How 2x2, 2x3, 3x3 configs affect WLAN performance

* Impact of more antennas can be negligible, depending on implementation

Most enterprise wireless LAN vendors have supported two spatial streams in their 802.11n access points. Yet their product pitches go something like this: "Our latest-and-greatest AP is an 802.11n 2x3 [or 3x3] MIMO system that greatly enhances data rates, range and performance." So what is the difference in the actual bottom-line impact on network performance between 2x2, 2x3 and 3x3 designs?

There isn't an answer with a hard and fast performance number, according to industry experts. On the transmission side, the performance impact of having more than two antennas depends on what protocols are in use. For example, Cyclic Shift Diversity (CSD) and Space-Time Block Code (STBC) optimize transmission in single-data-stream systems. Such protocols improve range and resiliency by sending one stream (not two), phase shifted, from all available transmitters to extend rate over range by about 1dB to 3dB, says Devin Akin, wireless guru and now chief wireless architect at Aerohive.

More antennas at the receiving end are likely to improve signal gain, experts say, but whether the impact on overall network performance is negligible or profound depends on the overall WLAN ecosystem.

For example, if there are only two transmitting antennas, only two diverse data streams can be sent. In this sense, opinions on the impact of the third antenna on the receiving side range from it being irrelevant to being a detriment (by adding cost to the system) to being just one contributing performance-improvement factor.

"If you add more horsepower to your car, does it guarantee faster speed? Not necessarily," depending on the car's transmission, tires and other components, observes Pen Li, product manager at Wi-Fi chipmaker Atheros, which recently announced 3x3 chips supporting three spatial streams. Products based on them are expected in the first half of next year.

"Additional antennas could improve or degrade performance, depending on implementation," he says.

Gartner analyst Tim Zimmerman agrees. "The fact that you can eke out a bit more by tuning one component in the ecosystem may not noticeably affect the user experience as a whole," he asserts.

Craig Mathias, principal at the Farpoint Group wireless consulting firm, notes, though, that "a third receiving antenna can help because it will give you better diversity [and signal gain]. Even though the raw data rate is same, the effective data rate [throughput] is higher."

He acknowledges, however, "We're talking a possible 5% to 10% improvement on an individual data stream" but the overall network performance improvement would be lower than that, he says.

Data rates also are a consideration, according to Akin. He says that 2x3 products and 3x3 products advertising 300Mbps data connect rates "are doing the same thing. They are six of one, half a dozen of the other."

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