Understanding MIMO

There is no technology more important to wireless than MIMO – which stands, by the way, for Multiple Input, Multiple Output. We’ll get to what that means in a moment, but I want to correct a misconception up front: MIMO is not just about wireless LANs, specifically those based on the IEEE 802.11n standard; it’s also critical to further developments in cellular and wireless broadband systems as well. MIMO is the key technology in getting the most out of the relatively limited radio spectrum. It’s not easy to understand, but let me try to explain.

A radio link has a transmitter and a receiver. There are all kinds of clever things we can do with antennas to make this link more reliable, and thus maximize throughput and/or range under a given set of conditions, but, perhaps surprisingly, one of the most clever things we can do is to provision multiple transmitters and multiple receivers (multiple distinct radios with their own antennas) on a given link, all in the service of a single logical transmission stream. If we put a different signal on each of those transmitters, and do so cleverly, the result is MIMO – multiple transmitters, on the same radio frequencies, simultaneously putting energy into the radio channel that connects the two endpoints, and multiple receivers taking that energy out on the other end. This sounds like it shouldn’t work – after all, won’t the transmitters interfere with each other? Don’t the natural collisions and reflections of radio waves as they move through the environment – known as multipath – cause the usual problems with decreased signal strength and reliability? Well, no, these aren’t problems at all in a properly designed MIMO system. Multipath is, in fact and quite counterintuitively, essential to the proper functioning of a MIMO link.

Because MIMO so dramatically improves reliability, capacity, range, and throughput, I would not recommend any wireless LAN product today that isn’t based on 802.11n. You need to be careful here, though, because MIMO is in fact an optional feature in 802.11n, and you’ll find many products on the market that don’t implement it. These are usually the ones limited to 150 Mbps, but it’s often very hard to tell. Look for 300 Mbps (or more, although these are rare today) and you’ll be all set. As for cellular and mobile broadband – you won’t have much control over the technology in your purchases here, and it will regardless be a while before MIMO is dominant in the wide area. But keep your eyes peeled – MIMO, like I said is one of the most significant developments in wireless – ever – and if you can get it, you want it.