The spectrum shift from the sub-6G band to the high-frequency band has posed an ever-increasing demand on the paradigm shift from narrowband beamforming to wideband beamforming. Despite recent research efforts, the problem of wideband beamforming design is particularly challenging in reconfigurable intelligent surface (RIS)-assisted systems, due to that RIS is not capable of performing frequency-dependent phase shift, therefore inducing high signal processing complexity. In this paper, we propose a simple-yet-efficient wideband beamforming design for RIS-assisted systems, in which a transmitter sends wideband signals to a desired target, through the aid of the RIS. In our proposed design, we exploit space-frequency Fourier transformation and stationary phase method to yield an approximate closed-form solution of the RIS phase shifts which significantly reduces the signal processing complexity, compared to the existing approaches. The obtained solution is then used to generate a large and flat beampattern over the desired frequency band. Through numerical results, we validate the effectiveness of our proposed beamforming design and demonstrate how it can improve system performances in terms of communication rate and sensing resolution. Beyond generating the flat beampattern, we highlight that our proposed design is capable of mimicking any desired beampattern by matching the RIS phase shift with the amplitude modulation function, thus providing valuable insights into the design of novel wideband beamforming for RIS-assisted systems.

翻译：频谱从sub-6G频段向高频段的迁移，对波束成形范式从窄带向宽带的转变提出了日益增长的需求。尽管已有诸多研究，但在可重构智能表面（RIS）辅助系统中，宽带波束成形设计问题尤为复杂，这主要是因为RIS无法执行与频率相关的相移，从而导致信号处理复杂度极高。本文针对RIS辅助系统提出了一种简洁而高效的宽带波束成形设计，其中发射机通过RIS的辅助向期望目标发送宽带信号。在我们提出的设计中，我们利用空间-频率傅里叶变换和驻相法，推导出RIS相移的近似闭式解，与现有方法相比，该解显著降低了信号处理复杂度。所得解随后被用于在目标频带上生成大而平坦的波束方向图。通过数值结果，我们验证了所提波束成形设计的有效性，并展示了其如何在通信速率和感知分辨率方面提升系统性能。除了生成平坦波束方向图之外，我们强调所提设计能够通过将RIS相移与幅度调制函数相匹配来模拟任何期望的波束方向图，从而为RIS辅助系统的新型宽带波束成形设计提供了有价值的见解。