802.11ax – Uplink and Downlink Operations

Wifi 6 enhances the 802.11ac technology by improving the Network Efficiency with higher data rates.

Here are the key features which 802.11ax provides over 802.11c.

Let’s dive in a little –

Parameters	802.11ac	802.11ax
Spectrum	No Support for 2.4 GHz Supports 5GHz only	Supports 2.4 GHz Supports 5 GHz Supports 6 GHz (extended)
Bandwidth	20 MHz, 40 MHz, 80 MHz, 160 MHz	20 MHz, 40 MHz, 80 MHz, 160 MHz
Modulation	BPSK to 256 QAM	BPSK to 1024 QAM
MIMO	Single-User MIMO Downlink-Multi-User MIMO	Single-User MIMO Uplink-Downlink-Multi-User MIMO
Guard Interval	Long and Short Guard Interval	Long, additional guard interval available for outdoor channels Short Guard not available
Backward Compatibility	802.11 a/n	802.11 a/b/g/n/ac
Reducing Power Consumption Mechanism	Not Available	TWT (Target Wake Time)
Spacial Streams	1 to 8	1 to 8
Pilot Sub-carriers	4/6/8/16	2/4/6/8/16
Sub-Carrier Spacing	312.5 kHz	78.125 kHz (smaller value to increase the coverage for OFDMA systems)

What is OFDMA and how is it different that previously used OFDM ?

Few key terms that we will be coming across.

Subcarrier – A Subcarrier is a signal frequency (low frequency) that carries additional information which is modulated with the carrier signals (higher frequency) to travel the medium.
Types of subcarriers,
1) Pilot Subcarriers – These are just used for synchronization between the receiver and the transmitter.
2) Data Subcarriers – These are used to carry data.
3) Guard Subcarriers – These are null sub-carriers which are present at the edge of the band to protect from adjacent channel interference.

Resource Units (RU) – Resource units are a group of subcarriers (also referred as ‘Tones’).
Each subcarrier has a bandwidth of 78.125 kHz and is used in Uplink and downlink transmissions.
The minimum size of a Resource Unit can carries up to 26 subcarriers. This is also referred as 26-Tone RU.
There can be a maximum of 9 Resource Units (RU’s) in 20 MHz bandwidth.

OFDM – It is a digital modulation scheme with the help of which the radio signals can carry useful information in a single channel. While a single high frequency carrier is sent, OFDM technique uses multiple subcarriers (at a low frequency) that carry data information to be sent in parallel. These subcarriers are modulated using modulation techniques (like QPSK or QAM).
Currently 802.11a,g,n and ac use OFDM mechanism for single user data transmission.
With 802.11a/g/n/ac, there is subcarrier spacing of 312.5 KHz.

With the OFDM technique, there are 64 subcarriers, from which 52 are data subcarriers, 4 are pilot subcarriers and 8 used as Guard bands.

With OFDM, there is a limitation that access points can only process one users’ data transmission at a time for each channel.

OFDMA – OFDMA is build on the concept of OFDM, but using a better mechanism by transmitting multiple user data at the same time.
OFDMA divides the existing 802.11 channels (20, 40, 80 and 160 MHz wide) into smaller sub-channels with a predefined number of subcarriers.

OFDMA uses a group of Resource Units for data transmission.

With 802.11ax, each subcarrier has a bandwidth of 78.125 kHz (subcarrier spacing) and is used in Uplink and downlink transmissions.
The minimum size of a Resource Unit can carries up to 26 subcarriers. This is also referred as 26-Tone RU.
There can be a maximum of 9 Resource Units (RU’s) in 20 MHz bandwidth.

Below table expands more on the number subcarriers and how many RU’s can be used in various bandwidth combinations.

RU Type	Pilot Subcarrier	Data Subcarrier	20 MHz Channel Bandwidth	40 MHz Channel Bandwidth	80 MHz Channel Bandwidth	80+80 MHz and 160 Channel Bandwidth
26-Tone RU	2	24	9	18	37	74
52-Tone RU	4	48	4	8	16	32
106-Tone RU	4	102	2	4	8	16
242-Tone RU	8	234	1	2	4	8
484-Tone RU	16	468	N/A	1	2	4
996-Tone RU	16	980	N/A	N/A	1	2
2×996-Tone RU	0	1960	N/A	N/A	N/A	1

Below diagrammatical representation shows how the subcarriers (in OFDM) and subcarrier block (called Resource Units – used in OFDMA) transmits data with respect to frequency and time.

MU-MIMO/OFDMA Uplink Operation

For Downlink Operation, AP can assign RU’s based on the fact that it knows which user is getting what so allocation decision can be made easily, but for uplink traffic how would it decide how many RU’s to be allocated for each user.

Depending on the User capabilities and usage, AP decides to assign different user with suitable RU’s.

Details on how the transmission takes place:

1. To coordinate uplink MU-MIMO or uplink OFDMA transmissions the AP sends a trigger frame to all users.
2. This Trigger frame specifies
   a) the length of the UL window
   b) the users that may send during the UL window
   c) Allocates resources for the UL-MU PPDUs
   – RU allocation
   – Spacial Stream allocation
   – MCS to be used by the user
   d) Power control information, such that individual users can increase or reduce their transmitted power, to equally maintain the power that the AP receives from all uplink users and improve reception of frames from devices farther away.
   With the multi-user uplink trigger frame, AP instructs all users when to start and stop transmitting the frames to maintain synchronization between the users.
3. Once the AP receives the frames from all users, it sends them back a block ACK to finish the operation.

In my testbed, I am using 2 .11ax supported clients,

Windows – 68:54:5a:3e:44:8c (AX201)
IPhone – de:c7:d3:cc:33:50
AP’s BSSID – 00:4e:35:c7:06:50

Here is the user table on the controller.

Here is the user association table on the controller.

Both these clients IPhone – de:c7:d3:cc:33:50 and Windows – 68:54:5a:3e:44:8c have association ID of 2 and 3 respectively.

Let’s take a closer look into the frames,

Basic Trigger Frame

Common info – This field will carry the key informations for stations to prepare the frames to be sent.

User Info Field – This field carries information of the client that will be participating in the OFDMA transmission.

Wireshark Basic Trigger Frame Filter – wlan.trigger.he.trigger_type eq 0

Common Info :

User Info –

Multi-STA Block ACK

AP Sends a Block ACK to acknowledge the Data received by multiple stations, using a Multi-STA block ACK.

Wireshark filter – (wlan.fc.type_subtype == 0x0019) and (wlan.ba.control == 0x0016)

MU-MIMO/OFDMA Downlink Operation

Both OFDMA and MU-MIMO enable simultaneous transmissions to multiple users but they are designed for different types of applications.
MU-MIMO results in higher speeds per user and is ideal for high-bandwidth applications.
Unlike the MU-MIMO which increases capacity, OFDMA increases efficiency, reduces latency and is ideal for low-bandwidth applications e.g. Voice applications.

HE MU PPDU:

To capture these frame you need to capture radiotap header.

Wireshark filter – radiotap.he.data_1.ppdu_format == 0x2

References:

https://en.wikipedia.org/wiki/Resource_Unit#:~:text=Resource%20Unit%20(RU)%20is%20a,be%20applied%20to%20different%20RUs.

CWAP – Study Guide

Aruba’s 802.1ax tech notes