Will MIMO work indoors?

This post is more of a question than an answer.

Many larger buildings (airports, shopping malls etc) have various forms of indoor coverage - active and passive distributed antenna systems (DAS), in particular. These usually involve connecting small base stations - often from multiple network operators - to a network of antennas, splitters and other paraphernalia around the building.

All of which is fine.... until we get to technologies like LTE and some variants of WiMAX and HSPA+, which use MIMO technology. Multiple-in, multiple-out technology uses a number of antennas.

I've recently asked a few people the question "So, how does MIMO work with DAS systems installed historically in large buildings?"

The usual response has been "Errr...... that's a good question. Not sure".

I bounced this one off a DAS vendor this morning (Commscope's Andrew division), and got an answer that it *should* all work with their recently installed systems. Asked about whether older systems, or other vendors' installs will need upgrading got a less-clear answer.

So, a set of questions for anyone who might have looked at this already:

- Do older DAS installations work OK with MIMO?
- Does LTE (or WiMAX or HSPA+) work properly when MIMO doesn't do what it's supposed to? What are the side-effects? (Slower speeds? Lower aggregate capacity?)
- Are the effects made worse when you go to 4x4 or more complex versions?
- How do you test all this?
- If certain installations don't work OK, how much will it cost to fix them?
- And while we're on the in-building topic, will the older implementations support new bands like 700MHz and 2600MHz OK as well?