Transmit Beamforming - TxBF

“MIMO” refers to a technique for sending and receiving more than one data signal on the same radio channel at the same time via multipath propagation.

802.11n/ac systems take advantage of multipath by sending multiple radio signals at the same time. Each of these signals, called a spatial stream, is sent from its own antenna using its own transmitter. Because there is some space between these antennas, each signal follows a slightly different and unique path to the receiver, a situation called spatial diversity. The receiver has multiple antennas as well, each with its own radio that independently decodes the arriving signals, and each signal is combined with the signals from the other receive radios. The result is that multiple data streams are received at the same time.

MIMO can be sub-divided into three main categories :-
precoding:
precoding is multi-stream beamforming. With this, same signal is emitted from each of the transmit antennas with appropriate phase and gain weighting such that the signal power is maximized at the receiver input.
- Precoding requires knowledge of channel state information (CSI) at the transmitter and the receiver.

spatial multiplexing:
Signal is split into multiple lower-rate streams and each stream is transmitted from a different transmit antenna in the same frequency channel. If these signals arrive at the receiver antenna array with sufficiently different spatial signatures, it can separate these streams into almost parallel channels. Spatial multiplexing can also be used for simultaneous transmission to multiple receivers, known as space-division multiple access or multi-user MIMO.
- Spatial multiplexing can be used without CSI at the transmitter, but can be combined with precoding if CSI is available.

diversity coding:
The signal is emitted from each of the transmit antennas with full or near orthogonal coding.
- Diversity coding can be combined with spatial multiplexing when some channel knowledge is available at the transmitter. Because there is no channel knowledge, there is no beamforming.

- Orthogonal Frequency Division Multiplexing (OFDM) is a modulation scheme that uses multiple subcarriers within the same single channel. Rather than transmit a high-rate stream of data with a single subcarrier, OFDM makes use of a large number of closely spaced orthogonal subcarriers (meaning that crosstalk between the subchannels is eliminated) that are transmitted in parallel.
- The advantages of using OFDM include reduced multipath effects in reception and increased spectral efficiency.
- OFDMA achieves multiple access by assigning subsets of subcarriers to individual users.

- multiple transmit antennas can be used for beamforming
- multiple receive antennas can be used for diversity

TxBF requires multiple copies of the same signal arriving in phase at the receiver.
- Implicit Feedback: AP detect from its multiple antennas the different phases of arrival of a signal from the client on each of the AP’s antennas. This is roughly analogous to the way human ears process sounds that arrive at each ear at different times and therefore give an indication of the direction from which the sound came.
- Explicit Feedback:

- it should be obvious that a system can do spatial multiplexing or phase-based beamforming, but not both at the same time.



Reference:
http://www.digitalairwireless.com/wireless-blog/t-80211n/transmit-beamforming-txbf-explained.html
http://theruckusroom.typepad.com/files/adaptive-antennas-txbf-wp-0521-1.pdf
http://rfmw.em.keysight.com/wireless/helpfiles/89600b/webhelp/subsystems/wlan-ofdm/Content/ofdm_basicprinciplesoverview.htm