To develop an millimeter-waves (mm-waves) channel model for in-room access scenarios, a double-directional channel measurement campaign conducted in a conference room environment is presented. In the measurements, a custom-developed channel sounder with a 12 • angle and 2.5-ns delay resolutions at 58.5 GHz was used. From the measured data, the multi-path components were extracted using a high-resolution path parameter estimation algorithm; then, they were clustered based on the actual physical propagation paths to identify the scattering processes explicitly. The cluster analysis revealed that the signal paths by line-of-sight, single-bounce, and double-bounce reflections were power-dominant and well predicted by ray-tracing, but the contribution of random clusters was not significant. In this study, to express diffuse scattering on the rough surface of an ambient reflector, an intra-cluster model of plasterboard wall reflection was parameterized. Furthermore, the proposed intra-cluster model was experimentally validated by analyzing the small-scale fading captured along the wall in the time domain which is caused by the constructive and destructive interference of the specular reflection and diffuse scattering components. INDEX TERMS Millimeter wave, indoor channel, intra-cluster, diffuse scattering, radio propagation measurement, ray-tracing, parameter estimation, direction-of-arrival.

For seamless communication in millimeter-wave (mm-wave) transmission systems, the robustness against link blockage and user mobility should be guaranteed. Cooperative joint network design over conventional microwave bands and mm-wave bands is essential in future mm-wave WLANs (e.g., IEEE 802.11ay) and 5G cellular networks, and hence understanding the discrepancy between the propagation properties at those frequency bands is crucial. In this letter, the angle-of-arrival characteristics at mm-wave band (60 GHz) and microwave band (2.4 GHz) in indoor environments are presented. From the measurement results, it was seen that the lineof-sight and first-order reflected paths agree well each other, but diffraction and scattering are observed only at microwave band. It was also shown that the angular spreads at mm-wave band was about 25 degrees smaller than those at microwave band.

The IEEE 802.11ay channel models have been developed based on the quasi-deterministic (Q-D) channel modeling that considers environment specific property of millimeter-wave (mm-wave) propagation by representing multi-path channels as superposition of deterministic (called D-ray) and random clusters (called R-ray). Based on the Q-D model, this letter presented the inter-cluster parameters of the random clusters which were obtained from the double-directional channel measurement in an indoor entrance hall environment. Comparison between the parameters obtained in this study and those of the existing IEEE 802.11ay/MiWEBA model showed that both are reasonable values inferred from each corresponding environment.