At the K.U. Leuven a software package (EPICS) that uses deterministic models to calculate the fields of antenna arrays has been developed. The software also allows one to compute important parameters of communication system like the power delay profile (PDP) and direction of arrival (DOA). Originally, it was designed to evaluate outdoor communication systems, but the emphasis has been moved towards the smaller and more difficult environments of indoor propagation. For outdoor communication the model was based only on three-dimensional physical optics (including diffuse scattering) reflection phenomena. This model has been extended by introducing penetration through walls and the uniform theory of diffraction (UTD). Also, new formulas containing Fresnel integrals and allowing closer distances between the reflecting surface and the receiver or transmitter were developed and introduced. These three aspects are also discussed. (4 pages)