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Satellite Systems
History Basics Localization
Handover Routing Systems
•Global Coverage without wiring costs
•Independent of population density
•Chiefly for broadcast TV
•Useful addition to exisiting services – e.g. with UMTS
History of satellite communication
1945 - Arthur C. Clarke “Extra Terrestrial Relays“1957 - first satellite USSR’s SPUTNIK1960- first reflecting communication satellite ECHO1962 – Telstar launched, an important step1963 - first geostationary satellite SYNCOM1965 - 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 broadcastmilitary satellites – espionage, warning systemnavigation and localization (GPS)
Telecommunication – ‘cable in the sky’global telephone connections & mobilesbackbone for global networksremote/rural areasextend 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
base stationor gateway
Classical satellite systems
Inter Satellite Link (ISL)
Mobile User Link (MUL) Gateway Link
(GWL)
footprint
small cells (spotbeams)
User data
PSTNISDN GSM
GWL
MUL
PSTN: Public Switched Telephone Network
Satellite NetworksSatellite 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
achievableOnly 10 bps may be achievablewith GEOs, compared to 10Kbps at 100 km, 2GHz carrier
24
c
frL
L: Lossf: carrier frequencyr: distancec: speed of light
Atmospheric attenuationExample: 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
Satellites - features
GEO: geostationary, ~ 36000 km from the earth
LEO (Low Earth Orbit): 500 - 1500 kmMEO (Medium Earth Orbit) or ICO
(Intermediate Circular Orbit): 6000 - 20000 kmHEO (Highly Elliptical Orbit) elliptical orbitsMicrowave, line of sight; GHz rangeUplink and downlink – different frequencies
Satellite orbit altitudes
Orbits II
earth
km
35768
10000
1000
LEO (Globalstar,
Irdium)
HEO
inner and outer VanAllen belts
MEO (ICO)
GEO (Inmarsat)
Inner & outer Van-Allen-Belts: ionized particles2000 - 6000 km, 15000 - 30000 km altitude
Table 17.1 Satellite frequency bandsTable 17.1 Satellite frequency bands
Band Downlink,
GHzUplink,
GHzBandwidth,
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 orbitTelephony, 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 km5-8 years lifetime90-120 min to orbit20000 – 25000 km/hr8000 km diameter footprintSystem of satellites = network of switches
Little Leos - < 1GHz, low data rate messagingBig Leos (1-3 GHz) – Globalstar, IridiumBroadband 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 - present66 satellites, 6 orbits, altitude
750 km.Originally for global voice, data,
fax, paging, navigationSpectrum - 1.6 G, ISL 23 G66 x 48 spot beams or cells2000 cells to cover the earth240 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 MbpsComplex 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 ISLBest to forward connections or data packets within the satellite network as long as possibleOnly 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 dataHLR (Home Location Register): static user dataVLR (Visitor Location Register): (last known)
location of the mobile stationSUMR (Satellite User Mapping Register):
satellite assigned to a mobile stationpositions of all satellites
Localisation of Mobiles
Registration of mobile stationsMobile’s signal received by several satellites,
reported to gateway(s)Localization of the mobile station is via the
satellite’s positionrequesting user data from HLRupdating VLR and SUMR
Calling a mobile station localization using HLR/VLR similar to GSMconnection setup using SUMR & the appropriate
satellite
Handover in satellite systems
More complex, due to motion of satellitesIntra satellite handover
handover from one spot beam to anothermobile station still in the footprint of the
satellite, but in another cellInter satellite handover
handover from one satellite to another satellite
mobile station leaves the footprint of one satellite
Handover (Contd.)
Gateway handoverHandover from one gateway to anothermobile station still in the footprint of a satellite,
but satellite moves away from the current gatewayInter 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 systemsIridium Globalstar ICO Teledesic
# satellites 66 + 6 48 + 4 10 + 2 288altitude(km)
780 1414 10390 ca. 700
coverage global 70° latitude global globalmin.elevation
8° 20° 20° 40°
frequencies[GHz(circa)]
1.6 MS29.2 19.5 23.3 ISL
1.6 MS
2.5 MS 5.1
6.9
2 MS
2.2 MS 5.2
7
19
28.8 62 ISL
accessmethod
FDMA/TDMA CDMA FDMA/TDMA FDMA/TDMA
ISL yes no no yesbit rate 2.4 kbit/s 9.6 kbit/s 4.8 kbit/s 64 Mbit/s
2/64 Mbit/s # channels 4000 2700 4500 2500Lifetime[years]
5-8 7.5 12 10
costestimation
4.4 B$ 2.9 B$ 4.5 B$ 9 B$