Misfocus-Reduction in RIS-Assisted Ultra-Wideband Wireless Communication

Misfocus or beam squint, is a fundamental prob-lem for reconfigurable intelligent surface (RIS)-assisted beam-forming. In this work, we investigate two methods for misfocus-reduction based on the stationary phase method (SPM) and the semi-definite relaxation (SDR), respectively. The proposed algorithms can provide a misfocus robust phase shift configuration. In the SPM-based algorithm, a closed-form expression is given, which reduces the computational complexity and has high efficiency when the RIS elements number is large. We use the circuit model to characterize the frequency selectivity of RIS elements. We investigate our methods under several critical factors: the number of RIS elements, the beamfoming bandwidth and the impact of the frequency selectivity of RIS itself.