On the Spectral Efficiency of Hybrid Relay/RIS-Assisted Massive MIMO Systems

Reconfigurable intelligent surfaces (RISs) play an important role in extending the connectivity and improving the data rate of future wireless communication systems. However, the conventional (passive) RISs have their limitations and, thus, a hybrid-relay RIS (HR-RIS) architecture is proposed to reap the benefits of relaying systems with high power consumption but higher throughput, and passive RIS systems with cascaded fading effects but low complexity. In this paper, we investigate the performance of HR-RISs in a massive multiple-input multiple-output (M-MIMO) system with zero-forcing (ZF) processing, where channel state information (CSI) is unavailable. We first model the uplink/downlink channels and derive the linear minimum mean square error (LMMSE) estimate of the effective channels. We, then, derive a closed-form expression for the signal to interference and noise ratio (SINR) and spectral efficiency (SE). Finally, we provide some useful engineering insights with our asymptotic analysis and numerical results.