  Accurate channel model and channel estimation are essential to empower
extremely large-scale MIMO (XL-MIMO) in 6G networks with ultra-high spectral
efficiency. With the sharp increase in the antenna array aperture of the
XL-MIMO scenario, the electromagnetic propagation field will change from
far-field to near-field. Unfortunately, due to the near-field effect, most of
the existing XL-MIMO channel models fail to describe mixed line-of-sight (LoS)
and non-line-of-sight (NLoS) path components simultaneously. In this paper, a
mixed LoS/NLoS near-field XL-MIMO channel model is proposed to match the
practical near-field XL-MIMO scenario, where the LoS path component is modeled
by the geometric free space propagation assumption while NLoS path components
are modeled by the near-field array response vectors. Then, to define the range
of near-field for XL-MIMO, the MIMO Rayleigh distance (MIMO-RD) and MIMO
advanced RD (MIMO-ARD) is derived. Next, a two stage channel estimation
algorithm is proposed, where the LoS path component and NLoS path components
are estimated separately. Moreover, the Cramer-Rao lower bound (CRLB) of the
proposed algorithm is derived in this paper. Numerical simulation results
demonstrate that, the proposed two stage scheme is able to outperform the
existing methods in both the theoretical channel model and the QuaDRiGa channel
emulation platform.
