UNDERSTANDING MIMO

The first step in clarifying the confusion is to understand the major approaches taken by the different MIMO camps. In general, all agree that MIMO uses multiple antennas to send multiple distinct signals across different spatial paths at the same time, increasing throughput.

They also agree that multipath reflection isn't the enemy. In any enclosed space, radio signals propagate at different speeds through different materials and are partially or fully reflected by some materials. If you took high school physics, you might remember an experiment with a laser beam and a tank of water that showed how light can be deflected through materials of different refractive index. A simpler experiment is to look through a bottle of water (or old window glass) to see distortions as light travels at slower speeds.

Radio signals are more susceptible than light to diffraction, reflection and absorption, which has traditionally limited speed and range. The higher the data rate, the more likely it is that multiple paths for transmitted signals will emerge and have to be reassembled at the receiver.

Enter OFDM (orthogonal frequency division multiplexing), which was the bridge technology that took wireless networking from the old 802.11b standard to 802.11a and g. Instead of having data symbols - constellations of information in a signal forming a retrievable chunk - spread across a whole Wi-Fi channel, OFDM subdivides a frequency into a set of slower subchannels. A Byte sent over each subchannel is much easier to recognize because it takes longer to transmit and thus many slightly time-offset versions can be reconciled more easily. One player at the heart of this debate is Airgo, a company that provides MIMO technology to WLAN equipment vendors such as Linksys and Belkin. The company's founder, Greg Raleigh, was an early voice in the wilderness about MIMO and he is still defending his definition of the technology. The company has even trademarked the term "true MIMO" to describe its approach.

Airgo's MIMO builds on OFDM by using spatial multiplexing in which different radio signals are sent over the same frequencies at the same time. Multipath reflection allows the transmitting and receiving antennas to essentially create a unique path in space for each signal using separate radios.

The MIMO delivered by Video54 - appearing first in equipment from Netgear - uses several antennas that can be switched off and on in 50 combinations on a per-packet basis but a single radio. Video54 says its technology dramatically improves the ability of a receiver to reassemble signals. This increases range and throughput.

Selina Lo, Video54's CEO, noted that "if you have a lot of redundant routes, you can always find the route that has the lowest latency or the least cost" in terms of signal usage. Atheros will offer similar technology to its OEM partner D-Link. Spatial multiplexing will be incorporated, at least in large part, into the 802.11n standard, and the folks pursuing just multiple antennas say that that's the time to add it: when both adapters and gateways can take advantage of multiple signal paths. And both approaches are currently available in the marketplace, retrofitted on top of 802.11g equipment and protocols. Besides Linksys and Belkin, SOHOware also uses the Airgo technology. Netgear has adopted the Video54 technology.

In addition, D-Link reportedly will use MIMO technology from a third vendor, Atheros. Atheros has reportedly developed a beam-forming MIMO chipset that offers features like Video54. All the technologies are different, none is standardized and each uses a different theory for delivering greater speed and range over wireless LANs.