OTFS Enabled RIS- Aided Localization: Fundamental Limits and Potential Drawbacks

In this paper, we consider a reconfigurable intelligent surface (RIS) aided orthogonal time frequency space (OTFS) system that can operate in either the near-field or far-field propagation regimes. The received signal expressions in the Doppler-delay (DD) are derived when the system experiences the effects of near-field or far-field propagation. Utilizing these received signal expressions and the Fisher information matrix (FIM), we derive the fundamental localization error limits of a user equipment (UE) that receives signals in the DD domain after reflections from the RISs. Through simulations, we show that the localization error decreases with the number of receive antennas and RIS elements. Subsequently, we note that RISs can become misoriented, i.e., their orientations can be disturbed after initial deployment. Hence, we investigate the possibility of exploiting the signals received at the UE in the DD domain to estimate the RIS orientation. Our simulation results indicate that the orientation of a certain RIS can only be estimated when the UE is experiencing the effects of near-field propagation with respect to that RIS. Through simulations, we show that a fundamental drawback of RIS-aided localization of a UE is that the RIS orientation offset is detrimental to the localization error of the UE.