Enhanced Physical Layer Security for Wireless Systems with Non-Diagonal IRS

The commonly used diagonal intelligent reflecting surface (D-IRS) architecture is favored for its simplicity in implementation and control link load in future wireless networks. However, a recent study has demonstrated that the non-diagonal IRS (ND-IRS) architecture offers significant performance improvements without adding complexity. In this paper, we focus on analyzing the secrecy performance of an ND-IRS assisted wireless communication setup in terms of ergodic secrecy rate and secrecy outage probability (SOP). We consider a practical uniform 4-quadrature amplitude modulation (QAM) based codebook. We have conducted simulations for N = 16, 64 IRS elements, as it is both practical and feasible. Our results show that the ergodic secrecy rate with a uniform codebook reaches a maximum at a specific transmit power level. This optimum power level is then used to analyze the secrecy outage probability (SOP) of the setup. Notably, when N = 64, the SOP of the ND-IRS scheme with a practical uniform 4-QAM-based codebook closely matches that of the Diagonal IRS. Furthermore, it outperforms the Diagonal IRS when N = 16 using an ideal Gaussian codebook. In other words, when N = 16, SOP (Gaussian codebook, D-IRS) is less than SOP (Uniform codebook, ND-IRS), and when N = 64, SOP (Gaussian codebook, D-IRS) is approximately equal to SOP (Uniform codebook, ND-IRS).