[Paper Review] Near-Field Wideband Beamforming for Extremely Large Antenna Array.

Mingyao Cui, Linglong Dai+2 more|arXiv (Cornell University)|Sep 21, 2021

Microwave Engineering and Waveguides33 references17 citations

TL;DR

This paper proposes a phase-delay focusing method using a piecewise-far-field model with piecewise-linear phase properties to mitigate the near-field beam split effect in extremely large antenna arrays, enabling high spectral efficiency for Tbps-wideband communications. It introduces the effective Rayleigh distance as a more accurate metric for near-field/far-field distinction.

ABSTRACT

Extremely large antenna array for wideband communications is a promising technology to achieve a Tbps data rate for next-generation communications. However, due to the extremely large bandwidth and antenna array aperture, the near-field beam split effect will severely decrease the actual transmission rates, which has not been investigated in existing works. To solve this challenging problem, we first reveal the near-field beam split effect and analyze the corresponding array gain loss. Then, a piecewise-far-field model with piecewise-linear phase property is proposed to approximate the near-field channel, based on which we propose a phase-delay focusing method to effectively mitigate the near-field beam split effect. Moreover, a new metric called effective Rayleigh distance is defined, which is more accurate than the classical Rayleigh distance to distinguish the far-field and near-field regions for practical communications. Finally, theoretical and numerical results are provided to demonstrate the effectiveness of our methods.

Motivation & Objective

• Address the unexplored challenge of near-field beam split effect in extremely large antenna arrays for tera-bit-per-second (Tbps) wideband communications.
• Analyze array gain loss caused by the near-field beam split effect in wideband systems with large aperture and wide bandwidth.
• Develop a practical model—piecewise-far-field with piecewise-linear phase properties—to approximate the complex near-field channel.
• Propose a phase-delay focusing method to effectively suppress beam splitting and restore array gain.
• Introduce the effective Rayleigh distance as a refined metric to better distinguish near-field and far-field regions in practical wideband systems.

Proposed method

• Formulate a piecewise-far-field model that approximates the near-field channel by dividing the array into sub-apertures, each treated as a far-field region with linear phase variation.
• Leverage the piecewise-linear phase property to model the spherical wavefronts in the near-field with high accuracy for wideband signals.
• Design a phase-delay focusing method that compensates for the phase errors introduced by the near-field geometry, minimizing beam splitting.
• Derive the effective Rayleigh distance as a function of bandwidth and array size, improving upon the classical Rayleigh distance for wideband systems.
• Use theoretical analysis and numerical simulations to validate the performance of the proposed model and beamforming method.

Experimental results

Research questions

• RQ1 How does the near-field beam split effect degrade array gain and spectral efficiency in extremely large antenna arrays with wideband signals?
• RQ2 To what extent can a piecewise-far-field model with piecewise-linear phase properties accurately represent the near-field channel in wideband systems?
• RQ3 Can a phase-delay focusing method effectively mitigate beam splitting and restore array gain in the near-field regime?
• RQ4 How does the proposed effective Rayleigh distance compare to the classical Rayleigh distance in distinguishing near-field and far-field regions for practical wideband communications?
• RQ5 What is the achievable spectral efficiency and array gain improvement using the proposed method in realistic wideband massive MIMO scenarios?

Key findings

• The near-field beam split effect causes significant array gain loss in wideband, extremely large antenna arrays due to the spatial variation of wavefronts.
• The proposed piecewise-far-field model accurately captures the near-field channel response with piecewise-linear phase approximation, enabling effective beamforming design.
• The phase-delay focusing method successfully mitigates beam splitting, restoring array gain and improving spectral efficiency in the near-field regime.
• The effective Rayleigh distance provides a more accurate criterion for near-field/far-field distinction than the classical Rayleigh distance, especially in wideband systems.
• Numerical results confirm that the proposed method achieves near-optimal array gain and spectral efficiency, even in the extreme near-field region.