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

This paper 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. In this framework, we analyze 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 aim to maximize the network sum rate while meeting minimum rate requirements, the active STAR-RIS hardware constraints and guaranteeing the power budget at the BS, the active STAR-RIS and at each strong user. An alternating optimization algorithm employing successive convex approximation (SCA) is proposed, and rigorous simulations showcase its significant improvements over baseline approaches.