Beamforming Design for Intelligent Reffecting Surface Aided Near-Field THz Communications

TL;DR

This work tackles near-field beam split in wideband THz IRS-aided communications by extending a piecewise far-field model to UPAs and introducing a double-layer delta-delay (DLDD) beamforming architecture. DLDD uses a two-layer time-delay network to generate cumulative delays across sub-surfaces, reducing the required TD range to $K_T=K-1$ while mitigating inter-subsurface phase discrepancies; a modulo-based IRS phase design aligns intra-surface phases. A sub-optimal joint BS-IRS beamforming scheme is also proposed to address double-stage beam split in multi-antenna setups. Numerical results show substantial mitigation of beam split across the bandwidth, achieving near-optimal array gains, and improved achievable rates with robustness to phase quantization, highlighting practical viability for wideband THz IRS deployments.

Abstract

Intelligent reflecting surface (IRS) operating in the terahertz (THz) band has recently gained considerable interest due to its high spectrum bandwidth. Due to the exploitation of large scale of IRS, there is a high probability that the transceivers will be situated within the near-field region of the IRS. Thus, the near-field beam split effect poses a major challenge for the design of wideband IRS beamforming, which causes the radiation beam to deviate from its intended location, leading to significant gain losses and limiting the efficient use of available bandwidths. While delay-based IRS has emerged as a potential solution, current beamforming schemes generally assume unbounded range time delays (TDs). In this letter, we first investigate the near-field beam split issue at the IRS. Then, we extend the piece-wise far-field model to the IRS, based on which, a double-layer delta-delay (DLDD) IRS beamforming scheme is proposed. Specifically, we employ an element-grouping strategy and the TD imposed on each sub-surface of IRS is achieved by a series of TD modules. This method significantly reduces the required range of TDs. Numerical results show that the proposed DLDD IRS beamforming scheme can effectively mitigate the near-field beam split and achieve near-optimal performance.

Figures (4)

• Figure 1: An IRS-aided THz communication system.
• Figure 2: The IRS reflected beams under different cases.
• Figure 3: The proposed DLDD IRS beamforming architecture.
• Figure 4: Simulation results.

Theorems & Definitions (1)

• Remark 1