Design and Analysis of an Ultra-Wideband Reconfigurable Intelligent Surface (RIS) at 28 GHz for mmWave Applications

This paper presents the design and performance analysis of a 1-bit reconfigurable intelligent surface (RIS) operating in the ultra-wideband (UWB) spectrum centered at 28 GHz, targeting next-generation mmWave communications. A novel elliptical patch unit cell integrated with a central horizontal slot is proposed. The structure is optimized for wideband reflection characteristics and includes a PIN diode for achieving dynamic phase switching. The unit cell is constructed on a Rogers RO5880 substrate with low dielectric loss to ensure minimal reflection loss and stable phase response over a wide frequency range.

Using full-wave electromagnetic simulations in CST Microwave Studio, we investigate the S-parameters, phase behavior, and reflection efficiency of the proposed structure across a bandwidth from 26 GHz to 32 GHz. The diode is modeled using a lumped RLC circuit, and both ON and OFF states are evaluated to confirm a near 180° phase shift essential for effective 1-bit RIS operation. In addition to unit cell characterization, we simulate RIS arrays of various sizes to examine the impact on radar cross-section (RCS) and beam shaping capabilities.

The results show that the RIS maintains good matching (S11 < –10 dB), low reflection loss (< 1.5 dB), and consistent phase control, making it suitable for UWB beamforming applications. Future work will explore 2-bit RIS architectures for enhanced resolution and reconfigurability.