802.11 Beamforming

Beamforming technology allows the antennas and controlling circuitry to focus the transmitted RF signal only where it is needed, unlike the omnidirectional antennas people are used to. Beamforming is PHY layer procedure.

Two types of beam forming:

1.    In implicit beamforming, the upstream wireless channel is measured by the beamformer, and the measurement used to derive the parameters for subsequent downstream beam formed transmission.

2.    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.

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.

Since 802.11n allowed multiple forms of beamforming and both ends of a wireless link need to use the same beamforming method to produce real benefit.

Sounding frames are known pattern of RF symbols sent from each antenna.

Following is the sequence of events that take place as part of Transmit Beamforming to get reports from clients.

1.    AP sends an announcement that its going to send out a sounding frame containing data to be evaluated by client.

2.    AP sends the data in the Null data packet.

3.    All the clients supporting TxBF receive it as start responding to the report request by sending back a Compressed V-Matrix to the AP.

4.    Based on the feedback received by the AP from clients, it re-calibrates the phase shift for each of the transmitted signal from each antenna so that the signal strength reaches its maximum at client

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