Finding MIMO

The fundamental innovation is the way multiple receive antennas can power phase processing, in effect using the phase differences from incoming signals to differentiate between them. Again, far from being a new trick, this has been around for the odd hundred million years: animals with two ears, including us, can not only tell what direction a sound is coming from but can concentrate on that sound even when there's a lot of extraneous noise appearing from other directions. We can even screen out sounds on the same frequency and with very similar information: the famous cocktail party effect is a good example, where curious socialites can earwig on interesting gossip coming from one direction while the rest of the room is also awash with drunken conversations.

What's really exciting about this is the ability to use space to differentiate between signals on the same frequency. Spatial multiplexing is a cracking way to get around the standard bandwidth limitations of only having so many megahertz of spectrum to use: if you can transmit multiple signals on the same frequency, you're able to cram much more in without breaking your licence restrictions. Moreover, if you encode the transmissions so that information on each can be used to help reconstruct the information on the others -- space time block coding -- you can increase robustness as well as pure throughput.

MIMO systems work best with lots of multipath, as they can treat each arriving signal as an independent link from which information can be extracted -- many of the benefits of the approach are lost if you've got a classically good line-of-sight link. Fortuitously, at the microwave frequencies used by wireless LANs and broadband data links, the world is stuffed full of multipath: it feels like MIMO is the right technique coming along at the right time.

There are restrictions. To work well, the antennas have to have a reasonable physical separation. Exactly what depends on the frequencies: you can easily build a MIMO system for, say, 30GHz broadband links, but there may not be the room in a PDA or mobile phone. A laptop with a MIMO-enhanced 2.4GHz or 5 GHz wireless LAN? Well, probably. They're working on it. Also, there's still plenty of work to be done in finding the best ways to model MIMO systems, finding out the best modulation and encoding schemes, and finding the most effective tradeoffs between cost and performance.

However, there's a good chance that MIMO will be incorporated in many forthcoming wireless standards, including 802.11n -- which will take 802.11-style networks up to as much as 300Mbps -- and 802.20 for wireless broadband. Toshiba is also showing an 802.11g network at the ITU Telecom World conference in Geneva this week, which uses MIMO techniques to increase the data rate to 100Mbps. Having found it, nobody's going to let MIMO go -- and the results should be more than worthwhile.