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The role of beam-forming in 11n

Improving signal strength with Transmit Beam Forming

Wireless Alert By Joanie Wexler, Network World
December 17, 2008 12:04 AM ET
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Industry analysis by expert Joanie Wexler, plus links to the day's wireless news headlines

Network World - In a couple of newsletters this month, I've discussed the effects of multiple input/multiple output (MIMO) and antenna technology in 802.11n primarily from the perspective of a "traditional" vendor, if there is such a thing in the nascent 802.11n market. Much of that insight came from Cisco and, indeed, is applicable to many early 11n products on the market today. That's because most do not use beam-forming technology, also called "smart antenna" technology. Rather, they support static antennas - albeit in a dynamically changing environment.

Transmit Beam Forming is an optional component to the as yet unratified 802.11n standard that can improve signal strength for the receiver by up to 400%, according to experts at Wi-Fi chipmaker Atheros. To do so, the technology uses an array of transmit antennas with the same signal, except that the magnitude and phase are adjusted at each transmitter to generate a focused beam. If information about the location of the target receiver is known, the transmitter points its focused beam in that direction, which will improve range because all of the radiated waves can be focused in the one direction of interest.

Startup Ruckus Wireless, which makes controller-based 802.11n Wi-Fi networks, is one shop that makes beam-forming smart antennas to use with its APs. The company says that antenna alignment can have a significant impact on the performance of 802.11n networks, yet most of the solutions on the market have few, if any, instructions on how to adjust the antennas.

Steve Martin, Ruckus VP of engineering, notes that most of today’s Draft N Wi-Fi systems use statically defined antennas in a dynamic client environment, which is less than optimal. “Alignment of antennas is a big deal. Nobody’s talking about what a big performance hit you can get; the differences in throughput can be up to 50%,” he says.

“Performance can change dramatically because of relatively minor changes in the client device’s position or in the environment, like the opening and closing of a door,” Martin explains.

For its part, however, Ruckus attempts to have MIMO antennas adjust to the dynamic environment around them. It uses many more antennas, providing 4,000 different antenna signal patterns from which a given client can choose. The system determines on a packet-by-packet and client-by-client basis which antenna(s) to use for a given transmission.

“We have lots of antennas to choose from and many, many paths to get data from an AP to a client” so that performance is dynamically optimized, Martin says.

He agrees with Cisco that in a static-antenna environment, the “best you can do is use omnidirectional antennas and provide a consistent environment. It’s the path of least resistance but provides the lowest gain.”

Read more about wireless & mobile in Network World's Wireless & Mobile section.

Joanie Wexler is an independent networking technology writer/editor in Silicon Valley.

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