Uplink Performance and Beamforming Design of SIM-enhanced Cell-Free Massive MIMO Systems

In this paper, we explore the integration of low-power, low-cost stacked intelligent metasurfaces (SIM) into cell-free (CF) massive multiple-input multiple-output (mMIMO) systems to enhance access point (AP) capabilities and address high power consumption and cost challenges. Specifically, we investigate the uplink performance of a SIM-enhanced CF mMIMO system. First, the closed-form expressions of the spectral efficiency (SE) are obtained using the unique two-stage signal processing framework of CF mMIMO systems. Second, a wave-based beamforming algorithm for SIM is proposed, based only on statistical channel state information, which effectively reduces the fronthaul costs. The results indicate that increasing the number of SIM layers and meta-atoms leads to significant performance improvements and allows for a reduction in the number of APs and AP antennas. Moreover, the proposed wave-based beamforming algorithm can enhance the SE performance of SIM-enhanced CF-mMIMO systems by 72%, significantly outperforming traditional CF mMIMO systems.