  With the increasing adoption of millimeter-waves (mmWave) over cellular
networks, outdoor-to-indoor (O2I) communication has been one of the challenging
research problems due to high penetration loss of buildings. To address this,
we investigate the practicability of utilizing reconfigurable intelligent
surfaces (RISs) for assisting such O2I communication. We propose a new notion
of prefabricated RIS-empowered wall consisting of a large number of chipless
radio frequency identification (RFID) sensors. Each sensor maintains its own
bank of delay lines. These sensors which are built within the building walls
can potentially be controlled by a main integrated circuit (IC) to regulate the
phase of impinging signals. To evaluate our idea, we develop a thorough
performance analysis of the RIS-based O2I communication in the mmWave network
using stochastic-geometry tools for blockage models. Our analysis facilitates
two closed-form approximations of the downlink signal-to-noise ratio (SNR)
coverage probability for RIS-based O2I communication. We perform extensive
simulations to evaluate the accuracy of the derived expressions, thus providing
new observations and findings.
