Performance Analysis and Reflection Optimization for Wideband THz Double-IRS Aided Wireless Communications

Intelligent reflecting surface (IRS) is deemed as a promising technology to improve the spectral and energy efficiency of wireless communications cost-effectively. In this paper, we investi-gate a double-IRS aided wideband terahertz (THz) communication system and the beam-squint effects at the two IRSs over their double-reflection line-of-sight (LoS) link. To gain useful insights into such beam-squint effects, we first analyze the performance loss incurred by applying the conventional narrowband cooperative passive beamforming (CPB) at the two IRSs in the considered wideband system, which unveils that signal nulling may frequently occur over frequency in the case of large-size IRSs. To resolve this issue, we propose in this paper a new max-min CPB design, aiming to maximize the minimum end-to-end channel power gain over the frequency band. However, this problem is non-convex and difficult to be optimally solved. To tackle this difficulty, we propose to combine the alternating optimization (AO) and alternating direction method of multipliers (ADMM) algorithms to obtain a high-quality suboptimal solution. Numerical results show that the proposed max-min CPB design can achieve much better performance than the conventional narrowband CPB design.