28-01-2013, 02:26 PM
Satellite Communications
[attachment=48606]
Useful Orbits (3):
The LEO Orbit
• Circular or inclined orbit with < 1400 km altitude
• Satellite travels across sky from horizon to horizon
in 5 - 15 minutes => needs handoff
• Earth stations must track satellite or have omnidirectional
antennas
• Large constellation of satellites is needed for
continuous communication (66 satellites needed to
cover earth)
• Requires complex architecture
Frequency Spectrum concepts:
• Frequency: Rate at which an electromagnetic wave reverts its
polarity (oscillates) in cycles per second or Hertz (Hz).
• Wavelength: distance between wavefronts in space. Given in
meters as: λ= c/f
Where: c = speed of light (3x108 m/s in vacuum)
f = frequency in Hertz
• Frequency band: range of frequencies.
• Bandwidth: Size or “width” (in Hertz) of a frequency band.
• Electromagnetic Spectrum: full extent of all frequencies from
zero to infinity.
Radio Frequencies (RF)
• RF Frequencies: Part of the electromagnetic spectrum
ranging between 300 MHz and 300 GHz.
– Efficient generation of signal power
– Radiates into free space
– Efficient reception at a different point.
Differences depending on the RF frequency used:
- Propagation effects (diffraction, noise, fading)
- Antenna Sizes
Spectrum Regulation
• International Telecommunication Union (ITU):
-Members from practically all countries in the world
-Allocates frequency bands for different purposes and
distributes them around the planet
-Creates rules to limit RF Interference (RFI) between countries
that reuse same RF bands
-Mediates disputes and deals with harmful RFI when it occurs
-Meets biannually at the World Radiocommunication
Conference (WRC) to discuss rules and allocations
Satellite vs. Terrestrial
• Satellite systems can be deployed in 4-5 years, whereas
terrestrial systems take longer
• One satellite can cover the same region that it would
take multiple base stations of a terrestrial system to
cover
– Better for covering sparsely populated areas
• Of course, satellite systems cost a lot
– Careful studies must be done to assure success – IRIDIUM
• A satellite failure can have catastrophic results, but:
– Launch reliability – 99 %
– Most satellites proven to be reliable beyond predicted lifetime
Future of Satellite Communications (1)
• Growth requires new frequency bands
• Propagation through rain and clouds becomes a
problem as RF frequency is increased
– C band (6/4 GHz)
• Rain has little impact; 99.99% availability is possible
– Ku band (10-12 GHz) & Ka band (20 - 30 GHz)
• Rain has significant impact, affects link availability
[attachment=48606]
Useful Orbits (3):
The LEO Orbit
• Circular or inclined orbit with < 1400 km altitude
• Satellite travels across sky from horizon to horizon
in 5 - 15 minutes => needs handoff
• Earth stations must track satellite or have omnidirectional
antennas
• Large constellation of satellites is needed for
continuous communication (66 satellites needed to
cover earth)
• Requires complex architecture
Frequency Spectrum concepts:
• Frequency: Rate at which an electromagnetic wave reverts its
polarity (oscillates) in cycles per second or Hertz (Hz).
• Wavelength: distance between wavefronts in space. Given in
meters as: λ= c/f
Where: c = speed of light (3x108 m/s in vacuum)
f = frequency in Hertz
• Frequency band: range of frequencies.
• Bandwidth: Size or “width” (in Hertz) of a frequency band.
• Electromagnetic Spectrum: full extent of all frequencies from
zero to infinity.
Radio Frequencies (RF)
• RF Frequencies: Part of the electromagnetic spectrum
ranging between 300 MHz and 300 GHz.
– Efficient generation of signal power
– Radiates into free space
– Efficient reception at a different point.
Differences depending on the RF frequency used:
- Propagation effects (diffraction, noise, fading)
- Antenna Sizes
Spectrum Regulation
• International Telecommunication Union (ITU):
-Members from practically all countries in the world
-Allocates frequency bands for different purposes and
distributes them around the planet
-Creates rules to limit RF Interference (RFI) between countries
that reuse same RF bands
-Mediates disputes and deals with harmful RFI when it occurs
-Meets biannually at the World Radiocommunication
Conference (WRC) to discuss rules and allocations
Satellite vs. Terrestrial
• Satellite systems can be deployed in 4-5 years, whereas
terrestrial systems take longer
• One satellite can cover the same region that it would
take multiple base stations of a terrestrial system to
cover
– Better for covering sparsely populated areas
• Of course, satellite systems cost a lot
– Careful studies must be done to assure success – IRIDIUM
• A satellite failure can have catastrophic results, but:
– Launch reliability – 99 %
– Most satellites proven to be reliable beyond predicted lifetime
Future of Satellite Communications (1)
• Growth requires new frequency bands
• Propagation through rain and clouds becomes a
problem as RF frequency is increased
– C band (6/4 GHz)
• Rain has little impact; 99.99% availability is possible
– Ku band (10-12 GHz) & Ka band (20 - 30 GHz)
• Rain has significant impact, affects link availability