RU vs. MRU: Key terminologies used in WiFi 6 and WiFi 7

The difference between RU (Resource Unit) in Wi-Fi 6 (802.11ax) and MRU (Multiple Resource Unit) in Wi-Fi 7 (802.11be) lies in how the channel is allocated to users for simultaneous transmission.

Before we dive into concepts of RU and MRU, let us understand OFDM and OFDMA. In OFDM, entire channel is occupied between AP and user regardless of data amount which is not spectrum effient as unused bandwidth is wasted. In OFDMA, channel resources are divided into multiple resource units (RUs) and different users are allocated these RUs which carry respective data between them. In OFDMA, data of multiple users can be transported on single channel simultaneously unlike OFDM. OFDMA achieves point to multi-point communication between AP and multiple users which greatly improves communication efficiency.

RU (Resource Unit) in WiFi-6 (IEEE 802.11ax)

As mentioned, AP assigns specific RUs to different users in OFDMA mode in 802.11ax. Here RU is fixed sub-channel portion of overall channel bandwidth (e.g. 20, 40, 80 or 160 MHz). RU is minimum transmission unit in OFDMA.

To simplify OFDMA based scheduling, WiFi 6 defines seven types of RUs which include 26-tone RU, 52-tone RU, 106-tone RU, 242-tone RU, 484-tone RU, 996-tone RU and 2x996 tone RUs. WiFi-7 introduced one more RU type due to addition of 320 MHz bandwidth that is 4x996 tone RU.

MRU (Multiple Resource Unit) in WiFi-7 (IEEE 802.11be)

MRU allows a device to use multiple non-contiguous RUs in the same transmission, combining them to increase bandwidth. This enables a single STA to transmit/receive using multiple RUs, either adjacent or spread across the channel. MRU concept supports better aggregation and adaptation to channel conditions (e.g., avoiding interference prone parts).

In WiFi-6 each user can be allocated only one RU in a period, Hence, if some RUs become idle, it can lead to wastage of resources and lack of flexibility. To overcome this limitation, multiple RUs are allocated to single user simultaneously in WiFi-7. An MRU can contain multiple RUs of different sizes. The figure depicts comparison between channel allocation concepts in WiFi-5, WiFi-6 and WiFi-7 versions.

Conclusion

• Wi-Fi 6 enables OFDMA-based scheduling, allowing the channel bandwidth to be split into smaller sub channels (i.e. RUs) for multiple users. This improves network efficiency and latency in crowded environments.
• Wi-Fi 7 extends this concept by allowing a single user to be assigned multiple, non-contiguous RUs. This leads to better utilization of fragmented spectrum and significantly higher data throughput.