We envision a future in which multi-antenna technology effectively exploits the spatial domain as a set of non-interfering orthogonal resources, allowing for flexible resource allocation and efficient modulation/demodulation. Reconfigurable intelligent surface (RIS) has emerged as a promising technology which allows shaping the propagation environment for improved performance. This paper studies the ability of three extended types of reconfigurable surface (RS), including the recently proposed beyond diagonal RIS (BD-RIS), to achieve perfectly orthogonal channels in a general multi-user multiple-input multiple-output (MU-MIMO) scenario. We propose practical implementations for the three types of RS consisting of passive components, and obtain the corresponding restrictions on their reconfigurability. We then use these restrictions to derive closed-form conditions for achieving arbitrary (orthogonal) channels. We also study the problem of optimal orthogonal channel selection for achieving high channel gain without active amplification at the RS, and we propose some methods with satisfying performance. Finally, we provide efficient channel estimation and RS configuration techniques such that all the computation, including the channel selection, may be performed at the base station (BS). The numerical results showcase the potential and practicality of RS channel orthogonalization, thus taking a step towards orthogonal spatial domain multiplexing (OSDM).

翻译：我们展望一个未来，其中多天线技术能够有效利用空间域作为一组无干扰的正交资源，从而实现灵活的资源分配与高效的调制/解调。可重构智能表面（RIS）已成为一种有前景的技术，能够塑造传播环境以提升性能。本文研究了三类扩展型可重构表面（RS）——包括近期提出的超对角RIS（BD-RIS）——在通用多用户多输入多输出（MU-MIMO）场景中实现完美正交信道的能力。我们提出了由无源器件构成的三类RS的实用实现方案，并获得了其可重构性的相应约束。基于这些约束，我们推导了实现任意（正交）信道的闭式条件。我们还研究了在RS无需有源放大的情况下，为实现高信道增益而进行最优正交信道选择的问题，并提出了几种性能令人满意的方法。最后，我们提供了高效的信道估计与RS配置技术，使得包括信道选择在内的所有计算均可在基站（BS）完成。数值结果展示了RS信道正交化的潜力与实用性，从而向正交空间域复用（OSDM）迈出了一步。