Orthogonal multi-user multiple-input multiple-output (MU-MIMO) channels allow for optimum performance with simplified precoding/equalization, and they achieve maximum multiplexing gain which is shared fairly among users. Reconfigurable intelligent surface (RIS) constitutes a promising cost-efficient solution to improve the wireless channel, since they consist of passive reflecting elements able to adjust the phases of the incoming waves. However, it is still widely unclear how these surfaces can improve spatial-multiplexing. In fact, the common RIS model cannot achieve perfect orthogonalization of MU-MIMO channels with a reasonable number of elements. Furthermore, efficient channel estimation algorithms for RIS, which are key for taking advantage of its benefits, are still a matter of research. We study two types of reconfigurable surfaces (RSs), namely amplitude-reconfigurable intelligent surface (ARIS) and fully-reconfigurable intelligent surface (FRIS), with extended capabilities over RIS. We show how these RSs allow for perfect channel orthogonalization, and, by minimizing the applied power, we show that they can potentially be implemented without the need of amplification. We also present an efficient channel estimation method for each of them that allows the base station (BS) to select the desired propagation channel.

翻译：正交多用户多输入多输出（MU-MIMO）信道允许通过简化的预编码/均衡实现最优性能，并达到用户间公平共享的最大复用增益。可重构智能表面（RIS）因其由能够调整入射波相位的无源反射单元组成，成为改善无线信道的一种有前景的性价比方案。然而，这些表面如何提升空间复用仍不明确。事实上，常见 RIS 模型无法以合理单元数量实现 MU-MIMO 信道的完美正交化。此外，作为利用 RIS 优势的关键，高效的 RIS 信道估计算法仍是研究课题。我们研究两种可重构表面（RS）类型：幅度可重构智能表面（ARIS）和全可重构智能表面（FRIS），其能力较 RIS 有所扩展。我们展示了这些 RS 如何实现信道的完美正交化，并通过最小化施加功率，证明它们可能无需放大即可实现。我们还为每种表面分别提出了一种高效信道估计方法，使基站（BS）能够选择所需的传播信道。