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The hybrid relay-reflecting intelligent surface (HR-RIS) has been recentlyintroduced as an efficient solution to overcome the double path loss andlimited beamforming diversity of the conventional fully passive reflectingsurface. This motivates us to investigate the application of the HR-RIS inimproving the secrecy capacity of millimeter wavemultiple-input-multiple-output (MIMO) systems with the presence ofmulti-antenna eavesdropper. The joint optimization of the transmit beamformerand the relay-reflecting coefficients at RIS is tackled via alternatingoptimization. In the proposed solution, a closed-form expression for theoptimal transmit beamformer at the transmitter is derived, and a metaheuristicsolution based on particle swarm optimization is proposed to optimize theactive and passive elements at the HR-RIS. The simulation results verify thatunder various scenarios, the HR-RIS provides significant improvement in thesecrecy capacity with respect to the conventional passive reflecting surface.

Secrecy Capacity Maximization for a Hybrid Relay-RIS Scheme in mmWave MIMO Networks