We present a novel, power- & hardware-efficient, multiuser, multibeam RIS (Reflective Intelligent Surface) architecture for multiuser MIMO, especially for very high frequency bands (e.g., high mmWave and sub-THz), where channels are typically sparse in the beamspace and LOS is the dominant component. The key module is formed by an active multiantenna feeder (AMAF) with a small number of active antennas, placed in the near field of a RIS with a much larger number of passive controllable reflecting elements. We propose a pragmatic approach to obtain a steerable beam with high gain and very low sidelobes. Then K independently controlled beams can be achieved by closely stacking K such AMAF-RIS modules. Our analysis includes the mutual interference between the modules and the fact that, due to the delay difference of propagation through the AMAF-RIS structure, the resulting channel matrix is frequency selective even in for pure LOS propagation. We consider a 3D geometry and show that ``beam focusing'' is in fact possible (and much more effective in terms of coverage) also in the far-field, by creating spotbeams with limited footprint both in angle and in range. Our results show that: 1) simple RF beamforming (BF) without computationally expensive baseband multiuser precoding is sufficient to practically eliminate multiuser interference when the users are chosen with sufficient angular/range separation, thanks to the extremely low sidelobe beams; 2) the impact of beam pointing errors with standard deviation as large as 2.5 deg and RIS quantized phase-shifters with quantization bits > 2 is essentially negligible; 3) The proposed architecture is more power efficient & much simpler from a hardware implementation viewpoint than standard active arrays with the same BF performance. We show also that the array gain of the proposed AMAF-RIS structure is linear with the RIS aperture.

翻译：本文提出了一种新颖、高功率效率及硬件效率的多用户多波束RIS（可重构智能表面）架构，专为多用户MIMO设计，尤其适用于极高频率频段（如毫米波高频段及亚太赫兹频段）。在这些频段中，信道通常在波束空间呈稀疏特性，且视距传播为最主要分量。该架构的核心模块由一个小型有源多天线馈源（AMAF）构成，该馈源具有少量有源天线，放置于一个拥有大量无源可控反射元件的RIS的近场区域内。我们提出了一种务实的方法，以获得具有高增益和极低旁瓣的可转向波束。随后，通过紧密堆叠K个这样的AMAF-RIS模块，可实现K个独立控制的波束。我们的分析涵盖了模块间的相互干扰，并指出由于通过AMAF-RIS结构传播的时延差异，即使在纯视距传播条件下，所得信道矩阵也具有频率选择性。我们考虑了三维几何结构，并展示在远场中，“波束聚焦”实际上也是可行的（在覆盖范围方面更为有效），其通过创建在角度和距离上具有有限足迹的定点波束来实现。研究结果表明：1）由于极低旁瓣波束的存在，当用户在角度/距离上具有足够分离度时，采用简单的射频波束赋形（BF）而无需计算复杂的基带多用户预编码，即可有效消除多用户干扰；2）标准差高达2.5度的波束指向误差以及使用量化位数>2的RIS量化移相器的影响基本可忽略；3）与具有相同波束赋形性能的传统有源阵列相比，所提架构在硬件实现角度上具有更高的功率效率且更为简单。我们还证明，所提AMAF-RIS结构的阵列增益与RIS孔径呈线性关系。