Hello, everyone!
As a supplement to the ONT basic knowledge, I begin to introduce a series of basic Wi-Fi knowledge.
Today, I will share knowledge about Beamforming.
When we drive to a place, the first thing is to open a navigation map which provides real-time traffic conditions, helping us select the optimal road and improve the travel efficiency. However, wireless channels are invisible and unpredictable. If channel conditions can be obtained before data transmission, as in a navigation map, can data transmission polices be optimized accordingly to improve the performance of a communications system? The answer is yes. Beamforming is a transmission method that uses channel information to preprocess signals to be transmitted.
What Is Beamforming?
Beamforming is a part of the IEEE 802.11n and 802.11ac protocols and is called Tx Beamforming in the protocols.
An AP obtains the channel state information (CSI) of a terminal through protocol interaction. A baseband chip computes the phase difference between different antenna spatial flows to the terminal according to the CSI. Multiple antennas are used to transmit the same data symbols, but the data symbols of different antennas use different phase amplitudes for transmission. In this way, phases of multiple antenna signals are superimposed in different directions to present different strength, so that signal power in a specific receive-end direction is maximized. Because each antenna transmits the same data symbol, the diversity gain is obtained.
Types of Beamforming
A prerequisite of beamforming is obtaining the CSI.
According to CSI obtaining modes, beamforming technologies are classified into explicit beamforming and implicit beamforming.
Explicit beamforming
Explicit beamforming requires a Wi-Fi terminal to provide feedback on channel information.
In the Wi-Fi 4 and Wi-Fi 5, the CSI detection and feedback process of explicit beamforming is as follows:
Implicit beamforming
Implicit beamforming is originally designed to not require a Wi-Fi terminal to provide feedback on channel information. It uses reciprocity of a time division duplex (TDD) system (that is, it is considered that upstream CSI and downstream CSI of the same frequency band are equal) to directly apply upstream CSI to the downstream channel for beamforming.
In the Wi-Fi 4, the CSI detection and feedback process of implicit beamforming is as follows:
The difference between implicit beamforming and explicit beamforming is as follows: In explicit beamforming, an STA returns the actual CSI information. In implicit beamforming, however, the downstream CSI is considered to be the same as the upstream CSI, not the actual downstream CSI.
In actual applications, because of differences between upstream and downstream channels, directly applying the upstream CSI to the downstream channel may be ineffective. To improve the accuracy, an STA still needs to obtain CSI of a downstream channel and reports the CSI to an AP to assist the AP in completing channel calibration between the upstream and downstream channels.
Because implicit beamforming is too complex, it is deleted from the Wi-Fi 5.
That's all for today's basic knowledge sharing.
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