The 802.11 Standards body defined Implicit feedback and explicit feedback as two mechanisms for calculating the channel characteristics Transmit weights that are utilized for Transmit Beamforming.
In Implicit Beamforming, The reciprocity of the channel characteristics between Transmitter and the receiver is considered to be equal compute the channel weights between the Transmitter and receiver.
However,
- The interference seen by the signal from transmitter to the receiver and vice versa is not reciprocal and
- The transmitter and receiver RF distortions are not reciprocal
Hence, a calibration procedure would be needed to compute accurately the channel characteristics between the transmitter station and the receiver station
In Implicit beamforming, normally an Access Point sends a beamformed signal to the 802.11 station without performing calibration with the 802.11 station. The 802.11 station might be incapable in partaking in a channel calibration procedure. Hence, the channel characteristics matrix computed might not be accurate. The use of implicit beamforming would enhance Link performance.
In Explicit feedback, The Transmitter station and the receiver station should be able to partake in a handshake and be able to provide the channel characteristics for beamforming. The transmitter WLAN station will just apply the transmit weights that are received from the receiver when transmitting a beamformed signal to the receiver.
The standard defined different kinds of feedback mechanisms
- channel state information feedback (CSI feedback)
- Non-compressed Beamforming feedback
- Compressed Beamforming feedback