Double-IRS Aided Secure Symbiotic Radio System with Inter-Surface Reflection

This paper investigates a novel double-intelligent reflecting surface (IRS) aided secure multi-user symbiotic radio system, in which one IRS serves as a backscatter device (BD) for the secondary signal transmission, and the other one helps to build up the wireless communication bridge between transmitter and the BD based IRS. A max-min secrecy rate maximization of worst case primary signal transmission under the guarantee of the minimum secondary signal transmission rate is formulated. Since there exists the inter-surface reflection between the two IRSs, the problem is complicated with coupled vairables. Then, an effective iterative block coordinate descent algorithm in combination with semi-definite relaxation, equal constraint relaxation and successive convex approximation is proposed to solve the non-convex problem. Simulations demostrate the effectiveness of the proposed algorithm.