KA BAND

Satellite Communications Glossary

Satellite CommunicationsSatellite Frequencies — Ka Band

 

 

 

 

 

 

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Information on Ka Band

As the C and Ku bands begin to become congested, significant interest has been generated in Ka band (30/20 GHz) for commercial satellite communications applications. Early research was conducted from the 1970s with satellites from Japan, US, Italy and the European Space Agency (ESA), followed most recently by NASA’s Advanced Communications Technology Satellite (ACTS) in 1993. While development of Ka-band satellites have been slow with only several in orbit in 2003 (Optus C1 was launched in June 2003), several hundred applications have been filed with ITU for Ka-band satellites. Iridium uses Ka-band frequencies for inter-satellite links as well as for links to Earth station gateways. The principal interest in the band is for the delivery of interactive multimedia services.
     Ka-band systems have a number of advantages. Perhaps the most significant is the increased bandwidth, in a band that does not have to be shared with terrestrial systems; Ka band offers 2-3 GHz of bandwidth, approximately twice that available in Ku band and some five times more than C band. Ka-band components are much smaller due to the smaller wavelength allowing more devices and smaller antennas on the same-sized platform. Footprints for Ka-band spot beams can also be made smaller, often from the same antenna, facilitating frequency re-use.
     The difficulty in Ka band is that rain attenuation becomes so severe that link margins of the order of 14 dB are required. These large losses can be accommodated during periods of heavy rain by increased satellite transmit power (possible because of the exclusive access to the band); Ka-band transmit powers may be 3-4 dB higher than X-band or C-band equivalents. Antennas can also be made larger; since gain of an antenna increases with the square of the frequency, the same size antenna as in C or Ku band will naturally have a higher gain (although this will be necessary to offset the free-space loss over the path, which will be higher by the same degree). Ka-band antennas must also be formed to much higher precision than for lower frequency bands to reduce loss. In any case, Ka-band systems are currently more expensive, although the full benefits may be realised over time as the price of components drops. Ka-band LNAs can achieve noise figures of 3-4 dB and TWTs are available to deliver 45-90 W.
     Reliable prediction of propagation impairments becomes critical in Ka band, as do mechanisms for overcoming them. Since increased antenna size is already necessary to compensate for additional free-space loss, beam shaping can be used to increase gain in those directions that are experiencing fading (although this is at the expense of gain in other directions). Up-link power control can be used at the Earth station which, based on measurement of signal strength, can increase and decrease the uplink power to accommodate fading. Additionally, site diversity can be used to overcome localised fading: provided sites are separated by more than about 10 km, the fading experienced in each location will be uncorrelated. Site diversity is expensive, however, and is really only an option in large cooperative networks where some traffic sharing arrangement can be made.


Other topics in our resources on Satellite Communications related to Ka Band include: 
 
  • P Band
  • L and S Band
  • Ku Band
  • C band
  • X Band
  • Extremely high frequency (EHF)
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