Metasurface Transmitters in RIS-based Wireless Networks

Future communication networks are demanding real-time control of the reflection and scattering of radio waves through the propagation medium. The concept of a new controllable scenario has encouraged research on the existing theoretical frameworks for channel characterization. New communication models account for the several states of an active channel, due to this generalization, the interaction with the environment is taking the shape of an optimization problem. At the physical layer, this implies the design of electromagnetic devices able to follow new specifications. Current electromagnetic devices can become a bottleneck in this new panorama, which aims to perform localization, joint communication, and sensing while enabling data rates of the order of 1 Tbps, end-to-end latencies of hundreds of milliseconds, and high energy efficiency. Current infrastructures cannot handle the expected increase in data traffic, which will rise the spectral efficiency demand. This leads to the need for a new architectural platform to dynamically control channel assignment and radio resources. This dissertation proposes technical solutions that contribute to this new environment. Key challenges associated with the physical layer in transmission for RIS-based wireless networks are addressed, whether concerning radiating structures based on traveling waves—particularly surface waves—or spatial waves. The investigation begins with a study on the potential and limitations of metasurface (MTS) antennas based on varactor diodes. The extent of this technology is clearly illustrated in the shape of two implementations that feature the possible scanning methods of this technology. First, we present a simple, low-profile, scanning antenna providing flexible control of both the pointing direction and leakage. Then, the study on metasurface antennas based on varactors is completed with a double layer metasurface antenna, which features low power consumption and a number of controls equal to the number of channels. However, these benefits are given for a reduced angular range. This leads to the second part of the thesis, where reconfigurable transmitters are based on the manipulation of spatial waves. Here, a metasurface radome, or metadome (MTD) with efficient full-range scanning is proposed. This finding relies on the discussion of the compensation technique used so far, which is based on a global compensation of the MTS response, in contrast to the predistortion based on local periodicity assumption. Globally compensating for the MTS response and antenna mutual coupling allows to design a MTD providing optimal performance across the entire angular range and effectively expands the scan range—an achievement not attained with MTS-based solutions in previous investigations—. Lastly, the previous scan range expansion technique has also been applied to sparse arrays. From a system-level perspective, the benefits of sparse transmitters can be preserved without increasing computational burden, as the proposed system avoids scan blindness and suppresses grating lobes. The entire research process has been conducted within the framework of a multi-partner training network, whose primary goal is to integrate both system-level and physical perspectives: Meta Wireless - Future wireless communications empowered by reconfigurable intelligent metasurfaces, H2020-MSCA-ITN-2020, grant agreement 956256. This dissertation contributes to Meta Wireless by orienting transmitter design towards the physical layer of smart radio environments, which is vital for the growth of telecommunications.