On Transmissive RIS for mm-Waves Outdoor-to-Indoor Coverage Enhancement

This paper investigates the use of large Reconfigurable Intelligent Surfaces (RIS), also known as “Smart Skins”, to improve outdoor-to-indoor millimetre-wave (mmWave) propagation, taking advantage of ventilation holes commonly found, or easily realizable, in European buildings. By using a transmissive, focusing RIS, the signal could be concentrated and routed through the opening, enhancing signal penetration into the indoor space even in modern, highly insulated buildings that particularly hinder signal penetration. Using Ray tracing simulations at 27 GHz we compare scenarios with and without RIS, demonstrating significant indoor signal coverage improvements in the first case. The study highlights the use of RIS - or smart-skins - as a cost-effective solution to address the challenges of mmWave propagation in modern building designs.