WiFi 802.11ax - what is Resource Unit?

A 20 MHz OFDMA channel consists of a total of 256 subcarriers (tones). These tones are grouped into smaller sub-channels, known as resource units (RUs). As shown in Figure 1, when subdividing a 20 MHz channel, an 802.11ax access point designates 26, 52, 106, and 242 subcarrier resource units (RUs), which equates roughly to 2 MHz, 4 MHz, 8 MHz, and 20 MHz channels, respectively. The 802.11ax access point dictates how many RUs are used within a 20 MHz channel, and different combinations can be used.

Figure 1- OFDMA resource units – 20 MHz channel

An 802.11ax AP may allocate the whole channel to only one client
 at a time, or it may partition the OFDMA channel to serve multiple clients simultaneously. For example, an 802.11ax AP could simultaneously communicate with one 802.11ax client using 8 MHz of frequency space while communicating with three additional 802.11ax clients using 4 MHz sub-channels. These simultaneous communications can be either downlink or uplink.

Figure 2 – OFDMA transmissions over time

In the example shown in Figure 2, the 802.11ax AP first simultaneously transmits downlink to 802.11ax clients 1 and 2. The 20 MHz OFDMA channel is effectively partitioned into two sub-channels. Remember that an ODFMA 20 MHz channel has a total of 256 subcarriers; however, the AP simultaneously transmitted to clients 1 and 2 using two different 106-tone resource units. In the second transmission, the AP simultaneously transmits downlink to clients 3, 4, 5, and 6. In this case, the ODFMA channel had to be partitioned into four separate 52-tone sub-channels. In the third transmission, the AP uses a single 242-tone resource unit to transmit downlink to a single client (5). Using a single 242-tone resource unit is effectively using the entire 20 MHz channel. In the fourth transmission, the AP simultaneously transmits downlink to clients 4 and 6 using two 106-tone resource units. In the fifth transmission, the AP once again transmits only downlink to a single client, with a single 242-tone RU utilizing the entire 20 MHZ channel. In the sixth transmission, the AP simultaneously transmits downlink to clients 3, 4, and 6. In this instance, the 20 MHz channel is partitioned into three sub-channels; two 52-tone RUs are used for clients 3 and 4, and a 106-tone RU is used for client 6.

For backward compatibility, 802.11ax radios will still support OFDM. Keep in mind that 802.11 management and control frames will still be transmitted at a basic data rate using OFDM technology that 802.11a/g/n/ac radios can understand. Therefore, the transmission of management and control frames will be transmitted using the standard 64 OFDM subcarriers of an entire primary 20 MHz channel. OFDMA is only for 802.11 data frame exchanges between 802.11ax APs and 802.11ax clients. Please check back every week and read future 802.11ax blogs, where we will discuss in more detail the mechanisms of ODMA including resource unit allocation and trigger frames. We also discuss the differences between downlink OFDMA and uplink OFDMA.

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OFDM & OFDMA in context of WiFi 802.11ax technology.

802.11a/g/n/ac radios currently use Orthogonal Frequency Division Multiplexing (OFDM) for single-user transmissions on an 802.11 frequency. 802.11ax radios can utilize orthogonal frequency-division multiple access (OFDMA) which is a multi-user version of the OFDM digital-modulation technology. OFDMA subdivides a channel into smaller frequency allocations, called resource units (RUs). By subdividing the channel, parallel transmissions of small frames to multiple users can happen simultaneously.

Think of OFDMA as a technology that partitions a channel into smaller sub-channels so that simultaneous multiple-user transmissions can occur. For example, a traditional 20 MHz channel might be partitioned into as many as nine smaller sub-channels. Using OFDMA, an 802.11ax AP could simultaneously transmit small frames to nine 802.11ax clients. OFDMA is a much more efficient use of the medium for smaller frames. The simultaneous transmission cuts down on excessive overhead at the MAC sublayer as well as medium contention overhead. The goal of OFDMA is better use of the available frequency space. OFDMA technology has been time-tested with other RF communications. For example, OFDMA is used for downlink LTE cellular radio communication.

To illustrate the difference between OFDM and OFDMA, please reference both Figures 1 and 2. When an 802.11n/ac AP transmits downlink to 802.11n/ac clients on an OFDM channel, the entire frequency space of the channel is used for each independent downlink transmission. In the example shown in Figure 1, the AP transmits to six clients independently over time. All 64 subcarriers are used when an OFDM radio transmits on a 20 MHz channel. In other words, the entire 20 MHz channel is needed for the downlink communication between the AP and a single OFDM client. The same holds true for any uplink transmission from a single 802.11n/ac client to the 802.11n/ac AP. The entire 20 MHz OFDM channel is needed for the client transmission to the AP.

Figure 1- OFDM transmissions over time

 As shown in Figure 2, an 802.11ax AP can partition a 20 MHz OFDMA channel into smaller sub-channels for multiple clients on a continuous basis for simultaneous downlink transmissions. In a future blog, you will learn that an 802.11ax AP can also synchronize 802.11ax clients for simultaneous uplink transmissions. It should be noted that the rules of medium contention still apply. The AP still has to compete against legacy 802.11 stations for a transmission opportunity (TXOP). Once the AP has a TXOP, the AP is then in control of up to nine 802.11ax client stations for either downlink or uplink transmissions. The number of resource units (RUs used can vary on a per TXOP basis.

Figure 2- OFDMA transmissions over time

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