Two-Fold Physical Layer Security for OTFS-Aided RIS Communication

The integration of orthogonal time frequency and space modulation technique (OTFS) and reconfigurable intel-ligent surfaces (RIS) provides a promising solution for better connectivity in high mobility scenarios. Physical layer security (PLS) by exploiting channel characteristics has recently emerged as a promising security solution. We propose a two-fold PLS scheme for communication via RIS using OTFS waveform. As a first fold of security, we design a channel-based precoder for rotating delay-Doppler (DD) information symbols of a legitimate user. Subsequently, an artificial noise (AN) is added with the precoded DD symbols to enable another layer of security. To project the AN onto the channel subspace of a legitimate user and to mitigate its effect on the legitimate user's receiver, we derive a projection matrix by using the superposition of direct and cascaded channels. The strongest channel maximization based phase optimization scheme is also designed. Our results show that, unlike the legitimate user, the eavesdropper is not able to decode the information at all. The simulation results demonstrate the improved bit-error-rate (BER) of the proposed phase optimization technique over the random phase selection and OTFS system without RIS.