Active RIS-Assisted mmWave Indoor Signal Enhancement Based on Transparent RIS

Due to the substantial path loss inherent to millimeter-wave (mmWave) frequencies, the signal sent by the outdoor base station is seriously attenuated when it reaches the indoors. Recent research has introduced a glass-based metasurface to enhance mmWave signals in indoor settings. While a transparent reconfigurable intelligent surface (RIS) can focus signals in specific areas, achieving ideal coverage is hindered by constraints such as building structures. To address this limitation, we propose a novel RIS-assisted mmWave indoor enhancement scheme in which a transparent RIS is deployed on the glass, and a reflection RIS is introduced to enhance signal connectivity, ensuring mmWave coverage across indoor spaces. Three distinct assisted transmission scenarios are considered in this proposed scheme: passive RIS (PRIS), active RIS (ARIS), and hybrid RIS (HRIS). 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 is presented in the PRIS and the ARIS-assisted transmission scenarios. In addition, the performance of the proposed scheme is analyzed under three assisted transmission scenarios. The results indicate that the ARIS-assisted transmission scenario achieves the highest data rate and energy efficiency under a smaller transmit power while demanding minimal unit cells.