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

TL;DR

This work tackles the reliability and spectral efficiency challenges of high-mobility wireless links by combining Orthogonal Time Frequency Space (OTFS) modulation with Faster-than-Nyquist (FTN) signaling and passive beamforming via Reconfigurable Intelligent Surfaces (RIS). It introduces a unified delay-Doppler domain model that captures the joint effects of RIS phase shifts, FTN-induced ISI, and the delay-Doppler channel, and derives closed-form expressions for average frame error rate, spectral efficiency, and PAPR/IBO. A practical RIS phase-adjustment algorithm with quantization and robustness to CSI errors is proposed, and extensive EVA-channel simulations demonstrate notable SE gains and energy efficiency improvements while maintaining reliability. The results reveal a favorable robustness-efficiency trade-off and establish RIS-OTFS-FTN as a viable solution for next-generation high-mobility and spectrum-constrained networks. Overall, the approach provides a rigorous analytical foundation and practical design insights for integrating RIS with FTN-enabled OTFS in terrestrial and non-terrestrial networks.

Abstract

High-mobility wireless communication systems suffer from severe Doppler spread and multi-path delay, which degrade the reliability and spectral efficiency of conventional modulation schemes. Orthogonal time frequency space (OTFS) modulation offers strong robustness in such environments by representing symbols in the delay-Doppler (DD) domain, while faster-than-Nyquist (FTN) signaling can further enhance spectral efficiency through intentional symbol packing. Meanwhile, reconfigurable intelligent surfaces (RIS) provide a promising means to improve link quality via passive beamforming. Motivated by these advantages, we propose a novel RIS-empowered OTFS modulation with FTN signaling (RIS-OTFS-FTN) scheme. First, we establish a unified DD-domain input-output relationship that jointly accounts for RIS passive beamforming, FTN-induced inter-symbol interference, and DD-domain channel characteristics. Based on this model, we provide comprehensive analytical performance for the frame error rate, spectral efficiency, and peak-to-average power ratio (PAPR), etc. Furthermore, a practical RIS phase adjustment strategy with quantized phase selection is designed to maximize the effective channel gain. Extensive Monte Carlo simulations under a standardized extended vehicular A (EVA) channel model validate the theoretical results and provide key insights into the trade-offs among spectral efficiency, PAPR, input back-off (IBO), and error performance, with some interesting insights.The proposed RIS-OTFS-FTN scheme demonstrates notable performance gains in both reliability and spectral efficiency, offering a viable solution for future high-mobility and spectrum-constrained wireless systems.

Figures (11)

• Figure 1: System model of the RIS-OTFS-FTN schemes.
• Figure 2: Signal processing of the RIS-OTFS-FTN scheme.
• Figure 3: Comparison between RIS-OTFS schemes with and without FTN in TF domain.
• Figure 4: PAPR (a & b) and CCDF (c & d) of the OTFS-FTN and OTFS schemes with $\alpha=0.8$, $M=32$ and $N=32$.
• Figure 5: IBO of the RIS-OTFS-FTN and OTFS-FTN schemes with $\alpha=0.8$ and $1$, $M=32$ and $N=32$.