On Channel Estimation for Group-Connected Beyond Diagonal RIS Assisted Multi-User MIMO Communication

TL;DR

This paper introduces an efficient channel estimation method for group-connected beyond diagonal RIS in multi-user MIMO systems, significantly reducing overhead and improving accuracy over existing schemes.

Contribution

It reveals a key property of cascaded channels in BD-RIS, enabling a two-phase estimation protocol that lowers complexity and pilot requirements.

Findings

Achieves lower estimation error than benchmarks.

Requires fewer pilot signals for accurate channel estimation.

Demonstrates effectiveness through numerical simulations.

Abstract

Beyond diagonal reconfigurable intelligent surface (BD-RIS) architectures offer superior beamforming gain over conventional diagonal RISs. However, the channel estimation overhead is the main hurdle for reaping the above gain in practice. This letter addresses this issue for group-connected BDRIS aided uplink communication from multiple multi-antenna users to one multi-antenna base station (BS). We first reveal that within each BD-RIS group, the cascaded channel associated with one user antenna and one BD-RIS element is a scaled version of that associated with any other user antenna and BD-RIS element due to the common RIS-BS channel. This insight drastically reduces the dimensionality of the channel estimation problem. Building on this property, we propose an efficient two-phase channel estimation protocol. In the first phase, the reference cascaded channels for all groups are estimated in parallel based on common received signals while determining the scaling coefficients for a single reference antenna. In the second phase, the scaling coefficients for all remaining user antennas are estimated. Numerical results demonstrate that our proposed framework achieves substantially lower estimation error with fewer pilot signals compared to state-of-the-art benchmark schemes.