What Does 802.11ax Change About The WLAN Standard?
Last time, we looked at the goals of the 802.11ax standard. Let’s break down some of the important changes 11ax introduces (in no particular order).
First introduced in 802.11ac, MU-MIMO technology theoretically allows the simultaneous transmitting of multiple frames to different receivers at the same time on the same channel using multiple RF streams to provide greater efficiency. 11ax supports up to 8x8x8 MU-MIMO in both downlink and uplink, which allows it to serve up to 8 users simultaneously for a significant capacity boost. 802.11ac only included downlink MU-MIMO, but we were experiencing a few issues:
- Many client devices are single antenna, and many two antenna clients switch to single stream mode for DL MU-MIMO for protection against interference
- Channel sounding responses for the users are transmitted serially in time resulting in high overhead
Uplink MU-MIMO was initially considered in 11ac, but not included due to implementation concerns. Because of all the issues, there are some significant 802.11ax MU-MIMO enhancements including the fact that sounding frames, data frames, etc. can be grouped among multiple users to reduce overhead and increase uplink response time. But just as MU-MIMO never fully came to fruition with 802.11ac, only time will tell if a real-world MU-MIMO scenario will become possible with 802.11ax.
Next, let’s talk QAM. If you think back to 802.11ac, that standard introduced 256 QAM which equates to 8 bits per symbol. The number of points in the modulation constellation determines the number of bits conveyed with each symbol. 802.11ax’s 1024 QAM equates to 10 bits per symbol. The use of 1024 QAM can result in a 25% increase in physical data rate. How? This is achieved using more efficient packaging of data for the same spectrum. This is similar to using a double-deck bus to make the best use of a bus lane, and is the main driver behind the speed gains.
The next change is related to OBSS- Overlapping Basic Service Set. To improve spatial reuse efficiency and performance, 11ax adjusts the carrier sense operation based on the ‘color’ of the BSS. Depending on the BSS the traffic is generated from, the station can use different sensitivity thresholds to transmit or defer. This results in higher overall performance.
802.11ax also introduces longer OFDM symbols. Very basically, 4x longer OFDM symbol times increase efficiency and also improves robustness, especially for transmission in outdoor scenarios.
The 802.11ax amendment also modifies the frame format. All 802.11 frames are preceded with a PHY layer header that contains a preamble. The preamble is used for synchronization between transmitting and receiving radios. The preamble consists of two parts: the legacy and the High Efficiency (HE) parts. The legacy preamble is easily decodable by legacy STAs and is included for backward compatibility with them.
Next up is TWT, or Target Wake Time. TWT allows the AP to schedule a series of times for a station to ‘wakeup’ at scheduled intervals to exchange data frames. This allows the station to ‘sleep’ longer and reduces energy consumption. This will be a key capability and enhancement for all mobile devices and especially IoT devices.
Lastly, the standard utilizes OFDMA- Orthogonal Frequency Division Multiple Access. This is a really exciting new feature, so I will go into this in much more detail in a later post.
Everything I have just stated sounds wonderful, but it should be taken with a grain of salt as it might not be replicable in a real world situation. Further testing will reveal what is actually usable!