Beamforming

Beamforming in 802.11ac

Excerpt from http://chimera.labs.oreilly.com/books/1234000001739/ch04.html.

In essence, multi-user MIMO works by taking advantage of beamforming to send frames to spatially diverse locations at the same time, building the first standardized version of an 802.11 “switch.” 
The beamforming in the downlink direction from the AP to the client was a ripe area for innovation in the 802.11ac standard.

Beamforming Basics

Traditionally, access points have been equipped with omnidirectional antennas, which are so named because they send energy in all directions. Frequently, omnidirectional coverage will be shown as a circle on an overhead-view map, centered on the AP. 

An alternative method of transmission is to focus energy toward a receiver, a process called beamforming. Provided the AP has sufficient information to send the radio energy preferentially in one direction, it is possible to reach farther. 

Beamforming increases the performance of wireless networks at medium ranges. At short ranges, the signal power is high enough that the SNR will support the maximum data rate. At long ranges, beamforming does not offer a substantial gain over an omnidirectional antenna, and data rates will be identical to non-beamformed transmissions.

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.

It is possible to measure the channel and determine how to best use the available transmit power to reach a client device.  The AP is sending higher-level data such as IP packets to a laptop as the recipient. The process begins by measuring the radio channel between the two devices in a calibration process. Although in general beamforming may be either explicit or implicit, depending on whether special channel measurement frames are used, in 802.11ac the standard form of beamforming requires the use of channel measurement frames and is only explicit.

Because 802.11ac beamforming is based on explicit channel measurements, both the transmitter and the receiver must support it.

Any device that shapes its transmitted frames is called a beamformer, and a receiver of such frames is called a beamformee. 802.11 defines new terms for the sender and receiver of beamformed frames because in a single exchange it is possible to have only one initiator and one responder, but a station may be both a beamformer and a beamformee.

In a frame exchange between two devices, either side may choose to calibrate the channel for beamforming purposes; when client devices have large amounts of data to transmit, the standard allows them to calibrate the channel to steer transmissions toward their serving AP.

Null Data Packet (NDP) Beamforming in 802.11ac

One of the biggest changes between 802.11n and 802.11ac is that beamforming has been dramatically simplified. To avoid a repeat with 802.11ac, engineers writing the specification settled on just one method of beamforming, called null data packet (NDP) sounding. 802.11ac’s second wave of products will introduce multi-user MIMO, an application of MIMO techniques that allows simultaneous transmission to multiple clients.

Channel measurement (sounding) procedures

Beamforming depends on channel calibration procedures, called channel sounding in the 802.11ac standard, to determine how to radiate energy in a preferred direction.  Mathematically, the ability to steer energy is represented by the steering matrix, which is given the letter Q in 802.11ac. Channel sounding consists of three major steps:

1. The beamformer begins the process by transmitting a Null Data Packet Announcement frame, which is used to gain control of the channel and identify beamformees. Beamformees will respond to the NDP Announcement, while all other stations will simply defer channel access until the sounding sequence is complete.
2. The beamformer follows the NDP Announcement with a null data packet. The value of an NDP is that the receiver can analyze the OFDM training fields to calculate the channel response, and therefore the steering matrix. For multi-user transmissions, multiple NDPs may be transmitted.
3. The beamformee analyzes the training fields in the received NDP and calculates a feedback matrix. The feedback matrix, referred to by the letter V in the 802.11ac specification, enables the beamformer to calculate the steering matrix.
4. The beamformer receives the feedback matrix and calculates the steering matrix to direct transmissions toward the beamformee.

With the steering matrix in hand, the beamformer can then transmit frames biased in a particular direction. If the transmitter applies a steering matrix, however, the array will send energy in a way that prefers one path. Channel sounding procedures do have a cost in airtime because the sounding exchange must complete before a beamformed transmission can be sent.

Single-User (SU) Beamforming

Single-user beamforming is readily understandable because its purpose is to shape a transmission from a single transmitter to a single receiver.
The beamformer sends a null data packet, which is a frame with a known fixed format. By analyzing the received NDP frame, the beamformee calculates a feedback matrixthat is sent in a reply frame. 

Channel Calibration for Single-User Beamforming

The channel calibration procedure is carried out as a single operation, in which the beamformer and beamformee cooperatively measure the channel to provide the raw data needed to calculate the steering matrix. The sounding procedure does not transmit the steering matrix directly, but instead works to exchange all the information necessary for the beamformer to calculate its own steering matrix.

NDP Announcement frame

The channel sounding process begins when the beamformer transmits a Null Data Packet Announcement frame, which is a control frame

The entire channel sounding process is carried out in one burst, so the duration set in an NDP Announcement corresponds to the length of the full exchange of three frames. In single-user MIMO beamforming, the NDP Announcement frame relays the size of the feedback matrix by identifying the number of columns in the feedback matrix.

The main purpose of the NDP Announcement frame is to carry a single STA Info field for the intended beamformee. The STA Info field is two bytes long and consists of three fields:

AID12 (12 least significant bits of the intended beamformee’s association ID)

Upon association to an 802.11 access point, client devices are assigned an association ID. The least significant 12 bits of the beamformee’s association ID are included in this field. When a client device acts as a beamformer, this field is set to 0 because the AP does not have an association ID.

Feedback Type

In a single-user NDP Announcement frame, this field is always 0.

Nc Index

This index describes the number of columns in the feedback matrix, with one column for each spatial stream. As a three-bit field it can take on eight values, which matches the eight streams supported by 802.11ac. This field is set to the number of spatial streams minus one.

NDP frame

Upon transmission of the NDP Announcement frame, the beamformer next transmits a Null Data Packet frame. The reason for the name “null data packet” should be obvious in looking at the frame;

a PLCP frame with no data field, so there is no 802.11 MAC frame. Channel sounding can be carried out by analyzing the received training symbols in the PLCP header, so no MAC data is required in an NDP. Within an NDP there is one VHT Long Training Field (VHT-LTF) for each spatial stream used in transmission, and hence in the beamformed data transmission.