Please use this identifier to cite or link to this item: https://hdl.handle.net/20.500.11851/12571
Title: Capacity Maximization for MIMO Channels Assisted by Beyond-Diagonal RIS
Authors: Björnson, E.
Demir, O.T.
Keywords: Beyond-Diagonal RIS
MIMO Capacity
Publisher: Institute of Electrical and Electronics Engineers Inc.
Abstract: Reconfigurable intelligent surfaces (RISs) can improve the capacity of wireless communication links by passively beamforming the impinging signals in desired directions. This feature has been demonstrated both analytically and experimentally for conventional RISs, consisting of independently reflecting elements. To further enhance reconfigurability, a new architecture called beyond-diagonal RIS (BD-RIS) has been proposed. It allows for controllable signal flows between RIS elements, resulting in a non-diagonal reflection matrix, unlike the conventional RIS architecture. Previous studies on BD-RIS-assisted communications have predominantly considered single-antenna transmitters/receivers. One recent work provides an iterative capacity-improving algorithm for multiple-input multiple-output (MIMO) setups but without providing geometrical insights. In this paper, we derive the first closed-form capacity-maximizing BD-RIS reflection matrix for a MIMO channel. We describe how this solution pairs together propagation paths, how it behaves when the signal-to-noise ratio is high, and what capacity is achievable with ideal semi-unitary channel matrices. The analytical results are corroborated numerically. © 2025 European Association on Antennas and Propagation.
URI: https://doi.org/10.23919/EuCAP63536.2025.10999443
https://hdl.handle.net/20.500.11851/12571
ISBN: 9788831299107
Appears in Collections:Scopus İndeksli Yayınlar Koleksiyonu / Scopus Indexed Publications Collection

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