Fronthaul Compression and RIS Beamforming Design for Cloud-RAN: Active RIS Versus Passive RIS

This letter investigates a multi-cell reconfigurable intelligent surface (RIS)-assisted cloud radio access network (Cloud-RAN), where multiple remote radio heads (RRHs) are connected to a central unit (CU) via limited-capacity fronthaul links, and multiple user equipments (UEs) in each cell are supported by dedicated RISs. In particular, we provide a unified comparative analysis of three RIS architectures - active RIS, passive RIS with continuous phase shifts, and passive RIS with discrete phase shifts - highlighting their fundamental performance trade-offs under practical fronthaul and RIS structural constraints. To mitigate the effects of fronthaul limitations and inter-cell interference, we propose a joint optimization framework for fronthaul compression, coordinated transmit power allocation, and RIS beamforming to maximize the system sum-rate. Numerical results verify that the proposed approach effectively balances fronthaul resource constraints and RIS configuration flexibility, providing key insights into the performance trade-offs of different RIS architectures for practical network deployments.