Near-Field and Far-Field Sub-Terahertz Beam Steering Using Electrically Controlled Reconfigurable Intelligent Surface for 6G Applications

This work presents the design and numerical validation of a sub-terahertz (sub-THz) reconfigurable intelligent surface (RIS) capable of beam steering in both the near-field and far-field regions. The RIS is specifically designed to enable efficient operation across both regimes without requiring significant modifications to its physical structure. The proposed RIS employs an H-shaped unit cell with a centrally placed graphene strip, operating at 0.27 terahertz (THz) and a periodicity of approximately$\lambda $/2 (free-space wavelength), enabling maximum reflection with a phase difference of$180^{\circ } \pm 10^{\circ }$between two graphene chemical potential ($\mu _{c}$) states:onat$\mu _{c} =1.0$eV andoffat$\mu _{c} =0.01$eV. The RIS is excited in near-field by a center-fed pyramidal horn antenna (HA) to achieve a broadside gain of 23.1 dBi and beam steering up to ±60°, with a scan loss of 4.8 dB. Moreover, in the far-field regime, plane-wave excitation demonstrates beam steering capability up to ±40°, with a scan loss of 4.6 dB, as verified numerically. An equivalent circuit model (ECM) based on transmission-line techniques is developed to further validate the full-wave simulation results. The simulated outcomes show close agreement with the numerical calculations. These results indicate that the proposed graphene-based RIS is a promising candidate for sixth-generation (6G) systems, beam steering, imaging, and sub-THz wireless communication applications.