  Extremely large-scale antenna array (ELAA) is promising as one of the key
ingredients for the sixth generation (6G) of wireless communications. The
electromagnetic propagation of spherical wavefronts introduces an additional
distance-dependent dimension beyond conventional beamspace. In this paper, we
first present one concise closed-form channel formulation for extremely
large-scale multiple-input multiple-output (XL-MIMO). All line-of-sight (LoS)
and non-line-of-sight (NLoS) paths, far-field and near-field scenarios, and
XL-MIMO and XL-MISO channels are unified under the framework, where additional
Vandermonde windowing matrix is exclusively considered for LoS path. Under this
framework, we further propose one low-complexity unified LoS/NLoS orthogonal
matching pursuit (XL-UOMP) algorithm for XL-MIMO channel estimation. The
simulation results demonstrate the superiority of the proposed algorithm on
both estimation accuracy and pilot consumption.
