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How does it help?

Satellite transmissions at frequencies above 10GHz suffer higher attenuation when passing through rain or cloud than those in lower frequency bands. It is possible that fade mitigation techniques such as spatial, or site diversity will offer an improvement over the simple allocation of a fixed fade margin and hence enable more efficient use of the spectrum.

Explanation of the technology

Existing satellite planning methods use a fixed fade margin. This results in an excessive margin being used for much of the time, and thus inefficient use of the spectrum. This approach becomes increasingly problematic at frequencies above 10GHz as margins increase.

For satellite systems operating at frequencies above 12 GHz, site diversity is a commonly proposed fade mitigation technique. However, the gains achievable through this approach are related to the climatology of the region, and most existing work has been undertaken in the US which has a different climatology to the UK. In the diversity approach two or more satellite ground stations are used to receive the same signal. Combining the signal received at both minimises the signal losses that may result from rainfall which will typically only affect one. This allows a reduction in the fade margin and hence the power needed to transmit to a satellite This enables reduced coordination distances between Earth stations and terrestrial links, or alternatively the benefit can be accrued as a capacity improvement of an earth-space link with the same transmit power.

Current state of the art and our research

The work here addresses medium and long-range site diversity in the UK to quantify the gains achievable in the UK making measurements over long timescales. This is necessary to give statistical confidence for slow varying effects, such as the annual and monthly variations in propagation due to weather changes.

Measurements are currently underway at 20.7GHz using the beacon carried on the US Department of Defense (DoD) satellite UFO-9. The research utilises three receivers. Two receivers are located in the South of England (at Sparsholt and Chilbolton, both in Hampshire) with a 7.5 km separation. A third receiver is located in Dundee in Scotland, allowing long distance site diversity measurements to be made. A meteorological radar, based at Chilbolton, provides associated data on the formation and movement of rain fields and clouds, enabling accurate models to be developed.

Measurements shown in Figure 1, suggest that when transmitting at 20 GHz with a 7.5 km separation distance between the two earth stations the diversity fade margin is up to 6 dB less than a single site for small time percentages which are those of significance for services where high availability statistics are required, for example 99.99% availability. As discussed above this benefit may be realised either in a reduction in the coordination distances between Earth stations and terrestrial links, freeing up more geographical space for other terrestrial services, or it may be realised as a capacity improvement over an earth-space link with the same transmit power. The benefit of experiencing 6 dB less fading is that a capacity increase of up to 35% over the earth-space link can be made without increasing the bandwidth of transmission. Initial analyses indicate that with a single standard satellite transponder (assuming a 54MHz bandwidth), the use of diversity may pay for itself after 4 years of usage.

Figure 1: Statistics of diversity gain relative to single sites (Sparsholt and Chilbolton, 7.5km separation distance) in comparison with the ITU-R model presented in Rec. 618-8. The diversity gain expresses, for a given time percentage, the difference between the single site and diversity attenuation values. For a given availability this provides a measure of the reduction that is possible in fade margin when using site diversity.

Conclusions

High frequency satellite based communications are one means of providing high bandwidth services. To make efficient and economic use of these high frequencies, new techniques to compensate for rain and cloud fades are required, commonly called fade mitigation techniques or fade countermeasures. This research has demonstrated that a fade mitigation technique called site diversity will offer an improvement over the traditional method of allocating a fixed fade margin and hence enable more efficient use of the spectrum.

• Spectrum exploitation for non-terrestrial systems operating above 10 GHz Final Report [pdf]
  This report is commissioned by Ofcom to provide an independent view on the efficient use of the radio spectrum in the UK. The assumptions, conclusions and recommendations expressed in this report are entirely those of the consultants and should not be attributed to Ofcom

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