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）因其能以经济高效的方式调控无线信道，被视作实现和增强未来无线通信的一项有前景技术。传统RIS 1.0采用对角移相矩阵，每个RIS元件连接至独立对地负载且与其他元件无连接，这一架构已获得广泛研究关注。然而，RIS 1.0的简单架构限制了其在无源波束赋形中的调控灵活性。为充分挖掘RIS潜力，本文引入超越对角移相矩阵的RIS 2.0，即超越对角RIS（BD-RIS）。我们首先基于散射参数网络分析阐释BD-RIS的建模方法，并根据散射矩阵的数学特征、支持模式及架构对BD-RIS进行分类。随后通过仿真评估不同模式/架构下BD-RIS的和速率性能。总结BD-RIS在波束调控高灵活性、扩大覆盖范围、简化部署以及降低分辨率比特与元件数量复杂度方面的优势。受BD-RIS优势启发，我们进一步探讨其在不同无线系统中的潜在应用。最后，列举BD-RIS在实际建模、设计与实现中的关键挑战，并指出未来可能的研究方向。