Spectral Efficiency Analysis for RISs-Aided Wireless-Powered Cell-Free Massive MIMO

Reconfigurable intelligent surface (RIS) is a cost-effective component to enhance wireless energy harvest and spectral efficiency (SE) of wireless communications systems. In the paper, we investigate the performance of a RISs-aided cell-free massive multiple-input multiple-output (CF-mMIMO) system over Rician fading channels in conjunction with wireless power transfer (WPT). User equipments (UEs) harvest energy from the energy signal transmitted from access points (APs) with the help of RISs during the downlink and use it for the uplink pilot and data transmission. Specifically, we derive a closed-form expression for the uplink SE when using a two-layer decoding method and verify the accuracy of the derived results by Monte Carlo simulation. The results show that increasing the number of APs or the number of RIS elements is always beneficial for the system SE improvement. Furthermore, increasing the number of RIS elements can lead to a reduction in the required number of APs without a decrease in the average SE, which means that significant hardware and energy costs can be greatly reduced.