RIS-aided OTFS with index modulation: performance analysis and phase optimization

Abstract

The rapid evolution of reconfigurable intelligent surfaces (RISs) has opened new frontiers in channel impairment mitigation, particularly through passive beamforming. However, when integrated with waveforms under doubly selective channels, a fundamental timescale mismatch exists: the existing RIS designs typically employ a single static phase configuration per frame due to hardware constraints, whereas the channel is time-varying. To address this issue, we propose a novel RIS-aided orthogonal time frequency space (OTFS) system integrated with index modulation (IM). IM serves as a generalized framework encompassing conventional modulation, enhancing the spectral efficiency (SE) and transmission flexibility. By analyzing the structure and energy distribution of time-varying channels in the delay-Doppler (DD) domain, we develop a diagonal phase alignment (DPA) strategy that aligns the RIS-reflected paths with the direct path, maximizing the achievable rate with negligible computational overhead. Unlike existing works, we explicitly consider the impact of imperfect biorthogonality in rectangular pulse-shaping waveforms, which alters the structure of the equivalent channel matrix. This paper provides detailed mathematical system models and RIS deployment scenarios. Furthermore, the closed-form upper bound of bit error rate (BER) is derived for Rician fading channels. Simulation results validate the theoretical analysis and demonstrate promising performance in terms of the achievable rate and BER.