  Reconfigurable intelligent surface (RIS) is envisioned as a promising
technology for next-generation wireless communications. Its deployment
introduces a RIS cascade link between the transmitter (Tx) and receiver (Rx),
which makes its channel model significantly different from the Tx-Rx direct
link. In this letter, a RIS cascade channel modeling method based on a 3D
geometry-based stochastic model (GBSM) is proposed. The model follows a 3GPP
standardized modeling framework and extends the traditional Tx-Rx channel to
Tx-RIS-Rx cascade channel. In the modeling process, we consider the non-ideal
phase modulation of the RIS element, so as to accurately characterize the
dependence of its phase modulation on the incoming wave angle. The differences
between the proposed cascade channel model and the channel model with ideal
phase modulation are investigated. The simulation results show that the
proposed model can better reflect the dependence of RIS on angle and
polarization.
