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 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：我们提出一种新颖、节能且硬件高效的多用户多波束RIS（反射智能表面）架构，专用于多用户MIMO系统，尤其适用于高频段（如毫米波高阶频段和亚太赫兹频段），其中信道通常在波束空间中呈现稀疏特性且视距（LOS）为主要分量。该架构的核心模块由主动多天线馈源（AMAF）构成，其包含少量有源天线，并置于具有大量无源可控反射单元的RIS近场区域。我们提出一种实用方法以实现高增益、极低旁瓣的可控波束。通过紧密堆叠K个此类AMAF-RIS模块，可产生K个独立可控的波束。我们的分析涵盖了模块间的互干扰，并指出由于AMAF-RIS结构中传播时延差异，即便在纯LOS传播条件下，所得信道矩阵仍具有频率选择性。基于三维几何模型，我们证明通过在角度和距离上生成有限覆盖范围的"点波束"，远场场景下亦可实现波束聚焦（且在覆盖范围上效果显著）。研究结果表明：1）得益于极低旁瓣波束特性，当用户间保持足够角度/距离间隔时，简单的射频波束赋形（BF）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）标准差达2.5°的波束指向误差及量化位数≥2的RIS相位量化器所造成的影响可忽略不计；3）与具有相同波束赋形性能的标准有源阵列相比，所提架构在硬件实现上功耗更低且复杂度显著降低。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。