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.

翻译：超对角可重构智能表面（BD-RIS）架构相较于传统对角型RIS具有更优的波束赋形增益。然而，信道估计开销是实际应用中实现上述增益的主要障碍。本文针对分组连接BD-RIS辅助的多天线用户至多天线基站（BS）上行通信场景，对该问题展开研究。我们首先揭示：由于RIS-BS公共信道的存在，在每个BD-RIS组内，任意用户天线与任意BD-RIS单元对应的级联信道，均可表示为该组内指定参考天线与对应RIS单元级联信道的缩放版本。这一特性显著降低了信道估计问题的维度。基于此性质，我们提出一种高效的两阶段信道估计方案。第一阶段基于公共接收信号并行估计所有分组的参考级联信道，同时确定单个参考天线的缩放系数。第二阶段估计所有剩余用户天线的缩放系数。数值结果表明：相较于现有先进基准方案，所提框架能以更少的导频信号实现显著更低的估计误差。