A Unified Optimization Framework for Multiple Access in STAR-RIS-Aided Full-Duplex Systems

This paper introduces a unified optimization framework for bidirectional communication in simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) aided full-duplex (FD) systems. By leveraging unique characteristics of STAR-RIS modes: mode switching (MS), energy splitting (ES), and time switching (TS) - and accounting for practical channel state information (CSI) and successive interference cancellation (SIC) imperfections, we aim to optimize bidirectional multiple access for four operational regimes: 1) quality-of-service (QoS) feasibility, 2) energy efficiency (EE), 3) STAR-RIS efficiency, and 4) max-min fairness. For MS and ES modes, we employ the non-orthogonal multiple access (NOMA) protocol to derive a optimal STAR-RIS partitioning and energy splitting coefficients that optimizes performance subject to uplink (UL) QoS demands, then fine-tune base station (BS) power allocation to maximize downlink (DL) performance while ensuring compliance with DL-QoS. On the other hand, TS mode exploits the entire STAR-RIS for a single UE at a time, follows an orthogonal multiple access (OMA) approach and optimizes time allocation parameters individually for both UL and DL scenarios. Our performance analysis accounts for CSI/SIC imperfections. Extensive simulations confirm high consistency between analytical and numerical results, with SIC imperfections proving more disruptive due to heightened interference. While MS and ES outperform TS in terms of max-min rate under perfect SIC conditions (e.g., 5.57 bits/s/Hz in MS/ES modes compared to 3.3 bits/s/Hz in TS mode given N “ 128 and ϵ “ 0), TS yields superior results when SIC imperfections are present (e.g., 3.3 bits/s/Hz in TS mode compared to 2.05 bits/s/Hz in MS/ES modes given N “ 128 and ϵ “ 0.1).