In the Pre-802.11n era, A WLAN device could have multiple antennas, but only transmit/receive the same signal on multiple antennas or use the best antenna for transmit or receive.
With the advent of MIMO, multiple data streams could in effect be transmitted on different antennas simultaneously. MIMO also provided a means of shaping the signal on the air to traverse a particular path instead of spreading in all directions. The shaping of the transmit signal so that the transmit energy was focussed in a particular direction was termed Transmit Beamforming. The below diagram would explain the concept
Fig: An Non-Beam-formed signal and a beam-formed signal
The method in which Beamforming was achieved was to compute the channel characteristic matrix between the transmitter and the receiver WLAN station and then to apply the matrix to the transmit signal so that it could be beam-formed. The channel characteristics matrix was termed as transmit weights. To refer to the mathematical concepts behind transmit beamforming – refer the book — <Next Generation Wireless LANs, 802.11n and 802.11ac by Eldad Perahia and Robert Stacey>.
The Standard defined two methods of obtaining the channel characteristics for Transmit Beamforming. They are
- Implicit feedback
- Explicit feedback