  Holographic multiple-input multiple-output (H-MIMO) is considered as one of
the most promising technologies to enable future wireless communications in
supporting the expected extreme requirements, such as high energy and spectral
efficiency. Empowered by the powerful capability in electromagnetic (EM) wave
manipulations, H-MIMO has the potential to reach the fundamental limit of the
wireless environment, and opens up the possibility of signal processing in the
EM-domain, which needs to be depicted carefully from an EM perspective,
especially the wireless channel. To this aim, we study the line-of-sight (LOS)
H-MIMO communications with arbitrary surface placements and establish an exact
expression of the wireless channel in the EM-domain. To further obtain a more
explicit and computationally-efficient channel models, we solve the implicit
integrals of the exact channel model with moderate and reasonable assumptions.
Numerical studies are executed and the results show good agreements of our
established approximated channel models to the exact channel model.
