  Technologies like ultra-massive multiple-input-multiple-output (UM-MIMO) and
reconfigurable intelligent surfaces (RISs) are of special interest to meet the
key performance indicators of future wireless systems including ubiquitous
connectivity and lightning-fast data rates. One of their common features, the
extremely large-scale antenna array (ELAA) systems with hundreds or thousands
of antennas, give rise to near-field (NF) propagation and bring new challenges
to channel modeling and characterization. In this paper, a cross-field channel
model for ELAA systems is proposed, which improves the statistical model in
3GPP TR 38.901 by refining the propagation path with its first and last bounces
and differentiating the characterization of parameters like path loss, delay,
and angles in near- and far-fields. A comprehensive analysis of cross-field
boundaries and closed-form expressions of corresponding NF or FF parameters are
provided. Furthermore, cross-field experiments carried out in a typical indoor
scenario at 300 GHz verify the variation of MPC parameters across the antenna
array, and demonstrate the distinction of channels between different antenna
elements. Finally, detailed generation procedures of the cross-field channel
model are provided, based on which simulations and analysis on NF probabilities
and channel coefficients are conducted for $4\times4$, $8\times8$,
$16\times16$, and $9\times21$ uniform planar arrays at different frequency
bands.
