Uplink Performance of Stacked Intelligent Metasurface-enhanced Cell-Free Massive MIMO Systems

Cell-free (CF) massive multiple-input multiple-output (mMIMO) stands as a key technological advancement poised to shape the forthcoming sixth-generation (6G) networks. However, the 6G demand for higher spectral and energy efficiency presents significant challenges to CF systems. To address this issue, we introduce stacked intelligent metasurfaces (SIM) to the CF mMIMO system. Specifically, we investigate the uplink performance of a spatially correlated SIM-enhanced CF mMIMO system. The local minimum mean squared error (L-MMSE)/maximum ratio (MR) combining are adopted at the APs and the closed-form expression for uplink spectral efficiency (SE) is obtained by using two-stage signal processing. Monte-Carlo simulations are employed to validate the accuracy of the derived analytical results. The results indicate that augmenting the number of meta-atoms per layer in the SIM consistently yields improvements. Interestingly, when the number of SIM meta-atoms is small, the increase in the number of the SIM metasurface layers results in a decrease in system performance. Furthermore, the deployment of SIMs equipped with extensive meta-atoms can function as an alternative to AP antennas even without phase shift design, contributing to the robustness of system performance.