Recommendation ITU
-R P.452 presents in its overall description a summary of the principal propagation
mechanismmechanisms.
It It explains that interference may arise through a range of propagation mechanisms whose individual dominance depends on climate, radio frequency, time percentage of interest, distance and path topography. At any one time a single mechanism or more than one may be present. The principal interference propagation mechanisms are as follows:
- Line-of-sight (Figure 462): The most straightforward interference propagation situation is when a line‑of‑sight transmission path line of sight (LoS) transmission path exists under normal (i.e. well‑mixed) atmospheric conditions. However, an additional complexity can come into play when subpath diffraction causes a slight increase in signal level above that normally expected. Also, on all but the shortest paths (i.e. paths longer than about 5 km) signal levels can often be significantly enhanced for short periods of time by multipath and focusing effects resulting from atmospheric stratification (see Figure 463).
- Diffraction (Figure 462): Beyond line-of-sight ( LoS ) and under normal conditions, diffraction effects generally dominate wherever significant signal levels are to be found. For services where anomalous short-term problems are not important, the accuracy to which diffraction can be modelled generally determines the density of systems that can be achieved. The diffraction prediction capability must have sufficient utility to cover smooth‑earth, discrete obstacle and irregular (unstructured) terrain situations.
- Tropospheric scatter (Figure 462): This mechanism defines the “background” interference level for longer paths (e.g. more than 100-150 km) where the diffraction field becomes very weak. However, except for a few special cases involving sensitive receivers or very high power interferers (e.g. radar systems), interference via troposcatter will be at too low a level to be significant.
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