  Due to the serious path loss of millimeter-wave (mmWave), the signal sent by
the base station is seriously attenuated when it reaches the indoors. Recent
studies have proposed a glass-based metasurface that can enhance mmWave indoor
signals. The transparent reconfigurable intelligent surface (RIS) focuses on
the mmWave signal to a specific location indoors. In this paper, a novel
RIS-assisted mmWave indoor enhancement scheme is proposed, in which a
transparent RIS is deployed on the glass to enhance mmWave indoor signals, and
three assisted transmission scenarios, namely passive RIS (PRIS), active RIS
(ARIS), and a novel hybrid RIS (HRIS) are proposed. This paper aims to maximize
the signal-to-noise ratio (SNR) of the received signal for the three assisted
transmission scenarios. The closed-form solution to the maximum SNR is
presented in the PRIS and the ARIS-assisted transmission scenarios. Meanwhile,
the closed-form solution to the maximum SNR for the HRIS-assisted transmission
scenario is presented for given active unit cells. In addition, the performance
of the proposed scheme is analyzed under three assisted transmission scenarios.
The results indicate that under a specific RIS power budget, the ARIS-assisted
transmission scenario achieves the highest data rate and energy efficiency.
Also, it requires very few unit cells, thus dramatically reducing the size of
the metasurface.
