Communications, Radar & Electronic Warfare (201.. - Index of

180 CommullIcations, Radar and Electronic Warfareaccuracy of reflections from over sea paths. Diffraction in the APM model iscalculated using the parabolic equation rather than the Fresnel method used byother models. This is effectively a finite element analysis technique rather thancalculating diffraction over the most dominant diffraction over edges. It also includesTropospheric scatter for paths beyond the diffraction region. The current version ofAPM does not however account for rainfall and other precipitation or vegetation andurbanisation clutter.10.4 Empirical Models10. 4. 1 Basic Form of Empirical ModelsEmpirically derived models are generated using data gathered from measurementsrather than being based on application of physical models to the scenario under test.The basic form of an empirical model is:whereE,. is the received field strength in dB).lV 1m.d is the path length in km.y is the inverse exponent used with path length.f is the frequency in MHz.K is a coefficient based on power (PBS, normally in dBW or dBm).hBS is the height of the base station in metres.hMS is the height of the mobile station in metres.The received power in dBm can be calculated for an isotropic receiver with animpedance of 50 n by:P( dBM) = Field strength (dB).l Vim) - 20 logf - 77.2Other corrections can be added for clutter and other factors relevant to specificapplications.The form of this type of equation shows that it does not require detailed path profileanalysis. Rather, it produces a generic result useful for network dimensioning andwhere the margin added is used as a mechanism to ensure the received signal is strongenough to provide a serviceable signal.10.4.2 Okwnura Ra ta and COST 231 RataThe Okumura-Hata model was introduced to model mobile phone applications,where the base station antenna is clear of clutter but the mobile station is embedded