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

CTS 072: Guide to Using External Antennas

Why should you use external antennas in your Wi-Fi environment? There are many reasons you should use them. In this episode, we talk about what to look for in external antennas, what to watch out for when deploying them, and what details to keep in mind.

We are used to internal antennas from our access points. Have you considered using external antennas to meet your Wi-Fi requirements? In this episode, we provide some educational material around external antennas. Yes, they do add some cost to a project but it’s not much compared to the solution they bring to your Wi-Fi network. There are use cases for external antennas and below are the topics we talk about during the episode.

This episode is proudly sponsored by AccelTex Solutions. They are providing a special offer just for our listeners. If you head over to their products page and use promo code CTS Podcast 72 they will send you a free antenna sample of your choice.

AccelTex Solutions

Episode Contents

  • Why use antennas, other than the ones built into the AP?
    • Aesthetics
    • Direct signal
    • To meet requirements
    • Use outdoor
  • Antenna types
    • Omni
      • Signal in 360 degrees
    • Semi-directional
      • Ranges in degrees such as 65 degrees, 120 degrees, etc
      • Patch
    • Directional
      • Dish
      • Yagi
  • Antenna details
    • Beamwidth
    • Azimuth and elevation pattern
      • Lobes
    • Gain
      • Passive
    • Connectors
      • RPTNC
      • RPSMA
    • Elements
      • Isolation
        • Antennas hearing each other
      • Horizontal
      • Vertical
    • Orientation
    • Antenna Polarization
      • Important for indoors?
        • MIMO
      • Outdoor PtP links
  • Design
    • When designing Wi-Fi be sure to use the right antennas
  • Deployment
    • Mount the antennas properly
    • Get the right angle
    • Consider spacing between antennas
    • Don’t connect two antennas to one access point, unless you’re using something like a DART connector on the Cisco 3802E
  • Personal experience
    • High density
    • Outdoor
    • Special use cases

Sample Radiation Pattern for AccelTex ATS-OP-245-47-6RPSP-36

Sample Radiation Pattern for AccelTex ATS-OP-245-47-6RPSP-36


RF Elements Antenna Propagation Patterns

RF Elements Antenna Propagation Patterns


Meraki MR53 2.4 GHz antenna pattern

Meraki MR53 2.4 GHz antenna pattern

Links and Resources

This Week In Wireless