A general path loss model for in-room radio channels is proposed. The model is based on experimental observations of the behavior of the delay power spectrum in closed rooms. In a given closed room, the early part of the spectrum observed at different positions typically consists of a dominant component (peak) that vanishes as the transmitter-receiver distance increases, while the late part decays versus distance according to the same exponential law regardless of this distance.
These observations motivate the proposed model of the delay power spectrum with an early dominant component and a reverberant component. The dominant component is modeled as a Dirac delta function weighted with a factor decaying with an inverse distance power law ($d^{-n}$). The reverberant component is a delayed exponentially decaying function in delay and the power decays exponentially with distance. The proposed model allows for the prediction of path loss, mean delay, and RMS delay spread versus distance. We use measurements to validate the proposed model and we observe good agreement of the model prediction for mean delay and RMS delay spread.