CTS 159: Wi-fi 6 (802.11ax) Overview

I decided to finally get myself a little familiar with 802.11ax. I’m not sure why but I’ve pretty much ignored it until now. In this episode, I’m going to provide my overview of 802.11ax, or Wi-Fi 6. This episode will be the start of a mini series diving into detail of the components of 802.11ax. 802.11ax = High Efficiency (HE) and the marketing term for it is Wi-Fi 6. Currently in draft, there are no devices yet to support 802.11ax. Laptop and Samsung phone coming this year to support 802.11ax draft. Wi-Fi Alliance has their certifications coming later in 2019 for 802.11ax, Aerohive is shipping 802.11ax APs, and I predict we will see ratification in early 2020. Main PHY features in 802.11ax (HE) not in 802.11ac (VHT) and 802.11n (HT) * Mandatory support for DL & UL OFDMA* Mandatory support for DL MU-MIMO* Optional support for HE sounding protocol for beam forming* Optional support for UL MU-MIMO Main MAC features in HE not in previous protocols * AP has optional support for two NAV operation* Client has mandatory support for two NAV operation* Mandatory AP support for TWT* Optional client support for TWT* Optional support for UL OFDMA-based random access* Optional support for spatial reuse operation What are the general topics I’ll talk about in this episode? Here they are in no special order: Channel access for 802.11ax. An HE BSS can use RTS and CTS for transmit opportunity. Clients use RTS and CTS to initiate transmit opportunity. MU-RTS and CTSThe Multi-user RTS and CTS lets an AP initiate transmit opportunity. The MU-RTS Trigger frame is used to solicit simultaneous CTS responses from multiple 11ax clients. MU-RTS and CTS from 802.11ax Draft 3.0 MU OperationHE allows simultaneous downlink transmissions from AP to client in both DL-OFDMA and DL MU-MIMO. OFDMA is the biggest enhancement in 802.11ax which creates a multi-user version of OFDM. It may seem like the same definition as MU-MIMO but it isn’t. OFDMA is multiple access for OFDM. In OFDMA, the channel is subdivided into small channels called resource units or RUs. On each channel can be a different transmission hence multiple access. 11ax allows UL MU operation by letting the AP solicit simultaneous responses from one or more 11ax clients. For an AP to use UL MU operation it must follow EDCA HCF procedure. OFDMA is not new. It is implemented in LTE technologies. We’re simply using it here for Wi-Fi 🙂 Subcarriers In OFDM, the channel was divided into multiple subcarriers. Specifically it was 64 subcarriers in which 52 carried data, 4 subcarriers for pilot, and 8 subcarriers for guard bands. The width of the subcarriers is 312.5 KHz. When it comes to OFDMA, the subcarriers are now much smaller, 78.125 KHz! That equates to 256 subcarriers for OFDMA. It will maintain the different types of subcarriers for data, pilot, and guard. Resource UnitsPrior to 802.11ax, AP will transmit or receive across the whole OFDM channel, the entire frequency, for a single client. In OFDMA, the 256 subcarriers are further divided into resource units (RUs). An 802.11ax AP can determine the allocation of RUs used for a client or multiple clients. Yes, the AP can service multiple clients simultaneously using resource units and various resource unit combinations. BSS Frame Determination802.11ax introduces BSS colors to determine if a frame is destined for the same BSS or not.