Simultaneously Transmitting and Reflecting Reconfigurable Intelligent Surface–Aided Orthogonal Time Frequency Space System for 6G and Beyond High-Mobility Wireless Networks

Unlike conventional reflection-only reconfigurable intelligent surfaces (RISs), simultaneously transmitting and reflecting RISs (STAR-RISs) constitute a paradigm shift in intelligent surface design by facilitating electromagnetic wave manipulation in both transmission and reflection domains, thereby achieving full-space coverage and mitigating the inherent directional limitations of traditional RIS implementations. Orthogonal time frequency space (OTFS) modulation has emerged as a highly promising waveform candidate for enabling reliable and efficient communication in high-mobility environments, owing to its resilience against severe Doppler effects and time-frequency channel variations. In this paper, a new STAR-RIS aided OTFS system (STAR-RIS-OTFS), which is a promising technique for high-mobility future wireless communication systems, is proposed. The combination of these two methodologies yields enhanced bit error rate performance over conventional OTFS systems. The performance of the proposed STAR-RIS-OTFS scheme is evaluated under M-ary quadrature amplitude modulation over both Rayleigh and Nakagami-m fading channels. In addition, a detailed computational complexity analysis is carried out for the maximum likelihood detector and the enhanced low-complexity detector, considering both the proposed STAR-RIS-OTFS architecture and the benchmark system. Moreover, the effects of imperfect channel state information (CSI) on the proposed STAR-RIS-OTFS system are investigated, and comparative performance results are provided under perfect and imperfect CSI assumptions. The proposed STAR-RIS-OTFS system is evaluated through Monte Carlo simulations over high-mobility Rayleigh channels for various system parameters. It is shown that the proposed STAR-RIS-OTFS scheme outperforms the traditional OTFS technique.