Last Post: November 11, 2014:
Could you please let me know the MCS table for 11AC ?
What is the significance of MCS 8 and 9 ?
Aruba has said:
The MCS 8 and MCS 9 rates are new and enabled by advances in chip technology. MCS 9 is not applicable to all channel width/spatial stream combinations.
From their whitepaper at:
It's worth getting hold of a copy of the Matthew Gast 802.11ac book to find out more about the standard.
Here is an extract in Google books which details what your after I believe: http://bit.ly/J8fEVb
Here is a nice tool for your referance:
I should have noted that VHT MCS 8 & 9 both use QAM-256. Just guessing, I would bet these both have ridiculously high SNR requirements.
I didn't follow the links; I hope this is not redundant.
MCS 8 and 9 are also different in that they are optional. Only 0 - 7 are required.
There are some combinations of modulation, coding, and channel width that don't work. It isn't just MCS9 with 256-QAM.
I highly recommend Gast's book. In fact, I recommend all of them.
@Howard Gast says 256-QAM requires about 5dB more signal than 64-QAM. I vaguely remember the IEEE saying that it was more like 6, but I have no idea where. Gast also says that we should get a one to two dB increase from LDPC, which is also optional.
If you understand the basics on modulation and coding, here is a great resource many overlook