{"ID":2878099,"CreatedAt":"2026-06-01T04:54:23.091178241Z","UpdatedAt":"2026-06-01T04:54:23.091178241Z","DeletedAt":null,"paper_url":"https://arxiv.org/abs/2508.19000","arxiv_id":"2508.19000","title":"Beyond-Diagonal RIS: Adversarial Channels and Optimality of Low-Complexity Architectures","abstract":"Beyond-diagonal reconfigurable intelligent surfaces (BD-RISs) have recently gained attention as an enhancement to conventional RISs. BD-RISs allow optimizing not only the phase, but also the amplitude responses of their discrete surface elements by introducing adjustable inter-element couplings. Various BD-RIS architectures have been proposed to optimally trade off between average performance and complexity of the architecture. However, little attention has been paid to worst-case performance. This paper characterizes novel sets of adversarial channels for which certain low-complexity BD-RIS architectures have suboptimal performance in terms of received signal power at an intended communications user. Specifically, we consider two recent BD-RIS models: the so-called group-connected and tree-connected architecture. The derived adversarial channel sets reveal new surprising connections between the two architectures. We validate our analytical results numerically, demonstrating that adversarial channels can cause a significant performance loss. Our results pave the way towards efficient BD-RIS designs that are robust to adversarial propagation conditions and malicious attacks.","short_abstract":"Beyond-diagonal reconfigurable intelligent surfaces (BD-RISs) have recently gained attention as an enhancement to conventional RISs. BD-RISs allow optimizing not only the phase, but also the amplitude responses of their discrete surface elements by introducing adjustable inter-element couplings. Various BD-RIS architec...","url_abs":"https://arxiv.org/abs/2508.19000","url_pdf":"https://arxiv.org/pdf/2508.19000v1","authors":"[\"Atso Iivanainen\",\"Robin Rajamäki\",\"Visa Koivunen\"]","published":"2025-08-26T12:57:51Z","proceeding":"eess.SP","tasks":"[\"eess.SP\"]","methods":"[]","has_code":false}