  Extremely large-scale massive MIMO (XL-MIMO) is a promising technique for
future 6G communications.However, existing far-field or near-field channel
model mismatches the hybrid-field channel feature in the practical XL-MIMO
system.Thus,existing far-field and near-field channel estimation schemes cannot
be directly used to accurately estimate the hybrid-field XL-MIMO channel. To
solve this problem, we propose an efficient hybrid-field channel estimation
scheme by accurately modeling the XL-MIMO channel.Specifically,we firstly
reveal the hybrid-field channel feature of the XL-MIMO channel, where different
scatters may be in far-field or near-field region.Then, we propose a
hybrid-field channel model to capture this feature, which contains both the
far-field and near-field path components. Finally, we propose a hybrid-field
channel estimation scheme, where the far-field and near-field path components
are respectively estimated. Simulation results show that the proposed scheme
performs better than existing schemes.
