Owing to the widespread use of smartphones and various cloud services, user traffic\nin cellular networks is rapidly increasing. Especially, the traffic congestion is severe in urban\nareas, and effective service-cell planning is required in the area for efficient radio resource usage.\nBecause many users are also inside high buildings in the urban area, the knowledge of propagation\nloss characteristics in the outdoor-to-indoor (O2I) scenario is indispensable for the purpose. The\nray-tracing simulation has been widely used for service-cell planning, but it has a problem that\nthe propagation loss tends to be underestimated in a typical O2I scenario in which the incident\nradio waves penetrate indoors through building windows. In this paper, we proposed the extension\nmethod of the ray-tracing simulation to solve the problem. In the proposed method, the additional\nloss factors such as the Fresnel zone shielding loss and the transmission loss by the equivalent\ndielectric plate were calculated for respective rays to eliminate the penetration loss prediction error.\nTo evaluate the effectiveness of the proposed method, we conducted radio propagation measurements\nin a high-building environment by using the developed unmanned aerial vehicle (UAV)-based\nmeasurement system. The results showed that the penetration loss of direct and reflection rays was\nsignificantly underestimated in the ray-tracing simulation and the proposed method could correct\nthe problem. The mean prediction error was improved from 7.0 dB to -0.5 dB, and the standard\ndeviation was also improved from 8.2 dB to 5.3 dB. The results are expected to be utilized for actual\nservice-cell planning in the urban environment.
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