Comment on diffraction loss of rooftop-to-street in COST 231-Walfisch-Ikegami model

As a small-cell path-loss prediction model, the COST 231-Walfisch-Ikegami model has been used extensively. It facilitates radio frequency (RF) path-loss predictions in typical suburban and urban environments where the building heights are quasi-uniform. However, it was found that the expression accounting for the diffraction loss from the last rooftop to the street was erroneously obtained. In this paper, we show why the error is present in the COST 231-Walfisch-Ikegami model and how it impacts on path-loss prediction.