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C.Aravindan Asst.Prof. SRM University
Mobile CommunicationsChapter 5: Satellite Systems
HistoryBasicsLocalization
HandoverRoutingSystems
C.Aravindan Asst.Prof. SRM University
History of satellite communication
1945 Arthur C. Clarke publishes an essay about „Extra Terrestrial Relays“
1957 first satellite SPUTNIK1960 first reflecting communication satellite ECHO1963 first geostationary satellite SYNCOM1965 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 communication1982 first mobile satellite telephone system INMARSAT-A1988 first satellite system for mobile phones and data
communication INMARSAT-C1993 first digital satellite telephone system 1998 global satellite systems for small mobile phones
C.Aravindan Asst.Prof. SRM University
Applications
Traditionally weather satellitesradio and TV broadcast satellites military satellitessatellites for navigation and localization (e.g., GPS)
Telecommunicationglobal telephone connectionsbackbone for global networksconnections for communication in remote places or underdeveloped areasglobal mobile communication
satellite systems to extend cellular phone systems (e.g., GSM or AMPS)
replaced by fiber optics
C.Aravindan Asst.Prof. SRM University
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
C.Aravindan Asst.Prof. SRM University
BasicsSatellites in circular orbits
attractive force on earth gravity Fg = m g (R/r)²centrifugal force Fc = m r ω²m: mass of the satelliteR: radius of the earth (R = 6370 km)r: distance from the satellite 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 orbitFg = Fc
The distance of a satellite to the earth’s surface depends on its rotation frequency f.
The satellite period equals 24 hours for a distance of 35,786 km.
32
2
)2( fgRrπ
=
C.Aravindan Asst.Prof. SRM University
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 distance35,786 km
C.Aravindan Asst.Prof. SRM University
Basics:
elliptical or circular orbitscomplete rotation time depends on distance satellite-earthInclination: angle between orbit and equatorElevation: angle between satellite and horizonLOS (Line of Sight) to the satellite necessary for connection
high elevation needed, less absorption due to e.g. buildingsUplink: connection base station - satelliteDownlink: connection satellite - base stationtypically separated frequencies for uplink and downlink
transponder used for sending/receiving and shifting of frequenciestransparent transponder: only shift of frequenciesregenerative transponder: additionally signal regeneration
The inclination angle: angle between the equatorial plane and plane by satellite orbit.Inclination angle 0 degrees - satellite above the equator.If satellite does not have circular orbit, the closest point to the earth is called the PERIGEE.The elevation angle: angle between centre of satellite beam and plane tangential to earth surface so called FOORPRINT.
C.Aravindan Asst.Prof. SRM University
C.Aravindan Asst.Prof. SRM University
Inclination
inclination δ
δ
satellite orbit
perigee
plane of satellite orbit
equatorial plane
C.Aravindan Asst.Prof. SRM University
Elevation
Elevation:angle ε between center of satellite beam and surface
εminimal elevation:elevation needed at leastto communicate with the satellite
C.Aravindan Asst.Prof. SRM University
Link budget of satellitesParameters like attenuation or received power determined by four
parameters:sending powergain of sending antennadistance between sender and receiverElevation angles
Problems:varying strength of received signal due to multipath propagation .The power of the received signal decreases with the square of the distance.(transmit power, antenna diameter, operating frequency)Attenuation of signal due to atmospheric condition, depending on elevation the signal has to penetrate smaller and larger % of atmosphere.Rain absorption quite strong in tropical areas.>10 degree is useless for comm.,
24⎟⎠⎞
⎜⎝⎛=
cfrL π
L: Lossf: carrier frequencyr: distancec: speed of light
C.Aravindan Asst.Prof. SRM University
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
C.Aravindan Asst.Prof. SRM University
Four different types of satellite orbits can be identified depending on the shape and diameter of the orbit:
GEO: geostationary orbit, ca. 36000 km above earth surface LEO (Low Earth Orbit): ca. 500 - 1500 kmMEO (Medium Earth Orbit) or ICO (Intermediate Circular Orbit): ca. 6000 - 20000 kmHEO (Highly Elliptical Orbit) elliptical orbits
Orbits I:
C.Aravindan Asst.Prof. SRM University
Orbits II
earth
km35768
10000
1000
LEO (Globalstar,
Irdium)
HEO
inner and outer VanAllen belts
MEO (ICO)
GEO (Inmarsat)
Van-Allen-Belts:ionized particles 2000 - 6000 km and15000 - 30000 km above earth surface
C.Aravindan Asst.Prof. SRM University
Geostationary satellitesOrbit 35,786 km distance to earth surface, orbit in equatorial plane
(inclination 0°)complete rotation exactly one day, satellite is synchronous to earth rotation
Advantages:fixed antenna positions, no adjusting necessary.GEOs are ideal for TV and Broadcasting.Lifetime are high about 15yrs.No handover due to the large footprint.
Disadvantages:Northern & southern regions more problems on receiving due to low elevation above a latitude of 60deg i.e greater antenna needed.Shading of signals in cities due to High buildings.high transmit power neededhigh 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
C.Aravindan Asst.Prof. SRM University
LEO systems
Orbit ca. 500 - 1500 km above earth surfacevisibility of a satellite ca. 10 - 40 minutesglobal radio coverage possible Provides omnidirectional antenna for mobile terminalsLow power transmit 1Wsmaller footprints, better frequency reusehandover 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)
Bankruptcy in 2000, deal with US DoD (free use, saving from “deorbiting”)
Globalstar (start 1999, 48 satellites)Not many customers (2001: 44000), low stand-by times for mobiles
Disadvanyages:
C.Aravindan Asst.Prof. SRM University
C.Aravindan Asst.Prof. SRM University
MEO systems
Orbit ca. 5000 - 12000 km above earth surfacecomparison with LEO systems:
slower moving satellites less satellites neededsimpler system designfor many connections no hand-over neededhigher latency, ca. 70 - 80 mshigher sending power neededspecial antennas for small footprints needed
Example: ICO (Intermediate Circular Orbit, Inmarsat) start ca. 2000
Bankruptcy, planned joint ventures with Teledesic, Ellipso – cancelled again, start planned for 2003
C.Aravindan Asst.Prof. SRM University
Routing
One solution: inter satellite links (ISL)reduced number of gateways needed 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:more complex focusing of antennas between satellites high system complexity due to moving routershigher fuel consumptionthus shorter lifetime
Iridium and Teledesic planned with ISLOther systems use gateways and additionally terrestrial networks
C.Aravindan Asst.Prof. SRM University
Localization of mobile stations
Mechanisms similar to GSMGateways maintain registers with user data
HLR (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
Registration of mobile stationsLocalization of the mobile station via the satellite’s positionrequesting user data from HLRupdating VLR and SUMR
Calling a mobile stationlocalization using HLR/VLR similar to GSMconnection setup using the appropriate satellite
C.Aravindan Asst.Prof. SRM University
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 handoverhandover from one spot beam to anothermobile station still in the footprint of the satellite, but in another cell
Inter satellite handoverhandover from one satellite to another satellitemobile station leaves the footprint of one satellite
Gateway handoverHandover from one gateway to anothermobile station still in the footprint of a satellite, but gateway leaves the footprint
Inter system handoverHandover from the satellite network to a terrestrial cellular networkmobile station can reach a terrestrial network again which might be cheaper, has a lower latency etc.
C.Aravindan Asst.Prof. SRM University
Overview of LEO/MEO systems
Iridium 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$