The propagation of radio waves within buildings at 433MHz and 900MHz was investigated where the wireless communication system relay is placed on the sidewall of the building. Statistical analysis of measurements obtained in a commercial building and in a residential building was used to analyze the influence of various parameters on the signal strength distribution in a room. The parameters include the distance from the relay, the relative position of the transmitter and the receiver, the window size, and the room area and structure. The results show that the signal in a room is mainly due to the windows.