Here are my detailed question:
As I can see, the H in HMIMO should stand for holographic and it should obey holographic principles.
But as I read more HMIMO papers, especially this one:Holographic MIMO Communications: Theoretical Foundations, Enabling Technologies, and Future Directions, it seems only a very small part of this technology is about holography principle, i.e. recording and reconstruction by reference wave. If that's the truth, why HMIMO is called holographic when holographic principle is actually not used in it?
Here is my guess:
In fact, more papers are taking HMIMO surfaces as "continuous surface" or "continuous MIMO", i.e. infinitesimal elements. And it makes me think that the continuous surface is the reason for the word "holographic" since the surface used in optical holography is continuous. This reason is far-fetched.
I also find the naming somewhat far-fetched, but the motivation is the one that you mentioned. The company Pivotal Commware has a trademark on Holographic beamforming (HBF), and they explain it like this in their white paper from 2017:
"HBFs do not use discrete phase shifters to accomplish beam steering by the antenna. Instead, beamforming is accomplished using a hologram. [...] The HBF has a single RF input port on the backside center of the antenna which is directly connected to an RF distribution network on the inner layers of the HBF. A travelling RF wave propagates from the input connector through the distribution network and is depicted as yellow lines fanning out from the central feed point in Figure 5b. In optical holography, this travelling wave is referred to as the ‘Reference Wave’ and it is this wave we wish to transform into a desired beam. The desired beam shape is referred to as the ‘Object Wave’ in optical parlance. The ‘Hologram’ is then the structure that will transfer energy from the Reference Wave into the Object Wave." and so on...
https://pivotalcommware.com/wp-content/uploads/2017/12/Holographic-Beamforming-WP-v.6C-FINAL.pdf
I believe that having large surfaces is more important in the future of MIMO than having continuous surfaces, which is why I prefer to analyzed and consider "extremely large aperture arrays (ELAA)" that might be created in the holographic beamforming way, but not necessarily.
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