A Novel 3D Non-Stationary Double-RIS-Assisted Channel Model for 6G Wireless Communication Systems

Nowadays, Reconfigurable intelligent surface (RIS) is regarded as one of the key technology of the sixth generation (6G) wireless communication systems. However, most of the current researches are based on single RIS. In this paper, we propose a three-dimensional (3D) double-RIS-assisted geometry-based stochastic model (GBSM) for massive multiple-input multiple-output (MIMO) communication systems. The channel model also supports the movements of transmitter, receiver, and clusters. For RIS, a new method is proposed for the joint design of reflection coefficients in MIMO channels based on cascaded RISs. In addition, different channel properties in the spatial domain, time domain, and frequency domain are studied to verify the validity and non-stationary properties of the model and explore the improvement of channel performance by double RISs.