MIMOing Your Way to Faster Wi-Fi

Linksys claimed that its new MIMO-enhanced 802.11g devices would enable users to have three times the range, and network speeds of up to eight times that of conventional 802.11g. This, if accurate, would give Linksys MIMO devices throughputs in the range of 400-Mbps.

More realistically, most vendors predict that users will see at least two to four times faster throughput and a doubling of range. Perhaps more importantly, MIMO Wi-Fi devices should deal better with structural issues that lead to dead spots in Wi-Fi coverage.

Best of all, while neither Netgear nor Linksys has announced prices yet, it is expected that MIMO adaptors and access points will cost little more than their existing 802.11 counterparts.

How can they do it? MIMO works by taking one of radio communication's oldest problems, multipath, and turning it into a solution.

Multipath is what happens when signals bounce off objects or structures and take multiple paths to the receiver. If you listen to your car radio, you run into the multipath problem every day. For example, if your favorite radio station fades out every day in a certain location, you're hearing an example of what's called multipath fading or "Rayleigh" fading.

What's happening is that your antenna is receiving both the transmitter's main signal and its reflections. When these signals arrive out of phase with each other, they cancel each other out, and your morning traffic report fades out.

But starting in the 1990s, a pair of Stanford University researchers showed that you could use each reflection, each multipath route, as a separate channel.

The engineering problem was that in order to make use of multipath this way, you need multiple antennas. However, it turns out that these antennas can be very close together—close enough to fit on a Wi-Fi card.

So it is that MIMO devices actually transmit and receive multiple data streams over their multiple antennas. These streams are then bonded together on the Wi-Fi device to create a higher-speed wireless connection.

If this sounds familiar, it should. Bonding was a trick we often used with ISDN (Integrated Services Digital Network) routers, and in modem devices that combined several 56K modem connections into a single, faster connection.

More recently, Wi-Fi chip maker Atheros Communications Inc. this week has used bonding with 802.11h to make Wi-Fi devices that could double the theoretical speed of 802.11g from 54Mbps to 108Mbps. Wi-Fi devices labeled "Super G" use this technology.

Unfortunately, these accomplish this speed increase by bonding together two or more of the 802.11g standard channels. Therefore, if you have multiple 802.11g networks, they can interfere with each other. This can result in a slowdown in the conventional 802.11g network.

MIMO avoids this problem by not bonding together 802.11 channels. Instead of sending one data stream down one channel and another stream down another channel, MIMO simultaneously transmits multiple data streams over the same channel.

As you might guess, this does cause signal interference. But MIMO receivers use algorithms to pull out the proper data streams and bond them in real time, resulting in a much faster throughput with longer range than conventional 802.11 technologies.

MIMO also uses SDM (Spatial Division Multiplexing). SDM multiplexes the multiple data streams, one per antenna, to transfer data simultaneously in each channel of bandwidth. This also results in faster traffic while still using any of the 802.11 networking protocols: 802.11a, 802.11b or 802.11g.

Airgo Networks Inc. was the first company to release a commercial version of MIMO. The groundbreaking firm's "True MIMO" AGN100 Baseband/MAC (Media Access Control) processor and AGN100RF transceiver are being adopted by such Wi-Fi equipment vendors as Belkin Corp., Linksys and SOHOware Inc.

Airgo, however, is far from the only chip OEM to be producing MIMO-capable chip sets. For example, Netgear is using Video54's RangeMax MIMO technology. Intel is also working on MIMO technology for its next-generation Centrino chips.

Of course, it would be too much to expect these varying MIMO implementations to be compatible with each other. Each of them, however, is backwards-compatible with existing Wi-Fi equipment. In addition, to that extent, pieces of MIMO equipment will be compatible with each other. So, for example, while you wouldn't get 100Mbps from a combination of Belkin MIMO-enhanced 802.11g access points and Netgear MIMO-enabled Wi-Fi cards, you would still get 802.11g's usual speeds.

In addition, MIMO Wi-Fi cards, even used with non-MIMO access points, should see extended range. That's because, by their very nature, these cards are better at picking up low-strength signals.

What all of this means for you as a reseller or an integrator is that MIMO technology deserves your attention today. Everyone loves Wi-Fi, especially now that it's getting more secure with the maturation of 802.11i. At the same time, everyone wants more range and faster throughput.

If at all possible, stick with one vendor, or at least one MIMO chip set, in your deployments. If you don't, your customers won't see significant performance gains.

Finally, if you're hoping that 802.11n, the next high-speed networking update standard, takes care of these incompatibility woes ... don't hold your breath. 802.11n is in the very early stages of development, and we're already seeing at least two main camps with different ideas on how to achieve a real-world 100Mbps Wi-Fi throughput.

On one side, you have the MIMO camp led by Airgo. They're grouped together under the name WWiSE (World-Wide Spectrum Efficiency). On the other, you have the TGn (Task Force 802.11n) Sync group. This group, which includes Atheros, Intel, Nokia and Sony, is taking the bigger-is-better approach by using existing wireless technologies over bigger, 40-Mhz channels.

The bottom line? Don't wait for 802.11n, but do test out the coming MIMO equipment, pick a vendor and start rolling it out. You and your customers will be glad you did.

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