DESIGN OF MONOLITHICALLY INTEGRATED RF-MEMS MULTI-FUNCTIONAL PASSIVES FOR HYBRID BEAMFORMING ARCHITECTURES IN BEYOND-5G AND 6G SCENARIOS

The recent years have witnessed an unprecedented growth in the number of connected devices and amount of bandwidth required by the multiple services offered by wireless devices. The current 5G standard addresses such issues by adopting higher carrier frequencies and antennas with a large number of radiating elements. The former solution enables to exploit larger bandwidths in the millimeter-wave (mmWave) portion of the spectrum, while the latter one allows access points to serve an increasingly higher number of users. Both find realization in the Multiple-Input-Multiple-Output (MIMO) antenna systems with their enhanced beamforming capabilities. While the adoption of the hybrid digital-analog beamforming architecture lightens the overall system complexity, the need of miniaturized, high-performance and broadband hardware components is still an open issue. Passive Radio Frequency (RF) components in MicroElectroMechanical-Systems technology (RF-MEMS) offer notable and broadband electrical performances, while maintaining the marked miniaturization required for the hardware to be employed in the MIMO antennas, characterizing the current and future telecommunications scenario. Whilst numerous examples of single RF-MEMS switches, attenuators and phase shifters are available in the literature since about two decades, still limited attention is dedicated to the development of MEMS-based multi-device monolithic networks embedding such devices. High-performance RF-MEMS networks of this kind could represent the base of future MIMO beamforming architectures. Given such a context, the fundamental core of this thesis is the design and the realization of ad hoc RF-MEMS devices to be integrated in a reconfigurable monolithic module, operating in the realistic scenario of the mm-Wave portion of the spectrum allocated to 5G in Europe (24.25–27.5 GHz). The resulting devices consist in a 3-bit attenuator, three 1-bit phase-shifting cells and a Single-Pole-Double-Throw (SPDT) switch, each relying on membranes featuring a reduced actuation voltage, in the 5–9 V range, for an easier interfacing with electronics based on Complementary Metal–Oxide–Semiconductor (CMOS). To this purpose, the ad hoc designed MEMS switching membranes, along with prototypes of the building blocks to be embedded in the final module, are designed, optimized and fabricated. The experimental measurements performed on the prototypes of membranes (i.e. micro-switches), attenuation cells, optimized resistors and a phase shifter are compared to FEM-based (Finite Element Method) simulated results. Such comparison validates the simulation approach, in both the electromagnetic and the electro-mechanical domains, by which the proposed module is then designed and optimized in its final layout. To the best of our knowledge, this project is among the first to investigate the development of a monolithic module, entirely based on RF-MEMS passives, implementing both the attenuation and the phase shifting functionalities that can be employed in hybrid beamforming architectures at each antenna element. More in detail, the module features at least 25 attenuation and phase shifting states, from -5.39 dB to -13.51 dB by variable steps, and from 10.59° to 158.46°, respectively. Concerning the SPDT switch, satisfying electrical performances have been demonstrated in terms of return loss (<-10 dB), insertion loss (<-1.2 dB) and isolation (<-25 dB) over the 0–30 GHz interval. Despite their increased complexity, appealing results have marked the proposed attenuator and the phase-shifting cells, whose return and insertion losses are always better than -10 dB and -3 dB, respectively, along the frequency interval of interest. With an overall footprint not exceeding 9.51x3.35 mm2, the designed module effectively combines the miniaturization, broadband, and linear electrical behavior of RF-MEMS, making it a suitable candidate for the MIMO antennas of the current and future telecommunications scenario.