Title: Analysis and Optimization of RIS: A Multiport Network Theory Approach

Abstract: Multiport network theory is a suitable abstraction model for analyzing and optimizing reconfigurable intelligent surfaces (RISs), especially for studying the impact of the electromagnetic mutual coupling among radiating elements that are spaced less than half of the wavelength. The representations in terms of Z-parameter (impedance) and S-parameter (scattering) matrices are widely utilized. In this talk, we elaborate on multiport network theory for analyzing and optimizing the reradiation properties of RIS-aided channels, with focus on four key aspects. (i) We offer a thorough comparison between the Z-parameter and S-parameter representations. This comparison allows us to unveil that typical scattering models utilized for RIS-aided channels ignore the structural scattering from the RIS, which results in an unwanted specular reflection. (ii) We develop an iterative algorithm for optimizing, in the presence of electromagnetic mutual coupling, the tunable loads of the RIS based on the S-parameters representation. We prove that small perturbations of the step size of the algorithm result in larger variations of the S-parameter matrix compared with the Z-parameter matrix, resulting in a faster convergence rate. (iii) We develop a new algorithm to suppress the specular reflection due to the structural scattering, while maximizing the received power towards the direction of interest, and analyze the effectiveness and tradeoffs of the proposed approach. (iv) Finally, we validate the theoretical findings and algorithms with numerical simulations and a commercial full-wave electromagnetic simulator based on the method of moments.

Title: Optimization and Signal Processing for Beyond-Diagonal RIS Architectures

Abstract: Reconfigurable intelligent surfaces can modify the signal propagation conditions in wireless networks. After careful modelling, design, and implementation of classical diagonal RIS, the huge potential of this technology is recognized. More differentiated use cases including interference management and signal neutralization benefit from so-called beyond-diagonal RIS architectures. The additional degrees of freedom to tune the reflected signals depend on the capabilities of the underlying hardware architecture. Currently, several options and versions of BD-RIS exist, including fully connected, sub connected, passive, and active. To understand the benefits and gains of the BD-RIS architectures, the chosen performance metric needs to be optimized. Therefore, optimization problems are formulated and solved under constraints which are determined by the BD-RIS hardware. In this talk, we will review the current RIS architectures, starting with passive diagonal and ending with active fully connected with global reflection, and discuss the resulting optimization problems and algorithms to tune the RIS parameters. The underlying programming problems are characterized, and algorithms are developed to solve them globally or locally. Finally, we discuss the interplay of BD-RIS with other interference management technologies including multi-carrier, rate-splitting, and improper signal processing. It is illustrated that BD-RIS can significantly support the interference management capabilities and improve the fairness rates. The talk concludes with open problems and future work.

The talk is based on the following publications:

[1] M. Soleymani, I. Santamaria, E. Jorswieck, and B. Clerckx, "Optimization of Rate-Splitting Multiple Access in Beyond Diagonal RIS-assisted URLLC Systems," IEEE Trans. Wireless Commun., 2023.

[2] I. Santamaria, M. Soleymani, E. Jorswieck, and J. Gutiérrez, "SNR Maximization in Beyond Diagonal RIS-assisted Single and Multiple Antenna Links," IEEE Signal Process. Lett., 2023.

Title: Exploring the Potential Advantages of Beyond Diagonal RIS for 6G

Abstract: Beyond-diagonal reconfigurable intelligent surface (BD-RIS) has been proposed recently as a novel and generalized RIS architecture that embraces the state-of-the-art RIS architectures such as diagonal-RIS (D-RIS), simultaneously transmitting and reflecting RIS (STAR-RIS) or intelligent omni-surface (IOS) as special cases. This makes BD-RIS highly promising for the upcoming 6G networks. In this talk, we focus on exploring the potential advantages of deploying BD-RIS at the transmitter side or among the distributed users, and assessing the performance improvements it can offer compared to conventional D-RIS. When BD-RIS is deployed at the transmitter side, we not only demonstrate its superiority in terms of spectral efficiency over D-RIS, but also show that fewer transmit antennas are required to achieve the same or even better performance as conventional massive MIMO with the assistance of BD-RIS. This implies a significant potential of BD-RIS to reduce the transceiver hardware costs. Moreover, by deploying BD-RIS at the user side and integrating it with rate-splitting multiple access (RSMA), we demonstrate that BD-RIS significantly enhances spectral efficiency. Throughout the presentation, we will also delve into efficient passive beamforming design algorithms for BD-RIS and explore its joint design with active beamforming at the transmitter.

Title: Implementing Reconfigurable Intelligent Surface Structures

Abstract: In this talk the focus is on the design, implementation and measurement of reconfigurable intelligent surfaces (RIS). Both RIS 1.0 and RIS 2.0 (BD-RIS) are considered. Novel passive and active RIS designs are described and experimental results are provided. In addition, new approaches to channel estimation when RIS are present is also given. Furthermore, approaches to the development BD-RIS are also discussed and preliminary results are provided. While these technologies and fundamental breakthroughs are promising, significant further research is needed to exploit the enormous potential of new RF wave technology for 6G. As background to the talk methods for linking electromagnetic design with wireless communication are also briefly summarized.

Title: Reconfigurable Intelligent Surfaces – State of Play from ETSI ISG RIS

Abstract: European Telecommunications Standards Institute (ETSI) Industry Specification Group (ISG) on Reconfigurable Intelligent Surfaces (RIS) was formed to coordinate and streamline pre-standardization research on RIS technology and pave the way for future standardization and eventual commercialization of the technology. This talk provides an overview of the research and standardization activities by ETSI ISG RIS since the launch of the group.

Title: Adversarial Attacks via Non-Reciprocal Beyond Diagonal Surfaces

Abstract: By now there are numerous applications that have been proposed for Reconfigurable Intelligent Surfaces (RIS), most of which are focused on using them to improve the quality of wireless communication links. However, in the hands of an adversary, an RIS can also wreak havoc on communication systems, nulling desired signals, increasing interference, enabling eavesdropping, and destroying the natural reciprocity of the RF propagation channels. In this talk we review some of these adversarial uses and potential countermeasures, and in particular we focus on the advantages of using non-reciprocal beyond diagonal surfaces for such attacks.

Title: Beyond Diagonal Reconfigurable Intelligent Surface Utilizing Scattering Parameter Analysis

Abstract: Beyond-diagonal reconfigurable intelligent surface (BD-RIS) is an emerging technology for enhancing the next-generation wireless communication system. Compared with conventional reconfigurable intelligent surface which is characterized by a diagonal scattering matrix, BD-RIS has a more general architecture characterized by a beyond diagonal scattering matrix and thus provides a higher capability for controlling the wireless propagation environment. In this talk, we focus on utilizing the scattering parameter network analysis for modeling, architecture design, and mode design for BD-RIS. We first introduce toy examples to demonstrate how to use scattering parameter to analyze the channel gain for a single-input single-output system and further show the general channel model for BD-RIS. Based on the scattering parameter network analysis, we introduce two architecture designs for BD-RIS, namely fully-connected and group-connected RIS, with the corresponding optimization and numerical results. In addition, we introduce the hybrid transmitting and reflecting mode and multi-sector mode design for BD-RIS. A discussion on the future research directions for BD-RIS will be also provided.

Title: Beyond Diagonal RIS: Non-Diagonal Single-Connected and Group-Connected RIS Architectures

Abstract: In this talk, we focus on the family of spectral-, hardware- and energy-efficient reconfigurable intelligent surface (RIS) architectures. Firstly, non-diagonal single-connected RIS architectures are presented, where the incident signal impinging on one element can be reflected from another element after an appropriate phase shift adjustment, which increases the flexibility in the design of the RIS phase shifts, potentially improving the system’s performance. Then, the non-diagonal group-connected RIS architecture is presented, where the signals impinging on specific elements can be reflected from other elements under some predefined routing. This routing is a group-to-group mapping, optimized according to the channel state information of the BS-RIS link and the RIS-UE link. Throughout the presentation, the beamforming design and performance will be considered, and we finally conclude with some future research ideas.