RIS-aided OTFS with Index Modulation

Reconfigurable intelligent surfaces (RIS) technology, orthogonal time frequency space (OTFS) modulation, and index modulation (IM) are promising physical layer techniques in wireless communications. An RIS system steers the electromagnetic wave towards the receiver using an electronically tunable surface. OTFS delivers robust performance in high mobility scenarios using information signalling in the delay-Doppler (DD) domain, and IM can offer improved spectral efficiencies and bit error performance. In this paper, we propose an RIS-aided OTFS scheme in which IM is carried out in the DD domain. We call the proposed scheme as RIS-aided OTFS-IM scheme. We develop an end-to-end DD domain input-output relation for the proposed RIS-aided OTFS-IM scheme and evaluate its bit error performance. Our results show that 1) the performance of RIS-aided OTFS improves significantly with the proposed DD domain indexing and increased number of RIS elements, 2) among the phase selection schemes considered at the RIS, Frobenius norm based phase selection achieves superior performance, and 3) RIS-aided OTFS-IM performs better than RIS-aided OFDM-IM.