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Satellite by gjmpzlaezgx

VIEWS: 20 PAGES: 30

									            Satellite Systems
•Global Coverage without wiring costs
•Independent of population density
•Chiefly for broadcast TV
•Useful addition to exisiting services – e.g. with UMTS


    History                 Handover
    Basics                  Routing
    Localization            Systems
    History of satellite communication
1945 - Arthur C. Clarke “Extra Terrestrial Relays“
1957 - first satellite USSR‟s SPUTNIK
1960- first reflecting communication satellite ECHO
1962 – Telstar launched, an important step
1963 - first geostationary satellite SYNCOM
1965 - first commercial geostationary satellite “Early
    Bird” (INTELSAT I 68 kg): 240 duplex telephone
    channels or 1 TV channel, 1.5 years lifetime
1967-69 – Intelsat II, III; 1200 phone channels
1976 - three MARISAT satellites for maritime
    communication; 40W power, 1.2 m antenna
                   History (Contd)
1982 first mobile satellite telephone system
  INMARSAT-A
1988 first satellite system for mobile phones and data
  communication INMARSAT-C; 600 bps, interface to
  X.25
1993 INMARSAT-M - first digital satellite telephone
  system; still very heavy equipment
1998 global satellite systems for small mobile phones –
  Iridium & Globalstar
Currently about 200 geo satellites.
                 Applications
 Traditional
    Weather, radio and TV broadcast
    military satellites – espionage, warning system
    navigation and localization (GPS)
 Telecommunication – „cable in the sky‟
    global telephone connections & mobiles
    backbone for global networks
    remote/rural areas
    extend cellular systems (AMPS, GSM UMTS),
     need low orbit satellites.
              Satellite Functions
 Transponder
    Receive on one frequency, repeat on
     another frequency (transparent
     transponder)
    May amplify or regenerate (regenerative
     transponder)
 Inter satellite routing
 Error correction is essential
       Classical satellite systems
                                        Inter Satellite Link
                                        (ISL)
            Mobile User
            Link (MUL)                                                 MUL
                                        Gateway Link
                                        (GWL)                  GWL

                          small cells
                          (spotbeams)



                                          base station
                                          or gateway
footprint



                              ISDN                              PSTN         GSM


  PSTN: Public Switched                    User data
  Telephone Network
Satellite Networks
           SATELLITE RECEPTION

 Footprint – area on earth‟s surface where signal can
  be received
 LOS (Line of Sight) to the satellite necessary for
  connection
 Attenuation depends on distance, elevation,
  frequency of carrier and atmosphere
 High elevation means less absorption due to rain,
  fog, atmosphere and buildings; at least 10 degrees
  needed.
    Signal Loss Calculation (qualitative only)

Attenuation or power loss is
  determined by
 gain of sending/receiving
  antennae
 distance between sender
  and receiver
 Carrier frequency
 This affects data rates
  achievable                      L: Loss
                                  f: carrier frequency
Only 10 bps may be achievable
                                  r: distance
with GEOs, compared to 10         c: speed of light
Kbps at 100 km, 2GHz carrier
Atmospheric attenuation
                  Attenuation of
                  the signal in %       Example: satellite systems at 4-6 GHz

                  50



                  40                      rain absorption



                  30
                                                  fog absorption
  e

                  20



                  10
                                    atmospheric
                                    absorption

                          5° 10°         20°        30°          40°   50°
                                    elevation of the satellite
               Satellites - features
 GEO:   geostationary, ~ 36000 km from the
  earth
 LEO (Low Earth Orbit): 500 - 1500 km
 MEO (Medium Earth Orbit) or ICO
  (Intermediate Circular Orbit): 6000 - 20000 km
 HEO (Highly Elliptical Orbit) elliptical orbits
 Microwave, line of sight; GHz range
 Uplink and downlink – different frequencies
Satellite orbit altitudes
Orbits II
                                                   GEO (Inmarsat)


             HEO                                   MEO (ICO)

             LEO                                   inner and outer Van
       (Globalstar,                                Allen belts
           Irdium)


                        earth

                                1000

                                   10000




                                           35768
                                             km




Inner & outer Van-Allen-Belts: ionized particles
2000 - 6000 km, 15000 - 30000 km altitude
       Table 17.1 Satellite frequency bands


          Downlink,         Uplink,      Bandwidth,
Band
            GHz              GHz           MHz
 L          1.5              1.6            15
 S             1.9             2.2             70
C                4             6              500
Ku              11             14             500
Ka              20             30             3500
Satellites in geosynchronous orbit
 Telephony, broadcast TV, Internet backbone
             Geostationary satellites
 35,786 km, equatorial (inclination 0°), 15 yrs life
 24 hr period, synchronous to earth rotation
 fix antenna positions, no adjusting necessary
 large footprint (up to 34% of earth), limited frequency
  reuse; 3 satellites are enough to cover
 bad elevations in areas with latitude above 60°
 high transmit power 10KW, high latency (0.25 s)
 not for global coverage for small mobile phones and
  data transmission,
 suitable for radio & TV
                  MEOs – used for GPS

18000 km altitude
24 to cover the earth
6 hrs to orbit
GPS based on
„triangulation‟ – need
distance from 4 points
Used widely by all sorts of
users
              LEO – global telephony

Polar orbits, 500-2000 km
5-8 years lifetime
90-120 min to orbit
20000 – 25000 km/hr
8000 km diameter footprint
System of satellites = network of switches
    Little Leos - < 1GHz, low data rate messaging
    Big Leos (1-3 GHz) – Globalstar, Iridium
    Broadband Leos (like fibre) - Teledesic
                     LEO systems

 visibility ~ 10 - 40 minutes, period of 95-120 min
 global radio coverage possible, 50-200 satellites
 latency similar to terrestrial long distance: 5 - 10 ms
 smaller footprints (i.e. cells), better frequency reuse
 handover necessary from one satellite to another
 High elevation even in polar regions
 more complex systems due to moving satellites
 Need for routing
   LEOS




ISL Inter Satellite Link
GWL – Gateway Link
UML – User Mobile Link
Iridium 1998 - present
                         66 satellites, 6 orbits, altitude
                           750 km.
                         Originally for global voice, data,
                           fax, paging, navigation
                         Spectrum - 1.6 G, ISL 23 G
                         66 x 48 spot beams or cells
                         2000 cells to cover the earth
                         240 channels of 41 KHz each,
                         can support 253 440 users.

  Applications – telephony ($7 per minute) and data
  2.4 kbps (10 kbps under new ownership)
  Inter satellite links for routing 25 Mbps
  Complex software for call routing via ISL
                Globalstar

48 Satellites, 6 orbits
Altitude of 1400 km
Relaying uses earth stations as well as
  satellites – „bent pipe‟.
Ground stations can create stronger signals
Voice and data at 4.8 kbps
  Teledesic – planned but never materialised

288 satellites, 12 polar orbits,1350 km
BB channels – Internet in the sky
8 satellites form a unit, earth stations are also used
Earth divided into several 10k‟s cells, each assigned a
   time slot to transmit
User terminals to communicate directly
155 M/1.2G up/down links – Ka band
     Routing between satellites, gateways, fixed
     networks: ISL or terrestrial?
Reduced   number of gateways needed with ISL
Best to 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 - ISL

 more complex focusing of antennas between satellites
 satellites need routing software
 high system complexity due to moving routers
 higher fuel consumption, shorter lifetime
 Iridium and Teledesic planned with ISL

Other systems use terrestrial gateways and also terrestrial
  networks
            Localization of mobile stations
Mechanisms similar to GSM, except „base stations‟ are
  satellites.
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
                Localisation of Mobiles
Registration of mobile stations
    Mobile‟s signal received by several satellites,
     reported to gateway(s)
    Localization of the mobile station is 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 SUMR & the appropriate
     satellite
Handover in satellite systems
  More   complex, due to motion of 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
 Handover (Contd.)

 Gateway    handover
    Handover from one gateway to another
    mobile station still in the footprint of a satellite,
     but satellite moves away from the current gateway
 Inter system handover
    Handover from the satellite network to a
     terrestrial cellular network
    mobile station can use a terrestrial network again
     which might be cheaper, have a lower latency.
Overview of LEO/MEO systems

								
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