Spatial Degrees of Freedom in Near Field MIMO: Experimental Validation of Beamspace Perspective

TL;DR

This paper investigates the spatial degrees of freedom in near-field MIMO systems, providing an analytical framework and experimental validation for the transition distances where multiple spatial DoF are supported, highlighting the advantages of near-field conditions.

Contribution

It introduces a new framework for characterizing the effective DoF in near-field MIMO, including analytical expressions and experimental validation of transition distances.

Findings

Experimental results match theoretical EDoF trends.

Defined effective MIMO Rayleigh distance and maximum spatial multiplexing distance.

Near-field MIMO supports multiple DoF at short ranges.

Abstract

Conventional far-field multiple-input multiple-output (MIMO) channels are limited to a single spatial degree of freedom (DoF) under a line-of-sight (LoS) condition. In contrast, the radiative near field (NF) supports multiple spatial DoF, enabled by spherical wavefronts and the reduced spatial footprint at short ranges. While recent research indicates that the effective DoF (EDoF) increases in NF, experimental validation and clear identification of the transition distances remain limited. In this letter, we develop an intuitive framework for characterizing the EDoF of a ULA-based MIMO system and derive two complementary analytical expressions: a closed-form formulation that relates the EDoF to the physical transmit beamwidth and receive aperture, and a discrete formulation based on the discrete Fourier transform (DFT) domain angular decomposition of the NF spherical wavefront, which is well suited for experimental evaluation. We further introduce the effective MIMO Rayleigh distance (EMRD) and the maximum spatial multiplexing distance (MSMD), which mark the distances where the EDoF reduces to one and attains its maximum, respectively. Experimental measurements using widely spaced phased arrays closely match the theoretical EDoF trends and validate the proposed distance metrics.