Optimized Design of Graphene-based RIS Unit Cell at THz with Reduced Specular Reflection

Inhalt:
Terahertz communication empowered by reconfigurable intelligent surfaces offers a promising solution for achieving ultra-high data rates in 6G networks. The design and modeling of RIS is challenged by electromagnetic effects such as mutual coupling and structural scattering, which can degrade performance if neglected. In particular, structural scattering results in an unwanted specular reflection that reduces the power transmitted in the desired directions. The paper focuses on graphene-based RIS, the reflection response of which can be changed by controlling the chemical potential of graphene. Usually, RIS are designed using a one-step approach where the unit cell (size, chemical potentials etc.) is designed to guarantee that the reflection coefficients of different unit cell states are characterized by a given phase difference and similar amplitude. The characterization is performed by applying local periodic boundary conditions, which mimic an infinite homogeneous surface of identical elements. This paper highlights that an optimized design of the unit cell can result in a RIS characterized by a better trade-off between specular reflection and power in the desired direction. Through full-wave simulations of a 24x24 RIS made of unit cells characterized by different chemical potentials, the paper proves that different chemical potentials have a different behaviour in terms of specular reflection. Notably, certain chemical potentials enable a RIS configuration that better trades power transmitted in desired and specular directions. Designing unit cells using such chemical potentials can lead to a noticeable improvement (e.g., 8 dB) in power transmission toward the desired angle, especially at low reflection angles.