MU-OFDMA Explained: Revolutionizing WiFi 6 Wireless Communication

Introduction

WiFi technology is constantly evolving, delivering faster speeds and improved efficiency to meet the growing demand for high-performance wireless connectivity. A key innovation in the WiFi 6 (802.11ax) standard is Multi-User Orthogonal Frequency Division Multiple Access (MU-OFDMA). This advanced feature revolutionizes the way data is transmitted, enhancing network efficiency and reducing latency, especially in dense environments. In this guide, we’ll explore what MU-OFDMA is, how it works, and its impact on wireless communication.

What is OFDMA?

We already know about OFDM, which stands for Orthogonal Frequency Division Multiplexing. In OFDM, all the subcarriers are assigned to an individual client all the time during its time interval.

In OFDMA, the total subcarriers are divided into groups known as subchannels. One or more subchannels are assigned to clients based on bandwidth requirements. OFDM has been used in previous WLAN standards such as 802.11a/g/n/ac, as well as fixed WiMAX (16d). OFDMA has been used in mobile WiMAX (16e), LTE, and 5G cellular systems.

This is the first time OFDMA has been adopted in the WLAN 802.11ax standard in order to increase the efficiency of the WiFi system. IEEE 802.11ax is the latest in a series of WiFi standards.

What is MU-OFDMA?

• OFDMA is a multi-user version of OFDM used in 802.11a/g/n/ac.
• It allows multiple client devices (i.e., users) with varying bandwidth to use the WiFi network simultaneously.
• OFDMA divides the WiFi channel into small frequency allocations known as RUs (Resource Units).
• The Access Point (AP) communicates with individually assigned clients simultaneously due to their unique allocations of RUs.
• Figure 1 depicts MU-OFDMA, in which channel bandwidth is divided into a number of channels. Each of these channels can be used by multiple clients at different time instants, unlike previous WiFi standards.

Image Courtesy: Aerohive Networks

• OFDM divides a single channel into multiple subcarriers using IFFT (Inverse Fast Fourier Transform). Subcarriers are orthogonal and, hence, do not interfere with one another. Moreover, it prevents ICI (Intercarrier Interference).
• OFDM has been employed in 802.11ac with 20 MHz bandwidth. This 20 MHz has 64 subcarriers with 312.5 kHz spacing between them. It has a symbol duration of about 3.2 microseconds. OFDMA in 802.11ax uses a larger symbol duration of about 12.8 µs. It has a subcarrier spacing of about 78.125 KHz.

Advantages of MU-OFDMA

Here are some of the benefits of MU-OFDMA:

1. It helps in better frequency reuse in the WiFi network across a broad region.
2. It helps in reducing latency between a WiFi client and the Access Point (AP).
3. It helps to increase network efficiency.
4. Narrow subcarrier spacing in 802.11ax allows better equalization and enhanced channel robustness.
5. Due to 78.125 kHz channel spacing, OFDMA with 20 MHz bandwidth consists of a total of 256 subcarriers as shown in Figure 2. Subcarriers are categorized into three types based on their functionalities, viz. data subcarriers, pilot subcarriers, and unused subcarriers.

Conclusion

MU-OFDMA is a cornerstone feature of WiFi 6 (802.11ax), designed to meet the challenges of modern wireless communication. By enabling simultaneous data transmission for multiple users, it significantly enhances network performance and user experience. Whether it’s streaming, gaming, or supporting IoT devices, MU-OFDMA ensures efficient, low-latency communication. As WiFi technology advances, features like MU-OFDMA will continue to shape the future of wireless connectivity.