Impact of Antenna Arrays Misalignment on the Near Field Distance in Terahertz Communications

TL;DR

This work tackles the limitation of traditional near-field boundaries that assume perfect alignment between THz AP and UE antenna arrays. It develops exact and approximate closed-form expressions for the near-field distance under misalignment for ULA--ULA and UPA--UPA links, employing a phase-difference threshold of $\Delta\phi \le \pi/8$ and an effective-plane concept to account for phase shifter delays. The key contributions are the new $d_F^{(ULA)}$, $d_F^{(UPA,1)}$, and $d_{F,approx}^{(UPA,2)}$ formulas, along with numerical validation at $f = 300$ GHz that reveals substantial reductions in the near-field region due to UE rotation. These results provide practical guidelines for deploying and optimizing THz systems with realistic mobility and misalignment, enhancing beam steering and wavefront engineering in near-field-aware designs.

Abstract

The extremely short wavelength of terahertz (THz) communications leads to an extended radiative near-field region, in which some canonical far-field assumptions fail. Existing near-field boundary formulations (Fraunhofer distance) for uniform linear/planar array (ULA/UPA) configurations assume ideal alignment between transceivers, overlooking practical misalignments caused by mobility or mechanical imperfections. This paper addresses this critical gap by analyzing the impact of spatial misalignment on near-field distance calculations in THz systems. We derive exact analytical expressions and simplified approximations for the near-field boundary in both ULA--ULA and UPA--UPA configurations under arbitrary misalignment offsets. Through numerical simulations, we validate our theoretical models and quantify how misalignment reshapes the near-field region. These findings provide essential guidelines for optimizing THz system deployment in realistic scenarios.

Figures (6)

• Figure 1: Modeled THz communication system with misalignment between the AP and the UE antennas caused by various UE rotations (UPA case presented, the ULA setup is similar but with an $N_1 \times 1$ AP and an $N_2 \times 1$ UE arrays).
• Figure 2: ULA scenario with antenna misalignment (analyzed in Sec. \ref{['sec:ULA']}).
• Figure 3: UPA setup with the UE rotation over one plane (Sec. \ref{['sec:UPA1']}).
• Figure 4: UPA setup with the UE rotation over both planes (Sec. \ref{['sec:UPA2']}).
• Figure 5: The phase mismatch with $D_1=0.1\text{ m}$ and $D_2=0.05\text{ m}$ and near-field distance under different array and rotation configurations.