Pilot-Domain NOMA for RIS-Assisted Communications

Due to variations in node mobility or differences in the scattering environment, wireless links in multi-user systems often experience non-identical coherence intervals; this is also true in systems assisted by reconfigurable intelligent surfaces (RIS). This paper studies RIS-assisted multi-user downlink systems under link coherence disparity. Since the RIS channel model has many parameters to be estimated, controlling the training overhead under coherence disparity has a strong impact on the practical operation of RIS. Thus motivated, we propose a novel pilot-domain non-orthogonal multiple access (NOMA) technique for RIS-assisted downlink systems. The transmit beamforming and RIS reflection coefficient vectors are jointly designed to maximize the achieved sum-rate. We analyze the resulting reduction in training overhead and the corresponding rate improvements. We investigate efficient pilot placement strategies for multi-user scenarios with arbitrary coherence intervals. Numerical results illustrate the effectiveness of the proposed technique.