Beamforming is one of the techniques for improving the data throughput of the system using multiple antennas. Beamforming applies different weightings of amplitude and phase to the signal on each antenna in order to maximize the signal at the receiver’s antenna(s) after the signals pass through a typical environment (the channel) with reflections and multipath. To determine the correct weighting values, the 802.11ac standard defines a process where the receiver provides explicit feedback to the transmitter about the channel conditions. The sounding process to determine the channel information involves these steps:
The transmitter (typically an access point) sends a Null Data Packet (NDP) Announcement frame with the address of the AP and the target recipients.
The transmitter sends a Null Data Packet to the target recipients.
Each target receiver uses the preamble in the NDP to measure the RF channel characteristics, and returns the measurements as a compressed beamforming steering matrix to the transmitter.
The transmitter uses the data from all recipients to beamform its transmissions.
The beamforming matrix is complex, and includes magnitude and phase information for each pair of transmit and receive antennas, for each OFDM subcarrier. Signal Studio allows a user to provide values for this beamforming matrix, which is then applied to the signals for each transmit antenna.
This technique is also adopted in the 802.11ah standard.