wi-fi 6

CTS 167: 802.11ax 1024-QAM & HE-MCSs

1024-QAM

Evolution of the modulation techniques we are using today with 802.11ax (256-QAM).

With 1024-QAM, we are now able to encode 10 bits per cycle on each subcarrier. The way we are able to do that is by increasing the number of different levels of amplitudes used to encode the data.

If you want to learn more about the different types of modulations used by Wi-Fi and how they work, there is a great video where Keith Parsons explains it on youtube: https://www.youtube.com/watch?v=W5DMfEuY2Vg&t=8s

Due to the addition of a new modulation technique (QAM-1024), 2 new MCS indexes are now available with 802.11ax:

  • Index 10: when the 1024-QAM modulation is used with a coding of 3/4
  • Index 11: when the 1024-QAM modulation is used with a coding of ⅚

FEC (Forward Error Correction). Send more than the data bits. If you lose some of the sequence, the remaining bits will help you to understand what you were supposed to be sent.

What is the challenge with more complex modulation techniques?

How well will 1024-QAM in real life?

Will we be able to take advantage to it?

Interesting talk on twitter around the subject (Troy Martin, Andrew, Hendrik Lüth & Jim Vajda). We tend to think that a smaller communication bandwidth will give us a better SNR. But it is not necessarily the case here since the receiver in 802.11ax will still be listening to the whole 20MHz wide channel even if its RU is smaller. Link here: https://twitter.com/VergesFrancois/status/1113779145731977216

HE-MCSs

Download the updated MCS Table

With 802.11ax, we are getting a whole new set of data rates (or MCSs). If we want to understand why, we need to understand how these data rates are calculated.

The amount of data we can transfer through a Wi-Fi link will depend on:

  • The channel width (or the number of subcarriers)
  • The modulation and coding use
  • The amount of spatial streams used
  • The guard interval used
  • The duration of the symbol

And we can actually take all these different variables and calculate the different data rates using the following formula:

802.11n/ac Data Rate Formula

You can take a look at the blog post and see which values can each of these variables can have for 802.11n and 802.11ac:

HT & VHT Parameters

Now, the reason why we have a new set of data rates for 802.11ax is because some of the key variables are changing:

  • A new symbol duration is used: 12.8µs
  • Different Guard Intervals are used: 0.8µs, 1.6µs and 3.2µs
  • The size and number of data subcarriers is not the same (especially with the different RU sizes introduced by ODFMA.

Also, with the introduction of OFDMA and the use of Resource Units, we might be using smaller numbers of subcarriers which impact the data rates.

This is why the draft identify the OFDMA and non-OFDMA MCS differently.

The draft even use specific variables related to each resource unit. And we can therefore define this new formula:

802.11ax ODFMA Data rate Formula

And here are the different values each of these variables can have for 802.11ax communications. The first table details the parameters used when OFDMA is not used. The second table details the parameters when OFDMA and resource units are used.

802.11ax OFDM Parameters
802.11ax OFDMA Parameters

Sections Talking about MCS in The Standards & Draft

  • 28.3.7 HE Modulation and coding schemes (HE-MCSs) (p.442)
  • 28.5 Parameters for HE-MCSs (p. 589)
  • 19.5 Parameters for HT MCSs
  • 21.5 Parameters for VHT-MCSs

Resources


CTS 166: 802.11ax HE Channel Access

There are multiple functions in which a client or AP can gain access to the channel. What we’re going to discuss is in addition to the HCF and DCF.

  • TXOP duration-based RTS/CTS
  • Intra-BSS or inter-BSS frame determination
  • SRG PPDU determination
  • Two NAVs
  • MU-RTS/CTS
  • EDCA using MU EDCA parameters

GOOD NEWS, CTS IS NOT GOING ANYWHERE!

It’s all new ways of thinking about channel access but with added complexity. You’re dealing with multi-access, TXOP, BSS coloring which complicates the channel access process.

TXOP Duration-based RTS/CTS

HE AP can use TXOP duration-based RTS/CTS exchanges to mitigate interference for dense environments.

RTS/CTS is now under TXOP Duration RTS Threshold.

There is a TXOP Duration RTS Threshold subfield within the HE Operation element. To disable TXOP Duration RTS Threshold, the AP can set this subfield to a value of 1023. A client receiving a non-zero value from the AP will set its TXOP Duration RTS Threshold to the value of the TXOP Duration RTS Threshold subfield.

A client shall use an RTS/CTS exchange to initiate a TXOP if the feature is enabled and:

  • The client intends to transmit unicast frames to the AP or to a TDLS peer client
  • The transmission opportunity duration is greater than or equal to 32 μs * RTS/CTS threshold. (Ex: RTS/CTS threshold = 300 & duration is 12345. Then 12345 μs > 32*300=9600)

The RTS and CTS frames contain a Duration field that defines the period of time that the medium is to be reserved to transmit the actual Data frame and the returning ACK frame.

The AP can also send a MU-RTS and simultaneous CTS responses by clients prior to the actual Data frames is another means of distribution of the medium reservation information.

Otherwise, the client follows the normal DCF rules used today by 802.11ac.

Intra-BSS and Inter-BSS Frame Determination

Client determines if received PPDU is an inter-BSS PPDU based on the following:

  • BSS_COLOR is not 0 and is not the same BSS color which the client is a member. Pretty much is the client doesn’t have the same BSS color.
  • BSS_COLOR is not 0 and if HE client is associated with a non-HE AP. If the client is not associated to a 802.11ax AP

Client determines if received PPDU is an intra-BSS PPDU based on the following:

  • BSS color is the same as the client
  • The frame carries an RA/TA/BSSID field value that is equal to the BSSID the client is associated with

Essentially, in order for a transmitter to determine whether it can obtain access to the shared medium, it must determine if a received frame is within its own BSS color or not.

SRG PPDU Identification

A client looks to the Spatial Reuse Parameter Set element to see if the SRG Information Present subfield has been set to 1. It’s used to identify BSSs that are members of the client’s SRG.

It will be used by the client to identify if the received inter-BSS frame is a SRG frame.

It is used during Spatial Reuse Parameter (SRP) OBSS (Overlapping Basic Service Set) PD operation. We will have a dedicated episode on Spatial Reuse.

Updating Two NAVs

HE clients must maintain two NAVs. A HE AP can maintain two NAVs. Those two NAVs are:

  • intra-BSS NAV
  • Basic NAV

Within the intra-BSS PPDU will be the intra-BSS NAV. The basic NAV, as we have known it previously, can be updated within the inter-BSS PPDU or a PPDU that is not classified as an intra-BSS or inter-BSS. That classification is covered under Intra-BSS or Inter-BSS frame determination – which we just talked about earlier.

The purpose of two NAVs is to protect frames from clients within the intra-BSS and inter-BSS in dense environments. It gives more control to the AP in order to decide which clients will contend to the channel within a given TXOP.

As HE AP obtains TXOP, the intra-BSS NAV prevents associated clients from contending for the channel. Because the basic NAV is not updated during AP’s TXOP, the client will not transmit even if it received a trigger frame from the AP.

If both NAVs equal zero, the medium is idle. If one of the two NAVs is nonzero, the virtual CS is that the medium is busy. EASY RIGHT? ^^

Intra-BSS NAV Updates:

The intra-BSS NAV will be updated if a client receives/sends a intra-BSS frame with a duration higher than the current value of the intra-BSS NAV within a given TXOP.

The intra-BSS NAV will also be updated if the client receives a trigger frame from the AP.

Basic NAV Updates

The basic NAV will be updated if the inter-BSS frame RA is not the address of the STA. Still using the Duration ID Field.

MU-RTS/CTS Procedure

The MU-RTS/CTS procedure allows an AP to initiate a TXOP and protect the TXOP frame exchange.

The AP transmits an MU-RTS/CTS trigger frame to solicit simultaneous CTS responses from one or more HE STAs.

MU-RTS

The STA will send a MU-RTS and expect to receive a CTS from 1 or multiple STA. Interesting note about MU-RTS, if the STA’s CTS Timeout interval reaches 0 it will assume the MU-RTS Trigger frame has failed and will invoke a backoff procedure.

CTS Response to a MU-RTS

The client shall response if the following conditions are met:

  • The medium is idle
  • The RU allocation is specified in the User Info field
  • There is a User Info field addressed to the client

Otherwise, the client show not send the CTS response.

The CTS will be carried in a non-HE PPDU so everyone can understand it. The data rate used should be 6Mbps.

It should be transmitted on the 20MHz wide channel specified in the RU Allocation subfield of the User Info field of the MU-RTS Trigger frame.

It’s important to know in MU-RTS, an AP can receive simultaneous CTS responses from clients.

EDCA Operation Using MU EDCA Parameters

A MU-EDCA Parameter Set Element might be included in the beacon frames, probe response and re(association) frames.

It can be a copy of the EDCA Parameters and can also be updated.

This is mainly used for QoS purposes. There is a QoS Info field within the (MU-)EDCA Parameter Set Element.

A client receiving a basic trigger frame containing addressed for itself will update various contention window values for the respective access categories.

The client station shall use the parameters set in the mostly recently received MU-EDCA. Including CWmin, CWmax, AIFSN and MUEDCATimer.

Closing Note (From Cisco 802.11ax White Paper)

“Because of this preamble-level compatibility, there is no inherent need for 802.11ax devices to precede their 802.11ax transmissions by CTS-to-self or RTS/CTS, although devices may still choose to implement and send them to protect longer PPDUs. However, 802.11ax adds the capability of multiuser RTS/CTS which allows the access point to reserve the channel (set the NAV) for multiple STAs simultaneously with a single MU-RTS PPDU that is then confirmed with simultaneous CTS PPDUs from multiple STAs. This scenario overcomes the inherent inefficiency of single-user RTS/CTS still prevalent in 802.11ac networks while adding protection to 802.11ax transmissions.”

Sections in the 802.11ax Draft Talking about HE Channel Access

  • 27.2 – HE channel access (p.253)
  • 10.3.1 DCF General

This Week in Wireless

CTS 164: 802.11ax Target Wake Time

Objectives of TWT

Objectifs of 802.11ax

  • Increase the performance of the Wi-Fi network by a factor of 4 while improving or not impacting power requirements
  • Provide power saving mechanisms for new emerging IoT devices

All the current power saving mechanisms defined today remain usable with ax. In addition, the 802.11ax draft defines a new mechanism called Target Wake Time or TWT.

Target wake time (TWT) is used to help minimize contention between clients and reduce the amount of time a client in power save mode to be awake.

Clients will operate in non-overlapping times and/or frequencies and the frame exchanges are coordinated.

TWT was introduced in 802.11ah and is particularly useful for battery-powered devices that communicate infrequently.

TWT Modes of Operations

There are three modes of operation:

  • Individual TWT
  • Broadcast TWT
  • Opportunistic PS

TWT Power-save options in 802.11ax from Aruba 802.11ax White Paper:

TWT Power-Save options in 802.11ax

Individual TWT

Client will be assigned specific times to wake up and exchange frames. The schedule is determined and delivered by the AP. There is a different mode of TWT such as explicit TWT. A client doesn’t need to know about another client’s TWT values.

Simple process:

  • A client wants to establish a TWT agreement
  • A client communicates its waking schedule information to the AP
  • The AP devises a schedule and delivers TWT values to the client
  • The client wakes up and transmit a frame according to the schedule
  • The AP send the client the next TWT information on when to wake up again (explicit mode)
  • The client wakes up again at the next scheduled time to send a frame and receive a new TWT information
  • When TWT implicit is used, the client calculates the Next TWT by adding a fixed value to the current TWT value

A client can go off of the AP’s TWT parameters. Or a client can “demand” a TWT with indicated parameters for agreement. If agreed upon, the AP will respond with “Accept TWT”. The AP can counter the offer with an Alternate TWT.

A client wanting to utilize TWT will indicate what channel to use as a primary channel during a TWT SP.

Broadcast TWT

The AP will be in charge. The AP will send TWT parameters in the Beacon frame using the TWT Element. The TWT Element might be sent in other management frames as well such as the (Re)Association frame or the probe response frame.

The clients will use the TWT parameters from the most recently received TWT element carried in the Management frames of its associated AP. The client is also called “TWT Scheduled STA” in this case in the draft. The AP is called “TWT Scheduling AP”

The AP will provide the schedule to all the clients that supports broadcast TWT.

The AP will send a trigger frame to discover which clients are awake. The AP will then send frames to these clients that will, then, be able to doze again. This is called a trigger-based TWT SP (Service Period).

Clients can device to join or leave a broadcast TWT. This is done by an exchange of frame that carry TWT elements.

Find the information in 802.11ax Frames

Subfields to identify client support of TWT modes:

  • TWT Requester Support
  • TWT Responder Support
  • Broadcast TWT Support

TWT Element

The Control field will indicate the negotiation type: Individual, Broadcast or Wake TBTT interval.

When Individual TWT is used, the TWT parameter Information field will have this format:

When Broadcast TWT is used, the TWT Parameter Information field will have this format:

Sections Talking About TWT in the 802.11ax Draft

9.4.1.60 – TWT Information field – p.118

9.4.2.200 – TWT element – p.139

9.6.25.9 – TWT Teardown frame format – p.185

10.43 – Target wake time (TWT) – p.234

27.7 – TWT Operation – p.312

Example

Here is an example of a beacon frame coming from a 802.11ax Aerohive AP (AP630). If we look into the “HE Capabilities” information element, we can drill down into the “HE MAC Capabilities Information” and validate both the “TWT Requester Support” and “TWT Responder Support” flags. In this case, they are set to 0, which means that the feature is not yet supported by the firmware used on this AP.

Then, we can take a look at the “TWT Required” flag located in the “HE Operation Parameters”. Here again, it is set to 0.

We should be able to show you example of TWT being supported later on this year as vendors update their 802.11ax AP firmware and start enabling the feature.


This Week In Wireless

CTS 162: 802.11ax OFDMA Subcarriers

With OFDMA in 802.11ax, the size of the subcarriers has been divided by 4. Going from 312.5KHz wide with OFDM to 78.125KHz wide.

The symbol duration has been increased by 4 times in the meantime. Going from 3.2 microseconds with OFDM to 12.8 microseconds.

Zooming into the subcarriers of a 20 MHz channel width

View the full image here

Advantages of having more subcarriers

  1. Allow OFDMA to extend to small sub-channels. Each sub-channel requires at least one (usually two) pilot subcarriers, and with a 2 MHz minimum sub-channel size, a smaller subcarrier spacing loses a much smaller percentage of the overall bandwidth to pilots.
  2. The number of guard and null subcarriers across a channel can be reduced as a percentage of the number of usable subcarriers, again increasing the effective data rate in a given channel. The figures above show a ~10% increase in usable subcarriers compared to 802.11ac, after allowing for the 4x factor. Example: OFDM: 64 subcarriers, 12 GuardNull subcarriers = 18.75%, OFDMA: 256 subcarriers. 22 GuardNull subcarriers = 8.5%.
  3. The longer OFDM symbol allows for an increase in the cyclic prefix length without sacrificing spectral efficiency, which in turn enables increased immunity to long delay spreads, especially in outdoor conditions. The cyclic prefix can be reduced to a smaller percentage of the symbol time, increasing spectral efficiency even while more robust to multipath conditions. And it reduces the jitter-sensitivity of uplink multi-user modes.

The smallest sub-channel is composed of 26 subcarriers.

Type of subcarriers:

  • Data subcarriers
  • Pilot subcarriers
  • DC subcarriers
  • Guard subcarriers
  • Null subcarriers

A 26-tone RU consists of 24 data subcarriers and 2 pilot subcarriers.

A 52-tone RU consists of 48 data subcarriers and 4 pilot subcarriers.

A 106-tone RU consists of 102 data subcarriers and 4 pilot subcarriers.

A 242-tone RU consists of 234 data subcarriers and 8 pilot subcarriers.

A 484-tone RU consists of 468 data subcarriers and 16 pilot subcarriers.

A 996-tone RU consists of 980 data subcarriers and 16 pilot subcarriers.

DC (Direct Current) subcarriers are used for the subcarriers located in the center of the channel. Depending on the channel width and the number of tone used, the number of DC subcarriers can vary (Ex: 3 or 7 for a 20MHz wide channel). Most of the time it will be 7 for the 20MHz and 80MHz wide channels and 5 for the 40MHz wide channels.

A 20MHz wide channels has 11 guard interval: the first 6 and the last 5 of the channel.

Here are the diagrams extracted from the 802.11ax draft document detailing the structure of the subcarriers for each channel width using different RUs sizes:

Links & Resources

Meraki MR55 and MR45 802.11ax (Wi-Fi 6) access points
Meraki MR45 802.11ax (Wi-Fi 6) access point
Meraki MR55 802.11ax (Wi-Fi 6) access point