While trying to learn more about 802.11ax (Wi-Fi 6) and the various data rates that it can offer, I found an excellent blog post written by François Vergès that explains the theory / math behind calculating the data rates for 802.11n, 802.11ac and 802.11ax. François generously even provided various links to the MCS tables to make it easier for everyone to access them.
To real understand the math behind it, I have decided to build an excel sheet for the MCS table. I felt this was a better option than just skimming through the formulas. Besides, the devil lies in the details…
While preparing the MCS Table Excel Sheet and reading some Wi-Fi 6 documents, I realized the following:
1) 802.11ax supports both OFDMA and OFDM technologies. This explains the two sections for 802.11ax MCS data rates in the excel sheet.
2) 802.11ax defines the use of two multi-user technologies: OFDMA and MU-MIMO. OFDMA allows multiple-user access by subdividing a channel into smaller Resource Units (RU) while MU-MIMO allows for multiple-user access by using different spatial streams. The majority of the efficiency benefits of 802.11ax are a result of multi-user OFDMA.
3) Not all MCS values are allowed:
3.1 ) François’s MCS table had some fields that were empty. 802.11ac supports up to 256-QAM so all 1024-QAM rows for 802.11ac should be blank. This is clear and easy to understand.
3.2) François’s table had some “N/A” values. For example, MCS index 9 for 20 MHz with 1, 2, 4, 5, 7, 8 spatial streams was “N/A” while it had values for MCS index 9 for 20 MHz and 3 and 6 spatial streams client.
This was a bit confusing. I searched online to better understand this and found the below explanation from 802.11ac: A Survival Guide. This clearly explained the above. Some MCS values are invalid as per the standard.
3.3 ) This brought my attention to the other columns. Are these values, that I have highlighted in yellow in the excel sheet valid MCS values? I found many resources online that listed them as valid values but the above resource from 802.11ac: A Survival Guide explained that these are not valid values as per 802.11ac standard. Also, the MCS table available at WLAN professionals website listed these values as blank.
At the end, I now have a better understanding of how the data rates are calculated. I realized that this MCS Table Excel Sheet might be useful for others as well in case they need to have a local MCS table excel sheet which they can tune / edit as needed. That’s why I am sharing it here. Feel free to share your comments.