An SFFT-Based Method for Wideband Beamforming of Reconfigurable Intelligent Surface

The spectrum shift to high-frequency bands 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 the space-frequency Fourier transformation (SFFT)-based wideband beamforming design for RIS-assisted systems. In the proposed design, we exploit SFFT and stationary phase method to yield an approximate closed-form solution of RIS phase shifts, which significantly reduces signal processing complexity. The obtained solution is then used to generate a large and flat beampattern over the desired frequency band to overcome the beam squint effect. Through numerical results, we validate the effectiveness of our proposed beamforming design and demonstrate how it can improve communication and sensing performance. Furthermore, the proposed method can be extended to generate any expected frequency-domain beampattern which provides valuable insights into the design of novel wideband beamforming for RIS-assisted systems.