RIS-Level SIC for Uplink Cascaded-RIS Assisted NOMA System

A brand-new framework for cascaded-reconfigurable intelligent surface (RIS) empowered multi-antenna non-orthogonal multiple access (NOMA) system is conceived, wherein RISs are deployed to serve their associated users and assist data transmission for the farther RISs as well. A new concept of RIS-level successive interference cancellation (RSIC) is developed to eliminate the inter-RIS interference. With the devised framework, a transmit power minimization problem is formulated by collaboratively designing each user's transmit power, the RISs' phase shifts, and the BS's equalizers. Specifically, a parallel iteration method is developed to solve the subproblem of jointly optimizing transmit power and equalizers, whereas a sequential phase-rotation algorithm is proposed to address the subproblem of phase shift optimization. It is showed by simulations that the proposed scheme outperforms the existing benchmark schemes in terms of total transmit power.