RIS-Empowered OTFS Modulation With Faster-Than-Nyquist Signaling in High-Mobility Wireless Communications

Faster-than-Nyquist (FTN) signaling offers a promising path to enhance the spectral efficiency (SE) of orthogonal time frequency space (OTFS) modulation in high-mobility scenarios. However, the inherent non-orthogonal pulse shaping induces severe structured interference and colored noise, leading to reliability degradation and error floors. This paper proposes a reconfigurable intelligent surface (RIS)-empowered OTFS-FTN framework, demonstrating that the RIS acts as a critical enabler rather than a generic SNR booster. By strategically configuring RIS phases, we reshape the cascaded delay-Doppler (DD) channel to mitigate FTN-induced impairments, thereby enhancing reliability and relaxing power amplifier (PA) back-off constraints. We establish a unified DD-domain model explicitly accounting for FTN-induced colored noise and derive analytical expressions for the average frame error rate (AFER) upper bound, SE, peak-to-average power ratio (PAPR), and input back-off (IBO). Moreover, a low-complexity RIS phase design is proposed, with quantified computational and control overheads. Extensive simulations against comprehensive baselines, including Nyquist OTFS and power-compensated benchmarks, validate that the proposed framework achieves a superior SE-reliability-IBO trade-off. Results show up to around 10 dB reduction in required SNR for equivalent SE and AFER, confirming gains that cannot be replicated by simply optimizing simpler systems.