  In this paper, we develop an effective degrees of freedom (EDoF) performance
analysis framework specifically tailored for near-field XL-MIMO systems. We
explore five representative distinct XL-MIMO hardware designs, including
uniform planar array (UPA)-based with point antennas, two-dimensional (2D)
continuous aperture (CAP) plane-based, UPA-based with patch antennas, uniform
linear array (ULA)-based, and one-dimensional (1D) CAP line segment-based
XL-MIMO systems. Our analysis encompasses two near-field channel models: the
scalar and dyadic Green's function-based channel models. More importantly, when
applying the scalar Green's function-based channel, we derive EDoF expressions
in the closed-form, characterizing the impacts of the physical size of the
transceiver, the transmitting distance, and the carrier frequency. In our
numerical results, we evaluate and compare the EDoF performance across all
examined XL-MIMO designs, confirming the accuracy of our proposed closed-form
expressions. Furthermore, we observe that with an increasing number of
antennas, the EDoF performance for both UPA-based and ULA-based systems
approaches that of 2D CAP plane and 1D CAP line segment-based systems,
respectively. Moreover, we unveil that the EDoF performance for near-field
XL-MIMO systems is predominantly determined by the array aperture size rather
than the sheer number of antennas.
