DPC-Coded MIMO Multiuser Communications Using RIS with Discrete Phase Shifts

A novel design of RIS-assisted MIMO multiuser communication system using dirty paper coding (DPC) is presented in this paper. The design is applicable to any numbers of transmit and receive antennas and to any number of users, and requires only discrete phase shifts for RIS elements. For any given phase-configuration of RIS, the proposed design always achieves the corresponding channel capacity. The proposed framework involves two alternating optimization steps. Initially, we optimize the input covariance matrices of DPC encoders to maximize the sum-rate of downlink communication while keeping the RIS coefficient matrix fixed. Subsequently, we employ the alternating direction method of multipliers to identify optimal discrete RIS phases that maximize an upper bound on the sum-rate, with the DPC covariance matrices fixed. Simulation results show that the proposed design is highly robust and delivers near optimal sum-rate performance using only quad-phases for RIS elements.