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.
 Above
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 recent 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 staff currently involved in post-graduate
research in this area.
To discuss how Aegis can help your organisation to solve its
spectrum sharing, interference analysis and propagation modelling
problems, contact us at enquiry-2008@aegis-systems.co.uk.
See also:
|
|
|
|
|
|
|
