Multi-Functional RIS-Aided Cell-Free Networks

In this paper, we investigate the utilization of multi-functional reconfigurable intelligent surfaces (MF-RIS) in cell-free networks to enhance capacity. Equipped with reflective and refractive circuits, as well as negative resistance-based amplifiers, the MF-RIS is able to simultaneously reflect, refract, and amplify incident signals. As such, the signal received by all users in the whole space can be enhanced significantly in a cost-efficient manner. To maximize the weighted sum-rate of the considered network, a non-convex problem is formulated by jointly optimizing the transmit precoding and MF-RIS beamforming. Then, we propose an alternating optimization algorithm to solve this challenging problem efficiently. Simulation results show that, through coordinated manipulation of the amplitude and phase shift of reflected and refracted signals, the MF-RIS improves the throughput of cell-free networks significantly compared to benchmarks with/without existing RIS.