Robust beamforming design for passive RIS-assisted NOMA systems

A transmission power minimization problem for a reconfigurable intelligent surface (RIS) aided non-orthogonal multiple access (NOMA) network has been studied here, which has assumed the channel state information is imperfect. The channel error in this paper has been modeled as two different error models, that is, bounded error model and statistical error model. First, a two-step algorithm is proposed to decouple the decoding order and optimization variables under bounded error model. Then, for the statistical error model, the alternating optimization method is adopted to decouple optimization variables. Finally, the performance difference is compared between the proposed beamforming scheme and the non-RIS scheme from two key parameters, the number of reflected elements and the error factor. Simulation results verify that passive RIS has the advantage on the system performance compared with the scheme without RIS. It shows the transmission power versus the error factor under statistical error model, which compares the difference of transmission power between random phase and optimal phase. Simulation results illustrate that RIS can provide a more noticeable gain with a optimal phase, compared with a random phase that also can provide a higher gain than the scheme of No-RIS. image