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Friday, March 05, 2010

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?


Dimitris said...

Hi Dean,
not that I am an expert in MIMO, but in such configurations, 2 or 4 antennas transmit/receive the same signal which is then processed to extract information IF there is a significant difference in propagation channel between the 2/4 branches.

So DAS systems may be rewired to enable MIMO but that is highly unlikely given than existing systems have been designed for coverage or capacity and not likely to be radically changed in favour of a new system.

Even if MIMO doesn't work (i.e. the channel is same for all branches) you get 1x1 performance - as if there is no MIMO.

There are MIMO testbeds (such as this but measuring performance in a real environment will be more complicated.

Not sure if existing installations will be "fixed". Probably new antennas will be introduced for HSPA+ or LTE. Mind you, some cell sites already have cross-polarised antennas (xpol) which can be used for 2x2 MIMO.

On older installations and new frequencies: Antennas have limited bandwidths for optimum performance at the carrier frequency so new frequencies almost always need new antennas.

Barney said...

Yes, MIMO air interfaces will work with nearly all DAS implementations (subject to standard caveats). MIMO is great at coping with channel impairments such as delay spread and multipath - but the DAS helps to overcome these anyway, so if you are reduced to 1x1 and you've designed for it you will be fine. You could think of it as an extension of diversity reception - with a DAS you don't have two inputs, so you design the system assuming no diversity gain.

Anonymous said...

My MIMO 802.11n access point works great. I get more throughput than if SIMO, so it's definitely working. The OFDM that is used in WiFI is similar to WiMAX/LTE. DAS vendors are full of crap.