Movable-Element STARS-Assisted Near-Field Wideband Communications

TL;DR

This paper introduces a movable-element STARS-assisted near-field wideband communication framework that dynamically adjusts element positions to mitigate beam squint, enhancing performance without costly delay components.

Contribution

It proposes a novel movable-element STARS design with four movement modes and a joint optimization algorithm for near-field wideband multi-user systems, addressing beam squint issues.

Findings

ME-STARS effectively mitigate beam squint in near-field wideband systems.

RB mode offers the most efficient beam squint mitigation.

Increasing ME-STARS elements or subcarriers improves system performance.

Abstract

A novel movable-element simultaneously transmitting and reflecting surface (ME-STARS)-assisted near-field wideband communication framework is proposed. In particular, the position of each STARS element can be adjusted to combat the significant wideband beam squint issue in the near field instead of using costly true-time delay components. Four practical ME-STARS element movement modes are proposed, namely region-based (RB), horizontal-based (HB), vertical-based (VB), and diagonal-based (DB) modes. Based on this, a near-field wideband multi-user downlink communication scenario is considered, where a sum rate maximization problem is formulated by jointly optimizing the base station (BS) precoding, ME-STARS beamforming, and element positions. To solve this intractable problem, a two-layer algorithm is developed. For the inner layer, the block coordinate descent optimization framework is utilized to solve the BS precoding and ME-STARS beamforming in an iterative manner. For the outer layer, the particle swarm optimization-based heuristic search method is employed to determine the desired element positions. Numerical results show that:1) the ME-STARSs can effectively address the beam squint for near-field wideband communications compared to conventional STARSs with fixed element positions; 2) the RB mode achieves the most efficient beam squint effect mitigation, while the DB mode achieves the best trade-off between performance gain and hardware overhead; and 3) an increase in the number of ME-STARS elements or BS subcarriers substantially improves the system performance.