Monday, July 9, 2012
Beamforming and How to survey for it?
Beamforming
and How to survey for it?
Beamforming is popular these days with certain WiFi
venders, this technology is maybe not a big mystery for the geeks in the WiFi
industry, although I see and hear a lot of misconceptions around beamforming.
What
is it?
Beamforming allows an access point to
concentrate RF energy in the direction of the WLAN client/receiver in order to
improve the Signal to Noise Ratio at the WLAN client/receiver and thereby
improving WLAN performance (Increased SNR = increased data rates & decreased
retransmissions).
So basically you could say beamforming affects the
radiation pattern of the access point, so with this in mind anything that
affects the RF pattern should be taken in consideration when doing a
pre-installation onsite site survey, correct?
Before we elaborate on this further let's dig
into beamforming first to grab a good understanding what it actually is (anyway
how I see it ;-)).
Beamforming comes in 3 versions, static beamforming, transmit beamforming and dynamic beamforming (sometimes they are
called different but for this blog we use these names to differentiate).
Static
Beamforming:
Under static beamforming we can understand that an
access point will use a fixed directional antenna to focus the RF energy (beam)
in a certain direction, the coverage area in this direction will extend in
comparison with a traditional omnidirectional antenna. Almost every vendor can
do static beamforming when their access point supports external (directional)
antennas. The directional antenna will be aimed (fixed) towards a certain area
to improve SNR for that area, keep in mind that all mobile WLAN clients have
small omnidirectional antennas on board that also need to send traffic back
towards the directional antenna of the access point (more on that later). So
static beamforming will be mostly used to improve SNR for a certain location or
to extend a coverage area.
Some vendors like Xirrus, use an array of static
directional antennas to improve SNR in 360 degrees, by assigning a different
channel to each beam they can be packed together within a single array.
Transmit
Beamforming (TxBF)
Under transmit beamforming we can understand that
an access point will transmit 2 or more phase-shifted signals towards a WLAN
client/receiver in order to achieve an constructive signal at the WLAN
client/receiver. The phase-shifted signals as they travel through the air
create constructive signal (in phase) points at various locations in time and
space, the goal here is to achieve this at close as possible to the WLAN
client/receiver and thereby increase SNR.
To do this correct, possible feedback is required
coming from the WLAN client/receiver to help pinpoint the location of the WLAN
client/receiver.
Explicit and implicit transmit beamforming are 2 forms
of optional components of the 802.11n standard and require feedback from
802.11n clients, as till now 802.11n chipsets do not support either form of
transmit beamforming.
Although Cisco has his own proprietary form of TxBF
called ClientLink, version 1.0 works only with legacy 802.11ag WLAN clients without
requiring feedback. Optimizing SNR values on the 802.11ag clients in a mixed
environment (802.11agn) will increase airtime for all 802.11 WLAN clients and
is beneficial for overall WLAN performance.
ClientLink 2.0 works for all 802.11agn clients
(release 7.2 on the 3600 series access points).
Dynamic
Beamforming
Under dynamic beamforming we can understand that an
access point will focus RF energy (beam) towards a certain WLAN client/receiver
in a way it's kinda like static beamforming but with a big difference that it
is not static and uses an dynamic antenna array that can change it's radiation
pattern on a frame by frame basis. It selects the optimum antenna pattern for
each communicating device in real time, while actively avoiding interference
and minimizing noise to nearby networks and devices.
Test results showed that dynamic beamforming had the highest SNR increase in an overall WLAN deployment of the 3 beamforming technologies as described above.
The only WiFi vendor currently offering dynamic beamforming technology is Ruckus Wireless.
Site
survey considerations
By understanding the 3 different types of beamforming, what do we need to take in consideration while doing a onsite pre-installation site survey?
This is how we see it.
Note that these are only additional considerations for the onsite site survey concerning beamforming technologies, during the actual site survey other considerations like capacity, throughput and so on need to be addressed also.
Static
beamforming:
Because static beamforming will cover only one
certain coverage area following points need to be taken in consideration.
• Always measure with the antenna type that will be
proposed in the bill of materials.
• Adapt your TX power settings before you measure
(see additional note at the end).
Transmit
beamforming:
In this case only cisco has a proprietary form of
TxBF, we recommend to follow their recommendations (I really appreciate all the
documentation Cisco put's out there)
• "Although ClientLink increases the SNR and
data rates of clients at the edges of the cell, it does not extend the maximum
range of a cell. This is because certain packets that must be heard by all
clients (such as beacons) are sent as broadcasts, which ClientLink cannot optimize.
These broadcast packets effectively become the limiting factor on the cell
size. Therefore cell spacing should not be increased when using
ClientLink. In fact, to be conservative, the ClientLink feature can be
turned off during a site survey." this makes sense to me I would
say ClientLink feature must be turned off during a site
survey.
• Take note that ClientLink 2.0 is enabled by
default, and needs to disabled by a cli command.
Dynamic
Beamforming:
In this case only Ruckus supports this technology, I
have a tiny issue with their recommendation to do onsite site surveys. I've got
this information from their technical guy at a Big Dog seminar.
• Ruckus recommends that onsite measurements need to
be done with the WLAN client/receiver in the beam and with the access point at
full tx power, note that the beacons are not broadcasted in the directional
pattern but in a omnidirectional pattern to communicate with other WLAN
clients/receivers. Also sending out traffic in a directional pattern at full
power (20dBm EIRP in Europe) raised an big question mark in my mind that
is related towards WiFi math and physics, certainly if you want to support
smart mobile devices (see additional note at the end).
• I would recommend doing the site survey with the
access point tx power settings at 14dBm IR.
Additional Note: this note refers to the
"antenna reciprocity theorem" (damm
that’s a hard one if you're native tongue is not English ;-)) which I refer to
when it comes to tx power settings and antenna options.
Basically what this theorem says is the following, If
an electromagnetic force of some particular magnitude is applied to the
terminals of antenna "A" and the received current is measured at some
other antenna "B" then an equal current (in both amplitude and phase)
will be obtained at the terminals of antenna "A" if the same
electromagnetic force is applied to the terminals of antenna "B".
In human terms, no matter with type of antenna is
used on both sides if the input power on both ends is the same then, if you can
hear me, I can hear you.
Explained in a real world scenario: if you're
laptop has a wireless NIC with a lousy omnidirectional antenna on board and is
sending at a tx power input of 17dBm (50mw), and I have the same laptop with
the same setup sitting 1km away from each other. In this case we would probably
not be able to set up a wireless communication because we don't hear each
other, if I connect a high gain directional antenna to my wireless NIC and aim
it towards you're direction whenever I can hear you, we will be able to set up
a wireless link even if you're lousy omnidirectional antenna don't reach me, as
long as we have the same tx power input on both laptops (tell this to all the
IT managers that for security reasons are eager to block WiFi signals so that
they don't bleed into the parking lot).
So lets put this in an access point and a mobile
wireless device relationship.
Consider what happens if an access point has a tx
power input of 17dBm with a directional antenna and the mobile
wireless device has an tx power input of 14dBm with a low gain omnidirectional
antenna, due to the fact that the access point is more powerful then the mobile
wireless device, then "reciprocity" is broken and around the edge of
the cell the mobile wireless device will attempt to associate with the access
point that it will never reach.
Consider what happens if an access point has a tx
power input of 14dBm with a directional antenna and the mobile wireless device
has an tx power input of 17dBm with a low gain omnidirectional antenna, due to
the fact that the access point is less powerful then the mobile wireless
device, also "reciprocity" is broken, but there is a big but here
;-), reciprocity is broken in a way that augments, rather than detracts.
So taking all of this in consideration, and if
you're WLAN needs to support smart mobile devices (BYOD, which have a limited
power input) it's better to have your access points on limited tx power
settings, so take this in account when doing the onsite site survey.
Please reply, if there are any remarks or somebody
has additional information regarding this topic.
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