Design of innovative transparent smart electromagnetic skins working in transmitting mode

The development of modern communication systems such as 5G, showing enhanced performance despite of no need of additional Base Stations and lower power supply, is made possible by employing among the other devices the so-called Smart Electromagnetic Skin (SES). Thanks to their use, the environment in which the propagation of the signal occurs represents no longer a limitation but a valuable resource and for this reason it is named Smart Electromagnetic Environment (SEE). The reason for relying on this approach is to enhance the performances of these technologies, which requires high data rates wide bandwidth and low latency as main goals. SES are planar or curved surfaces discretized with a suitable number of unit-cells with resonant or sub-wavelength size, designed in such a way to provide anomalous reflection or transmission, thanks to which an incident field can be redirected in not Line of Sight (LoS) regions. Strictly speaking, SES are full passive surfaces, able to provide a pre-defined coverage but also reconfigurable configurations, identified as Reconfigurable Intelligent Surfaces (RIS), can be obtained by adding active elements in the unit-cells that can be dynamically adjusted. While the usage of these surfaces in reflection mode with the aim to cover blind spots or a pre-defined area and overcome obstacles has been widely investigated in literature, their application in transmitting mode is still almost unexplored. A transmitting SES has the aim to mitigate one of the most problematic issues in dealing with the last communication technologies, i.e. the high loss experienced to the signal due to the penetration through a building, establishing an Outdoor to Indoor Link. This thesis project has the aim to implement this idea, considering a passive innovative SES that can be integrated into a building window. The design of such a structure is challenging, since it has not only to satisfy the requirements relative to its performance, but also to minimally affect the window optical transparency. While the few configurations already discussed in literature are made up with unit cells adopting thin metallizations, the idea exploited here is to design a Dielectric-Only SES, that could be manufactured for instance using an Additive Manufacturing (AM) technique. The starting point of the carried-out activity is the design of a suitable unit cells; different structure are analysed, characterized by one or more geometrical parameters to control both the provided transmission and reflection coefficients, but always including a layer of glass, that takes into account the presence of the window. While the transmission coefficient is the parameter adopted for the design of the SES, since each unit cell must provide the value of its phase that guarantees the focusing of the transmitting field in the desired direction for a giving incidence, a low value of the reflection coefficient assures reduced back radiation. The most promising unit cells are then adopted for the design of different SESs: their simulation gives values for the Radar Cross Section at the design frequency of 27GHz near to 19dbm2. In view of these promising results and for their further validation, a Transmitarray (TA) is designed (it differs from a transmitting SES only in the position of the feeding structure, that in TA is close to the transmitting surface) and then experimentally characterized.