Rapidly rising performance requirements on enterprise and
carrier Wi-Fi networks dictate squeezing every available Mbps out of
infrastructure gear – naturally driving increased interest in using any and all
advances in RF technology.
One in particular, so-calledtransmit beamforming(commonly
abbreviated as TxBF), is getting much more attention these days.
While this is a potentially useful tool, be careful not to be fooled by vendor
claims. As always, the devil is in the details.
Rf building block
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 them 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.
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. There’s simply no way around it.
Today 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.
Incompatibility with Spatial Multiplexing. The
explanation for this one is definitely best left to our 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 net-net 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 – the basis for Ruckus BeamFlex
technology – 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 2 to 3x the performance improvement of TxBF,
through better leverage of multipath and statistical optimization techniques.
What’s it all mean?
In short, while vendors are now marketing TxBF as THE solution
to the RF performance problem all by itself, it’s not going to do much if any
good any time soon (see figure below).
There are circumstances when TxBF will be useful when
client support emerges and in combination to adaptive antenna switching. Since
it’s available in the next generation of Wi-Fi chipsets TxBF in combination with
BeamFlex adaptive antennas, in a form of BeamFlex 2.0″, offers a
best-of-both-worlds solution that yields higher SINR gain with less
interference than TxBF alone.
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.
Sudarshan Boosupalli, General Manager-
India & SAARC Operations, Ruckus Wireless