The secret sauce of WiFi 6

WiFi 6 will transition WiFi from a ‘best-effort’ endeavour to a deterministic wireless technology that is now the de-facto medium for connectivity. This is made possible by a range of technologies that optimise spectral efficiency, increase throughput and reduce power consumption.

These include Orthogonal Frequency-Division Multiple Access (OFDMA), BSS Colouring, Target Wake Time (TWT), 1024-QAM and MU-MIMO.

In this Industry Insight, I will take a closer look at OFDMA and how 802.11ax wireless APs can utilise this mechanism to help achieve a 4X increase in network throughput.

OFDMA is arguably the most important new WiFi 6 capability. Legacy 802.11a/g/n/ ac radios currently use orthogonal frequency division multiplexing (OFDM) for single-user transmissions on any given channel. WiFi 6 radios utilise OFDMA, which is a multi-user version of the OFDM digital-modulation technology. OFDMA subdivides a WiFi channel into smaller frequency allocations, called resource units (RUs). By subdividing the channel, small frames can be simultaneously transmitted to multiple users in parallel.

For example, a traditional 20MHz channel might be partitioned into as many as nine smaller sub-channels. Using OFDMA, a WiFi 6 AP could simultaneously transmit small frames to nine WiFi 6 clients.

The AP can allocate the whole channel to a single user, or partition it to serve multiple users simultaneously, based on client traffic needs. OFDMA is most useful when multiple connections transmit limited amounts of data – which allows the protocol to squeeze smaller data packets through multiple sub-channels.

Think of a delivery truck (see Figure 1) that can only send a package to one house at a time (prior to 802.11ax), versus that same truck able to carry multiple packages to multiple houses (802.11ax).

When an 802.11n/ac AP transmits downstream 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 2, the AP transmits to six clients independently over time. When using a 20MHz OFDM channel, all of the 64 subcarriers are used for each independent transmission.

In other words, the entire 20MHz channel is needed for the communication between the AP and a single OFDM client. The communications are single-user. The same holds true for any uplink transmission from a single 802.11n/ac client to the 802.11n/ac AP. The entire 20MHz OFDM channel is needed for the client transmission to the AP.

An OFDMA channel consists of a total of 256 subcarriers (tones). These tones can be grouped into smaller sub-channels known as RUs. As shown in Figure 3, when subdividing a 20MHz channel, a WiFi 6 access point can designate 26, 52, 106 and 242 subcarrier RUs, which roughly equates to 2MHz, 4MHz, 8MHz and 20MHz channels. The WiFi 6 AP dictates how many RUs are used within a 20MHz channel and different combinations can be used.

The AP may allocate the whole channel to only one client at a time, or it may partition the channel to serve multiple clients simultaneously. For example, a WiFi 6 AP could simultaneously communicate with one WiFi 6 client using 8MHz of frequency space while communicating with three other WiFi 6 clients using 4MHz sub-channels. These simultaneous communications can be either downlink or uplink.

In the example shown in Figure 4, the WiFi 6 AP first simultaneously transmits downlink to WiFi 6 clients 1 and 2. The 20MHz OFDMA channel is effectively partitioned into two sub-channels.

Remember that an ODFMA 20MHz channel has a total of 256 subcarriers; however, the AP is 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 different 52-tone resource units.

In the third transmission, the AP uses a single 242-tone resource unit to transmit downlink to a single client (client 5). Using a single resource unit is effectively using the entire 20MHz 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 only transmits downlink to a single client with a single RU utilising the entire 20MHZ channel. In the sixth transmission, the AP simultaneously transmits downlink to clients 3, 4 and 6. In this instance, the 20MHz channel is partitioned into three sub-channels; two 52 RUs are used for clients 3 and 4 and a 106-tone RU for client 6.

The WiFi 6 AP dictates how many RUs are used within a 20MHz channel and different combinations can be used. In addition to 20MHz channels, 40, 80 and even 160MHz channels can also be partitioned into various combinations of RUs as shown in Table 1. For example, if an 80MHz channel was subdivided using strictly 26 subcarrier RUs, 37 WiFi 6 clients could theoretically communicate simultaneously using their OFDMA capabilities.

OFDMA is ideal for most network applications and results in better frequency reuse, reduced latency and increased efficiency.