发表机构
Italian Aerospace Research Centre; University Federico II; University of Sannio（意大利航空航天研究中心; 那不勒斯费德里科二世大学; 桑尼奥大学）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
提出物理一致的LPTV模型统一建模RIS的频率选择性与周期时变，推导广义CP条件，经全波仿真验证带内色散及对OFDM的影响。
AI 中文摘要
可重构智能表面（RIS）的单元通常被建模为频率选择性时不变反射器或瞬时时变反射系数。然而，在实际中，时间调制超表面同时表现出频率选择性和周期性时间变化。我们开发了一种物理一致的线性周期时变（LPTV）模型，该模型联合捕获这些效应，并表征它们对宽带正交频分复用（OFDM）通信的影响。从通用RIS单元的典型等效电路出发，我们推导出单谐振模型，其物理意义明确的参数同时决定了频率选择性反射系数和有效冲激响应持续时间，即单元的有限记忆。周期切换的色散响应随后通过谐波传递函数表示，从而得到闭式的每子载波OFDM输入输出关系。由此产生的耦合通常是非对角线的：每个接收子载波通过RIS谐波从多个发射子载波收集贡献，每个贡献由单元在相应绝对输入频率下的响应加权。我们进一步推导了广义循环前缀（CP）条件，要求保护间隔同时容纳传播信道延迟扩展和RIS记忆。在该条件下，符号间干扰和栅格外频谱泄漏被抑制，而确定性的栅格上谐波耦合仍然存在。针对5G NR信道设计的OpenRIS单元的全波仿真验证了所提出的谐振模型，并揭示了尽管近乎理想的二进制相位切换，仍存在明显的带内色散。在3GPP抽头延迟线（TDL）信道上的仿真证实了广义CP条件，并显示了其对高品质因数RIS单元的相关性。
英文摘要
The elements of a reconfigurable intelligent surface (RIS) are commonly modeled either as frequency-selective time-invariant reflectors or as instantaneous time-varying reflection coefficients. In practice, however, time-modulated metasurfaces exhibit both frequency selectivity and periodic time variation. We develop a physically consistent linear periodically time-varying (LPTV) model that jointly captures these effects and characterizes their impact on wideband orthogonal frequency-division multiplexing (OFDM) communications. From a canonical equivalent circuit of a generic RIS element, we derive a single-resonance model whose physically meaningful parameters determine both the frequency-selective reflection coefficient and the effective impulse-response duration, i.e., the finite memory of the element. The periodically switched dispersive responses are then represented through harmonic transfer functions, leading to a closed-form per-subcarrier OFDM input-output relation. The resulting coupling is generally non-diagonal: each received subcarrier collects contributions from multiple transmitted subcarriers through the RIS harmonics, each weighted by the element response at the corresponding absolute input frequency. We further derive a generalized cyclic-prefix (CP) condition requiring the guard interval to accommodate both the propagation-channel delay spread and the RIS memory. Under this condition, intersymbol interference and out-of-grid spectral leakage are suppressed, while deterministic on-grid harmonic coupling remains. Full-wave simulations of an OpenRIS unit cell designed for a 5G NR channel validate the proposed resonant model and reveal appreciable in-band dispersion despite nearly ideal binary phase switching. Simulations over 3GPP tapped-delay-line (TDL) channels confirm the generalized CP condition and show its relevance for high-quality-factor RIS elements.
Comments13 pages, 6 figures, 2 tables
