Sun Shu, Tao Meixia
Department of Electronic Engineering, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China.
Shanghai Key Laboratory of Digital Media Processing and Transmissions, Shanghai 200240, China.
Sensors (Basel). 2022 Jul 15;22(14):5297. doi: 10.3390/s22145297.
As a prospective key technology for the next-generation wireless communications, reconfigurable intelligent surfaces (RISs) have gained tremendous research interest in both the academia and industry in recent years. Only limited knowledge, however, has been obtained about the channel eigenvalue characteristics and spatial degrees of freedom (DoF) of systems containing RISs, especially when mutual coupling (MC) is present between the array elements. In this paper, we focus on the small-scale spatial correlation and eigenvalue properties excluding and including MC effects, for RISs with a quasi-continuous aperture (i.e., holographic RISs). Specifically, asymptotic behaviors of far-field and near-field eigenvalues of the spatial correlation matrix of holographic RISs without MC are first investigated, where the counter-intuitive observation of a lower DoF with more elements is explained by leveraging the power spectrum of the spatial correlation function. Second, a novel metric is proposed to quantify the inter-element correlation or coupling strength in RISs and ordinary antenna arrays. Furthermore, in-depth analysis is performed regarding the MC effects on array gain, effective spatial correlation, and eigenvalue architectures for a variety of element intervals when a holographic RIS works in the radiation and reception mode, respectively. The analysis and numerical results demonstrate that a considerable amount of the eigenvalues of the spatial correlation matrix correspond to evanescent waves that are promising for near-field communication and sensing. More importantly, holographic RISs can potentially reach an array gain conspicuously larger than conventional arrays by exploiting MC, and MC has discrepant impacts on the effective spatial correlation and eigenvalue structures at the transmitter and receiver.
作为下一代无线通信的一项潜在关键技术，可重构智能表面（RISs）近年来在学术界和工业界都引起了极大的研究兴趣。然而，对于包含RISs的系统的信道特征值特性和空间自由度（DoF），人们目前了解有限，特别是当阵列元件之间存在互耦（MC）时。在本文中，我们聚焦于具有准连续孔径的RISs（即全息RISs）在排除和包含MC效应情况下的小尺度空间相关性和特征值特性。具体而言，首先研究了无MC的全息RISs空间相关矩阵的远场和近场特征值的渐近行为，通过利用空间相关函数的功率谱解释了元件越多DoF越低这一违反直觉的现象。其次，提出了一种新颖的指标来量化RISs和普通天线阵列中的元件间相关性或耦合强度。此外，分别深入分析了全息RISs在辐射和接收模式下工作时，各种元件间距下MC对阵列增益、有效空间相关性和特征值结构的影响。分析和数值结果表明，空间相关矩阵的相当一部分特征值对应于消逝波，这对于近场通信和传感很有前景。更重要的是，全息RISs通过利用MC可能实现明显大于传统阵列的阵列增益，并且MC对发射机和接收机处的有效空间相关性和特征值结构有不同的影响。
