What is the physical meaning of coherence bandwidth in mmWave massive MIMO channel? I am doing some research about millimeter wave massive MIMO channel now.
We first take account about static MIMO channel, cause we only have SISO measurement system now.
We use scanner to move our antenna to simulate it as array antenna to measure the static channel.
Afterwards, using signal processing method to simulate static MIMO channel.
There are some paper that doing the similar research, they analysis the coherence bandwidth at different antenna element. But I can't realize what the purpose of studying that issue. Is that related to some other MIMO  techniques?

As far as I know, coherence bandwidth is corresponding to multi-path delay spread, and is an indicator to distinguish whether the channel is flat-fading or not.

In the other point side of view, I also study multi-path MIMO channel model. The model is represent as below


It shows that multi-path would cause difference phase shift owing to the steering array. However, I can't realize what the model is associated with coherence bandwidth at each element.

Can someone please help me or provide some idea. Thanks in advance 

reference ： R. W. Heath, N. González-Prelcic, S. Rangan, W. Roh and A. M. Sayeed, "An Overview of Signal Processing Techniques for Millimeter Wave MIMO Systems," in IEEE Journal of Selected Topics in Signal Processing, vol. 10, no. 3, pp. 436-453, April 2016.
You are right, coherence bandwidth is the frequency domain counterpart of delay spread.

However, to achieve diversity across antennas the spacing between the antennas is important "relative to the environment".Let me explain that a bit more. In a user mobile phone you would typically find the order of wavelengths seperation between antennas sufficient to get independent fading accross them. However, the same is not true at the base station. Typically at the base station the seperation between the antennas has to be a 10s of order of wavelength. Why so? Because a user equipment near to ground sees a much richer scattering environment, which are enough to cause phase changes within a wavelength. However at the base station (usually placed high up in the air) the scattering environmental is not that rich and hence more separation is required.

So, to study the effect of independent fading with antenna separations one can use the methodology to see if other antennas are seeing the same coherence bandwidths or not. It is all part of channel profiling whenever new frequencies are deployed.