Development of 2D and 3D structures for millimeter-wave communications and future applications

Abstract

Over the last few decades, the exponential growth of smart devices has drastically increased the demand for high-capacity communication links. The rapid increase in the data transmission, especially for streaming applications, has driven to the search for wireless links operating at higher frequencies. Consequently, the primary goal in current Fifth Generation (5G) and future Sixth Generation (6G) mobile networks is to utilize channels in the millimeter-wave bands (from 30 to 300 GHz) and even the sub-THz band (up to 1 THz) to achieve greater bandwidth and capacity in communication links. However, at these frequencies, propagation losses in the channels and antenna directivity become more pronounced. In this context, broadcasting, where a single user sends information to multiple users, becomes challenging. Thus, new communication systems focusing on point-to-point links with beam-steering capabilities have emerged as promising solutions. Additionally, frequency mixing, which allows antennas to transmit information across different frequency bands (such as for telephony or geolocation), presents a viable alternative to design more efficient devices. This Doctoral Thesis focuses on the development of radiating systems and metastructures that address these technological challenges. In particular, we research the use of periodically-modulated metamaterials to modify the properties of antennas in the millimeter-wave bands. We divide this extensive topic into three main categories: spatial modulations, time modulations and spacetime modulations. In each of them, we propose different devices that present interesting capabilities in the millimeter-wave bands. On the one hand, concerning spatial modulations, we propose several passive metamaterials/metasurfaces, including: a Fresnel lens-antenna designed to enhance directivity and introduce beam-steering capabilities to a conventional waveguide at 60 GHz, and a metasurface for polarization control in reflection at K/Ka satellite bands. Additionally, we develop analytical frameworks based on Floquet-Bloch expansions of the electromagnetic fields and integral-equation methods to simulate these spatially-modulated metamaterials from a circuit-based perspective. Specifically, we use the circuit theory, previously applied to static spatial metasurfaces, in a 3D metadevice and a 2D metagrating loaded with lumped elements (diodes, resistors, capacitors and inductors). On the other hand, regarding time modulations, we propose a theoretical framework to simulate a time-varying metasurface that alternates its electrical properties (air, metal and grating states). Due to the lack of commercial full-wave software to simulate this kind of devices in an efficient manner, this solution arises as a good alternative in this topic. Furthermore, we implement a finite-difference time-domain (FDTD) method to test the obtained analytical results. These works lead to the development of a comprehensive theoretical tool for simulating spacetime-modulated metasurfaces. The proposed spacetime device demonstrates both beamforming and frequency mixing capabilities in the microwave bands, offering promising and intriguing features for future telecommunications systems. Additionally, a setup for electromagnetic characterisation of materials is proposed with the aim of understanding the constitutive parameters of homogeneous dielectrics that can be useful for the design of metamaterials in the millimeter-wave bands.