Reconfigurable intelligent surface (RIS) has been envisioned as a promising technique to enable and enhance future wireless communications due to its potential to engineer the wireless channels in a cost-effective manner. Extensive research attention has been drawn to the use of conventional RIS 1.0 with diagonal phase shift matrices, where each RIS element is connected to its own load to ground but not connected to other elements. However, the simple architecture of RIS 1.0 limits its flexibility of manipulating passive beamforming. To fully exploit the benefits of RIS, in this paper, we introduce RIS 2.0 beyond diagonal phase shift matrices, namely beyond diagonal RIS (BD-RIS). We first explain the modeling of BD-RIS based on the scattering parameter network analysis and classify BD-RIS by the mathematical characteristics of the scattering matrix, supported modes, and architectures. Then, we provide simulations to evaluate the sum-rate performance with different modes/architectures of BD-RIS. We summarize the benefits of BD-RIS in providing high flexibility in wave manipulation, enlarging coverage, facilitating the deployment, and requiring low complexity in resolution bit and element numbers. Inspired by the benefits of BD-RIS, we also discuss potential applications of BD-RIS in various wireless systems. Finally, we list key challenges in modeling, designing, and implementing BD-RIS in practice and point to possible future research directions for BD-RIS.
翻译:可重构智能表面(RIS)被视为一种有前途的技术,可通过成本效益的方式在无线通信中实现和增强,因为它具有工程化无线信道的潜力。传统的RIS 1.0使用对角相位移位矩阵,已经引起了广泛的研究关注,其中每个RIS元素都连接到自己的负载到地,但不连接到其他元素。然而,RIS 1.0的简单架构限制了其操纵被动波束形成的灵活性。为了充分利用RIS的好处,在本文中,我们引入了超越对角相位移位矩阵的RIS 2.0,即超越对角RIS(BD-RIS)。我们首先解释了基于散射参数网络分析的BD-RIS建模,并通过散射矩阵的数学特征、支持的模式和结构对BD-RIS进行分类。然后,我们提供了不同模式/结构的BD-RIS的总速率性能模拟。我们总结了BD-RIS提供的高灵活性,扩大覆盖范围,促进部署,并要求分辨率位和元素数量低的好处。受到BD-RIS的好处的启发,我们还讨论了BD-RIS在各种无线系统中的潜在应用。最后,我们列出了BD-RIS建模、设计和实施中的关键挑战,并指出了可能的BD-RIS未来研究方向。