Reconfigurable Intelligent Surfaces (RISs) are an emerging technology for future wireless communication systems, enabling improved coverage in an energy efficient manner. RISs are usually metasurfaces, constituting of two-dimensional arrangements of metamaterial elements, whose individual response is commonly modeled in the literature as an adjustable phase shifter. However, this model holds only for narrow communications, and when wideband transmissions are utilized, one has to account for the frequency selectivity of metamaterials, whose response follows a Lorentzian profile. In this paper, we consider the uplink of a wideband RIS-empowered multi-user Multiple-Input Multiple-Output (MIMO) wireless system with Orthogonal Frequency Division Multiplexing (OFDM) signaling, while accounting for the frequency selectivity of RISs. In particular, we focus on designing the controllable parameters dictating the Lorentzian response of each RIS metamaterial element in order to maximize the achievable sum-rate. We devise a scheme combining block coordinate descent with penalty dual decomposition to tackle the resulting challenging optimization framework. Our simulation results reveal the achievable rates one can achieve using realistically frequency selective RISs in wideband settings, and quantify the performance loss that occurs when using state-of-the-art methods which assume that the RIS elements behave as frequency-flat phase shifters.
翻译:重新配置的智能表面(RIS)是未来无线通信系统的一种新兴技术,能够以节能的方式改善覆盖面。RIS通常是元表面,构成元物质元素的二维安排,这些元物质元素的个别反应通常在文献中建模,作为可调整的阶段转换器。然而,这一模型只用于狭小的通信,在使用宽带传输时,必须说明元材料的频率选择,其反应以Lorentzian为背景。在本文中,我们考虑将宽频带多用户多用户多输出输出(MIMO)无线系统与Orthoonial频率司多输出(OFDM)信号的双维组合连接,同时考虑可调整的阶段转换器的频率。特别是,我们侧重于设计可控参数,指定每个IRS元元素的Lorentzian反应频率,以便最大限度地实现可实现的总和率。我们设计了一个组合组合组合组合组合组合,以协调刑罚双向脱钩,从而应对具有挑战性的优化框架。我们的模拟结果显示,当使用可实现的频度变化率时,在可实现的系统变化状态时,在可实现的状态中,一个可实现可实现的频率变化的状态的状态中,一个可量化的状态将可实现的状态的状态的状态的状态将可实现的状态的状态的状态设定,即:在使用可实现可实现的周期性能变率。