Exploring the Advantages of Sparse Arrays in Near-Field XL-MIMO Systems: Beam Analysis and EDoF Function

TL;DR

This paper analyzes the beamforming and degrees-of-freedom in near-field XL-MIMO systems with sparse arrays, deriving closed-form expressions and proposing an efficient EDoF estimation algorithm.

Contribution

It introduces new closed-form expressions for beam power, lobe length, and EDoF estimation in sparse XL-MIMO systems, enhancing understanding and system design.

Findings

Enhanced beam focusing with decreased focal distance

Increased grating lobe suppression with larger antenna spacing

Proposed EDoF estimation algorithm with high accuracy and low complexity

Abstract

This paper investigates near-field XL-MIMO systems with sparse uniform planar arrays (UPAs). Based on the Green's function-based channel model, the paper derives closed-form expressions for the signal beam power when the distance coordinate or the angular coordinates varies with respective to the focused position. Based on that, closed-form expressions for the lobe length and the suppressing ratio are obtained, indicating that both the distance-focusing property and the grating lobe behavior can be enhanced as the focal distance decreases or the antenna spacing increases. Furthermore, the paper introduces a crucial constraint on system parameters, under which effective degrees-of-freedom (EDoF) of XL-MIMO systems with sparse UPAs can be precisely estimated. Then, the paper proposes an algorithm to obtain a closed-form expression, which can estimate EDoF with high accuracy and low computational complexity. The numerical results verifies the correctness of the main results of this paper.