How Do 20 MHz-Only Clients Operate In 802.11ax?

By David Coleman, Aerohive Networks.

Much of the 802.11ax draft amendment has been written with Internet of Things (IoT) devices in mind. The radio chipsets used for IoT devices generally need to be low-cost and consume very little power. Traditionally, Bluetooth and Zigbee chipsets have been cheaper to manufacture for IoT devices than Wi-Fi radio radios. However, 802.11ax lays the groundwork for vendors to manufacture low-cost Wi-Fi radios with better power-saving capabilities for sensors and other IoT devices.

Target wake time (TWT) is a power saving mechanism originally defined in the 802.11ah-2016 amendment. A TWT is a negotiated agreement, based on expected traffic activity between clients and the AP, to specific a scheduled target wake-up time for the clients in power-save (PS) mode. TWT could theoretically allow client devices to sleep for hours. TWT is an ideal power-saving method for IoT devices that need to conserve battery life. We will provide more detail about TWT in a future blog.

Another 802.11ax enhancement is a 20 MHz-only operational mode for 802.11ax clients. Via management frames, client stations will be able to inform an 802.11ax AP that they are operating as 20 MHz-only clients. A 20 MHz-only device can transmit/receive in either the 2.4 or the 5 GHz band.

An OFDMA 20 MHz channel consists of a total of 256 subcarriers (tones). These tones are can be grouped into smaller sub-channels know as resource units (RUs). As shown in Figure 1, when subdividing a 20 MHz channel, an 802.11ax access point can designate 26, 52, 106, and 242 subcarrier resource units (RUs), which equates roughly to 2 MHz, 4 MHz, 8 MHz, and 20 MHz channels. By utilizing these sub-channels, an AP can orchestrate simultaneous downlink or uplink communications with multiple clients.

Figure 1

An 802.11ax AP could assign any of these RUs to 20 MHz-only devices when the AP has also been configured for a 20 MHz channel. For example, the AP might assign 26-tone RUs and communicate with 9 IoT devices at the same time. If the AP is transmitting on a standard 20 MHz channel, the 20 MHz-only clients will obviously be able to support tone mappings of a 26-tone RU, 52-tone RU, 106-tone RU, and 242-tone RU within a 20 MHz channel.

As shown in Figure 2, a 20 MHz-only client can still operate within a 40 MHz or 80 MHz channel width. However, with one rare operational exception, the 20 MHz-only clients must communicate via RUs the primary channel.

Figure 2

If the AP is transmitting on a 40 MHz channel, the 20 MHz-only clients may support the 40 MHz tone mappings for a 26-tone RU, 52-tone RU or 106-tone RU. Very specific 26-tone RU, 52-tone RU or 106-tone mappings are also supported for a 20 MHz-only client if the AP is transmitting, within an 80 MHz channel. For any channels larger than 20 MHz, a 242-tone RU assignment is optional for 20 MHz-only clients.

The whole purpose behind these rules is to ensure that a 20 MHz-only client is only assigned the proper OFDMA tone mappings and RU allocations that the client can support even if larger channels are being used. The 20 MHz-only operational mode is ideal for IoT clients that could take advantage of the 802.11ax power-saving capabilities but not necessarily need the full capabilities that 802.11ax has to offer. This will allow client manufacturers to design less complex chipsets at a lower cost which is ideal for IoT devices.