What is MU-OFDMA in WLAN 802.11ax

What is MU-OFDMA in WLAN 802.11ax | MU-OFDMA basics

This page describes MU-OFDMA concept used in WLAN 802.11ax standard. It mentions benefits or advantages of MU-OFDMA (Multi-User OFDMA).

Introduction:
We already know about OFDM which stands for Orthogonal Frequency Division Multiplexing. In OFDM, all the subcarriers are assigned to individual client all the time during its time interval. In OFDMA, total subcarriers are divided into groups which is known as subchannel. The one or more subchannels are assigned to the clients based on bandwidth requirement. 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. First time OFDMA has been adopted in WLAN 802.11ax standard in order to increase efficiency of the wifi system. IEEE 802.11ax is the latest in series of wifi standards.

What is MU-OFDMA in WLAN 802.11ax ?

• OFDMA is multi-user version of OFDM used in 802.11a/g/n/ac.
• It allows multiple client devices (i.e. users) with varying BW to use wifi network simultaneously.
• OFDMA divides wifi channel into small frequency allocations known as RUs (Resource Units).
• AP communicates with individually assigned clients simultaneously due to their unique allocations of RUs.
• The figure-1 depicts MU-OFDMA in which channel bandwidth is divided into 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 single channel into multiple subcarriers using IFFT. Subcarriers are orthogonal and hence they do not interfere with one another. Moreover it prevents ICI (Intercarrier Interference).
• OFDM has been employed in 802.11ac with 20 MHz Bw. This 20 MHz has 64 subcarriers with 312.5 kHz spacing between them. It has symbol duration of about 3.2 microsecond. OFDMA in 802.11ax uses larger symbol duration of about 12.8 µs. It has subcarrier spacing of about 78.125 KHz.

Refer difference between OFDM and OFDMA >>.

Benefits or Advantages of MU-OFDMA

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

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