Downlink Spectral Efficiency of RIS-Assisted Cell-Free Massive MIMO-NOMA Systems With CSI Errors

We consider a reconfigurable intelligent surface (RIS) assisted cell-free massive multiple-input multiple-output (MIMO) non-orthogonal multiple access (NOMA) system, where each access point (AP) serves its users with the aid of the RIS. We practically model the system by assuming imperfect channel state information (CSI) and employing imperfect successive interference cancellation (SIC) at the users’ end. We first obtain the channel estimates using linear minimum mean square error (LMMSE) approach considering the spatial correlation at the RIS and then derive a closed-form downlink spectral efficiency (SE) expression using the statistical CSI. We numerically show that i) the RIS-assisted link is more advantageous at lower transmit power region where the direct link between AP and user is weak, ii) SE with RIS significantly increases with the increase in the number of APs and RIS elements, and iii) NOMA outperforms OMA in terms of SE for the RIS-assisted cell-free system.