Asymmetrically Layered Unit Cell Topology to Reduce Cell Gap of Liquid Crystals for 5G Millimeter-Wave Transmissive Reconfigurable Intelligent Surface Applications

This study presents transmissive reconfigurable intelligent surface (TRIS) unit cells (UCs) with slimmed liquid crystals (LCs), where the effective cell gap is electromagnetically tailored to control the LC using empirically practical voltage values. Prior LC-based transmissive UCs utilize thickened cell gaps at millimeter-wave frequencies due to limited parametric degrees of freedom, aiming for enhanced performance, but this approach is unfavorable in terms of LC controllability. Therefore, a coupled resonator-loaded UC featuring a dumbbell-shaped complementary loop with intrinsically reduced coupling strength is proposed, despite the close proximity due to the slimmed cell gap. Moreover, a cleverly designed asymmetrical topology is proposed, where the resonators are positioned apart from each other in the perpendicular direction of the coupled dominant current. The designed UC achieves a simulated phase tuning range (PTR) of 123° by employing only a single-layered LC with a cell gap of 0.21 mm, which is improved more than two times than that of the previously studied counterpart. In addition, an operating range of 27.9–29 GHz with reasonable loss sustainable for entire range of LC dielectric constant is attained successfully, contrary to previous works where low loss is maintained only for a single tone (ST) frequency.

I. Introduction

The millimeter-wave (mmWave) frequency has been commercialized for 5G due to the demand for high-speed and low-latency wireless communications; however, many wireless network operators face challenges in providing affordable services to users due to the narrowed coverage caused by the abrupt increase in high-frequency loss. Therefore, the sub-6 GHz band is unintentionally utilized as the primary frequency band, and the 3GPP defines various use cases as part of a transitional process to effectively disseminate advanced quality. Among the various cases defined, reconfigurable intelligent surfaces (RISs) have been evaluated as a core technology for next-generation communication. The RIS relays arbitrary signals in a manner that spatially controls the wavefront of the incident waves and has the potential for economic popularization. Among the reconfigurable components, liquid crystals (LCs) are actively chosen for RIS over other candidates due to their operability at high frequencies with a reasonable loss. However, research on LC-based RIS is primarily limited to the reflective type rather than the transmissive type [1], [2], [3], [4], [5]. This limitation arises from the technical challenge of achieving a shared frequency band where high transmittance is preserved despite fluctuations in the LC dielectric constant () and simultaneously achieving a practical phase tuning range (PTR) within that band.