The antennas used by wireless devices can have a major impact on coverage, security and performance. In fact, new 802.11n access points (APs) utilize multiple-input multiple-output (MIMO) antennas and advanced signal processing techniques to increase network footprint, available bandwidth, and resilience to the multi-path issues that often crippled older 802.11a/b/g networks.
Interference welcome? Spacial multiplexing overcomes the problem
All 802.11 devices transmit radio waves by using antennas to distribute (propagate) them through the air. A basic dipole antenna spreads those waves out in all directions, just as dropping a pebble into a pool generates ripples. Now picture those ripples hitting the sides of the pool. Some waves actually bounce back from the wall, colliding with other waves still moving in the outbound direction. When waves bump into one another, some ripples get higher while others get smaller or disappear altogether.
A similar thing ...
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happens to radio waves that encounter doors, windows and other objects between an 802.11 transmitter and the receiver. This phenomenon, known as multi-path, causes many reflections of the same data transmission to reach the receiver at slightly different times. In the process, some of those reflections will add to, subtract from, or drown out one another.
You have probably experienced multi-path on your old analog TV as "ghosting." But your mind is a very sophisticated input processor -- you can ignore a little ghosting, while a lot of ghosting renders a TV show unwatchable. Similarly, with a high degree of multi-path fading and delay, an 802.11 receiver can have trouble making sense of reflections -- the signal may even be so degraded as to prevent meaningful communication.
The new 802.11n "high throughput" standard turns multi-path reflections into an advantage, however. Specifically, each 2.4 GHz or 5 GHz channel can carry only so many bits of information. But if you split an 802.11 frame into multiple pieces and transmit each of those pieces over a different spatial path, the entire frame can actually reach the receiver in a shorter time. Of course, the receiver must know how to recombine pieces to reconstitute the original frame. Since each frame takes less time, more information can be sent overall. This technique to improve speed and capacity is called spatial multiplexing.
MIMO antennas: Many paths to signal quality
To tap the power of spatial multiplexing, 802.11n devices must be able to send and receive more than one signal stream simultaneously; 802.11a/b/g devices lack this capability. Even APs with diversity antennas transmit or receive through only a single antenna at any time. However, all 802.11n devices use MIMO antennas to make more of each available channel.
MIMO devices are described by the number of transmit (M) and receive (N) antennas they use simultaneously. For example, a 2 x 2 MIMO client transmits and receives through two antennas, while a 3 x 3 MIMO AP transmits and receives through three antennas. Although there are further subtleties at work here, two transmit antennas generally cannot provide as much bandwidth as three transmit antennas can. MIMO antenna configuration is therefore an important factor when purchasing 802.11n products.
MIMO antennas not only allow different information to be sent along M spatial paths, they can also be used to transmit the same information on M paths. This can be done to improve signal quality at the receiver. By analogy, if you had an urgent message for a colleague, you might attempt to deliver it several ways -- email, office voicemail, cell phone -- because doing so increases the chances of getting through. Similarly, some 802.11n APs can use MIMO to improve signal quality by sending redundant transmissions, thereby increasing the WLAN's speed and/or reach.
Beamforming improves performance for 802.11n wireless antennas
Thus far, we have seen how multiple antennas and more powerful signal processing work together to improve 802.11n performance. The 802.11n standard also employs many other broadly supported techniques, such as bonding two regular 20 MHz-wide 802.11a/b/g channels into a single 40 MHz-wide 802.11n channel to support higher-throughput applications.
In the future, some 802.11n products may use another option called transmit beamforming. Without beamforming, each factory-issued 802.11n antenna radiates signals in all directions, more or less like the omni antennas found on older 802.11a/b/g APs. With beam-forming, a transmitter adjusts the signals emitted from each MIMO antenna to improve reception by an individual client. This technique can logically "aim" transmissions without requiring directional antennas, but beam-forming requires close coordination between transmitter and receiver that has not yet been finalized in the 802.11n standard.
Finally, aftermarket antennas can be used with some 802.11n APs to focus output power on a desired coverage area. Like hearing aids, 802.11n signal processors are better at making sense of what they hear, but a bullhorn (high-gain antenna) can still boost sound (signal) at the source. For example, you might still connect patch antennas to an 802.11n AP mounted on the back wall of a retail store. However, those antennas must be specifically designed and oriented to work with -- not against -- multi-path and MIMO.
Today, hundreds of business-grade WLAN products have passed Wi-Fi Alliance tests based on draft 2.0 of the emerging 802.11n standard. By achieving up to four times the speed and covering twice the distance, those "draft-n" products clearly illustrate the positive impact of better antenna design and more advanced signal processing.
About the author:
Lisa A. Phifer is vice president of Core Competence Inc. She has been involved in the design, implementation and evaluation of data communications, internetworking, security and network management products for more than 20 years and has advised companies large and small regarding security needs, product assessment and the use of emerging technologies and best practices.
