Orthogonal Frequency Division Multiple Access increases the efficiency of a crowded Wi-Fi channel. The access point is in control, managing who gets bandwidth and when.
In The basics of 5G’s modulation, OFDM, we looked at how Orthogonal Frequency Division Multiplexing (OFDM) is used in wireless communications channels. OFDM is an effective way to produce high data rates with excellent spectral efficiency. Orthogonal Frequency Division Multiple Access (OFDMA) is a further enhancement to OFDM that lets multiple devices share a wireless channel, improving the channel’s overall use.
OFDMA is already deployed in 4G and 5G. In the world of wireless LAN, the latest Wi-Fi standard (IEEE 802.11ax) uses OFDMA for both uplink and downlink data transfer.
Wi-Fi Naming
The IEEE 802.11 standard defines the commonly used wireless LAN technologies, with 802.11ax recently being released into the marketplace. The Wi-Fi Alliance has rolled out a new naming scheme that simplifies the terminology for consumers. IEEE 802.11ax will be referred to as Wi-Fi 6, while 802.11ac and 802.11n are retroactively named Wi-Fi 5 and Wi-Fi 4, respectively. Of course, Wi-Fi 6 devices also support the legacy Wi-Fi modes of operation (802.11a, b, g, n, and ac) to maintain backwards compatibility. Table 1 summarizes the most recent three Wi-Fi generations.
| Wi-Fi 4 | Wi-Fi 5 | Wi-Fi 6 | |
| IEEE standard | 802.11n | 802.11ac | 802.11ax |
| Frequency Bands | 2.4 GHz & 5 GHz | 5 GHz | 2.4 GHz & 5 GHz |
| Channel Size | 20, 40 MHz | 20, 40, 80, 160 MHz | 20, 40, 80, 160 MHz |
| Frequency Multiplexing | OFDM | OFDM | OFDM & OFDMA |
| Modulation | BPSK, QPSK, 16-QAM, 64-QAM | BPSK, QPSK, 16-QAM, 64-QAM, 256-QAM | BPSK, QPSK, 16-QAM, 64-QAM, 256-QAM, 1024-QAM |
| Symbol Time, max | 4 µs | 4 µs | 16 µs |
Table 1. A comparison of the last three generations of Wi-Fi.
Wi-Fi 6 makes use of both the 2.4 GHz and 5 GHz unlicensed bands. The FCC recently announced the availability of an additional 1200 MHz of unlicensed spectrum in the 6 GHz band. Wi-Fi equipment that uses this new band will be called Wi-Fi 6E [Ref 2].
Wi-Fi 6 channel widths can be 20, 40, 80 or 160 MHz, which is similar to Wi-Fi 5. Wi-Fi 6 adds 1024-QAM as a higher speed modulation option. Wi-Fi 6’s real contribution, however, is improved channel efficiency through OFDMA. The focus is on maximizing the capacity to serve multiple users in a dense environment, not banner spec data rates.
OFDM and OFDMA
A typical Wi-Fi network has an access point (AP) that serves multiple client devices such as smartphones, tablets and computers (Figure 1). The AP normally services multiple client devices which are growing in number as more electronic devices include Wi-Fi connections.
Figure 1. A Wi-Fi 6 access point provides wireless access to multiple client devices.
With OFDM (as used in Wi-Fi 4 and 5), the wireless channel is shared by multiple clients with only one client transmitting in any given time slot (Figure 2). The devices use Carrier-Sense Multiple Access (CSMA) to minimize and manage collisions on the channel. CSMA requires each device to listen to the channel for a prescribed time to determine that the channel is available. Then the device transfers its data packet, hopefully with no other device transmitting at that same time. The transmitting device then waits for an acknowledgement that the data packet was received, else it tries to resend the data packet. On a congested channel, the number of collisions can be large and cause poor network performance. Also, each client device uses the full bandwidth of the channel whether it needs it or not.
Figure 2. OFDM lets one client transmit at a time.
OFDMA provides for the dynamic assignment of channel bandwidth to multiple users. The channel is subdivided into smaller subchannels called Resource Units (RUs). Figure 3 shows a chart of a simple four RU channel serving three client devices. In each time slot, the AP assigns the RUs to each client based on their bandwidth needs. This is done dynamically, and each time slot can have a different set of RU assignments.
Figure 3. OFDMA provides for multiple clients using the channel simultaneously.
RUs are assigned on the uplink and downlink. On the downlink the AP transmits across the whole channel and the client devices know which RUs are intended for them. On the uplink, the clients all transmit at the same time on their assigned RUs.
Example: 20 MHz Channel
In Figure 4, a 20 MHz channel has 256 tones available, each spaced by 78.125 kHz. The AP can subdivide this channel by designating 26, 52, 106, or 242 tones into RUs.
Figure 4. OFDMA has many subcarriers (or tones) that carry the information in the wireless channel.
While there are 256 tones present in the system, some of these are used for pilot tones and guard tones, leaving a maximum of 242 tones to be assigned to an RU. The bandwidth of the narrowest RU is 26 tones × 78.125 kHz = 2.03 MHz. Similarly, 52, 106, and 242 tones map to approximate bandwidths of 4, 8, and 20 MHz.
Figure 5 shows how a 20 MHz channel can be subdivided for use by multiple client devices. The finest granularity supported is 26 tones, which allows nine clients on a 20 MHz channel. Fig. 5 shows other possible configurations such as 4 clients each allocated 52 tones (~4 MHz bandwidth). The entire 20 MHz channel (242 tones) can be assigned to one client. Various combinations of RU sizes can be mixed to optimize the channel usage.
Figure 5. There are several different ways a 20 MHz Wi-Fi 6 channel can be subdivided using OFDMA.
We looked at the smallest channel (20 MHz) with Wi-Fi 6 but wider channels (40 MHz, 80 MHz and 160 MHz) can support wider bandwidth RUs and higher numbers of RUs per channel. The widest RU size available is 1992 tones (~156 MHz bandwidth). The smallest possible RU size is always 26 tones, so up to 74 clients could fit into one 160 MHz channel.
Scheduling
The AP schedules the uplink transmission by sending out a Trigger Frame to all client devices telling them when and where the client devices should transmit.
All the client devices transmit back at the same time using their assigned RUs. The AP can adjust the downlink transmit power of each RU individually and direct the client devices to adjust their uplink power, too. The clients must transmit data for the same time interval on the uplink so some clients may need to send additional padded data to fill in the required time slot.
To determine the proper use of RUs on the uplink, the AP can send a Buffer Status Report Poll (BSRP) that asks the clients to respond with a Buffer Status Report (BSR). The BSR indicates how much data the client has available to transmit. Clients with more data will have more RUs assigned to them to optimize the channel throughput. Once the network is up and running, the clients can also send buffer information as part of its normal uplink packets. It is critical that the AP and client devices remain synchronized in time, with the clients following the various link instructions from the AP.
The Wi-Fi 6 standard anticipates there will be legacy clients on the channel, so the AP needs to operate accordingly. Wi-Fi 4 and Wi-Fi 5 clients will continue to operate using OFDM on the full channel bandwidth. This may mean that the full benefits of Wi-Fi 6 will be achieved by upgrading most or all of the clients to Wi-Fi 6.
OFDMA is a major step forward in the world of wireless LAN. The AP is in charge, controlling the transmit times and bandwidth usage of the wireless channel and the efficiency of the channel, especially in dense environments, is greatly improved.
This is a brief introduction to OFDMA in Wi-Fi 6. See References 3, 4, & 5 for more detail.
References
1. “Wi-Fi CERTIFIED 6™: A new era in wireless connectivity (2019),” Wi-Fi Alliance, https://www.wi-fi.org/file/wi-fi-certified-6-a-new-era-in-wireless-connectivity-2019.
2. “Wi-Fi 6E cometh: FCC opens 1200 MHz of spectrum to unlicensed communications,” Martin Rowe, 5G Technology World, April 23, 2020, https://www.5gtechnologyworld.com/wi-fi-6e-cometh-fcc-opens-1200-mhz-of-spectrum-to-unlicensed-communications/.
3. “802.11ax – OFDMA Overview,” Chris Radford, Cradtech, https://cradtech.com/2018/10/25/802-11ax-ofdma-overview/, October 25, 2018.
4. “IEEE 802.11ax Technology Introduction,” Lisa Ward, Rohde & Schwarz GmbH & Co., April, 2020, https://www.rohde-schwarz.com/us/solutions/test-and-measurement/wireless-communication/wireless-connectivity/wlan-wifi/wlan-ieee-802.11ax-testing/white-paper-ieee-802.11ax-technology-introduction_253050.html.
5. “OFDMA: The Key Technology of 802.11ax,” Devin Akin, video of technical presentation, Feb 20, 2019, https://youtu.be/owBrkFk9afM.
6. “802.11ax and OFDMA,” Perry Correll, Wireless LAN Professionals, Prague 2018, video https://youtu.be/uM6y_vt630Q/
Bob Witte is President of Signal Blue LLC, a technology consulting company. Bob has held many positions in R&D, technology planning, strategic planning, and manufacturing for Keysight Technologies, Agilent Technologies and Hewlett-Packard Company. Inside, he is just an engineer that loves to see innovative products solve real customer problems. Bob is the author of two books on test and measurement instrumentation: Electronic Test Instruments (2nd Edition) and Spectrum and Network Measurements (2nd Edition).
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This is an awesome article and a great learning tool to understand how WiFi works in the networking world as we know it. I think by referencing the word MU-MIMO in the article which I was thinking about when reading the article would help people understand more of what is being said. Just my observation that’s all.
Very good article and easy to understand. Thank a million