Wireless LANs could soon get more than twice the throughput and range if a smart antenna technology dubbed MIMO, pans out as its backers anticipate.
Multiple-input-multiple-output, or MIMO (pronounced "my-moh" or "mee-moh"), has made its way into chipsets and could be in WLAN devices by year-end.
"Everybody is working on this," says Craig Mathias, principal with wireless consultancy Farpoint Group. "It's the most important radio technology for the next few years."
MIMO algorithms in a radio chipset send information out over two or more antennas. The radio signals reflect off objects, creating multiple paths that in conventional radios cause interference and fading. But MIMO uses these paths to carry more information, which is recombined on the receiving side by the MIMO algorithms.
Many WLAN vendors expect that some form of MIMO will be the basis of work just starting in the IEEE 802.11n Task Group, which is creating a specification for WLANs having at least 100 Mbit/s throughput. The 3rd Generation Partnership Project, a collaboration of telecom standards groups, is also evaluating MIMO techniques for use in cellular networks.
Part of the enthusiasm for MIMO is based on the conviction that it can dramatically boost performance and range, and still handle existing 802.11a/b/g radios, with only a slight initial increase in price over those products.
This will make it more easy to implement than more radical radio technologies, such as UWB, which operate across a broader range of spectrum.
The Holy Grail in radio technology is increased spectral efficiency, or how many bits per second per hertz pass through the air, Mathias says. MIMO doubles the spectral efficiency compared with that of current WLANs. The maximum data rate for 802.11g and 802.11a networks is 54 Mbit/s, though actual throughput is closer to 20 to 30 Mbit/s. Current MIMO techniques can boost raw WLAN throughput to 108 Mbit/s, supporters say.
So far, the only company with MIMO chipsets is Airgo Networks, which launched its products in August 2003. Former Stanford University researchers Greg Raleigh and V.K. Jones, who hammered out some mathematical proofs for MIMO and multipath, founded this Palo Alto radio chip designer.
One leading WLAN vendor has tested Airgo products for six months and has decided to use MIMO in products due out by year-end. "Our engineers found a 200 percent to 400 percent increase compared to the performance of other (802.11) products," says a vice president with this vendor who requested anonymity. "They found a 150 percent to 300 percent increase in range."
Right now, the decisive feature for this anonymous vendor is MIMO's range increase, which allows a single WLAN access point to cover an entire office or home. The Airgo chipset will support both the 802.11b/g 2.4-GHz band and the 802.11a 5-GHz band, the vice president says. The chipset will deliver full MIMO benefits with MIMO clients. With existing 802.11b/g and 802.11a clients, the chipset will work like a conventional WLAN radio, but with somewhat longer range.
MIMO is just one form of smart antenna. The others are mainly arrays which use the interference between multiple antennae to control the direction and strength of a transmitted signal, so that signals can be focused towards a given client and interference with other networks minimized.
Array networks come in two kinds: adaptive arrays, which use feedback to produce the optimum signal strength, and phased arrays where interference between the different signals produces a strong beam in the required direction.
Motia has designed Javelin, an adaptive array system, as an add-on for existing 2.4GHz 802.11 systems. A chipset is coupled with a four-antenna adaptive array, as an add-on for existing 802.11 radio transceivers. The chipset combines signals to shape an optimal radio beam, and like MIMO uses the multipath method, says Robert Warner, vice president of sales and marketing. Motia plans to have chip samples available shortly, with volume production by June.
Wireless switch vendor Vivato Networks uses phased array. This approach packs a lot of individual antennas, each with a slightly different directional pattern, into a single panel. Algorithms steer the radio beam to the appropriate WLAN client. The result is a big increase in range, though this technique has been most successful in outdoor applications.
MIMO is unique because it multiplies bandwidth by essentially providing multiple channels between devices, says Ben Manny, director of the radio communications laboratory in Intel's corporate technology group. "MIMO is the one antenna (approach) that gives you higher point-to-point data rates," he says.
Intel is running a range of MIMO projects, with an eye toward moving more wireless radio functions into complementary metal-oxide semiconductor (CMOS) silicon.
Two other WLAN chip makers, Atheros Communications and Broadcom, are researching MIMO, but both declined to specify the resources they're devoting to it or when they will have chips available. Executives at both companies say MIMO is relatively expensive to create in silicon and that buyers will resist paying even a small premium for more WLAN throughput or range. They add that buyers will resist products that fall outside the IEEE standard.
Atheros uses a technique called channel-bonding to double data rates for its 802.11g and 802.11a/g chipsets. This technique combines 54 Mbit/s channels to create one 108M bit/sec channel.
Airgo's Raleigh says channel bonding comes with its own high price: It uses up scarce radio spectrum. A better approach, he says, is to increase the data through a given channel, which MIMO does.
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