The existing phase shifter models adopted for reconfigurable intelligent surfaces (RISs) have ignored the electromagnetic (EM) waves propagation behavior, thus cannot reveal practical effects of RIS on wireless communication systems. Based on the equivalent circuit, this letter introduces an angle-dependent phase shifter model for varactor-based RISs. To the best of our knowledge, this is the first phase shifter model which reveals that the incident angle of EM waves has influence on the reflection coefficient of RIS. In addition, the angle-reciprocity on RIS is investigated and further proved to be tenable when the reflection phase difference of adjacent RIS unit cells is invariant for an impinging EM wave and its reverse incident one. The angle-dependent characteristic of RIS is verified through full-wave simulation. According to our analysis and the simulation results, we find that the angle-reciprocity of varactor-based RIS only holds under small incident angles of both forward and reverse incident EM waves, thus limits the channel reciprocity in RIS-assisted TDD systems.

中文翻译：

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可重构智能表面的角度相关移相器模型：角度互易成立吗？

可重构智能表面（RIS）采用的现有移相器模型忽略了电磁（EM）波的传播行为，因此无法揭示 RIS 对无线通信系统的实际影响。在等效电路的基础上，这封信为基于变容二极管的 RIS 介绍了一种与角度相关的移相器模型。据我们所知，这是第一个揭示电磁波入射角对 RIS 反射系数影响的移相器模型。此外，研究并进一步证明了当相邻 RIS 晶胞的反射相位差对于入射电磁波及其反向入射波不变时，RIS 上的角互易性是成立的。通过全波仿真验证了 RIS 的角度相关特性。