标题： 主动RIS：建模与优化 显示英文标题

标题： Active RISs: Modeling and Optimization

摘要： 可重构智能表面（RIS）增强的通信已成为下一代无线网络的变革性技术，使传播环境的可编程塑造成为可能。 然而，传统的RIS在双路径损耗效应上存在根本性的限制，这严重衰减了反射信号。 为克服这一问题，已经提出了能够放大入射信号的主动RIS架构。 本章研究了主动RIS的建模、性能分析和优化，重点关注两种硬件设计：一种是使用单个功率放大器（PA）的双RIS结构，另一种是在单元级别使用隧道二极管的反射放大结构。 对于基于PA的设计，开发了一个全面的数学模型，并推导出了接收信噪比、误码概率和能量效率（EE）的闭式表达式。 提出了一种优化框架，用于配置相位偏移和放大器增益，以在功率约束下最大化系统容量。 关于第二种设计，通过分析其I-V特性，仔细研究了将隧道二极管集成到单元中的方法，从而推导出负阻范围和功耗模型。 此外，通过紧凑的线性代数公式表征了反射系数的内在相位-幅度耦合，实现了对主动RIS的实际优化。 大量的数值仿真验证了理论分析，表明主动RIS可以有效克服双路径损耗限制，并与被动RIS相比实现有利的能量效率权衡。 最后，研究了可用功率预算与主动元件数量之间的权衡，结果表明更多的主动元件并不总能带来最佳性能。 显示英文摘要

摘要： Reconfigurable Intelligent Surfaces (RIS)-empowered communication has emerged as a transformative technology for next generation wireless networks, enabling the programmable shaping of the propagation environment. However, conventional RISs are fundamentally limited by the double path loss effect, which severely attenuates the reflected signals. To overcome this, active RIS architectures, capable of amplifying impinging signals, have been proposed. This chapter investigates the modeling, performance analysis, and optimization of active RISs, focusing on two hardware designs: a dual-RIS structure with a single Power Amplifier (PA), and a reflection amplification structure at the unit cell level using tunnel diodes. For the PA-based design, a comprehensive mathematical model is developed, and closed-form expressions for the received signal-to-noise ratio, bit error probability, and Energy Efficiency (EE) are derived. An optimization framework for configuring the phase shifts and amplifier gain is proposed to maximize system capacity under power constraints. Regarding the second design, the integration of a tunnel diode into the unit cell is carefully studied by analyzing its I-V characteristic, enabling the derivation of the negative resistance range and the power consumption model. Furthermore, the intrinsic phase-amplitude coupling of the reflection coefficient is characterized through compact linear algebra formulations, enabling practical optimization of active RISs. Extensive numerical simulations validate the theoretical analyses, demonstrating that active RISs can effectively overcome the double path loss limitation and achieve favorable EE trade-offs compared to passive RISs. Finally, the trade-off between the available power budget and the number of active elements is examined, revealing that a higher number of active elements does not always lead to optimal performance.