01-12-2012, 04:15 PM
Mobile Communications
Mobile Communications.ppt (Size: 338 KB / Downloads: 30)
Applications
Traditionally
weather satellites
radio and TV broadcast satellites
military satellites
satellites for navigation and localization (e.g., GPS)
Telecommunication
global telephone connections
backbone for global networks
connections for communication in remote places or underdeveloped areas
global mobile communication
Basics
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