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

This letter presents an early investigation of reconfigurable intelligent surfaces (RIS)-aided orthogonal time-frequency space (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.