[论文解读] Digital Reconfigurable Intelligent Surfaces: On the Impact of Realistic Reradiation Models

Marco Di Renzo, Abdelhamed Ahmed+5 more|arXiv (Cornell University)|May 19, 2022

Advanced Wireless Communication Technologies被引用 11

一句话总结

本文研究了真实电磁再辐射模型对可重构智能表面（RIS）的影响，表明亚波长单元间距、相位-振幅耦合、量化以及干涉显著影响性能。该文提出通过优化完整的再辐射模式来最大化期望信号功率并最小化泄漏，识别出具有近似恒定振幅的2-bit RIS、单元间距为1/8至1/4波长时，是性能与复杂度之间理想的折衷方案。

ABSTRACT

Reconfigurable intelligent surface (RIS) is an emerging technology that is under investigation for different applications in wireless communications. RISs are often analyzed and optimized by considering simplified electromagnetic reradiation models. In this chapter, we aim to study the impact of realistic reradiation models for RISs as a function of the sub-wavelength inter-distance between nearby elements of the RIS, the quantization levels of the reflection coefficients, the interplay between the amplitude and phase of the reflection coefficients, and the presence of electromagnetic interference. We consider both case studies in which the users may be located in the far-field and near-field regions of an RIS. Our study shows that, due to design constraints, such as the need of using quantized reflection coefficients or the inherent interplay between the phase and the amplitude of the reflection coefficients, an RIS may reradiate power towards unwanted directions that depend on the intended and interfering electromagnetic waves. Therefore, it is in general important to optimize an RIS by considering the entire reradiation pattern by design to maximize the reradiated power towards the desired directions of reradiation while keeping the power reradiated towards other unwanted directions at a low level. Our study shows that a 2-bit digitally controllable RIS with an almost constant reflection amplitude as a function of the applied phase shift, and whose scattering elements have a size and an inter-distance between (1/8)th and (1/4)th of the signal wavelength may be a good tradeoff between performance, implementation complexity and cost. However, the presented results are preliminary and pave the way for further research into the performance of RISs based on accurate and realistic electromagnetic reradiation models.

研究动机与目标

• 分析真实电磁再辐射模型对RIS性能的影响，突破简化假设的局限。
• 研究亚波长单元间距、相位-振幅耦合及量化对再辐射模式的影响。
• 通过考虑完整的再辐射模式而非仅主波束，优化RIS设计。
• 在近场和远场用户场景下评估性能。
• 识别一种在性能、复杂度和成本之间实现平衡的实际RIS设计方案。

提出的方法

• 本研究采用全波电磁模型计算每个RIS单元的再辐射场，考虑互耦效应和单元几何结构。
• 建立约束优化问题，以在目标用户处最大化接收功率的同时最小化向非期望方向的辐射功率。
• 优化过程采用交替算法，依次更新反射系数，考虑来自有限字母表的离散相移。
• 模型包含自由空间路径损耗，近场采用球面波前，发射端和接收端均采用方向性增益模式。
• 在不同单元间距（1/8至1/4波长）、量化级别（2-bit）以及振幅-相位耦合条件下评估性能。
• 基于球面波前和sinc基波束成形因子的信号模型，对比分析远场与近场条件下的RIS配置性能。

实验结果

研究问题

• RQ1 亚波长单元间距如何影响数字可重构RIS的再辐射模式？
• RQ2 相位量化和振幅-相位耦合对非期望再辐射及系统性能有何影响？
• RQ3 近场与远场传播条件如何改变最优RIS设计与性能？
• RQ4 忽略完整再辐射模式在多大程度上导致RIS部署与性能的次优？
• RQ5 在真实电磁约束下，哪种RIS配置在性能、实现复杂度和成本之间提供了最佳折衷？

主要发现

• 具有近似恒定振幅的2-bit数字可重构RIS在性能与硬件复杂度之间实现了良好平衡。
• RIS单元间采用亚波长间距（1/8至1/4波长）显著影响再辐射模式，必须在设计中予以考虑。
• 相位量化和振幅-相位耦合导致非理想再辐射，造成能量泄漏至非期望方向。
• 近场条件需采用球面波前建模，因为平面波前近似无法准确捕捉真实信道行为。
• 在RIS设计过程中必须优化完整再辐射模式，以最大化期望信号功率并抑制干扰。
• 所提出的优化框架通过考虑真实电磁耦合与散射效应，实现了性能增益。