This structure is used for the definition of the parameters for the Intelligent Ray Tracing Model. The structure can be passed to the OutdoorPlugIn_ComputePrediction function but this is optional. It can be initialized with default values by using WinProp_Structure_Init_Model_UrbanIRT().Determination of Diffraction Loss using the Empirical Interaction Model If the empirical interaction model is turned on (DiffractionModel = 'e') for the determination of the diffraction loss the following parameters are considered:
LossDiffractionMin (LInc,min)
LossDiffractionMax (LInc,max)
LossDiffractionOut (LDiff)
For the empirical diffraction model the total loss of the diffracted rays is computed depending on the angles Phi and Phi' using the following relations indicated in the next figures:
Figure 1. Empirical Diffraction Model
The empirical diffraction model computes the total diffraction loss in a two step approach based on the three parameters LInc,min, LInc,max and LDiff. In the first step the loss depending on the angle of incidence is determined (see left figure above). For this purpose the first two parameters LInc,min and LInc,max are evaluated. The corresponding loss increases with decreasing angle of incidence (i.e. increasing grazing incidence). Based on this first result the second curve (shown in the right figure above) is evaluated, leading to the total diffraction loss. Basically the diffraction loss increases with increasing interaction angle. However for a difference of 180 degrees, the total diffraction loss is fixed to 6 dB as in this special case the incident wave propagates straight forward while half of the space is shadowed according to the given obstacle. Therefore the range of possible total diffraction losses is given by [6 dB; LInc,max + LDiff]. The given angle dependence is derived from the uniform diffraction theory (UTD) by the evaluation of measurements with different materials (brick, concrete) in an anechoic chamber and can be varied with the parameters LInc,min, LInc,max and LDiff within appropriate limits.
