This article presents a generalized indoor-to-outdoor signal propagation model based on ray-tracing and exhaustive measurements campaign with the aim of developing a unified model for the interference in fifth-generation (5G) networks with femtocells underlying macrocells. It considers large buildings and typical residential houses. The objective is to capture the complex effects of the architectural layout, fading, shadowing, and building materials as well as to develop a practical nonsite-specific composite model to be used in the interference analysis and planning of next-generation networks, where not all information about the position of the transmitters or the building layouts is available. To that end, many different floor plans are considered, and the positions of the indoor transmitter are changed randomly. The obtained results reveal that the signal attenuation in the outdoor environment can be best characterized by a Gaussian-mixture (GM) distribution model.