Wideband Beamforming with RIS: A Unified Framework via Space-Frequency Transformation

TL;DR

This paper introduces a unified, low-complexity wideband beamforming method for RIS-assisted systems using space-frequency transformation, enabling flat or custom beampatterns across broad frequency ranges.

Contribution

It proposes a novel closed-form RIS phase shift design leveraging Fourier transformation and stationary phase, reducing complexity and enabling flexible wideband beamforming.

Findings

Significantly reduces signal processing complexity.

Achieves flat beampattern over desired frequency band.

Can mimic arbitrary beampatterns through amplitude modulation.

Abstract

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.