Covert Communication for Intelligent Reflecting Surface based Symbiotic Radio Systems

Intelligent reflecting surface (IRS) based symbiotic radio (SR) is a novel technology to improve the performance of SR by enhancing the backscatter link through passive beam-forming. To cope with the challenge to guarantee information security in the IRS-based SR system, an IRS-based SR covert communication system which utilizes the IRS as a second transmitter for the covert transmission without being detected by the warder is proposed. To demonstrate the covertness requirements, the optimal detection threshold and the detection error probability of the warder are derived which investigates the feasible regions of system parameters. On this basis, an IRS phase optimization problem is proposed to maximize the covert rate while guaranteeing a specific covert level. Numerical results show the parameters’ impacts on the system performance. Besides, compared with the traditional single-antenna backscatter covert system, the proposed system can provide a higher covert rate and more deployable regions for the second transmitter.