For prototyping purposes, the following white paper discusses methods and systems for analyzing MIMO signals: http://www.ni.com/white-paper/52382/en/.
But since you linked to a VSA, will you be needing a calibrated instrument for your MIMO signal analyzer?
Regardless, if you are looking to build a MIMO test system, I would highly encourage you to call in and speak with someone to ensure you're getting the device that best fits your application and need: http://www.ni.com/contact-us/en-us
The PXIe-5668R could work but could also potentially be overkill depending on how you are wanting to analyze the signals. Besides that, there are also a lot of other considerations like frequency range, the need for synchronizing multiple analyzer channels, desired MIMO setup (2x2, 4x4, etc.?), testing a specific standard or creating custom waveforms, etc.
If you are able to give more information on your application/needs either here or when you call in, that will help determine which device would work best for you.
Thank you for detailed explanation. We are at the starting points for our application and its testbed desgin. I will reach out you in the future. Thanks.