Downlink Sum-Rate Maximization for Joint Active and Passive Stacked Intelligent Metasurfaces

Stacked intelligent metasurfaces (SIM) have recently emerged as a novel technology that enables the implementation of signal-processing algorithms in the electromagnetic wave domain. However, existing works employing SIM in wireless scenarios rely solely on nearly passive structures that control only the phase of the meta-atoms in each layer. We propose a SIM-aided downlink multiuser transmission scheme, where the SIM at the base station (BS) is designed by combining nearly passive layers, i.e., with phase-only reconfiguration capabilities, and active layers integrated with amplifier chips to enable amplitude control. Our design aims at maximizing the sum rate for the best group of users by jointly optimizing the transmit power allocation at the BS and the wave-based beamforming at the SIM. Monte Carlo simulations demonstrate that using both nearly passive and active layers in the SIM entails a significant enhancement in terms of capacity compared to previously reported phase-only coding SIM.