Resource Unit
A Resource Unit (RU) in the IEEE 802.11ax wireless standard, commonly known as Wi-Fi 6, refers to the fundamental sub-channel unit in Orthogonal Frequency Division Multiple Access (OFDMA), comprising a contiguous group of subcarriers that enables simultaneous data transmission to or from multiple client devices.[1] RUs are designed to divide the available channel bandwidth—typically 20, 40, 80, or 160 MHz—into smaller, flexible portions, allowing access points to allocate resources dynamically based on client needs and traffic conditions.[2] Introduced to enhance efficiency in dense environments such as offices, stadiums, and urban areas, RUs support multi-user OFDMA (MU-OFDMA) for both uplink and downlink operations, reducing latency, minimizing collisions, and improving overall throughput by up to four times compared to previous standards.[3] The subcarrier spacing in 802.11ax is 78.125 kHz, with RUs formed from adjacent tones (subcarriers), excluding reserved DC, guard, and null tones to prevent interference.[2] Available RU sizes vary by channel width and include 26, 52, 106, 242, 484, and 996 subcarriers, corresponding to approximate bandwidths of 2 MHz, 4 MHz, 8 MHz, 20 MHz, 40 MHz, and 80 MHz, respectively; for example, a 20 MHz channel can support up to nine 26-subcarrier RUs for a maximum of nine simultaneous users.[1] Access points schedule RUs using trigger frames to coordinate transmissions, ensuring power levels are adjusted for path loss and enabling features like target wake time (TWT) for better battery efficiency in client devices.[2] This granular allocation contrasts with single-user OFDM in prior Wi-Fi generations, where the entire channel was dedicated to one user at a time, making RUs a cornerstone of 802.11ax's high-efficiency enhancements in the 2.4 GHz and 5 GHz bands. The RU concept was extended in later standards, such as IEEE 802.11be (Wi-Fi 7), which supports multi-resource unit (MRU) allocations to individual clients and operates in the 6 GHz band as well.[4][3]Overview
Definition and Purpose
A Resource Unit (RU) in IEEE 802.11ax (Wi-Fi 6) orthogonal frequency-division multiple access (OFDMA) systems is defined as a contiguous group of subcarriers, or tones, spaced at 78.125 kHz intervals, serving as the basic allocation for spectrum resources.[3] This structure allows the channel bandwidth to be partitioned into smaller, flexible segments that can be independently assigned.[1] The primary purpose of the RU is to enable multi-user access by dividing the available channel bandwidth into assignable units that can be allocated to multiple stations simultaneously, thereby enhancing spectral efficiency in dense environments.[3] Unlike the orthogonal frequency-division multiplexing (OFDM) employed in prior standards such as IEEE 802.11n and 802.11ac—which supported multi-user MIMO (MU-MIMO) in 802.11ac but dedicated the entire channel bandwidth to a single transmission opportunity without per-user frequency subdivision—OFDMA with RUs supports concurrent transmissions to or from multiple devices with granular frequency allocation, reducing contention and improving overall throughput per user.[1][5] As the smallest schedulable unit in 802.11ax OFDMA, an RU facilitates data transmission in both downlink and uplink directions, allowing access points to dynamically tailor resource assignments based on user needs and channel conditions.[3] This foundational role underpins the standard's ability to handle high-density scenarios more effectively than legacy Wi-Fi technologies.[1]Historical Development
The concept of Resource Units (RUs) in Wi-Fi draws its origins from cellular technologies, specifically the Orthogonal Frequency Division Multiple Access (OFDMA) framework in LTE, where spectrum is divided into resource blocks comprising subcarriers for multi-user allocation. This cellular-inspired approach was adapted for WLANs in IEEE 802.11ax to enable simultaneous transmissions to multiple devices, addressing limitations in traditional Wi-Fi by providing finer-grained frequency division and scheduled access.[6][7] Before 802.11ax, the IEEE 802.11ac standard (Wi-Fi 5) utilized OFDM with both single-user MIMO (SU-MIMO) and multi-user MIMO (MU-MIMO), assigning the full channel bandwidth to a group of devices per transmission opportunity without subdividing the frequency domain among users, which resulted in high contention, increased latency, and inefficient spectrum use in dense multi-device environments. These constraints became particularly evident as wireless traffic surged, with applications like video streaming and early IoT deployments overwhelming networks in shared spaces.[6][8] The push for RUs emerged from the need to support high-density scenarios, such as IoT networks and large venues like stadiums, where thousands of devices require low-latency, reliable connectivity without excessive interference. Development accelerated with the formation of the IEEE 802.11 Task Group ax (TGax) in May 2014, following the High Efficiency WLAN Study Group established in 2013, which identified RU-based OFDMA as a key mechanism to boost area throughput by up to four times over 802.11ac.[9][10][8] Key milestones included the Wi-Fi Alliance's initiation of Wi-Fi CERTIFIED 6 testing in September 2019, allowing certification of devices based on draft specifications, and IEEE ratification of 802.11ax on September 1, 2020. Extensions followed with Wi-Fi 6E in late 2020, integrating RU allocations into the 6 GHz band to expand capacity and reduce congestion in unlicensed spectrum.[11][12][13]Technical Specifications
Subcarrier Structure
In IEEE 802.11ax, the subcarrier structure underlying Resource Units (RUs) employs orthogonal frequency-division multiple access (OFDMA) with subcarriers spaced at 78.125 kHz to enable fine-grained frequency division for multi-user transmissions. This spacing arises from a 256-point fast Fourier transform (FFT) applied to a 20 MHz channel, yielding a total of 256 subcarriers, of which 234 carry data, 16 serve as pilots for phase tracking and channel estimation, and 6 are designated as DC or null subcarriers to maintain signal integrity at the center frequency and channel edges.[6] Pilot subcarriers are strategically placed across the bandwidth to allow receivers to estimate channel conditions, compensating for frequency-selective fading and timing errors during demodulation. Null subcarriers, positioned as guard bands primarily at the channel boundaries, suppress spectral regrowth and reduce adjacent channel interference by leaving those frequencies unmodulated. The DC subcarriers, located near the carrier frequency, are nulled to avoid direct-current offset issues in the baseband signal processing. Mathematically, the subcarrier spacing is given by \Delta f = \frac{20 \, \text{MHz}}{256} = 78.125 \, \text{kHz}, which determines the useful OFDM symbol duration as the reciprocal, T_s = \frac{1}{\Delta f} = 12.8 \, \mu\text{s}. A cyclic prefix is prepended to each symbol to mitigate inter-symbol interference, with supported lengths of 0.8 \mus, 1.6 \mus, or 3.2 \mus, selected based on the propagation delay spread; the longer options enhance performance in dense, reflective environments.[3] This design contrasts with legacy IEEE 802.11 OFDM implementations, such as in 802.11a/n/ac, where subcarriers are spaced at 312.5 kHz using a 64-point FFT for 20 MHz channels, resulting in fewer subcarriers (52 data tones) and coarser frequency granularity. The narrower spacing in 802.11ax accommodates more RUs per channel, facilitating efficient resource sharing among multiple devices without increasing overall bandwidth.[6]RU Sizes and Allocations
In IEEE 802.11ax, Resource Units (RUs) are defined in discrete sizes based on the number of tones, or subcarriers, they encompass, enabling flexible partitioning of the channel bandwidth for multi-user transmissions. The available RU sizes are 26, 52, 106, 242, 484, and 996 tones, where each size supports a specific number of data subcarriers and dedicated pilot subcarriers for channel estimation and synchronization. For instance, the smallest 26-tone RU includes 24 data subcarriers and 2 pilots, while the largest 996-tone RU comprises 980 data subcarriers and 16 pilots, fully occupying an 80 MHz channel without further subdivision.[14] These RU sizes are mapped to supported channel bandwidths of 20, 40, 80, and 160 MHz (or 80+80 MHz), with the maximum number of RUs determined by the total usable tones in each bandwidth and the need to maintain guard bands and DC subcarriers. Narrower RUs like 26 tones allow for finer granularity in serving more users, whereas larger RUs such as 484 or 996 tones are suited for higher-throughput single-user or fewer-user scenarios. The following table summarizes the RU configurations, including tones per size, pilot distribution, and maximum RUs per bandwidth:| RU Size (Tones) | Data Subcarriers | Pilots | Max RUs in 20 MHz | Max RUs in 40 MHz | Max RUs in 80 MHz | Max RUs in 160 MHz |
|---|---|---|---|---|---|---|
| 26 | 24 | 2 | 9 | 18 | 37 | 74 |
| 52 | 48 | 4 | 4 | 8 | 16 | 32 |
| 106 | 102 | 4 | 2 | 4 | 8 | 16 |
| 242 | 234 | 8 | 1 | 2 | 4 | 8 |
| 484 | 468 | 16 | N/A | 1 | 2 | 4 |
| 996 | 980 | 16 | N/A | N/A | 1 | 2 |