Wide Incident Angle Digital Coding Metasurface Applied to Reconfigurable Intelligent Surfaces

This paper presents a wide incident angle metasurface unit cell element applied to a reconfigurable intelligent surface (RIS) for beamforming and beam-steering applications in the 26 GHz frequency band from the fifth generation of mobile communications (5G) frequency range 2 (FR2). Each metasurface unit cell is based on a printed frequency selective surface (FSS) loaded with a varactor diode. The FSS-based structure is based on a circular loop at the top and a slot-based ground plane at the bottom resulting in a
$\boldsymbol{0.25 \mathrm{x} 0.25 \lambda_{0}}$
total area. The complete unit cell element encompasses four conducting layers, in which the first two ones form the FSS. RF chokes are printed at the middle layer to isolate the DC circuit, and the bias lines are routed at the fourth layer. The unit cell has been conceived using the full-wave electromagnetic solver ANSYS HFSS. Its numerical results demonstrate a reflection phase shift up to
$180^{\circ}$
and reflection magnitude higher than 0.4 at the 26 GHz frequency band for incident wave angle from 0 to
$50^{\circ}$
. The proposed reconfigurable intelligent surface might be applied to future wireless communication systems, planar antenna reflectors, and vortex beam generation.