This empirical model is a combination of the models from J. Walfisch and F. Ikegami. It was further developed by the COST 231 project. It is now called Empirical COST-Walfisch-Ikegami Model.
The model considers only the buildings in the vertical plane between the transmitter and the receiver. The accuracy of this empirical model is quite high because in urban environments especially the propagation over the rooftops (multiple diffractions) is the most dominant part. Only wave guiding effects due to multiple reflections are not considered.
The main parameters of the model are:
- Frequency f (800...2000 MHz)
- Height of the transmitter hTX (4...50 m)
- Height of the receiver hRX (1...3 m)
- Distance d between transmitter and receiver (20...5000 m)
Parameters depending on the buildings in the vertical plane between transmitter and receiver:
- Mean value of building heights hROOF
- Mean value of widths of streets w
- Mean value of building separation b
The orientation of the road with respect to the Tx-Rx line is not considered in the WinProp implementation, because the orientation of the road cannot be determined for all pixels (e.g. in courtyards, on crossings,...) .
As only these characteristic values are considered for the computation, the Walfisch-Ikegami model is a statistical model.
But the model distinguishes between two situations, the "line of sight" (LOS) and the "none line of sight" (NLOS) situation.
LOS situation:
For the LOS-case the prediction is very easy, as only one equation with two parameters is necessary.
This LOS equation is similar to the free space loss equation. It was modified after evaluating measurements in European cities. If the distance is d = 20 m , the loss is almost equal to the free space loss at the same distance. The following graph shows the comparison between free space loss and the transmission loss with the LOS equation over the full range of the distance d.
NLOS situation:
The NLOS equations are more complicated. The loss in the NLOS case is the sum of the free space loss l0, the multiple screen diffraction loss lmsd and the rooftop-to-street diffraction loss lrts:
The free space loss:
The rooftop-to-street diffraction loss term lrts determines the loss which occurs on the wave coupling into the street where the receiver is located. The origin of this loss comes from the Ikegami model, but COST 231 has extended this equation:
with
The width of the roads w, the rooftop height hROOF, the receiver height hRX and the road orientation φ are the parameters in this equation (but the road orientation φ is not considered in WinProp).
The orientation loss lOri is an empirical correction term obtained from the calibration with measurements.
An approximation for the multi-screen diffraction loss was published by Walfisch and Bartoni. COST 231 modified this approximation to be used also for base station antenna heights below rooftop level. The building heights hROOF and the building separation b are taken into account additionally:
with
