Joint Physical Layer Security and Information Freshness of RIS-Assisted SPC System: Analysis and Optimization

In order to maintain information freshness in passive eavesdropping scenarios of reconfigurable intelligent surface (RIS) assisted short packet communications (SPC), this paper proposes a physical layer security communication scheme that considers information freshness. Specifically, with the moment generating function in stochastic network calculus (SNC), the approximate closed-form expression for the upper bound of secrecy age outage probability (SAOP) under statistical channel state information (CSI) is derived. Besides, the effects of blocklength and decoding error probability on SAOP are analyzed. The average blocklength and decoding error probability satisfying minimum SAOP are derived. Furthermore, a SAOP minimization problem is proposed by considering reliability, security and RIS unit modulus constraints. For the non-convex problem with coupling variables, we decompose it into blocklength and RIS phase shift optimization subproblem through the alternating optimization algorithm. In particular, for the stochastic RIS phase shift optimization, a momentum stochastic successive convex approximation (M-SSCA) algorithm is proposed to obtain the optimal solution, which introduces a momentum disturbance factor to accelerate convergence. In simulation results, the effects of main system parameters on SAOP are given. Compared with the traditional convex optimization schemes, the algorithm proposed in this paper reduces SAOP more effectively and has better convergence performance according to simulation results.