Max-Min Secure Energy Efficiency Optimization for STAR-RIS-Assisted Near-Field Wideband THz Communication

We investigate a simultaneous transmitting and reflecting (STAR)-RIS-assisted near-field wideband terahertz communication system featuring multiple users and eavesdroppers on both sides of the STAR-RIS. Our objective is to maximize the minimum secure energy efficiency (EE) of the system while adhering to constraints related to transmit power, unit-modulus analog beamforming, STAR-RIS energy conservation, and the time-delay of true time delay units. This problem is challenging due to several factors, including the non-convex nature of the fractional EE objective function, the difference of the two rate expressions in the secrecy rate that appear in the numerator of the EE, the presence of multiple eavesdroppers, and the coupling of optimization variables. To address it, we employ a penalty dual decomposition method, where the inner loop optimizes a penalized objective function using a block coordinate descent approach, while the outer loop updates dual variables and the penalty factor. Our simulation results show that the proposed algorithm outperforms various benchmarks, including fully-digital beamforming design, conventional hybrid beamforming approach, simple RIS-assisted design, and state-of-the-art solutions that do not account for eavesdroppers. This underscores the significance of a novel secure EE optimization design for the STAR-RIS-assisted near-field wideband terahertz communication system in the presence of eavesdroppers.