Input-Output Relation and Performance of RIS-Aided OTFS with Fractional Delay-Doppler

Reconfigurable intelligent surfaces (RIS) and orthogonal time-frequency space (OTFS) modulation have gained attention in recent wireless research. RIS technology aids communication by reflecting the incident electromagnetic waves towards the receiver, and OTFS modulation is effective in high-Doppler channels. This paper presents an early investigation of RIS-aided OTFS in high-Doppler channels. We derive the end-to-end delay-Doppler (DD) domain input-output relation of a RIS-aided OTFS system, considering rectangular pulses and fractional delay-Doppler values. We also consider a Zak receiver for RIS-aided OTFS that converts the received time-domain signal to DD domain in one step using Zak transform, and derive its end-to-end input-output relation. Our simulation results show that $i)$ RIS-aided OTFS performs better than OTFS without RIS, $ii)$ Zak receiver performs better than a two-step receiver, and $iii)$ RIS-aided OTFS achieves superior performance compared to RIS-aided OFDM.