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, entire 20 MHz has been used for communication between client and
AP (Access Point). The drawback of this concept is that entire frequency space is used.
To avoid this and to make use of available frequency space efficiently OFDMA is used
in WLAN 802.11ax system.
What is RU (Resource Unit) in WLAN 802.11ax ?
• Using OFDMA technique, channel consisting of 256 subcarriers
are grouped into smaller subchannels. This subchannels are known as RUs (Resource Units).
• WLAN 802.11ax AP divides 20 MHz channel into RUs of different sizes
as per 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 can take 20 MHz respectively.
• WLAN 11ax AP dictates use of RUs within 20 MHz channel and different combinations.
• Based on these RU allocations, AP can communicate with one client using entire 20 MHz bandwidth or
it can communicate with one 8 MHz client and three 4 MHz client.
Image Courtesy: Aerohive Networks
• Using 26 subcarriers RU (i.e. RU-26), 802.11ax AP can communicate with maximum of nine clients in
MU-OFDMA transmissions per transmission opportunity.
• Using above RU concept, 802.11ax AP can transmit downlink to multiple
clients. Moreover multiple clients can transmit to AP simultaneously.
• 802.11ax uses other bandwidths viz. 40 MHz, 80 MHz and 160 MHz other than
20 MHz as explained above.
• 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
Following table-2 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 |
WLAN 802.11ac and 802.11ax related links
WLAN 802.11ax tutorial
advantages and disadvantages of 802.11ac
advantages and disadvantages of 802.11ax
Difference between 802.11ac Wave1 and Wave2
Difference between 802.11n,11ac,11ad
WLAN 802.11ac tutorial
WLAN 802.11ac frame structure
WLAN 802.11ac Physical Layer
802.11ac MAC Layer
802.11ax basics
WLAN 11a,11b,11n,11ac frame structures
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