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802.11ac Transmit Beamforming and VHT NDP sounding procedure

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Transmit beamforming requires the knowledge of the channel state to compute a steering matrix that is applied to the transmitted signal to optimize reception at one or more receivers. The STA transmitting using the steering matrix is called the VHT beamformer and the STA for which the reception is optimized is called the VHT beamformee.
 
Beamforming is directly enabled by the support of ‘sounding’. Sounding is the term used to denote the process performed by the transmitter to acquire CSI from each of the different users by sending training symbols and waiting for the receivers to provide explicit feedback containing a measure of the channel. This feedback is then used to create a weight or steering matrix that will be used to pre-code the data transmission by creating a set of steered beams to optimize the reception at one or multiple receivers.
 
Any device that shapes its transmitted frames is called a beamformer, and a receiver of such frames is called a beamformee. A single device may act both as a beamformer and a beamformee. The process of beamforming involves measurement of the MIMO channel, and as a result of the channel measurement, a derivation of the steering matrix is done. The steering matrix is a precise mathematical description of how the antenna array should use each individual element to select spatial path for the transmission.
 
Types of feedback mechanism -
 
For an HT beamformer to calculate the appropriate steering matrix for transmit spatial processing when transmitting to a specific HT beamformee, the HT beamformer needs to have an accurate estimate of the channel over which it is transmitting. There are two methods which can be used :-
 
  • Implicit feedback: in this method the HT beamformer receives long training symbols transmitted by the HT beamformee. This allows the MIMO channel between the HT beamformer and HT beamformee to be estimated. If the channel is reciprocal, the HT beamformer can use the training symbols it receives from the HT beamformee to make a channel estimate suitable for computing the transmit steering matrix.
  • Explicit feedback: When using explicit feedback, the HT beamformee makes a direct estimate of the MIMO channel from the training symbols sent to the HT beamformee by the HT beamformer. The HT beamformee may prepare CSI or steering feedback based on an observation of these training symbols. The HT beamformee quantizes the feedback and sends it to the HT beamformer. The HT beamformer can use the feedback as the basis for determining transmit steering vectors.

 

In 802.11ac, only explicit beamforming is used, hence both the transmitter and receiver must support it.
 
VHT NDP sounding procedure -
 
A VHT beamformer shall initiate a sounding feedback sequence by transmitting a VHT NDP (Null Data Packet) Announcement frame followed by a VHT NDP after a SIFS. The VHT beamformer shall include in the VHT NDP Announcement frame one STA info field for each VHT beamformee that is expected to prepare VHT compressed beamforming feedback and shall identify the VHT beamformee by including the VHT beamformee’s AID in the AID subfield of the STA Info field. The VHT NDP Announcement frame shall include at least one STA info field.
 
Sounding protocol with a single VHT beamformee
 
Sounding protocol with more than one VHT beamformee
 
VHT NDP Announcement frame format (single user)
 
Upon transmission of the VHT NDP Announcement frame, the beamformer next transmits a Null Data Packet frame shown below. Figure shows 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 needed in a NDP. Within a NDP there is one VHT Long Training Field (VHT-LTF) for each spatial stream used in transmission, and hence in the beamformed data transmission.
 
More specifically, upon reception of the VHT NDP frame each beamformee removes the space-time stream CSD (cyclic shift diversity) applied to the signals transmitted. The CSD consists of a signal shaping technique where different phase shifts are applied to the same signal across different transmit chains. After removing the CSD, the targeted beamformees are required to reply with a VHT compressed beamforming frame. The first intended stations replies immediately whereas the others have to wait to be polled by the beamformer (by using the Beamforming Report Poll). The most relevant information carried by the VHT Compressed Beamforming Frame is as follows -
 
  • The VHT MIMO Control Field which contains the dimension of the matrix, an indicator of the width of the channel in which the measurements used to create the feedback matrix were taken, and information indicating the size of the codebook entries.
  • The VHT Compressed Beamforming Report containing the compressed beamforming feedback matrix in the form of two angels, as well as SNR of each space-time stream averaged over all subcarriers used.
  • MU Exclusive Beamforming Report carrying explicit information used by a multi-user beamformer in order to create the steering matrices.

 

VHT NDP frame format
 
Calculating the feedback matrix -
  • calculating the feedback matrix can begin only after receiving the NDP from the beamformer. Once the NDP is received, each OFDM subcarrier is processed independently in its own matrix that describes the performance of the subcarrier between each transmitter antenna element and each receiver antenna element. The contents of the matrix are based on received power and phase shifts between each pair of antennas.
  • feedback matrix is transformed by matrix multiplication called Givens rotation, which depends on parameters called ‘angels’. Rather than transmitting the full feedback matrix , the beamformee calculates the angels based on the matrix rotation. 802.11ac protocol specifies the order in which these angels are transmitted so that the beamformer can receive a long string of bits and appropriately delimit each angels.
  • Having calculated the angels, the beamformee assembles them into compressed feedback form and returns them to the beamformer. Only one set of angels is required to summarize the radio link performance for all of the OFDM subcarriers. The set of angels can be quite large with a wider channels.
  • The beamformer receives the feedback matrix and uses it to calculate the steering matrix for transmissions to the beamformee.

 

One feedback matrix is sent by each beamformee. In SU beamforming, there is one feedback matrix from the beamformee and one steering matrix used. In MU beamforming, each beamformee send a feedback matrix and the beamformer needs to maintain a steering matrix for each client.
 
When transmitting the feedback matrix, there are three main factors that determine its size. First, wider channels have more OFDM subcarriers, so the feedback matrix must be larger to accommodate them. Second, higher the number of pairwise combinations of transmitter and receiver antennas is, the larger the matrix will be. Finally, 802.11ac allows two different representations of the angels value to enable devices to use higher resolution when necessary. MU MIMO requires higher resolution because of the need to avoid inter-user interference.
 
802.11ac compressed V-Matrix feedback report sizing -
 
References:
802.11ac IEEE standard, 802.11ac: A Survival Guide, Aruba Networks whitepaper (WP_80211acInDepth.pdf)
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