The expectation is that XG communications will have a disruptive impact in their applications in smart cities, industrial automation, agriculture, e-health, smart grids, domotics, and autonomous driving. However, such scenarios imply the availability of technology that cannot be
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The expectation is that XG communications will have a disruptive impact in their applications in smart cities, industrial automation, agriculture, e-health, smart grids, domotics, and autonomous driving. However, such scenarios imply the availability of technology that cannot be met resorting to incremental changes in present techniques. Specifically, it is now apparent that the ultrawideband massive MIMO in both the microwave and sub-THz bands will be needed.

By exploiting larger bandwidths, unprecedented data rates will be achieved for MIMO applications in the microwave band. Before this work, wideband arrays have been considered unsuitable for MIMO applications due to the high levels of inter-elements mutual coupling. However, in this work it has been proven that if the entire array is coherently excited, multiple orthogonal beams can be generated, regardless of the inter-element mutual coupling. The orthogonality levels depend solely on the beam overlap and, therefore, on the beam width, the side lobes, and the position of the nulls.

Moreover, a wideband phased array has been designed for sub-8 GHz MIMO communications. The array is realized in the form of a dual-polarized connected slot array with interchangeable Artificial Dielectric Layers (ADLs) radome. This allows the array to scan up to 60° in every azimuthal cut while being matched between 6 and 8GHz with the first radome, and 2 and 8 GHz with the second one. Finally, an 8x8 prototype is manufactured and tested.

Due to the long wavelength at microwave bands, the antenna size is the largest constraint when it comes to MIMO applications, especially for wideband operations. To this aim, a new metric is developed to assess the signal and the interference of MIMO antennas constrained within a given volume. This allows us to compare the performance of an intended antenna design or even a realized prototype to the one of the maximum gain antenna located within the given volume. By means of these concepts, it is possible to link the MIMO performance with the antenna size to optimize the space.

By exploiting larger bandwidths, unprecedented data rates will be achieved for MIMO applications in the microwave band.

The second road to high data rates for MIMO application is the use of higher frequencies (sub-THz) communications. The main hindrance to integrated antennas for this regime is the technological challenge due to microfabrication and the integration with the electronics. This calls for accurate simulations that enable optimal designs.

For this purpose, an integral equation solver was developed to study dielectric lenses together with their feeds. At high frequency, the thickness of the metal plays an important role and cannot be neglected. Due to the different scales involved in the feed and in the lens, the required computational effort might be prohibitive. Therefore, a method has been devised to combine the numerical solution with analytical results to enable large-scale simulations. The impedance of an integrated antenna can be seen as composed of the reactance of the feed, the impedance of the feed radiating in a semi-infinite space without reflections, and an impedance associated with the reflections. While the former two can be evaluated analytically or with fast numerical simulations, the latter requires a time-intensive full-wave simulation of the entire problem. However, this can be simplified by synthesizing a much coarser feed, which radiates equivalently into the semi-infinite medium. Having this much coarser discretization it allows us to simplify the simulation of the entire problem and to isolate the reflections conveniently. Then, all the components can be combined together, and the input impedance can be estimated accurately.

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