Transmissive RIS application to outdoor-to-indoor coverage improvement

This paper investigates the use of large Reconfigurable Intelligent Surfaces (RISs), also known as "Smart Skins," to improve outdoor-to-indoor millimeter-wave (mmWave) propagation, taking advantage of ventilation holes commonly found - or easily implemented - in European buildings. By using a transmissive, focusing RIS, the signal can 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, we compare scenarios with and without RIS at 29 GHz, 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.