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Propagation Modelling

At the lowest radio frequencies, below about 100 kHz, energy propagates in the waveguide formed between the Earth and the ionosphere, and suffers little attenuation over great distances. As the frequency increases, separate ‘groundwave’ and ‘skywave’ components can be distinguished, the former influenced largely by the conductivity of the ground and the latter reflected from the ionosphere. At the frequencies used for AM broadcasting (the ‘long wave’ and ‘medium wave’ bands), interference between these two components limits useful service areas, particularly after dark when skywave signals are strongest.

As frequencies increase into the shortwave region (3-30 MHz), the groundwave component diminishes rapidly, leaving the skywave component, which is widely used for global communications and international broadcasting. The daily and seasonal variation in the ionisation of the upper atmosphere affects the range of usable frequencies, so users of this part of the spectrum must have multiple assignments.

Coverage MapAbove around 30 MHz, at the frequencies used for radio and television broadcasting and local communications, the ionospheric component rapidly becomes negligible, leaving a ‘space wave’, which is influenced largely by the topography and clutter (buildings, trees) over which it passes, and by refractive effects in the lower atmosphere. These propagation effects are familiar to anybody listening to an FM car radio in mountainous areas, or suffering co-channel interference from distant stations to television reception during high-pressure weather conditions in the summer. The Aegis Spectrum Engineering Toolkit uses UK Ordnance Survey and other terrain data to accurately calculate the effects of diffraction at such frequencies. It also interfaces to UK census data, enabling us to predict population coverage with great accuracy.

As frequencies increase into the microwave region (greater than around 3 GHz) the same influences apply, but the atmosphere itself can no longer be considered transparent to radio waves. The molecules of constituent gasses, particularly oxygen and water vapour, exhibit strong, absorbent resonances at certain frequencies leading to high values of specific attenuation. In the oxygen absorption line at around 60 GHz, for example, radio waves are attenuated by some 10-20 dB per kilometre. Such attenuation limits link distances, but it allows intensive re-use of the same frequency. In a project, aimed at investigating the potential interference into Earth-sensing satellite radiometer systems operating at above 100 GHz, we modelled the atmospheric attenuation on Earth-space slant paths, using the line-by-line model developed by Liebe, and taking into account a variety of temperature, pressure and humidity height profiles.

At frequencies above around 10 GHz, attenuation due to rainfall becomes a significant factor in determining link budgets for both terrestrial and satellite systems. The prediction of the temporal, spatial, climatic and frequency dependence of such attenuation is an active research field, and is assuming ever-greater importance as the frequencies used for civil telecommunications systems extend beyond 40 GHz.

At Aegis, we have over a decade of experience modelling the effects described above. In the past, when modelling spectrum-sharing scenarios, it was often sufficient to make the worst-case assumption of free-space propagation: with the increasing congestion of the spectrum, this is no longer appropriate, and more realistic models are required. Study Group 3 of the ITU-R is continuing to develop suitable models and algorithms based on the latest academic research, and we have implemented the majority of these in the Aegis Spectrum Engineering Toolkit. For cases not covered by existing ITU-R Recommendations, we have developed models tailored to specific situations.

We place a high priority on ensuring awareness of the latest developments and have contributed to the development of the ITU-R propagation models through measurement campaigns and inputs into and attendance at Study Group 3 meetings. For example Aegis were involved in the development of the Recommendation P.1812 giving a propagation model for use in point-to-area predictions at frequencies between 30 MHz and 3 GHz.

To discuss how Aegis can help your organisation to solve its spectrum sharing, interference analysis and propagation modelling problems, contact us at enquiry-2015@aegis-systems.co.uk.

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See also:

Aegis Spectrum Engineering Toolkit | Interference Analysis | Software Development | Measurement

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