Transparent Amplifying Intelligent Surface for Multi-User Uplink Enhancement in Indoor-to-Outdoor Communications

This paper presents a new transparent amplifying intelligent surface (TAIS) architecture for improving multi-user uplink performance in indoor-to-outdoor communications. Unlike passive reconfigurable intelligent surfaces (RIS) that primarily manipulate phase shifts, TAIS operates as an amplifier-based transmissive intelligent surface. It possesses the unique ability to refract and amplify signals from all users. Utilizing indium tin oxide coating and cutting-edge printing techniques, TAIS can be fabricated on windows without causing any visible alterations. This paper focuses on leveraging the TAIS to enhance the uplink spectral efficiency (SE) of multiple users in indoor-to-outdoor communications. Our analysis builds upon key assumptions, including the availability of perfect channel state information (CSI) and the use of a third-order memoryless polynomial model for tractable nonlinear PA characterization. The Bussgang decomposition is applied for nonlinear performance analysis, which remains approximately accurate for practical non-Gaussian signals. By collaboratively optimizing the refraction coefficient matrix of the TAIS and the combiner of the base station, the multi-user uplink SE maximization is achieved with consideration of the nonlinearity in the amplification process of TAIS. An efficient alternating optimization framework is developed to solve the non-convex problem approximately. Another important aspect is that we develop a zero-forcing-based successive convex approximation (ZF-SCA) algorithm to solve the problem with lower computational complexity. By employing the zero-forcing combiner at the BS, the problem is reduced to the refraction coefficient optimization at TAIS, using SCA techniques. Simulations demonstrate that the proposed TAIS system can significantly enhance SE by up to 39.1%, as compared to its alternative methods.