Discrete Reflection Coefficients RIS for Interference Suppression in Multicast Transmissions

Reconfigurable Intelligent Surface (RIS) introduces a novel strategy for mitigating channel interference. However, the assumption of continuous reflection coefficients (CRC) in most existing studies on interference neutralization utilizing RIS is unrealistic due to hardware constraints. This work centers on interference neutralization in RIS-supported multicast transmission with the more common discrete reflection coefficients (DRC). Here, an RIS comprised of reflectors is utilized to improve the multicast channel and diminish the interference channel by designing DRC. In this context, a problem that maximizes the signal-to-interference-plus-noise ratio is defined as the attainable rate maximization by choosing the feasible DRC and a beamforming vector. First, an alternating optimization approach iteratively updates the DRC and beamforming vector. It combines Gaussian randomization with bisection search to solve feasibility problems, neutralizing interference by adjusting the interference channel without cooperator involvement. Then, for RIS-assisted multicast under millimeter-wave interference, the asymptotic behaviors of achievable rate is analyzed via the Toeplitz structure of channel covariance matrices. Last, this demonstrates that the RIS can greatly enhance the multicast rate in the presence of interference.