Getting the best Wi-Fi performance from recent RF advances
- 29 June, 2012 19:31
This vendor-written tech primer has been edited by Network World to eliminate product promotion, but readers should note it will likely favor the submitter's approach.
Rapidly rising performance requirements on enterprise and carrier Wi-Fi networks makes it necessary to squeeze every available megabit per second out of infrastructure gear, which in turn is driving interest in advances in RF technology.
One in particular, so-called transmit beamforming, commonly abbreviated as TxBF, is getting much attention, and while this is a potentially useful tool, be careful not to be fooled by vendor claims. As always, the devil is in the details.
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Before delving in, it's important to have a sense of the basic RF building blocks. Many of the technologies available to help improve radio performance come from the broad category known as "smart antennas." There are many variations on the theme, but the idea common to all is using more than one antenna on one or both ends of the link to send and/or receive radio signals in a more controlled manner to increase signal quality and throughput.
There's now a whole family of multi-antenna techniques that can be employed to achieve RF performance gains in Wi-Fi. In a properly designed Wi-Fi system, all of these tools can be used in combination to maximize results.
Note that both Adapative Antennas and TxBF are often referred to loosely as "beamforming," since many in the industry consider the term to mean generically "shaping radio energy in space to focus on the target recipient." There are fundamental differences in how these two technologies operate; however, yielding very significant differences in the performance improvements they can deliver in the real world.
Assessing transmit beamforming
Transmit beamforming allows an access point to concentrate energy in the direction of a particular client using signal processing techniques (phasing or timing the signals differently) at the baseband chipset. Explicit client feedback is required for APs to determine the correct phasing for each client.
While a promising potential addition to the RF toolkit, in reality, TxBF is subject to a number of constraints and disadvantages:
* No client support. This is a complete show-stopper. To achieve any real performance gains with TxBF in Wi-Fi, clients must support the optional feature in the 802.11n standard that provides explicit feedback to the AP about how to do beamforming effectively for each client. This feature has zero support in the market today and none on the way in the foreseeable future.
* Incompatibility with spatial multiplexing. The explanation for this one is definitely best left to a beefy whitepaper, since it requires looking under the hood of how spatial multiplexing in 802.11n really works. The bottom line is that with any commercially practical number of radio chains, it's impossible to achieve the higher data rates in 802.11n and use TxBF at the same time.
* Lots of self-interference. With only 3 or even 4 radio chains to work with, TxBF makes very symmetric beam patterns, generally sending as much energy away from the client of interest as it does toward it. This increases self-interference in the multi-AP networks that are critical to success in today's high-demand-density venues, reducing spectrum re-use and overall system capacity.
* Incompatibility with polarization diversity. There's a technical subtlety at work here, too. The upshot is that TxBF will fail frequently when used with today's mobile clients with arbitrary orientation.
* Modest gains at best. Even when it works, the Wi-Fi chipset engineering community predicts that performance gains in practice will be modest, on the order of 2-3 dB.
Assessing adaptive antennas
Adaptive antennas involve manipulating the inherent directionality and polarization of the physical antenna structure itself. This is achieved by electronically switching a subset of a large number of small antenna elements into use with each radio chain for each packet sent. Element selection is optimized client by client, based on achieved throughput, relying on the ACK packet that all clients send as a 100% standard part of the Wi-Fi protocol.
As a result of this unique "layer zero" role in the system, adaptive antennas have none of the operational limitations of TxBF. Specifically, adaptive antennas:
• Require no special client behavior beyond mandatory elements in the 802.11 standards (for b, g, or n).
• Can be used simultaneously with spatial multiplexing and polarization diversity.
• Mitigate interference through highly asymmetric beam patterns.
• Deliver 2x to 3x the performance improvement of TxBF, through better leverage of multipath and statistical optimization techniques.
In short, while vendors are now marketing TxBF as the solution to RF performance problems, it's not going to do much if any good any time soon.
So don't return to the old omni-antenna reference-design implementations that continue to pollute both the enterprise and carrier network landscape with such mediocre Wi-Fi performance.
Ultimately combining TxBF with adaptive antenna technology will simply deliver the best of both worlds yielding what no other Wi-Fi supplier can provide and every customer wants: pervasive performance.
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