Wi-Fi 6 and OFDMA

Within the cellular world, OFDMA (Orthogonal Frequency Division Multiple Access) has been in operation since the introduction of 4G or more specifically, LTE (Long Term Evolution) as part of the Release 8 3GPP technical specifications. However, in the case of Wi-Fi this approach of allowing multiple devices to access the radio channel at the same time is relatively new, as it has only just been introduced in the IEEE 802.11ax or Wi-Fi 6 standard.

Pre Wi-Fi 6 (802.11a/g/n/ac), the entire radio channel was assigned to one specific device at a time, be that on the uplink (device to access point) or downlink (access point to device). In 802.11ac Wave 2, multiple devices could operate on the radio channel at the same time, but this used a technique termed MU-MIMO (Multi User – Multiple Input Multiple Output), in which the devices were separated by their use of streams – i.e. each device would still use the full radio channel when transmitting or receiving information. However, the introduction of OFDMA in Wi-Fi 6 now enables portions of the radio channel to be assigned to different devices, as illustrated in the following diagram. This of course implies the scheduling of resources to every device wishing to send data, so they know when and where to transmit.

Figure 1 OFDM v OFDMA

The subcarriers or tones are now divided into multiple groups, each termed a RU (Resource Unit). These will then be allocated to the devices depending upon a number of parameters; channel conditions, service requirements, device capabilities etc. Furthermore, subcarrier spacing (distance between adjacent subcarriers) has been reduced to 78.125kHz to minimize guard interval overhead and improve frequency selection gain. In addition, new guard intervals of 1.6µsec and 3.2µsec have been added to the original 0.8µsec time interval to counter ISI (Inter Symbol Interference) in both indoor and outdoor environments.

Wi-Fi 6 is also capable of supporting differing channel bandwidths; 20MHz, 40MHz, 80MHz and 160MHz, with OFDMA being used across each. As such, the number of RUs that can be supported will differ depending on the channel bandwidth. Not surprisingly, the larger channel bandwidths support greater numbers of RUs and thus devices operating at the same time. However, the smaller the RU, the less data the devices will be able to send or receive. The following table sets out the maximum number of RUs that can be allocated per channel bandwidth and RU size.

Figure 2 Maximum Number of RUs in Channel Bandwidths

Thus, for a 20MHz channel, nine devices could operate simultaneously when a 26 tone (subcarrier) RU is configured, four devices using a 52 tone RU, two devices each using a 106 tone RU and finally a single device taking all 242 tones. This is illustrated in Figure 3.

Figure 3 RU Options in 20MHz Channel Bandwidth

At the other end of the scale when operating across a 160MHz channel, it would be possible to support up to 74 devices when using 26 tone RUs or a single device using a 1992 tone RU.

OFDMA and Power
On the downlink, an access point may raise the transmission power on some of the RUs whilst reducing power on others to account for the signal strength between the devices and access point itself. For example, devices further away from the access point would typically require a higher power transmission whereas those closer would receive a lower power transmission in order to minimize interference. The aggregated transmitted power across the whole band would however remain constant.

On the uplink, where devices typically transmit at a lower power than the access points, there is a power difference or asymmetry that limits the coverage area of the Wi-Fi network. As such, the use of RUs enables this difference to be compensated for by allocating smaller RUs to weaker devices, enabling them to raise their SNIR (Signal to Noise plus Interference Ratio) i.e. less in band noise and interference.

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