Transmit Beamforming (TxBF)

Basics of Beamforming

Beamforming is a technique that focuses a wireless signal towards a specific receiving device, rather than having the signal spread in all directions.
We know that mostly the internal APs are equipped with omnidirectional antennes, so they send the signals in all directions. So, in order to make the signalling more efficient we use beamforming to direct energy towards specific clients and favorably improves their connection rates.

Beamforming uses antenna arrays to dynamically alter the transmission pattern of the AP, and the transmission pattern can be changed on a per-frame basis. Broadcast and multicast traffic is designed to be received for multiple stations, so a beamforming AP will use traditional omnidirectional transmission methods for broadcast packets to maintain coverage throughout the designed coverage area. TxBF uses multiple antennas to transmit a signal by varying phases so that the signal received at the receiver can be with increased signal strength.

The transmitter is capable enough to understand channel quality around itself but needs to how it looks near the receiver’s end to improve further transmissions.

Beamforming participants:

  1. Beamformer: A device that directs the transmitted frames
  2. Beamformee: A device that receives the frames transmitted by a beamformer

There are two methods of transmit beamforming

  1. Implicit feedback beamforming :

In this method, the Beamformer (transmitter) analyzes the RF signals (special training symbols) it receives from the Beamformee (receiver) to make an estimate of the MIMO channel between them.
This estimate will be valid if the channel is reciprocal such that the multipath effect observed by the frames transmitted to the Beamformee will be similar to that encountered by the frames received from the Beamformee. But it needs client feedback to confirm reciprocity or make calibrations to it’s estimate of the channel.
The Beamformer then uses the channel estimate for controlling transmissions in a way that focuses RF signals at the receiver.

Delailed explanation

2. Explicit feedback beamforming :

In this method, the way of determining the channel estimate relies on the Beamformee (receiver).
The Beamformee will derive the MIMO channel estimate from the RF signals (training symbols) sent by the Beamformer.
The Beamformee then will send the channel estimate back to the Beamformer which uses it for beamformed transmissions to the receiver.
Since the channel calculations are done at the receiver, and is a function of the actual RF path from the transmitter to the receiver, it is expected to be more accurate than implicit beamforming.

Detailed Explanation

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