Transmit beamforming (Implicit feedback)

Before we start, few terms that we need to be clear with:

MIMO channel sounding : In many cases, it is required to obtain a full characterization that details the measurement of the MIMO channel.
It requires the transmission of sufficient number of High Throughput Long Training Fields (HT-LTFs) to sound the full dimensionality of the channel. 

Sounding PPDU : These sounding requests and responses helps the receiver to understand the characteristics of a MIMO channel, in order to generate a steering matrix and steer the next transmissions to the receiver more efficiently.

HT LTF : The High Throughput Long Training field helps to estimate the MIMO channel measurement between transmit and receive chains.

Moving on to the details,

Transmit beamforming with implicit feedback (TxBF) uses only HT, non-HT PPDU’s and HTC (High Throughput Control) field.

It can operate in 2 ways,

1) Unidirectional – Only the HT beamformer sends beamformed transmissions.
2) Bidirectional – Both STAs send beamformed transmissions, i.e. a STA may act as both HT beamformer and HT beamformee.

The performance of Tx Beamforming can be improved by calibrating the transmit and receive chains, and in case of implicit TxBF, only the Beamformer sending the beamformed transmissions needs to be calibrated.

Here are the STA capabilities required to support TxBF (Implicit Feedback):

STA Capabilities	Required support
Beamformer	In the Transmit Beamforming Capability Field > HT Capabilities Element > HT Capabilities Element – Set the Implicit TxBF Capable subfield to 1 – Set the Implicit TxBF Receiving Capable subfield to 1 – Set the Calibration subfield to 3 Capable of receiving Sounding PPDU (SOUNDING parameter and NUM_EXTEN_SS should be 0 in the RXVECTOR of PHY-RXSTART.indication primitive)
Beamformee	In the Transmit Beamforming Capability Field > HT Capabilities Element > HT Capabilities Element – Set the Implicit TxBF Receiving Capable subfield to 1 Capable of transmitting Sounding PPDU (by setting SOUNDING parameter and NUM_EXTEN_SS should be 0 in the TXVECTOR of PHY-TXSTART.request primitive)

Here are the transmit beamforming support required with Implicit Feedback :

Role	Required Support
An initiator in a calibration exchange	Can receive and transmit sounding PPDUs. Can receive a CSI (Channel State Information) frame sent by a calibration responder.
A responder in a calibration exchange	Can receive and transmit sounding PPDUs. Can respond with a CSI (Channel State Information) frame that contains channel measurement information obtained during reception of a sounding PPDU.
HT beamformee: A receiver of transmit beamformed PPDUs	Shall transmit sounding PPDUs as a response to TRQ=1.
Beamformer: A transmitter of beamformed PPDUs	Can receive sounding PPDUs. Can compute steering matrices from MIMO channel estimates obtained from long training symbols in sounding PPDUs received from the HT beamformee.

Unidirectional implicit transmit beamforming :

STA A is the beamformer that initiates the PPDU exchange, and STA B is the beamformee.

Flow representation –

a) STA A initiates the frame exchange sequence by sending an un-steered PPDU including long training request (TRQ= 1) to STA B .

b) STA B sends a sounding PPDU in response to the long training request from STA A.

c) On receiving the sounding PPDU from STA B, STA A uses the resulting channel estimate to compute steering matrices and uses these matrices to send a steered PPDU back to STA B.

d) The steered PPDUs transmitted by STA A will include training requests (TRQ=1).
In response to each training request, STA B returns a sounding PPDU to STA A, which enables STA A to update its steering vectors.

If the steering vectors resulting from step c) or subsequent sounding PPDUs are deemed stale due to delay, the sequence may be restarted by returning to step a).

Bidirectional implicit transmit beamforming

STA A initiates the frame exchange, and STA A and STA B alternate in the roles of HT beamformer and HT beamformee.

Flow Representation:

The PPDU exchange can be summarized as follows:

a) STA A initiates the frame exchange sequence by sending an un-steered PPDU which includes a training request (TRQ= 1) to STA B.

b) STA B sends a sounding PPDU in response to the training request.
In addition, this PPDU includes a training request to enable implicit transmit beamforming.

c) On receiving the sounding PPDU, STA A uses the resulting channel estimate to compute steering matrices and uses these matrices to send a steered PPDU back to STA B.
This steered PPDU is also a sounding PPDU in response to the training request from STA B.

d) On receiving the sounding PPDU, STA B uses the resulting channel estimate to compute steering matrices and uses these matrices to send a steered PPDU back to STA A. The steered PPDU transmitted in previous step (c) and further subsequent steered PPDUs transmitted by STA A would include training requests. In response to each training request, STA B returns a sounding PPDU to STA A, which enables STA A to update its steering vectors.

e) The steered PPDU transmitted in previous step (d) and subsequent steered PPDUs transmitted by STA B may include training requests in HTC. In response to each training request, STA A returns a sounding PPDU to STA B, which enables STA B to update its steering vectors.

Note: If the steering vectors resulting from step c) or d) or subsequent sounding PPDUs are deemed stale due to delay, the sequence may be restarted by returning to step a).

Attaching a snippet of the TxBF Capabilities in an Association Request frame:

References:

CWNP.com
CWAP Study Guide
802.11 IEEE Standard