First generation 11ax chipsets will have:

• 125KHz subcarrier spacing, so 256 subcarriers per 20MHz channel.
• 13.6uS symbol time, with 0.8uS guard interval time.
• 1024QAM modulation
• 26, 52, 106, and 242 subcarrier Resource Units (RUs), which equates roughly to 2MHz, 4MHz, 8MHz, and 20MHz channels.
• UL & DL OFDMA with Buffer Status Reports (BSRs)
• Mandatory use of Low Density Parity Checking (LDPC)
• MCS10 and MCS11 options
• 4 new PHY frame formats for 11ax
• Target Wake Time (TWT) for enhanced power save operation

This may cause some confusion at times because someone may ask the question, “what’s my data rate?” The answer depends on which RU you’re using during that 200, 357, or 421ms TxOP. In a nutshell, your 11ax data rate is changing constantly, both in the UL and DL directions, depending on what RU the AP assigns to your client.

11ax APs will not, in any way, help the performance or range of your legacy (11a/b/g/n/ac) clients. 11ax APs will not be more sensitive, and if they were, that would be a bad thing.

Good article - http://divdyn.com/802-11ax-not/