RIS assisted dual-function radar and secure communications based on frequency-shifted chirp spread spectrum index modulation

Reconfigurable intelligent surface (RIS) assisted dual-function radar communications (DFRC) system is a promising integrated sensing and communication (ISAC) technology for future 6G. In this paper, we propose a scheme of RIS-assisted DFRC system based on frequency shifted chirp spread spectrum index modulation (RDFI) for secure communications. The proposed RDFI achieves the sensing and transmission of target location information in its radar and communication modes, respectively. In both modes, the frequency-shifted chirp spread spectrum index modulation (FSCSS-IM) signal is used as the baseband signal for radar and communications, so that the signal sent by the radar also carries information. This scheme implements the RIS-assisted beamforming in the communication mode through the azimuth information of the target acquired in the radar mode, so that the signal received from the eavesdropper is distorted in amplitude and phase. In addition, this paper analyzes the radar measurement accuracy and communication security of the FSCSS-IM signal using ambiguity function and secrecy rate (SR) analysis, respectively. Simulation results show that RDFI achieves both excellent bit error rate (BER) performance and physical layer security of communications.