STAR-RIS-Assisted OTFS-NOMA with Residual Phase: Outage Analysis & Throughput Optimization

Abstract

Orthogonal Time Frequency Space (OTFS) modulation provides robust communication in high-mobility environments by operating in the delay-Doppler domain. In this paper, we analyze a downlink STAR-RIS-assisted OTFS-NOMA system without a direct source-user link. A practical multielement STAR-RIS model is considered, incorporating imperfect successive interference cancellation (ip-SIC) and residual phase errors due to imperfect phase adjustment. Under a worst-case phase uncertainty model, closed-form expressions for the outage probability are derived using the eigenvalue statistics of the effective delay-Doppler channel. High-SNR asymptotic analysis is further developed to characterize the outage behavior and reveal fundamental performance limits. In addition, a sumthroughput maximization problem is formulated under QoS constraints, and optimal NOMA power allocation and STAR-RIS coefficients are obtained. Numerical results validate the analysis and demonstrate the impact of phase uncertainty, showing that improved phase control significantly enhances system performance.