30-01-2012, 02:18 PM
Mobile Communications Chapter 5: Satellite Systems
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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
Basics
Satellites in circular orbits
attractive force Fg = m g (R/r)²
centrifugal force Fc = m r ²
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)
: angular velocity ( = 2 f, f: rotation frequency)
Stable orbit
Fg = Fc
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
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
