Cisco Cisco Aironet 3700i Access Point 백서

 

 

© 2014 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public Information. 

Page 8 of 10 

Standards-Based Beamforming for 802.11n 

There are two types of standards-based beamforming: explicit beamforming and implicit beamforming. 

In explicit beamforming, information about the wireless channel is fed back to the transmitter by the receiver. In 

order for the receiver to measure the wireless channel, the transmitter first sends a special sounding packet from 

all transmit antennas. The receiver examines the sounding packet at each receive antenna, extracts the wireless 

channel information, and sends that information back to the transmitter. 

Explicit beamforming is an optional mode defined in 802.11n and requires the support of the client (receiver). 

The channel sounding protocol incurs some overhead. Still, explicit beamforming provides the most accurate 

knowledge about the whole channel, from all transmit chains to all receive chains. When available, it should be 

used judiciously. 

Implicit beamforming does not require that a sounding packet be sent. Instead, the channel information is obtained 

by using the symmetry or reciprocity of the channels that is characteristic of Wi-Fi systems. The transmit and 

receive chains at the access point share the same set of antennas, so when an access point receives the uplink 

signal from the client, the access point extracts the c

hannel information from the client’s transmit chains to the 

access point’s receive chains. Further, due to channel reciprocity, the same channel information applies equally 

well to the downlink transmit beamforming. 

Standards-based implicit beamforming augments these capabilities to address two issues. First, implicit 

beamforming is most straightforward when a MIMO client transmits out of all its antennas, which is not always the 

case. When the client transmits out of fewer antennas, the access point cannot measure the wireless channel fully 
and cannot maximize beamforming gain. Second, implicit beamforming requires that a device’s transmit hardware 

and receive hardware be well matched. According to implementations, this may be possible during manufacturing, 

or via internal calibration in the field, or in the worst case may require over-the-air assistance from the client. For 

these reasons, 802.11n defined optional hardware modes where: 

● 

The client can send additional sounding information from its additional t

ransmit chains even if they weren’t 

being used to send actual data. This mode, when supported, has very low overhead. 

● 

The client can assist with calibration of the access point. 

Standards-Based Beamforming for 802.11ac 

In theory, any device with multiple antennas can beamform to any other device at any time. What 802.11ac 

specifies is the opportunity for the receiver to help the beamforming transmitter do a better job of beamforming. 

This is called 

“sounding,” and it enables the beamforming transmitter to precisely align its transmitted energy 

toward the receiver. 802.11ac defines a single, though optional, protocol for one 802.11ac device to sound other 

802.11ac devices. The protocol selected closely (but not completely) follows the 802.11n Explicit Compressed 

Beamforming Feedback (ECBF) protocol, as described below.