802.11N Transmit Beamforming TxBF - Explained

So I thought that I would put a short bog post together to try and explain it and make it simple. Am not surprise that it can get confusing especially when vendors give it marketing terms such as clientlink.

In its simplest terms, beamforming allows an access point to effectively concentrate its signal at the clients location. This results in a better signal, SNR and potentially a great throughput.

The 802.11n standard defines a few methods of beamforming. One method, referred to as "explicit" beamforming, requires the downstream channel to be measured at the receiver, or beamformee, and relayed back to the transmitter, or beamformer. The beamformer uses the measured channel information to derive the transmit beamforming parameters.

A second technique defined in the standard is "implicit" beamforming. In this implementation, the upstream wireless channel is measured by the beamformer, and the measurement used to derive the parameters for subsequent downstream beam formed transmission.

"Implicit" beamforming has the advantage that the beamformee does not need to measure and send the channel state information to the beam former. However, 11n standard implicit beamforming requires a calibration exchange between the beamformer and beamformee, which can complicate the transceiver design.

By carefully controlling the time (or phase) of the signal transmitted from multiple antennas, it is possible to shape the overall pattern of the received signal, emulating a higher-gain, or directional antenna in the direction of the target. The same implicit and explicit feedback mechanisms used to characterize the MIMO channel allow beamforming.

The image below shows in simple terms how a shift in phase on an antenna can "focus" its signal so that it arrives at the client at the same time as the other antennas signal thus giving aditional gain overalll to the client.

So when is transmit beamforing actually used in an access point? Well some vendors do not support this feature in their access point. I don't have a list of those that do, but i know of two that do; Cisco and Ruckus Wireless. (Cisco with their ClientLink and Ruckus as of the 7982 AP).

So when does an access point stop using spatial divison multiplexing and choose to use tramsmit beamforming. Your asking yourself why cant they do both? Well lets explain:

To perform spatial division multiplexing ( SDM ) you need to send different spatial streams to the client. For transmit beamforming you are sending the SAME spatial stream to the client so if you are a 2X2:2 AP/Client then you cant do both.

An 3X3:3 AP/Client could in theory be sending two SDM streams and 1 beamforming stream; thus improving one of the SDM streams but your data rate would be 300mbps and not 450mbps.

For a Cisco 3600 4x4:3 AP then again in theory you can deliver 3 SDM streams and one beamforming stream; thus improving one of the SDM streams but most importantly you will be loosing one of the SDM streams so your max data rate will be 450mbps.

If your a 3x3:3 which most access point are, then when you use transmti beamforming instead of SDM then you data rate is 300mbps not 450mbps as most people think. Remember the 3rd antennas is taken up sending one of the other antennas spatial streams and is not sendign one of its own...

Cisco's explanation of spatial streams..

For an explanation of Cisco ClientLink 2.0 follow this link: http://www.cisco.com/en/US/prod/collateral/wireless/ps5678/ps11983/at_a_glance_c45-691984.pdf