Satellite Communications: The Magic of Ka-band

The electromagnetic spectrum is a scarce commodity with a host of wireless technologies vying for the same spectrum. Today's applications demand higher throughput and correspondingly higher use of the spectrum. Satellite technology has undergone progressive elaboration and has moved from the traditional C band to the Ku-band, providing higher capacities on the way.

To overcome the constraints of spectrum and at the same time provide higher throughput is a daunting challenge, which the evolving satellite technology has mastered very well. The other equally challenging task is to reduce cost per bit and make satellite technology further affordable to the plethora of users across businesses and individuals. Both these one-time seemingly insurmountable challenges have been very well answered by the emergence of Ka-band satellites on the horizon. Today satellites like Hughes Jupiter are designed to deliver 100+ Gbps of throughput, something unheard of a few years back.

Technology Enablers

As the frequency moves towards the higher part of the spectrum, the challenges of dealing with the atmospheric vagaries and weather effects become critical. Ka-band is no exception to the rule and is also affected by the atmospheric absorption and rain. However a look at the atmospheric effects indicates that the maximum attenuation that can happen at Ka-band is around 1 dBm, something that can be easily handled.

Fade Mitigation Techniques

Over the years a number of fade mitigation techniques have developed and used in the present Ku- and C-band satellite technology, combined with strong prediction algorithms and advanced coding these techniques have extended to the Ka-band. Terminals like the Hughes HN9940 cater for the both the Ku-and-Ka bands and hence provide an easy option to migrate to high-throughput satellites without any cost impact. The frequencies being used for Ka-band uplink are in the range of 27-30 GHz, while the down links are in the range of 18-21 GHz. The down-link frequencies are nearer to the Ku-band and hence less prone to fade. Fade mitigation techniques (FMT) can be broadly classified under the following heads:

• Power Control: In this the output, power is increased to overcome the fade
• Adaptive Wave Forms: Adaptive coding, adaptive modulation, and data rate control form part of this
• Diversity: The link is shifted to alternate frequency in case the frequency in use faces degradation
• Layer 2 Algorithms: This method does not mitigate the fade affects directly but depends on repeat transmission either on request or automatically based on link-prediction algorithms.

To address the rain fade and other atmospheric conditions that may arise, all the above techniques would need to be applied together as any one technique would not give the desired results. The advent of DVB-S2 with adaptive coding and modulation combined with an advanced LDPC algorithm has been very effective in handling fade. Hughes HN9400 effectively uses these features both on the uplink and down link.

Typical Architecture

Ka-band satellites typically employ the multiple spot beams to achieve high throughput. The spot beams give flexibility of frequency re-use, something akin to the mobile cell architecture. This opens the possibility of having group and sub-group based content. A typical architecture of 48 spot beams and 4 sub-bands would provide 12 times more capacity than a normal single-beam satellite.

Conclusion

Ka-band high-throughput satellites have arrived and will be the main stay of tomorrow's communication needs, creating a revolution in satellite services. As the technology evolves further, satellites will move into higher realms of the spectrum and will progress beyond the 40 GHz band into the Q/V bands, thereby providing still higher throughput at lower costs. These would be the super high-throughput satellites.

Vidyut Kak
senior director-operations, Hughes Communications India
vadmail@cybermedia.co.in