Beamforming technology allows an AP to concentrate energy in the specified direction to clients. It helps improve the receive signal quality and the throughput.

In a multi-antenna system, if signals transmitted from different antennas have the beams with the same attenuation but opposite directions at a position, a wireless hole may be generated. 802.11n provides standard beamforming technology that pre-compensates the phase of a transmit antenna to superpose beams for the optimal effect. 

Beamforming facilitates signal reception by weighting transmission signals. The weighting coefficient is obtained based on the transmission environment or channel state information (CSI). Based on the structure and settings, beamforming is classified into explicit beamforming and implicit beamforming.

Explicit Beamforming

Explicit beamforming requires STAs to provide channel information feedback to APs.

1. An AP sends probe data (training symbols) to a STA. 802.11n uses the Null Data Packet (NDP) and staggered preamble to probe channels, while 802.11ac uses the NDP.

   • NDP

     An NDP is a packet without data or loads. After receiving a probe advertisement and an NDP from an AP, a STA provides channel information feedback to the AP.

   • Staggered preamble

     A staggered preamble sends a MAC frame carrying the probe channel.

2. The STA provides channel information feedback to the AP.
   • 802.11n defines three feedback formats:

     • CSI

       The STA sends original channel information to the AP which will calculate the final beamforming weight.

     • Non-compressed

       When receiving a probe frame, the STA calculates the beamforming weight and sends it to the AP. This mode increases system overhead.

     • Compressed

       When receiving a probe frame, the STA calculates the beamforming weight and sends it to the AP. Certain measures are taken to reduce system overhead.

   • 802.11ac enables STAs to provide channel information feedback in compressed V matrix mode.

3. Based on weight information, the AP performs beamforming so that multipath signals are aggregated on the STA, forming gains.

Implicit Beamforming

The initial assumption of implicit beamforming is to free STAs from providing channel information feedback. Implicit beamforming leverages the reciprocity of a time division duplex (TDD) system. Reciprocity states that the uplink and downlink CSIs on an frequency band are identical. In this way, uplink channel information is directly applied to the downlink channel for beamforming.

 

NOTE:

To simplify the design, 802.11ac does not support implicit beamforming, which is complex.

In explicit beamforming, STAs provide actual CSI feedback, while in implicit beamforming, the uplink and downlink CSIs are considered identical but are not actual channel information.

In practice, beamforming is not well supported by STAs, especially some legacy STAs. To address such issues, legacy beamforming is introduced, which does not require STA involvement.

Legacy Beamforming

1. An AP receives three uplink signals with different phases and amplitudes.
2. The AP corrects phases of the signals using MRC, weights them based on a proper variable gain, sends the signals to the detector for coherent detection, and combines them into one signal, as shown in the following figure.
   
   
3. After obtaining the phases and weights of uplink signals after optimization, the AP sends the signals with the same phases and amplitudes (related to weights) to channels of the same downlink STA. The beamforming is achieved on the STA, generating gains.