WI-FI 6: some technical notes on OFDMA

The OFDMA multiple access technique is the trump card of the new 802.11ax standard commonly called Wi-Fi 6.

Wi-Fi 6

What’s new in Wi-Fi 6

Wireless networks have become more and more widespread: more devices, more connections, and applications that require more bandwidth. Wireless networks for the future will need more and more capacity and reliability. The current evolution is confidentially called Wi-Fi 6.

More formally referred to as the IEEE 802.11ax standard, this is the latest evolution of WLANs. Built on the strengths of 802.11ac, it is promising greater flexibility and scalability to next-generation applications.

IEEE 802.11ax enables access points to support multiple clients, even in congested environments, and provides a better user experience for WLAN users. It also provides more stable performance for advanced applications such as 4K video, Ultra HD, smart offices, and the Internet of Things (IoT). By offering more flexible management of sleep/wake-on-lan times, the new standard allows client devices to stay on standby much longer than the previous 802.11ac and reactivate with less contention. This is to the benefit of the battery life of smartphones, IoT, and other mobile devices.

To achieve these results, the IEEE 802.11ax standard acts in particular on three technical specifications:

  • Denser modulation using 1024 quadrature amplitude modulation (1024-QAM), which allows a 35% higher burst rate;
  • Scheduling based on OFDMA (Orthogonal Frequency Division Multiple Access) to reduce overload and latency;
  • More robust and high-efficiency signaling technique with a significantly lower RSSI (Received Signal Strength Indication).

OFDMA is the most significant innovation

Many simultaneous requests from WLAN users cause network congestion and slowdowns: in fact, clients, in these overcrowded conditions, must queue in order to complete transmissions. OFDMA is a step forward in solving the congestion problem. It is able to manage multiple users at the same time and assign bandwidth to each one more efficiently.

OFDMA (Orthogonal Frequency Division Multiple Access), as mentioned, allows multiple clients to transmit or receive simultaneously from an Access Point by sharing the available bandwidth. The spectral efficiency of OFDMA improves transmission latency or delay in RF environment, which has a moderate to high level of congestion. In addition, it will also increase throughput in some Wi-Fi 6 implementations, due to reduced collisions and contention times.

In more detail, OFDMA allows, given a certain channel bandwidth, to group the subcarriers into smaller segments called “Resource Units” (RU). These RUs are assigned to different stations, which allows Access Points to serve them simultaneously during uplink and downlink transmissions.

OFDMA by Cisco
Differences in bandwidth allocation between OFDM and OFDMA. Credits: Cisco.

The subcarriers are further divided into smaller components, called tones. We can say that an RU consists of several tones. In Wi-Fi 6, the spacing between the subcarriers is 78.125 kHz, which is a quarter of the 312.5 kHz spacing of the 802.11ac standard.

Based on this, we can build a formula to calculate the number of tones for different bandwidths. Or:

Number of tones = (BW in MHz) ÷ (0.078125 MHz). Applying this formula for bandwidths of 20 MHz, 40 MHz and 80 MHz results in a number of tones of 256, 512 and 1024 respectively. However, not all of these tones can be used for data as some must be reserved for other operations. In summary, each RU can contain 26, 52, 106, 242 or 996 useful tones for the data.

In relation to the bandwidth (see figure below), taking into account what was said above or that the separation between the subcarriers is 78.125 kHz, it can be easily calculated that each RU with 26 tones corresponds to about 2 MHz, with 52 tones at about 4 Mhz, with 106 tones at about 8 Mhz and so on.

RU tones bandwidth. Credits Cisco
Physical layer protocol data unit (PPDU) and different RU. Credits: Cisco.

Correlation between RU and channel bandwidth

The following table represents the relationship between the number of subcarriers and channel bandwidth. We can say, simplifying, that the table represents the number of OFDMA users for a particular tone for a given bandwidth. For example, for a band of 80 Mhz, a maximum of 37 users with 26-tone RU are supported. At 40 Mhz, a maximum of 18 users are supported with the same RU. At 20Mhz, a maximum of 9 users. Where it reads 1 is the case of a single user (SU), in which the entire spectrum is allocated to him.

The relationship between tones, bandwidth, and number of users. Credits: Cisco.
The relationship between tones, bandwidth, and number of users. Credits: Cisco.

The allocation of RU can be done by choosing different combinations of tones. For example, if three stations are associated, the Access Point can assign 106 tones to the first two users and 26 tones to the third user. Or the AP could assign 52 tones to the third user. RU assignment decisions are made dynamically by the AP based on the type of client traffic and the amount of data to be transmitted. The AP can check the status of the client buffer using a periodic polling mechanism. Real-time audio/video applications, which are highly sensitive to latency, are the main candidates for OFDMA.

RU allocation and channel access

Wi-Fi Access Points 6 must compete for channel access with non-Wi-Fi 6 stations, and this is done using EDCA (Enhanced Distributed Channel Access). EDCA provides contention-free access to the channel for a period known as Transmit Opportunity (TXOP), during which a station can transmit the maximum number of frames. RU allocations in both downlink and uplink directions are performed by the AP on a TXOP basis. During this time, the Access Point can satisfy multiple Wi-Fi 6 users using a single MU PPDU or data packets.

EDCA also incorporates a tag called access category (AC): voice, video, best effort, and background. Stations that send data in the same access category (AC) are served together using multi-user OFDMA (MU) packets. Stations with different AC tags are served using different MU data packets.

Conclusion

Traffic-dense wireless situations with latency-sensitive applications are obvious candidates for Wi-Fi 6 adoption. IOT devices will also benefit, with improved performance and efficiency in both the 2.4 GHz and 5 GHz bands. OFDMA, in particular, is the best answer to overcrowding problems with unstable audio/video in offices, schools, shopping malls, airports and even in homes with multiple streaming devices.

Other resources

For the 802.11ax frame structure and its PPDU format, see this article.

For PPDU comparison between different WLAN standards, see this article.

Some nice summary pictures of the benefits of Wi-Fi 6 from TP-LINK site.

About TG Team 60 Articles
We are the TG magazine editorial team, a unique pool of copywriters and engineers to get you through technologies and their impact on your business. Need our expertise for an article or white paper?