802.11ax – Explicit Beamforming (TxBF)

As we know, that Omnidirectional antennas will send out signals in all directions, but it’s important to increase the efficiency of the signal transmission by keeping them focussed where it’s needed. The coherent way to doing this is by using beamforming, which directs the energy towards the users by making use of sounding signal, channel measurements and feedback metrics.

802.11ac and 802.11ax devices perform explicit beamforming, by initiating NDP (Null Data Packets) frame, so the clients can perform channel sounding, and send feedbacks accordingly.

Below frame sequence gives an idea of how HE and VHT channel sounding would look in a Multi-User scenario:

In this blog, I am discussing on how 802.1ax AP’s requests for the feedback to steer the signals.

HE Channel Sounding:

Beamforming uses channel calibration process to steer a beam in a particular directions.

Below are the steps involved in sounding procedure,

  1. The HE sounding process is initiated by an AP, by sending an NDP Announcement frame, so AP gets more details of the channel and available clients.
    After SIFS period, AP then sends an HE NDP frame, for the clients to analyze the OFDM training fields calculate the steering matrix.

Sample Frame – NDPA

2. The client then analyzes the training fields and calculates a feedback matrix.


An HE AP can use the Trigger frame called Buffer Status Report Poll (BSRP) to request HE beamforming feedback from a STA, if the UL MU Capable field of the last received HE Capabilities element from that STA is 1.
To the BSRP Trigger frame, client should reply back with a Buffer Status Report (BSR)

Aruba controller’s view/command to see the NDP frames

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