RIS-Aided Cell-Free Massive MIMO Systems With Channel Aging

Cell-free (CF) massive multiple-input multiple-output (MIMO) and reconfigurable intelligent surface (RIS) are two disruptive technologies for enabling the sixth-generation network. In this paper, we investigate the effect of channel aging on the RIS-aided CF massive MIMO systems over spatially correlated channels. We consider a minimum mean square error channel estimator that facilitates the derivation of closed-form spectral efficiency expressions. For the uplink, we compare the large-scale fading decoding method with the centralized decoding method. For the downlink, we adopt the maximum ratio precoding and propose a practical fractional power control method to improve the performance. Based on the analytical framework, we unveil the interplay between channel aging, spatial correlation, and signal processing methods. Our results demonstrate that the RIS-aided CF massive MIMO system outperforms the conventional CF counterpart system in both static and mobile scenarios. Moreover, when the link is weak, the introduction of RIS can lead to even more substantial performance enhancement. Also, RIS can be deployed to compensate for system performance degradation caused by channel aging and potentially lead to energy savings, while still achieving excellent performance. Intriguingly, due to the mutual coupling of RIS element and AP antenna correlations, RIS can effectively mitigate the negative effects caused by spatial correlation among AP antennas. Furthermore, we provide a practical rule-of-thumb guideline for designing resource block length, accounting for the impacts caused by channel aging.