6/25/24, 4:58 PM Ever Wonder How OFDMA Works?
Ever Wonder How OFDMA Works?
Manoj kumar Kancharana + Follow
Wireless Systems Engineer at Candela Technologies,Inc. |Wi-Fi|802.11|TR-
398| Talks about #wifi
Published Jun 25, 2024
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
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In Wi-Fi 6, the subcarrier spacing is 78.125
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kHz, which is four times narrower than
Ever Wonder How OFDMA Works?
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
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transmission from multiple devices, optimizing spectrum usage and reducing latency
compared
6/25/24, 4:58 PM to OFDM. Ever Wonder How OFDMA Works?
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
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