A question that I keep getting asked about 802.11ax technology is: “Will 802.11ax make 80 MHz and 160 MHz channels usable in the enterprise?” In theory, BSS color combined with the spatial reuse operation could provide the capability to take advantage of 80 MHz channels in the enterprise. However, this would be practical only if legacy clients were not in the same physical location.
The 802.11ax draft amendment does define an optional capability called preamble puncturing which could allow an 802.11ax AP to operate at 80 MHz or 160 MHz without interference from a legacy BSS. Preamble puncturing allows an 802.11ax access point to transmit a “punctured” 80 MHZ channel or 160 MHz channel if some of the secondary channels are already in use by nearby legacy radio.
As shown in Figure 1, four 20 MHz channels can be bonded to create an 80 MHz channel. In this example, channel 58 is the 80 MHz channel with channel 52 as the primary channel. For backward compatibility reasons, 802.11 management and control frames are transmitted only on the primary channel. Two radios that both support 80 MHz channels could use the entire 80 MHz of frequency space for data frame exchanges.
Although not often implemented, 802.11ac radios are capable of a dynamic bandwidth operation where an AP can change the channel-width on a per frame basis. In the example shown in Figure 2, an 802.11ac access point that is transmitting on an 80 MHz channel might hear a nearby AP or client station of an overlapping basic service set (OBSS) on channel 56. Instead of deferring transmission on the entire 80 MHz channel, the AP might fall back to transmitting only on its primary 20 MHz channel 52.
As shown in Figure 3, an 802.11ax access point transmitting on an 80 MHz channel might hear a nearby legacy AP or client station of an overlapping basic service set (OBSS) on channel 56. If preamble puncturing is enabled, the 802.11ax AP would not defer and would “puncture” the 80 MHz channel. The 802.11ax AP would transmit across the entire 80 MHz channel except in the secondary frequency space that is already tied up due to the transmission from the legacy radios.
For preamble puncturing to work, both the 802.11ax AP and the receiving 802.11ax clients would have to support the capability. Please note that an 80 MHz or 160 MHz can only be punctured on secondary channels and never on the primary channel. As previously mentioned, this is considered to be an optional capability and it remains to be seen how widely it will be supported.
Do I think that preamble puncturing and BSS color will allow us to immediately use 80 MHz channels in the enterprise? No, certainly not anytime soon, however, you can at least see that the IEEE is thinking about better efficiency for large channels. That stated, the biggest gain in efficiency with 802.11ax will be the result of smaller sub-channels. In future blogs, we will discuss OFDMA partitioning of a 20 MHz channel into very small sub-channels for simultaneous communications.