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
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:
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:
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:
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.
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
- MCS Table Updated with 802.11ax Data Rates: http://www.semfionetworks.com/blog/mcs-table-updated-with-80211ax-data-rates
- Different Types of 802.11 Modulation – Video by Keith Parsons: https://www.youtube.com/watch?v=W5DMfEuY2Vg&t=8s
- Understand Coding in 802.11 communications – Video by Jérôme Henry: https://www.youtube.com/watch?v=vTIy-rjopY8
- “802.11 ODFM Data Rates – The Math Behind The Numbers” by Renzo Notter: http://dot11.exposed/2018/11/29/802-11-ofdm-data-rates-the-math-behind-the-numbers/
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