  Extremely large-scale massive multiple-input multiple-output (MIMO) has shown
considerable potential in future mobile communications. However, the use of
extremely large aperture arrays has led to near-field and spatial
non-stationary channel conditions, which result in changes to transceiver
design and channel state information that should be acquired. This letter
focuses on the channel estimation problem and describes the non-stationary
channel through mapping between subarrays and scatterers. We propose
subarray-wise and scatterer-wise channel estimation methods to estimate the
near-field non-stationary channel from the view of subarray and scatterer,
respectively. Numerical results demonstrate that subarray-wise method can
derive accurate channel estimation results with low complexity, whereas the
scatterer-wise method can accurately position the scatterers and identify
almost all the mappings between subarrays and scatterers.
