Mobile Communications Satellite Systems

Transcript

Mobile Communications Satellite Systems q  History q  Basics q  Orbits Mobile Communications q  LEO, MEO, GEO q  Examples q  Handover, Routing Satellite Systems 1 History of satellite communication 1945 1957 1960 1963 1965 1976 1982 1988 1993 1998 Arthur C. Clarke publishes an essay about „Extra Terrestrial Relays“ first satellite SPUTNIK first reflecting communication satellite ECHO first geostationary satellite SYNCOM first commercial geostationary satellite „Early Bird“ (INTELSAT I): 240 duplex telephone channels or 1 TV channel, 1.5 years lifetime three MARISAT satellites for maritime communication first mobile satellite telephone system INMARSAT-A first satellite system for mobile phones and data communication INMARSAT-C first digital satellite telephone system global satellite systems for small mobile phones Mobile Communications Satellite Systems 2 Applications q  Traditionally q  q  q  q  q  weather satellites radio and TV broadcast satellites military satellites satellites for navigation and localization (e.g., GPS) Telecommunication q  q  q  q  global telephone connections replaced by fiber optics backbone for global networks connections for communication in remote places or underdeveloped areas global mobile communication è satellite systems to extend cellular phone systems Mobile Communications Satellite Systems 3 Classical satellite systems Inter Satellite Link (ISL) Mobile User Link (MUL) Gateway Link (GWL) MUL GWL small cells (spotbeams) base station or gateway footprint ISDN PSTN: Public Switched Telephone Network Mobile Communications PSTN GSM User data Satellite Systems 4 Basics Satellites in circular orbits q  q  q  q  q  q  q  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 q  Fg = Fc 2 r=3 Mobile Communications gR 2 (2π f ) Satellite Systems 5 Satellite period and orbits 24 satellite period [h] velocity [ x1000 km/h] 20 16 12 8 4 synchronous distance 35,786 km 10 Mobile Communications 20 radius 30 40 x106 m Satellite Systems 6 Basics q  q  q  q  q  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 è high elevation needed, less absorption due to e.g. buildings q  q  q  Uplink: connection base station - satellite Downlink: connection satellite - base station typically separated frequencies for uplink and downlink q  q  q  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 plane of satellite orbit satellite orbit perigee δ inclination δ equatorial plane Mobile Communications Satellite Systems 8 Elevation Elevation: angle ε between center of satellite beam and surface minimal elevation: elevation needed at least to communicate with the satellite Mobile Communications ε Satellite Systems 9 Link budget of satellites Parameters like attenuation or received power determined by four parameters: L: Loss q  sending power f: carrier frequency q  gain of sending antenna r: distance c: speed of light q  distance between sender 2 and receiver ⎛ 4π r f ⎞ L = ⎜ ⎟ q  gain of receiving antenna ⎝ c ⎠ Problems q  varying strength of received signal due to multipath propagation q  interruptions due to shadowing of signal (no LOS) Possible solutions q  Link Margin to eliminate variations in signal strength q  satellite diversity (usage of several visible satellites at the same time) helps to use less sending power Mobile Communications Satellite Systems 10 Atmospheric attenuation Attenuation of the signal in % Example: satellite systems at 4-6 GHz 50 40 rain absorption 30 fog absorption ε 20 10 atmospheric absorption 5° 10° 20° 30° 40° 50° elevation of the satellite Mobile Communications Satellite Systems 11 Orbits I Four different types of satellite orbits can be identified depending on the shape and diameter of the orbit: q  GEO: geostationary orbit, ca. 36000 km above earth surface q  LEO (Low Earth Orbit): ca. 500 - 1500 km q  MEO (Medium Earth Orbit) or ICO (Intermediate Circular Orbit): ca. 6000 - 20000 km q  HEO (Highly Elliptical Orbit) elliptical orbits Mobile Communications Satellite Systems 12 Orbits II GEO (Inmarsat, Thuraya) HEO MEO (ICO, GPS) LEO (Globalstar, Irdium) inner and outer Van Allen belts earth 1000 10000 Van-Allen-Belts: ionized particles 2000 - 6000 km and 15000 - 30000 km above earth surface Mobile Communications 35768 km Satellite Systems 13 LEO systems Orbit ca. 500 - 1500 km above earth surface q  visibility of a satellite ca. 10 - 40 minutes q  global radio coverage possible q  latency comparable with terrestrial long distance connections, ca. 5 - 10 ms q  smaller footprints, better frequency reuse q  but now handover necessary from one satellite to another q  many satellites necessary for global coverage q  more complex systems due to moving satellites Examples: q  Iridium (start 1998, 66 satellites) q  Globalstar (start 2000, 48 satellites) Mobile Communications Satellite Systems 14 MEO systems Orbit ca. 5000 - 12000 km above earth surface comparison with LEO systems: q  slower moving satellites q  less satellites needed q  simpler system design q  for many connections no hand-over needed q  higher latency, ca. 70 - 80 ms q  higher sending power needed q  special antennas for small footprints needed Example: q  ICO (Intermediate Circular Orbit, Inmarsat) q  GPS, GALILEO Mobile Communications Satellite Systems 15 MEO systems: GPS (Global Positioning System) Basic concept of GPS q  GPS receiver calculates its position (latitude, longitude, and altitude) by precisely timing the signals sent by GPS satellites high above the Earth q  Each satellite continually transmits messages that include q  q  q  q  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 à  q  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” q  Accuracy: „some meter“ with Wide Area Augmentation System WAAS Adopted from Wikipedia Mobile Communications Satellite Systems 16 MEO systems: GPS (Global Positioning System) Structure: three major segments 1.  space segment (SS) q  2.  control segment (CS) q  q  q  q  3.  orbiting GPS satellites, or Space Vehicles (SV) master control station (MCS), alternate master control station, four dedicated ground antennas and six dedicated monitor stations user segment (U.S.) q  user devices US Air Force develops, maintains, and operates space & ctrl segments Adopted from Wikipedia Mobile Communications Satellite Systems 17 MEO systems: GPS (Global Positioning System) q  Space segment (SS) q  q  orbiting GPS satellites, or Space Vehicles (SV) 24 SVs: six planes with four satellites each (plus some extra) approximately 55° inclination l  orbits are arranged such that >= 6 satellites are always within LOS à  four satellites are not evenly spaced (90 degrees) within each orbit, but 30, 105, 120, and 105 degrees l  q  q  q  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) •  of 18.10.2001, 00:00 h to 19.10.2001, 00:00 h •  orbit time is slightly shifted (about 4 minutes) in 24 h •  21:30 zone of sight Mobile Communications Satellite Systems 19 MEO systems: GPS (Global Positioning System) Position of the monitor stations and the master control station (Earthmap:NASA; http://visibleearth.nasa.gov/) q  q  q  “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°) è complete rotation exactly one day, satellite is synchronous to earth rotation q  fix antenna positions, no adjusting necessary q  satellites typically have a large footprint (up to 34% of earth surface!), therefore difficult to reuse frequencies q  bad elevations in areas with latitude above 60° due to fixed position above the equator q  high transmit power needed q  high latency due to long distance (ca. 275 ms) è 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 Mobile Communications Satellite Systems 21 GEO Systems: Example Thuraya q  regional satellite phone provider q  q  q  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: q  q  ~ 250,000 (March 2006) “sold in excess of 600,000 satellite handheld phones” (2013) Structure of Thuraya spot beams Mobile Communications Satellite Systems 22 GEO Systems: Example Thuraya Mobile Communications Satellite Systems 23 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 •  high-speed data transfer via a notebook-sized terminal (ThurayaDSL with 144 kbit/s and ThurayaIP with 444 kbit/s) •  GPS is supported by all handsets •  value-added services, e.g., news, call back / waiting, missed calls •  one-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 Mobile Communications Satellite Systems 24 GEO Systems: Examples SkyDSL, getinternet Internet access via satellite q  Interesting for users in areas where no other broadband Internet access available q  q  q  q  q  GEO satellite for downlink uplink (also for requests) via modem/telephone or other mobil comm. or also via satellite data rate up to max. 36000 KBit/s # users q  q  has technically not much to do with DSL ca. 100.000 in Germany large latency due to GEO q  already signal propagation for distance of 2 * 36000 km: ~240 ms Mobile Communications Satellite Systems 25 Routing One solution: inter satellite links (ISL) q  reduced number of gateways needed q  forward connections or data packets within the satellite network as long as possible q  only one uplink and one downlink per direction needed for the connection of two mobile phones Problems: q  more complex focusing of antennas between satellites q  high system complexity due to moving routers q  higher fuel consumption q  thus shorter lifetime Iridium and Teledesic planned with ISL Other systems use gateways and additionally terrestrial networks Mobile Communications Satellite Systems 26 Localization of mobile stations Mechanisms similar to GSM Gateways maintain registers with user data q  q  q  HLR (Home Location Register): static user data VLR (Visitor Location Register): (last known) location of the mobile station SUMR (Satellite User Mapping Register): l  l  satellite assigned to a mobile station positions of all satellites Registration of mobile stations q  q  q  Localization of the mobile station via the satellite’s position requesting user data from HLR updating VLR and SUMR Calling a mobile station q  q  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 q  Intra satellite handover l  l  q  Inter satellite handover l  l  q  handover from one satellite to another satellite mobile station leaves the footprint of one satellite Gateway handover l  l  q  handover from one spot beam to another mobile station still in the footprint of the satellite, but in another cell Handover from one gateway to another mobile station still in the footprint of a satellite, but gateway leaves the footprint Inter system handover l  l  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. Mobile Communications Satellite Systems 28