 km and 15000 - 30000 km above earth surface

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LEO systems

Orbit ca. 500 - 1500 km above earth surface

 visibility of a satellite ca. 10 - 40 minutes  global radio coverage possible  latency comparable with terrestrial long distance

connections, ca. 5 - 10 ms

 smaller footprints, better frequency reuse  but now handover necessary from one satellite to another  many satellites necessary for global coverage  more complex systems due to moving satellites

Examples:

 Iridium (start 1998, 66 satellites)  Globalstar (start 2000, 48 satellites)

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MEO systems

Orbit ca. 5000 - 12000 km above earth surface 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)  GPS, GALILEO

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MEO systems: GPS (Global Positioning System)

Basic concept of GPS

 GPS receiver calculates its position (latitude, longitude, and altitude) by

precisely timing the signals sent by GPS satellites high above the Earth

 Each satellite continually transmits messages that include

 the time the message was transmitted  precise orbital information  the general system health and rough orbits of all GPS satellites

 Receiver uses the received messages to determine the transit time of

each message and computes the distance to each satellite

 Trilateration  Due to errors (inprecise clocks), not three but four or more satellites are

used for calculations

Position useful in mobil communications for “Location based services”

 Accuracy: „some meter“ with Wide Area Augmentation System WAAS Mobile Communications Satellite Systems 16

Adopted from Wikipedia

MEO systems: GPS (Global Positioning System)

Structure: three major segments

1.

space segment (SS)

 orbiting GPS satellites, or Space Vehicles (SV)

2.

control segment (CS)

 master control station (MCS),  alternate master control station,  four dedicated ground antennas and  six dedicated monitor stations

3.

user segment (U.S.)

 user devices

US Air Force develops, maintains, and operates space & ctrl segments

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Adopted from Wikipedia

MEO systems: GPS (Global Positioning System)

 Space segment (SS)

 orbiting GPS satellites, or Space Vehicles (SV)  24 SVs: six planes with four satellites each (plus some extra)

 approximately 55° inclination  orbits are arranged such that >= 6 satellites are always within LOS  four satellites are not evenly spaced (90 degrees) within each orbit,

but 30, 105, 120, and 105 degrees

 rotation time approx. 12 hours  orbit 20200 km  ~ 9 satellites are visible from any point on ground at any one time Mobile Communications Satellite Systems 18

MEO systems: GPS (Global Positioning System)

Ground-Track (sub satellite path) of the Satellite GPS BIIR-07 (PRN 18)

• f 18.10.2001, 00:00 h
• to 19.10.2001, 00:00 h
• rbit time is slightly shifted (about 4 minutes) in 24 h

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21:30 zone of sight

MEO systems: GPS (Global Positioning System)

Position of the monitor stations and the master control station (Earthmap:NASA; http://visibleearth.nasa.gov/)

 “master control station” (Schriever AFB)  plus additional monitoring stations for monitoring the satellites  every satellite can be seen from at least two monitor stations Mobile Communications Satellite Systems 20

Geostationary satellites

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 (ca. 275 ms)

 not useful for global coverage for small mobile phones and data transmission, typically used for radio and TV transmission, but some for mobile communications as well

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GEO Systems: Example Thuraya

 regional satellite phone provider

 shareholders are mixture of Middle Eastern and North African telcos and

investment companies

 coverage area most of Europe, Middle East, North, Central and East

Africa, Asia and Australia

 subscribers:

 ~ 250,000 (March 2006)  ~ 360,000 Thuraya handsets in service since launch in 2001

Structure of Thuraya spot beams

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GEO Systems: Example Thuraya

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GEO Systems: Example Thuraya

Services

• Voice communications with hand held or fixed terminals
• Short message service
• 9.6 kbit/s of data & fax service
• 60 kbit/s downlink and 15 kbit/s uplink "GMPRS" mobile data service
• 144 kbit/s high-speed data transfer via a notebook-sized terminal

(ThurayaDSL)

• GPS is supported by all handsets
• value-added services, e.g., news, call back / waiting, missed calls
• ne-way 'high power alert' capability that notifies users of incoming

call, when signal path to satellite is obstructed (e.g. inside building)

• Marine Services: a combination of a special (fixed) base station and

subscription offering voice, fax, data and always on internet-access

• Also an emergency service: sends multiple SMS messages containing alarm-status

and actual position to pre-defined destinations

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GEO Systems: Example SkyDSL

Internet access via satellite

 Interesting for users in areas

where no other broadband Internet access available

 has technically not much to do with DSL

 GEO satellite for downlink  uplink (also for requests) via modem/telephone or other mobil comm.

• r also via satellite

 data rate up to max. 36000 KBit/s  # users

 ca. 100.000 in Germany

 large latency due to GEO

 already signal propagation for distance of 2 * 36000 km: ~240 ms Mobile Communications Satellite Systems 25

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 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 Mobile Communications Satellite Systems 27

Handover in satellite systems

Several additional situations for handover in satellite systems compared to cellular terrestrial mobile phone networks caused by the movement of the satellites

 Intra satellite handover

 handover from one spot beam to another  mobile station still in the footprint of the satellite, but in another cell

 Inter satellite handover

 handover from one satellite to another satellite  mobile station leaves the footprint of one satellite

 Gateway handover

 Handover from one gateway to another  mobile station still in the footprint of a satellite, but gateway leaves the

footprint

 Inter system handover

 Handover from the satellite network to a terrestrial cellular network  mobile station can reach a terrestrial network again which might be

cheaper, has a lower latency etc.

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