RIS-Assisted Mobile Channels With Directional Transmission: Modeling and Characteristic Analysis

In this paper, a 3D geometry-based reconfigurable intelligent surface (RIS)-aided channel model is proposed with directional antenna at base station (BS) side. Different from traditional RIS-assisted channel model, visible area (i.e., range of observed scatters) and radiated power of directional antenna with uniform, cos and Gaussian patterns are considered. The time-varying large- and small-scale parameters of complete communication links including BS-receiver, BS-RIS, and RIS-receiver links are modeled considering intensively line-of-sight (LoS) and non-LoS (NLoS) paths. Three transmission cases at BS antenna side, that is, LoS Case, Mixed Case, and RIS Case are defined by directivity and number of beams. The principle of case selection is to maximize absolute value of CIR as optimization objective. Based on the proposed channel model and directional transmission cases, some second-order statistical characteristics, such as space-time correlation function and Doppler power spectrum density are derived, and conversion between different cases causes the simulation results to vary discontinuously in time or space domains. Furthermore, impacts of different cases and switching of direction antenna, radiated patterns of directional beam, RIS optimization phase, and environment conditions on channel characteristics are investigated. The results show that the proposed case selection method can well enhance correlation and lead to correlation taking on a sawtooth shape. In addition, it is also able to reduce the Doppler power spectrum curve. Furthermore, the proposed model is validated by comparing root mean square delay spread with measurement. These observations can be used to present a reference for designing and evaluating directional RIS-assisted communication systems.