Electromagnetic Effective Degree of Freedom of a MIMO System in Free Space

TL;DR

This paper develops an electromagnetic framework to evaluate the effective degrees of freedom in MIMO systems considering full polarizations, including near-field effects, providing insights into system performance beyond traditional models.

Contribution

It introduces an EM-based EDOF model using dyadic Green's functions that accounts for full polarizations and near-field effects, extending traditional single-polarization approaches.

Findings

EM EDOF calculated for arbitrary MIMO configurations

Relations between EDOF limits and source/receiver counts clarified

Near-field benefits demonstrated with longitudinal polarization contributions

Abstract

Effective degree of freedom (EDOF) of a multiple-input-multiple-output (MIMO) system represents its equivalent number of independent single-input-single-output (SISO) systems, which directly characterizes the communication performance. Traditional EDOF only considers single polarization, where the full polarized components degrade into two independent transverse components under the far-field approximation. However, the traditional model is not applicable to complex scenarios especially for the near-field region. Based on an electromagnetic (EM) channel model built from the dyadic Green's function, we first calculate the EM EDOF to estimate the performance of an arbitrary MIMO system with full polarizations in free space. Then, we clarify the relations between the limit of EDOF and the optimal number of sources/receivers. Finally, potential benefits of near-field MIMO communications are demonstrated with the EM EDOF, in which the contribution of the longitudinally polarized source is taken into account. This work establishes a fundamental EM framework for MIMO wireless communications.