802.11ac and a bit of 802.11ax #CWAP11

This blog post will be focusing on 802.11ac in particular. We visited the aspects of 802.11n in the last blog post.

802.11ac introduced the VHT (Very High Throughput) along with some core technological advancement like MU-MIMO, 256 QAM addition & support for 80MHz/160MHz channels. One of the key differences also lie in the support of only 5GHz band. So there is still a dependency on 802.11n for 2.4Ghz support, however the upcoming 802.11ax will support both 2.4GHz and 5GHz.

Multi-user MIMO

• One of the greatest potential of 802.11ac
• Prior to this all the 802.11 standards used single user.
• If there are two receivers located in sufficiently different directions, a beamformed transmission may be sent to each of them at the same time.
• Enables better spatial reuse. As per the below example, the MU-MIMO builds on small-cell approach by enabling even more tightly packed networks. As a result AP can send independent transmissions within its own coverage area. Just as 802.3(Ethernet) reduces collision domains, MU-MIMO intends to reduce spatial contention of transmissions.

802.11ac Wave 1 and 2 – The first wave of 802.11ac products will be driven by the enthusiasm for higher speeds. APs will typically have three stream capabilities, but with 802.11ac providing 80 MHz channels and 256-QAM modulation, the speed will go from 450 Mbps to 1.3 Gbps. The second wave of 802.11ac products will add even wider channels and possibly even multi-user MIMO support, as outlined in the figure below.

The PHY

#Channels

• OFDM based transmission, 802.11ac divides the channel into OFDM sub carriers each 312.5kHz
• To increase throughput, 802.11ac introduces two new channel widths. Supports 80MHz and further added 160MHz channel option for even higher speeds.
• 802.11ac channels have exactly the same shape as previous OFDM channels (802.11a,g,n)
  

MCS & GI

• MCS Index tends to be much simpler than 802.11n. First 7 are mandatory and others are supported.

• 802.11ac retains the ability to select a shortened OFDM guard interval if both Tx and Rx are capable of processing it. The GI shrinks from 800ns to 400ns, providing a 10% boost in the throughput.

VHT Signal Fields

The purpose of the Signal Field is to help the receiver decode the data payload, which is done by describing the parameters used for transmission. 802.11ac separates into Signal A and Signal B fields. For CWAP purposes this has not been dealt in depth. There are 2 parts in VHT Signal A field are referred as VHT-SIG-A1 & VHT-SIG-A2.

SIGNAL A

• Bandwidth
  • 0 – 20MHz, 1 – 40MHz, 2- 80MHz & 3 – 160MHz
• STBC
  • If the payload is encoded with STBC (Space-time block coding may be used when the number of radio chains exceed the number of spatial streams, it tx a single data stream across 2 spatial streams.) for extra robust-ness, this field is set to 1, otherwise will be 0.
• Group ID
  • Frames to AP > group ID =0
  • Frames sent to STA Client > group ID = 63
• Number of space-time streams
  • Starts from 0, e.g. if field is set to 3, then there are 4 space time streams.
• Partial AID
  • Last 9 bits of the BSSID.
• Transmit power save forbidden
  • Field will be 0, if AP in network allows client to power off radios when they have opportunity to transmit frames. Otherwise will be 1.
• Short GI – Field set to 1 for 400ns, 0 for otherwise.
• Short GI disambiguation – Extra symbol may be required denoting 1 or 0 for not required.
• Coding – Field is 0 when convolutional coding is used to protect the data field, 1 when LDPC is used.
• LDPC Extra Symbol – Field is set to 1 if extra symbol is required.
• MCS – MCS Index value of the payload.
• Beamformed – If matrix is applied to the transmission, the bit is set to 1 otherwise set to 0.
• CRC – Error correction
• Tail – 6 zeros are included to terminate the convolutional coder that protects the Signal A field.

SIGNAL B

• Used to setup the data rate, as well as tune in the MIMO reception.
• VHT Signal B Length (17, 19 or 21 Bits)
• Reserved bits – Set to 1.
• Tail bits

Air Magnet Pro can help you scan through the PHY frames

The MAC

Frame aggregation was introduced in 802.11n, 802.11ac however adds an interesting new take on the aggregation. All frames transmitted use the aggregated MPDU (A-MPDU) format. Even the single frame transmitted in one shot is transmitted as aggregate frame.

Management Frames

• VHT Capabilities Information element.

• VHT Operations Information element

NOTE: Greenfield mode was offered with 802.11n. The efficiency gains from greenfield mode were often lost because airtime-devouring CTS-to self

messages were required before transmitting in the greenfield mode. As a result, greenfield mode was removed from 802.11ac.

Beamforming Basics

• As 802.11ac beamforming is based on explicit channel measurements, both the transmitter and receiver must support it.
• Any device that shapes its transmitted frames is called beamformer, receiver of such frames is called beamformee.
• The AP initiates frame exchange with the STA, which helps it to measure the channel. The result of the channel measurement is a derivation of the steering matrix.
• Steering Matrix describes how to setup each element of transmitter’s antenna system to precisely overlap transmissions to reach farther.
• To steer transmissions in a particular direction, a beamformer will subtly alter what is transmitted by each array. A simple phase shift can alter/steer the transmission.

Null Data Packet (NDP) – Standardizes beamforming methods. 802.11ac method of beamforming is termed as null data packet sounding. Sounding is the term used to denote the process  performed by the transmitter to acquire channel state information (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.

VHT beamformer shall initiate a sounding feedback sequence by transmitting VHT NDP announcement frame followed by a VHT NDP after a SIFS.

SU Beamforming

• Begins with the beamformer sending a NDP announcement packet followed by NDP. The NDP has fixed known format. The beamformee receives the NDP, analyzes it and computes back in form of feedback matrix. The feedback matrix is sent in reply to the NDP in the form of compressed beamforming frame (CBF).

MU Beamforming

• As opposed to Tx to one device, MU-MIMO Aps are capable of simultaneously transmitting data to multiple device groups.
• The key distinction between them is that with MU-MIMO beamforming and beamformer requires a response from all beamformees in order to conclude channel sounding.
• The CBF packet is 802.11 action frame which contains a channel matrix that specifies the CSI for each client. The CBF is the largest contributor to the overhead caused by MU-MIMO transmission and is size is determined by
  • Channel Width
  • Number of radio chain pairs
  • Bit count of each CSI unit

Recommended Reading

Cisco 802.11ax White Paper
Wifi Certified 6 Highlights
802.11 Framing in Detail
802.11ac Channel Planning
802.11ac VHT PHY
Research Paper on VHT MU-MIMO
802.11ac – A Survival Guide