  By integrating a nearly infinite number of reconfigurable elements into a
finite space, a spatially continuous array aperture is formed for holographic
multiple-input multiple-output (HMIMO) communications. This three-part tutorial
aims for providing an overview of the latest advances in HMIMO communications.
As Part I of the tutorial, this letter first introduces the fundamental concept
of HMIMO and reviews the recent progress in HMIMO channel modeling, followed by
a suite of efficient channel estimation approaches. Finally, numerical results
are provided for demonstrating the statistical consistency of the new HMIMO
channel model advocated with conventional ones and evaluating the performance
of the channel estimators. Parts II and III of the tutorial will delve into the
performance analysis and holographic beamforming, and detail the interplay of
HMIMO with emerging technologies.
