Wi-Fi 6 OFDMA Vs Wi-Fi 7 OFDMA

This article mainly focuses the key differences between Wi-Fi 6 OFDMA and Wi-Fi 7 OFDMA. To truly understand this, it's important to know when and why OFDMA was introduced, as it played a key role in advancing and optimizing wireless communication technologies.

OFDM - Orthogonal Frequency Division Multiplexing

OFDMA - Orthogonal Frequency Division Multiple Access

In previous Wi-Fi amendments, OFDM was used to transmit data over the single communication channel and typically used in single user requirements. Each user will occupy the entire bandwidth of their transmission and leading to less efficiency whenever multi users want to transmit the data.

Hence, OFDMA was introduced from Wi-Fi 6 to solve this problem for simultaneous multi-user transmission of data with different Unit (RU) sizes based on real-time user requirements which leads to the usage of high network efficiency and low latency applications.

From the above figure, you would have understood the key differences between OFDM and OFDMA. OFDMA is just a extended version of OFDM with multiple access.

OFDMA impact in Wi-Fi 6 and Wi-Fi 7?

OFDMA allows multiple users to share the same wifi channel by dividing the available bandwidth into smaller subcarriers. Each subcarrier can be assigned to different users, allowing for simultaneous data transmission. These subcarriers are grouped together to form Resource Units (RU's).

Subcarriers are also called tones. Some may use tones, don't get confused with the term use.

OFDMA in Wi-Fi 6 has OFDM symbol duration Ts (12.8 microseconds) is four times higher than previous wifi standards symbol duration Ts (3.2 micro seconds) which leading to high efficiency and contibuting to higher data rates.

RU's size in Wi-Fi 6 supported are 26, 52, 106 and 242 subcarriers.

Modulation: BPSK, QPSK, 16-QAM, 64-QAM, 256-QAM, 1024-QAM (10 bits/symbol)

Bandwidth: 20 MHz, 40 MHz, 80 MHz, 160 MHz

Coding Rate: 1/2, 2/3, 3/4, 5/6 supported for different modulation techniques.

Guard Interval (GI) for Wi-Fi 6 and Wifi 7 are 0.8 micro seconds.

No. of subcarrier spacing is calculated as 1/(Ts) implies 1/(12.8 micro seconds) = 78.125 KHz which is four times narrower than 802.11 ac's 312.5 KHz (1/3.2 micro secs).

RU mappings for Channel Bandwidth 20MHz:

Total Number of subcarriers = Data subcarriers + Pilot subcarriers + Unused subcarriers.

From the above figure which depicts the different RU size allocations for 20MHz depending on the no. of users. For example, 20MHz can use 9x26 or 4x52 or 2x106 or 1x242 RUs depending on the no. of users participation to transmit data simultaneously.

The small gap in every RU's in the above figure is called unused subcarriers.

For 20MHz, Total no. of subcarriers are calculated with the formula of 20MHz/(0.078125MHz) = 256 subcarriers. Same formula is applicable for all channel bandwidths.

Data, Pilot and unused subcarriers for 20MHz with 26 RU's = 9 x26 =234 data subcarriers + 8 Pilot subcarriers + 14 unused subcarriers.

Figure 3, shows allows multiple users by using various RU tones depending on the real-time user's requirements.

Below example shows 26-tone RU and multiple users across all Bandwidths for Wi-Fi 6:

• 20 MHz Channel: 9 RUs (each RU contains a certain number of subcarriers, depending on the RU size).
• 40 MHz Channel: 18 RUs.
• 80 MHz Channel: 37 RUs.
• 160 MHz Channel: 74 RUs.

Figure 4, its just a example for calculation of Data subcarriers using 26-tone RU for multiple users across all bandwidths. But, depending on the real-time user needs, user may use higher RU size for higher date rate applications that leads to low latency.

OFDMA in Wi-Fi 7 serves the same purpose as previous wi-fi amendments doing it. But with the Dynamic Resource Unit size allocation also called Multi-Resource Units (MRU) enhancing the efficiency of data transmission and improves network performance in high density environments.

This more allocation of RU sizes serves all type of multi-user requirements covering from low-bandwidth applications to bandwidth-intensive applications which provides Extremely High Throughput (EHT) data rates whereas OFDMA in Wi-Fi 6 has fixed RU sizes which leading to congestion issues in higher bandwidth scenario and when more multiple users transmitting data.

Wi-Fi 7 OFDMA also called as Multi-Resource Units (MRU) OFDMA.

RU size supported in Wi-Fi 7 are 26, 52, 106, 242, 484, 996, 2x996, 4x996 tones.

Bandwidth: 20 MHz, 40 MHz, 80 MHz, 160 MHz, 320 MHz

Modulation: BPSK, QPSK, 16-QAM, 64-QAM, 256-QAM, 1024-QAM, 4096-QAM (12 bits/symbol)

Coding Rate: 1/2, 2/3, 3/4, 5/6 supported for different modulation techniques.

Wi-Fi 7 introduces multiple Subcarrier Spacing (312.5 KHz, 156.25 KHz, 78.125 KHz) with different OFDM symbol duration (Ts) as 3.2, 6.4, 12.8 micro seconds respectively.

When using higher Bandwidth applications like 4K/8K video streaming. If Bandwidth (320MHz) increases, then No. of data subcarriers also increases which is influencing for EHT data rates in Wi-Fi 7.

For example, 320 MHz Total no. of subcarriers calculated as 320 MHz/(0.078125 MHz) = 4096 subcarriers. Data subcarriers are calculated depending the no. of users. If suppose, 4 x 996 tone RU + 2 x 52 tone RU = 4088 data subcarriers. Here, Unused/Null subcarriers are very less compared to Wi-Fi 6 OFDMA, because Wi-Fi 7 OFDMA chooses Multi-RU size allocation for different users leading to higher performances, so that unused/null subcarriers are not there.

Data Rates (Mbps) Calculations:

Many of us may wonder how this data rates are calculated. Here is the formula for calculating data rates (Mbps). Now you can calculate the theoretical data rates on your own by using below formulae.

Wi-Fi 6 Vs Wi Fi 7 Data rate calculations:

Taking example as 80MHz, 5/6 coding rate, Ts = 12.8 micro seconds, GI 0.8 micro seconds.

No. of data subcarriers = 37 x 26 = 962 data subcarriers.

No. of data subcarriers = 37 x 26 + 1 x 52 = 1014 (or) 1 x 996 + 1 x 26 = 1022 data subcarriers. This is purely based on the buffer sizes allocated and users requirements.

Figure 7, 8, shows the data rate (Mbps) numbers for Wi-Fi 6 and Wi-Fi 7 APs. It is calculated for one Spatial stream (SS).

Wi-Fi 7 which has 320 MHz BW and Multi-RU (MRU) size allocation for multi-users and 4096-QAM Modulation leading to Extremely High Throughput (EHT) data rates.

This Multi-RU size in Wi-Fi 7 OFDMA has a potential to increase the no. of Data subcarriers which contributes for higher data rate applications and for simultaneous multi-user data transmission.

If anyone has more insights about OFDMA in Wi-Fi 6 and Wi-Fi 7, or if there are any points I might have missed, please let me know. Additionally, I encourage you to check my other articles and share your thoughts as well. Your input is highly valued in expanding our understanding of these exciting new wireless technologies in this wireless world.

Thanks for reading!!!

Email ID: [email protected]

Sudharsan E.