Hybrid intelligent reflective surfaces and relay assisted secure transmission scheme with power allocation

To improve the security and reliability of communication transmissions, this study proposes a novel hybrid secure scheme that combines a decode-and-forward (DF) relay and an intelligent reflecting surface (IRS) for downlinking multiple-input single-output systems. The proposal maximizes the minimum achievable secrecy rate by utilizing alternating optimization algorithms to derive the closed-form solution of the beamforming vector, obtain the optimal power allocation factor with the successive convex approximation method, and obtain the optimal phase-shift matrix with the semi-definite relaxation method. The simulation results demonstrate that our approach outperforms state-of-the-art solutions using only the IRS or DF relay. Moreover, performance improves at a high signal-to-noise when increasing the number of IRSs. Notably, a proper power allocation is important to achieve optimal performance.