  To realize holographic communications, a potential technology for spectrum
efficiency improvement in the future sixth-generation (6G) network, antenna
arrays inlaid with numerous antenna elements will be deployed. However, the
increase in antenna aperture size makes some users lie in the Fresnel region,
leading to the hybrid near-field and far-field communication mode, where the
conventional far-field channel estimation methods no longer work well. To
tackle the above challenge, this paper considers channel estimation in a
hybrid-field multipath environment, where each user and each scatterer can be
in either the far-field or the near-field region. First, a joint angular-polar
domain channel transform is designed to capture the hybrid-field channel's
near-field and far-field features. We then analyze the power diffusion effect
in the hybrid-field channel, which indicates that the power corresponding to
one near-field (far-field) path component of the multipath channel may spread
to far-field (near-field) paths and causes estimation error. We design a novel
power-diffusion-based orthogonal matching pursuit channel estimation algorithm
(PD-OMP). It can eliminate the prior knowledge requirement of path numbers in
the far field and near field, which is a must in other OMP-based channel
estimation algorithms. Simulation results show that PD-OMP outperforms current
hybrid-field channel estimation methods.
