This paper develops a deterministic model for evaluating the influence of building windows upon the outdoor-to-indoor propagation path in cellular systems. This prediction model is based on the Aperture-field method of Huygens-Fresnel wave theory. Penetration losses and indoor signal characteristics are analyzed. It is found that the window frames of the building play an important role in determining the indoor field intensities. In order to verify this model's accuracy, numerical results are compared with measurement values. The calculations agree well with the measurements.