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MIMO opens lanes for wireless highway

By Carl Temme
Network World, 07/26/04

Corporations are beginning to use wireless LANs to carry voice and video, increasing the need for speed, capacity and reliability. But because WLANs share a finite allocation of frequency spectrum, without increased spectral efficiency they will consume all the available frequency channels and interfere with one another - becoming victims of their own success.

 

Multiple Input Multiple Output (MIMO) is a smart antenna technique that increases speed, range, reliability and spectral efficiency for wireless systems. Given the demands that applications are placing on WLANs, MIMO chipsets will figure prominently in new access points and network interface cards.

MIMO is one technology being considered for 802.11n, a standard for next-generation 802.11 that boosts throughput to 100M bit/sec. In the meantime, proprietary MIMO technology improves performance of existing 802.11a/b/g networks.

A conventional radio uses one antenna to transmit a datastream. A typical smart antenna radio, on the other hand, uses multiple antennas. This design helps combat distortion and interference. Examples of multiple-antenna techniques include switched antenna diversity selection, radio-frequency beam forming, digital beam forming and adaptive diversity combining.

These smart antenna techniques are one-dimensional, whereas MIMO is multi-dimensional. It builds on one-dimensional smart antenna technology by simultaneously transmitting multiple datastreams through the same channel, which increases wireless capacity.

You can think of conventional radio transmission as traveling on a one-lane highway. The speed limit governs the maximum allowable flow of traffic through that lane. Compared with conventional radios, one-dimensional smart antenna systems help move traffic through that lane faster and more reliably so that it travels at a rate closer to the speed limit. MIMO helps traffic move at the speed limit and opens more lanes. The rate of traffic flow is multiplied by the number of lanes that are opened.

During the 1990s, Stanford University researchers Greg Raleigh and VK Jones showed that a characteristic of radio transmission called multipath, which had previously been considered an impairment to radio transmission, is actually a gift of nature. Multipath occurs when signals sent from a transmitter reflect off objects in the environment and take multiple paths to the receiver. The researchers showed that multipath can be exploited to multiplicatively increase the capacity of a radio system.

If each multipath route could be treated as a separate channel, it would be as if each route were a separate virtual wire. A channel with multipath then would be like a bundle of virtual wires.

To exploit the benefits the virtual wires offer, MIMO uses multiple, spatially separated antennas. MIMO encodes a high-speed datastream across multiple antennas. Each antenna carries a separate, lower-speed stream. Multipath virtual wires are utilized to send the lower-speed streams simultaneously.

But wireless is not as well behaved as a bundle of wires. Each signal transmitted in a multipath environment travels multiple routes. This makes a wireless system act like a bundle of wires with a great deal of leakage between them, causing transmitted signals to jumble together. The MIMO receiver uses mathematical algorithms to unravel and recover the transmitted signals.

Temme is vice president of product marketing for Airgo Networks. He can be reached at carltemme@airgonetworks.com.

 

802.11n - A nascent IEEE proposal for a 100M bit/sec wireless standard.

By "100M bit/sec," the IEEE means throughput, what users see when they transfer a file, for example, as distinct from the data rate, which is the raw speed before you subtract the overhead associated with the protocol.

This is an important distinction because wireless overhead can add up to a significant percentage of the data - in the case of 802.11, typically more than half of the data rate. An 802.11b access point, rated at 11M bit/sec, typically gives a throughput of less than 6M bit/sec, often far less. The 802.11a and 802.11g hardware can give users about 18M to 22M bit/sec. The data rate for both is 54M bit/sec.

Silicon makers have been boosting WLAN throughput to around 100M bit/sec for some time. The catch is: You have to have the same chips in both the client and the access point, and high throughput sacrifices conformity to the 802.11 specification. Network executives already seem to be discounting high-throughput claims that are based on their WLAN experience. "Unless you are sitting right under the access point, you just don't get the maximum throughput," says Dewitt Latimer, deputy CIO and CTO at University of Notre Dame in South Bend, Ind.

WLAN throughput falls off more or less rapidly the farther a client device moves from an access point. The drop depends on how much metal, wood, concrete and other construction materials is between the two devices. In addition, in almost every case today, an access point is a shared medium: whatever throughput it can deliver is divvied up among however many users connect to that one access point.

"Most practical applications, such as three students sitting under a tree working on a paper [with wireless notebooks], tend to be insensitive to bandwidth. I don't think high throughput WLANs will be a big driver until we see things like streaming media applications being untethered."

From Wireless LAN throughput on the rise, Network World Fusion, 09/29/03.

 

 

   
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