Reconfigurable intelligent surface (RIS) architectures not limited to diagonal phase shift matrices have recently been considered to increase their flexibility in shaping the wireless channel. One of these beyond-diagonal RIS or BD-RIS architectures leads to a unitary and symmetric RIS matrix. In this letter, we consider the problem of maximizing the signal-to-noise ratio (SNR) in single and multiple antenna links assisted by a BD-RIS. The Max-SNR problem admits a closed-form solution based on the Takagi factorization of a certain complex and symmetric matrix. This allows us to solve the max-SNR problem for SISO, SIMO, and MISO channels.
@article{arxiv.2307.10707,
title = {SNR Maximization in Beyond Diagonal RIS-assisted Single and Multiple Antenna Links},
author = {Ignacio Santamaria and Mohammad Soleymani and Eduard Jorswieck and Jesus Gutierrez},
journal= {arXiv preprint arXiv:2307.10707},
year = {2023}
}