Spatial Multiplexing Characterization With Effective Degrees of Freedom Metrics for Near-Field MIMO Systems

This letter presents a comparative analysis of effective degrees of freedom (EDoF) metrics for near-field multiple-input–multiple-output (MIMO) systems. The study investigates three prominent EDoF metrics: Ψ _e (EDoF based on the eigenvalue spread of the channel Gramian matrix), N _H [EDoF based on geometry of transmit (Tx) and receive (Rx) antenna arrays], and K _e (EDoF based on effective multiplexing gain). The behavior and applicability of these metrics are examined across various near-field MIMO configurations and channel conditions. The analysis includes a detailed examination of how these metrics respond to changes in array geometry, signal-to-noise ratio, antenna spacing, and user antenna configurations in the near-field regime. Novel analytical frameworks and simulation methodologies specifically tailored for near-field scenarios are introduced, where spherical wave propagation and solid angle (Ω) are used to characterize the spatial relationships between Tx and Rx arrays. The results demonstrate the unique strengths and limitations of each EDoF metric in near-field conditions, providing important insights for the design and optimization of near-field, high-capacity MIMO systems.