Ever Wonder How OFDMA Works?

Understanding OFDMA

?OFDMA (Orthogonal Frequency Division Multiple Access) is a key technology that significantly enhances the efficiency of wireless networks, especially in environments with high user density. But how exactly does it work?

The Basics of OFDMA

?OFDMA allows subcarriers within a channel bandwidth to be grouped into smaller segments called resource units (RUs). These RUs are allocated to different devices, enabling access points to serve multiple devices simultaneously during both downlink and uplink transmissions. This multi-user capability is crucial for modern wireless networks, where numerous devices demand seamless connectivity.

?Breaking Down RUs and Tones

?Each subcarrier in OFDMA is further divided into smaller units called tones. Essentially, an RU is composed of a collection of these tones.

?In Wi-Fi 6, the subcarrier spacing is 78.125 kHz, which is four times narrower than the 312.5 kHz spacing used in 802.11ac. This narrower spacing increases the number of subcarriers available within a given bandwidth.

?Calculating the Number of Tones

?Using the subcarrier spacing, we can derive a formula to calculate the number of tones for different bandwidths:

Number of tones = (BW in MHz) ÷ (0.078125 MHz)

?Applying this formula, we get:

- 20 MHz bandwidth: 256 tones

- 40 MHz bandwidth: 512 tones

- 80 MHz bandwidth: 1024 tones

?Usable Tones for Data Transmission

Not all of these tones are used for data transmission. Some are reserved for Guard, and unused (Null Subcarriers). The usable RU tones, which include both data and pilot subcarriers, are 26, 52, 106, 242, and 996.

?Practical Implications

To summarize, a single RU can consist of a minimum of 26 tones and a maximum of 996 tones. This flexibility allows for efficient spectrum utilization, tailored to the needs of different devices and applications.

?Visualizing Bandwidth Allocation

?In terms of bandwidth, it’s evident that:

- Each 26-tone RU corresponds to approximately ~2 MHz

- 52 tones correspond to ~4 MHz

- 106 tones correspond to ~8 MHz

- And so on.

OFDM divides a channel's bandwidth into subcarriers for single-user transmission, while OFDMA further allocates these subcarriers dynamically to multiple users, enhancing network efficiency and capacity. OFDMA enables simultaneous data transmission from multiple devices, optimizing spectrum usage and reducing latency compared to OFDM.

In OFDMA, multiple devices communicate with a single access point simultaneously. The Rus is crucial for ensuring smooth and efficient data transmission. But how do the stations know that the AP is allocating Rus to them?

Enter trigger frames,

What is a trigger frame and how does it works?

?In OFDMA, a trigger frame is a control frame sent by an access point to manage and co-ordinate uplink transmission from the multiple stations.

Trigger frames specify which RUs each Station should use for transmission.

?The AP sends a trigger frame to all associated STAs. This frame contains information about which Stations should transmit, the specific RUs they should use, the timing of the transmission and power levels.

Please find the link below for sample OFDMA report

Report link : TR-398 OFDMA report