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Why do researchers talk about MU-MIMO being "less sensitive to line of sight" / "LOS Okay"? For example in this video, time=[00:40,00:42]

Similarly, "User separation based on spatial channel properties is particularly difficult in situations where the users are located close to each other and experience LOS propagation conditions to the BS antenna array"

My perception is that LOS is always preferred to the opposite, multipath propagation.

Can someone please explain this? Thank you.

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There is no mention of light-of-sight (LOS) on the time you mentioned. There is a mention of point-to-point, however. Point-to-point channels could be LOS or multipath.

MU-MIMO is better than point-to-point MIMO because users are usually well-separated in space. MU-MIMO is particularly better when the channels are correlated at the base station (BS). The reason for that is that the spatial separation of the users make the channel correlation matrices from different users to be different. In the ideal case where these correlation matrices eigenspaces are orthogonal, the data from different users can be perfectly separated without any interference. In point-to-point MIMO, on the other hand, the data from different antennas from the multiple-antenna transmitter will have the same correlation matrix at the BS, and thus there will be more interference from other data streams.

Read section 2.4 from the book Massive MIMO Networks: Spectral, Energy, and Hardware Efficiency.

EDIT: In the paper, they are talking about separating users that have LOS channels with the BS in massive MIMO, that are difficult to separate in traditional MIMO.

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For MIMO to have any advantage, you need uncorrelated paths between the multiple antennas.

If all the users have LOS, then their path is just a complex number with no way to become uncorrelated; that makes the channel matrices non-separable using MIMO methods.

In other words: MIMO uses the fact that different antennas see different channel realizations. When effectively all channels are the same, good channel, then you don't get to use these differences.

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  • $\begingroup$ Thank you, I did not think about it that way but it makes sense now when you say it! $\endgroup$ – Tompa Jan 5 at 17:57

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