Refractivity Effects in Propagation

Atmospheric multipath on LOS paths:

• Recommendation P.530

  • geoclimatic factor, path geometry, frequency
  • outage probability on digital links (frequency selective effects)

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• Another important propagation effect due to super-refractivity is that of atmospheric multipath. This is particularly important on LOS links where it can give rise to severe amplitude fading and, for wide-band digital links, to frequency selective fading; the latter can produce outages through increases in the BER.

• Rec. P.530 provides a empirically-based prediction procedure by which fading due to atmospheric multipath can be evaluated. The refractivity behaviour for the region of interest is described by a geoclimatic parameter.

• In wide-band digital systems, the channel is usually modelled by assuming that the signal follows several paths, or rays, from TX to RX. These correspond to the direct path and one or more additional paths due to reflection and/or atmospheric refracted paths. Performance prediction methods make use of such a multi-ray model by integrating the various variables such as delay and amplitude distributions along with a proper model of the equipment elements, e.g. modulator, equalizer, FEC schemes. The signature approach makes use of a laboratory two-ray simulator model, and connects this to other information such as multipath occurrence and link characteristics.