RFUnderstanding 802.11ax Resource Units (RUs)
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This page explains WLAN 802.11ax RU (Resource Unit). There are various 802.11ax RUs based on their bandwidth viz. RU-26, RU-52, RU-106, RU-242, RU-484 and RU-996.
Introduction
As we know, OFDM is used in 802.11n and 11ac as well as previous legacy wifi systems. In OFDM, the entire 20 MHz has been used for communication between the client and the AP (Access Point). The drawback of this concept is that the entire frequency space is used. To avoid this and to make use of available frequency space efficiently, OFDMA is used in the WLAN 802.11ax system.
What is RU (Resource Unit) in WLAN 802.11ax?
- Using OFDMA technique, channels consisting of 256 subcarriers are grouped into smaller subchannels. These subchannels are known as RUs (Resource Units).
- WLAN 802.11ax AP divides a 20 MHz channel into RUs of different sizes as per the number of subcarriers viz. 26, 52, 106 and 242 subcarriers. The 26 subcarriers can approx. take up 2 MHz, 52 subcarriers can take 4 MHz, 106 subcarriers take 8 MHz and 242 subcarriers take 20 MHz respectively.
- WLAN 11ax AP dictates the use of RUs within the 20 MHz channel and different combinations.
- Based on these RU allocations, the AP can communicate with one client using the entire 20 MHz bandwidth, or it can communicate with one 8 MHz client and three 4 MHz clients.
Image Courtesy: Aerohive Networks
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Using the 26 subcarriers RU (i.e. RU-26), the 802.11ax AP can communicate with a maximum of nine clients in MU-OFDMA transmissions per transmission opportunity.
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Using the above RU concept, the 802.11ax AP can transmit downlink to multiple clients. Moreover, multiple clients can transmit to the AP simultaneously.
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802.11ax uses other bandwidths viz. 40 MHz, 80 MHz and 160 MHz other than 20 MHz as explained above.
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Following table mentions usable subchannels, pilot and data subcarriers of OFDMA with 20 MHz channel BW.
| Channel BW (MHz) | Subchannels | BW of largest subchannel | Data subcarriers with pilots | Data subcarriers w/o pilots | Lower end guard subcarriers | Higher end guard subcarriers | DC subcarriers | Pilot subcarriers | Null Subcarriers | Total Subcarriers |
|---|---|---|---|---|---|---|---|---|---|---|
| 20 MHz | 9 x 26 | 1.88 MHz | 234 | 216 | 6 | 5 | 7 | 18 | 4 | 256 |
| 4x 52, 1x 26 | 3.91 MHz | 234 | 216 | 6 | 5 | 7 | 18 | 4 | 256 | |
| 2x 106, 1x26 | 7.97 MHz | 238 | 228 | 6 | 5 | 7 | 10 | 0 | 256 | |
| 1 x 242 | 18.3 MHz | 242 | 234 | 6 | 5 | 3 | 8 | 0 | 256 |
What are RU-26, RU-52, RU-106, RU-242, RU-484, RU-996 in 802.11ax?
The following table mentions RU-26, RU-52, RU-106, RU-242, RU-484, RU-996 with their respective subcarriers and support of clients for different bandwidths.
| Resource Unit with subcarriers | 20 MHz channel | 40 MHz channel | 80 MHz channel | 160 MHz channel | 80+80 MHz channel |
|---|---|---|---|---|---|
| RU-996 (2x),(1992 subcarriers) | N/A | N/A | N/A | 1 client | 1 client |
| RU-996,(996 subcarriers) | N/A | N/A | 1 client | 2 clients | 2 clients |
| RU-484,(484 subcarriers) | N/A | 1 client | 2 clients | 4 clients | 4 clients |
| RU-242,(242 subcarriers) | 1 client | 2 clients | 4 clients | 8 clients | 8 clients |
| RU-106,(106 subcarriers) | 2 clients | 4 clients | 8 clients | 16 clients | 16 clients |
| RU-52,(52 subcarriers) | 4 clients | 8 clients | 16 clients | 32 clients | 32 clients |
| RU-26,(26 subcarriers) | 9 clients | 18 clients | 37 clients | 74 clients | 74 clients |
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