如何计算MCS Data Rates

First we need to understand how the MCS data rates are calculated prior 802.11ax. I am only going to focus on 802.11n (HT) and 802.11ac (VHT) here. 

 

Here is the formula we can use to calculate which data rate is used for both 802.11n and 802.11ac:

 

 

Let's detail each of these variables and which value they can have for both 802.11n and 802.11ac:

 

HT and VHT OFDM Parameters

Now, the formula doesn't change much with 802.11ax. However, some new features will impact the way we calculate data rate for 802.11ax:

• 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 OFDMA.

 

Even though the formula doesn't change much, the IEEE does define 2 different formulas depending on if OFDMA is used or not. When OFDMA is not used, we can used the formula previously presented above.

 

Here is the formula we can used when OFDMA is used (it is pretty much the same except that we define the number of data subcarriers per RU and not per channel):

 

Let's now details each of these variables and which values they can have when HE (802.11ax) is used. 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.

 

HE OFDM Parameters

 

HE OFDMA Parameters

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 5/6

Example

So now that we have this information, let's try to understand the data rate that my phone was using.

The phone is a Samsung GS10 which supports 802.11ax and up to 2 spatial streams. The AP used is an Aerohive AP630. I have configured it with an 80MHz wide channel. OFDMA is not used here because ODFMA was not activated at the time of this capture.

 

So based on this information, we can determine some of the variables required to calculate the data rate and narrow down the data rates that will be used by this device:

• Number of Data Subcarriers for an 80MHz wide channel: 980
• Number of Coded bit per subcarrier (Modulation): we don't know yet
• Coding: we don't know yet
• Number of Spatial Streams: 2
• OFDM Symbol Duration: 12.8µs
• Guard Interval: we don't know yet

So here is the list of possible data rates used by this device when connecting to this AP:

 

Because we know that the data rate used was 1200.95 Mbps (as indicated on the picture above), we can now determine that:

• MCS 11 was used
• 1024QAM with a coding of 5/6 ​was being used
• A guard interval of 0.8µs was used