For Transmit Beamforming to function properly, The channel characteristics between the transmitter and the receiver should be known accurately. To allow channel characteristics to be determined the concept of Null Data Packet (NDP) was introduced in the 802.11n specification.

The Null Data packet does not contain any data field and purely comprises of the preamble and the legacy and HT tranining fields. The Receiver WLAN station recieiving the  Null Data Packet will deduce the channel characteristics from the HT-LTF in the preamble portion of the packet. The NDP is shown below

Fig Courtesy: Next Generation Wireless LAN – 802.11n and 802.11ac

The total number of streams that can be used for transmit beamforming determines the entire channel. That is, if two streams are to be used for Transmit Beamforming, the channel characteristics for both the streams need to be calculated. In order to achieve this, the MCS rate chosen will signify the number of streams that are supported. That is, for a two stream channel calculation in 802.11n – MCS rate between 8-15 are set.

A receiver station should look at the MCS rate and deduce the channel characteristics for both the streams.

In addition to the MCS rate, The “not sounding bit” is set to zero to indicate that the frame is a null data packet meant for sounding.

• Another Method that was used to determine the  channel characteristics was to expand the number of HT-Long training symbols (HT-LTF). The HT-LTF contains in this case Data HT-LTF (DLTF) which are needed at the receiver to demodulate the data portion of the packet. This could be 1, 2 or 4 based on the number of streams supported for the transmission
• Extension HT-LTFs, which are added to gain channel estimate of additional dimensions on which Tx beamforming might be desired.

The preamble is termed as a staggered preamble. By adding additional HT-LTF, equivalent to the number of streams that are to be supported, channel characteristics for the entire channel could be computed.

Implicit Feedback and Explicit Feedback for Transmit Beamforming