RIS-ASSISTED CELL-FREE MASSIVE MIMO NOMA SYSTEMS FOR 5G AND BEYOND COMMUNICATION

Abstract

We consider a cell-free massive multi-input multi-output system with non-orthogonal multiple access (NOMA) assisted by re-configurable intelligent surface (RIS) with rayleigh fading, correlated channels. We derive the MMSE estimation of two types of precoders namely, Accumulator which considers all user channels inside a each noma cluster and strongest user channel precoder which depends only on strongest user in a cluster. We also evaluate closed form downlink spectral efficiency (SE) lower bound for NOMA assuming imperfect successive interference cancellation (SIC). We compare perfromance of RIS link in NOMA when weak direct link exist between AP and users. We also investigate SE performance of both NOMA and OMA and We numerically prove that i) NOMA system through RIS link giving superior performance when number of UEs are more. ii) using more number of RIS elements help to increase in downlink SE under both NOMA and OMA scenarios.