A Wideband Reconfigurable 1-Bit Unit Cell for mmWave RIS with Enhanced Angular Stability and Phase Difference Error

This paper presents the design of a novel millimeterwave reflective unit cell for integration in reconfigurable intelligent surfaces (RIS). The proposed unit cell exhibits excellent reflection characteristics and angular stability throughout the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$24-30 \mathbf{~ G H z}$</tex> frequency range, making it an excellent candidate for utilization in wide bandwidth current mmWave 5G and future 6G systems. The proposed unit has a total size of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$3 \times 2.2 \text{mm}^{2}$</tex>. It comprises two diamond-shaped, out-of-phase patches connected via a p.in diode to enable reconfigurability and serve as a binary phase modulator. Full-wave simulations demonstrated an average phase difference error of only <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\pm 1.5^{\circ}$</tex> at an angle of incidence of 30° off the RIS boresight throughout the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$24-30 \mathbf{~ G H z}$</tex> band. The proposed unit cell also maintained good reflecting properties at angles of incidence of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$40^{\circ}, 20^{\circ}$</tex>, and 10°. It reflects at least 77% of the energy and has an average phase difference of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$180^{\circ} \pm 20^{\circ}$</tex> between its two binary states. The performance of the proposed unit cell allows for minimal loss and beam steering errors when utilized in RIS development.