Physical Layer Security Design for FDD IM-OTFS Transmissions Based on Secure Mapping

In this paper, a physical layer security scheme is proposed for index modulation aided orthogonal time frequency space (IM-OTFS) modulation systems working in Frequency Division Duplex (FDD) mode. In FDD systems, the channel reciprocity, which is the basic assumption for most physical layer security (PLS) techniques, is not applicable. To deal with the security challenge in FDD IM-OTFS systems, Chaos sequence is generated by exploiting the angular reciprocity of the legitimate link to construct the secure mapping rule between the information bits and the modulation symbols as well as the symbol indices. A PLS scheme based on the proposed secure mapping is introduced. The secrecy performance of the proposed PLS scheme is analyzed in terms of Bit Error Rate (BER) and the ergodic secrecy rate. Furthermore, a close-form approximated ergodic secrecy rate is derived to reduce the computational complexity. A truncation method is proposed to obtain robust secure transmissions when the angles of the uplink and downlink legitimate channels are estimated with errors. Simulation results show that the theoretical BER curve approaches the simulated BER curve closely for medium and high signal to noise ratio (SNR) regions, and the eavesdropper's BER is always around 0.5 for all SNRs. The closed-form approximated ergodic secrecy rate fits the ergodic secrecy rate exactly in low and high SNR regions, and can work as a lower bound of the ergodic secrecy rate in medium SNR region. The secure mapping at the legitimate users is not affected by the angle estimation errors by using the truncation method, while the eavesdropper's BER is influenced and the influence is analyzed.