Cooperative Rate Splitting Multiple Access for Active STAR-RIS Assisted Downlink Communications

This letter explores the influence of an active Simultaneously Transmitting and Reflecting Reconfigurable Intelligent Surface (STAR-RIS) in supporting Cooperative Rate Splitting Multiple Access (C-RSMA) in a downlink wireless network. We consider a scenario where a base station (BS) with multiple antennas utilizes the active STAR-RIS to aid weak and strong users. Strong users also serve as full-duplex relays to transmit the common stream to weak users. By jointly optimizing BS beamformers, active STAR-RIS reflection and transmission, stream split, and strong users' transmit power, we maximize the network sum rate while satisfying minimum rate requirements, active STAR-RIS hardware constraints and power budgets at the BS, the active STAR-RIS and at each strong user. An alternating optimization algorithm employing successive convex approximation is proposed, and simulations showcase its significant gains over baseline approaches.