802.11ax (Wi-Fi 6)

Wi-Fi 6: Designed for Density

Previous Wi-Fi generations focused on increasing peak throughput for a single user. 802.11ax (Wi-Fi 6) shifts the design objective to efficient operation in dense environments — airports, stadiums, lecture halls, enterprise offices with hundreds of devices per AP. The key innovation is bringing cellular-style scheduled multi-user access (OFDMA) to the unlicensed band, while retaining backward compatibility with CSMA/CA. Wi-Fi 6 targets a $4\times$ improvement in average per-user throughput in dense scenarios, even though the peak single-user rate improvement over 802.11ac is modest ( $\sim$ 30% from 1024-QAM and longer OFDM symbols).

Definition:
OFDMA in 802.11ax

802.11ax introduces OFDMA (Orthogonal Frequency Division Multiple Access) to Wi-Fi, allowing the AP to divide the channel bandwidth into Resource Units (RUs) and assign each RU to a different station:

Resource Unit sizes (number of data + pilot subcarriers):

RU size	Tones	Approx. BW	Use case
26-tone	24+2	2 MHz	IoT, ACKs, small packets
52-tone	48+4	4 MHz	Low-rate users
106-tone	102+4	8 MHz	Medium-rate users
242-tone	234+8	20 MHz	Single-user legacy
484-tone	468+16	40 MHz	High-rate users
996-tone	980+16	80 MHz	Highest-rate users
2 $\times$ 996	1960+32	160 MHz	Peak throughput

Key rules:

RUs must tile the bandwidth without overlap.
A 20 MHz channel supports up to 9 users (nine 26-tone RUs).
RU assignment is signalled via the trigger frame (for UL) or the HE-SIG-B field (for DL).
Each user can have a different MCS and number of spatial streams.

The total number of subcarriers in a 20 MHz channel is 256 (FFT size), with 234 data + 8 pilot + 3 DC null + 11 guard = 256.

OFDMA fundamentally changes Wi-Fi from a contention-based system (one user at a time) to a scheduled system (multiple users simultaneously). However, the AP still uses CSMA/CA to acquire the channel before scheduling OFDMA transmissions.

Definition:
Trigger-Based Uplink OFDMA

In 802.11ax, the AP coordinates uplink multi-user transmissions using a trigger frame:

The AP wins channel access via CSMA/CA.
The AP sends a trigger frame specifying:
- The list of target STAs and their assigned RUs
- The allowed MCS and spatial stream configuration
- The target RSSI (for UL power control)
- The UL transmission duration
After SIFS, all triggered STAs transmit simultaneously on their assigned RUs, synchronised to the AP's timing.
The AP receives and decodes all UL transmissions, then sends a multi-STA BlockAck to acknowledge each STA.

This replaces the traditional contention-based uplink where each STA independently contends for the channel. The trigger-based mechanism enables UL MU-MIMO (multiple STAs on the same RU with spatial separation) and UL OFDMA (different STAs on different RUs).

The trigger frame mechanism requires the AP to know the buffer status of each STA (via Buffer Status Reports) to make efficient scheduling decisions. This makes the AP role similar to a cellular base station scheduler.

Definition:
Target Wake Time (TWT)

TWT is a power-saving mechanism in 802.11ax that allows the AP and STA to negotiate specific times for the STA to wake up and communicate:

Individual TWT: The AP and a single STA agree on a schedule (wake interval, wake duration, trigger-enabled flag).
Broadcast TWT: The AP announces TWT schedules for groups of STAs, enabling coordinated wake-up patterns.

Between TWT service periods, the STA can enter deep sleep, reducing power consumption by up to $10\times$ compared to legacy power save.

TWT also serves a contention reduction role: by scheduling different groups of STAs to wake at different times, the AP reduces the number of simultaneously contending stations.

TWT is particularly valuable for IoT devices with small, periodic payloads (e.g., sensors reporting every few seconds). A sensor can negotiate a TWT schedule that matches its reporting interval, sleeping for $> 99$ % of the time.

Definition:
BSS Coloring and Spatial Reuse

BSS coloring assigns a 6-bit color (0--63) to each BSS, included in the PHY preamble of every frame. Stations use the color to distinguish between intra-BSS and inter-BSS frames:

Same color (intra-BSS): Defer as usual (virtual carrier sense).
Different color (inter-BSS): Apply OBSS/PD (Overlapping BSS Preamble Detection) threshold — if the received signal strength is below the OBSS/PD threshold (up to $-62$ dBm), the station may ignore the frame and transmit, enabling spatial reuse.

The OBSS/PD threshold is constrained by: $\text{OBSS/PD} \leq \max(-82 + (P_{\mathrm{ref}} - P_{\mathrm{TX}}), -62) \text{ dBm}$

where $P_{\mathrm{ref}} = 21$ dBm. This ensures that a station using a higher OBSS/PD threshold reduces its transmit power proportionally, limiting interference to other BSSs.

BSS coloring is one of the most impactful 802.11ax features for dense deployments. In a building with many APs on the same channel, legacy STAs defer for every detected frame (even from distant APs), severely limiting throughput. BSS coloring allows selective deferral, significantly improving spatial reuse.

802.11ax OFDMA Resource Allocation

Visualise how the AP allocates resource units to multiple users in an 802.11ax OFDMA transmission. Adjust the number of users and channel bandwidth to see the RU assignment. The plot shows the frequency-domain allocation of RUs, with each user assigned a different colour. Observe how the RU granularity allows efficient packing: a 20 MHz channel supports up to 9 users with 26-tone RUs, while an 80 MHz channel supports up to 37 users.

Parameters

Number of users4

Channel bandwidth (MHz)80

Example: OFDMA vs. CSMA/CA for Small Packets

Compare the channel efficiency for 10 stations each sending 100-byte packets, using (a) CSMA/CA (one at a time) vs. (b) OFDMA with 26-tone RUs on a 20 MHz channel.

Assume: 802.11ax, MCS 3 (16-QAM, $R = 1/2$ ), DIFS = 34 $\mu$ s, SIFS = 16 $\mu$ s, preamble = 40 $\mu$ s.

Solution

CSMA/CA (sequential)

Per frame:

DIFS: 34 $\mu$ s
Average backoff: $W_{\min}/2 \times \sigma = 8 \times 9 = 72$ $\mu$ s
Preamble: 40 $\mu$ s
Data (100 bytes at MCS 3 on 242-tone RU): $800 / (234 \times 4 \times 0.5) = 1.71$ symbols $\to$ 2 OFDM symbols $= 2 \times 13.6 = 27.2$ $\mu$ s
SIFS + ACK: $16 + 40 = 56$ $\mu$ s

Total per frame: $34 + 72 + 40 + 27.2 + 56 = 229$ $\mu$ s. For 10 frames: $10 \times 229 = 2{,}290$ $\mu$ s.

Total data: $10 \times 800 = 8{,}000$ bits. Efficiency: $8{,}000 / (2{,}290 \times 10^{-6}) = 3.5$ Mbps.

OFDMA (simultaneous)

All 10 stations use 26-tone RUs (9 fit in 20 MHz, need 2 OFDMA transmissions or use 40 MHz with space for all 10).

Using 40 MHz (up to 18 users with 26-tone RUs):

DIFS + backoff: $34 + 72 = 106$ $\mu$ s (once for the AP)
Trigger frame: $\sim 40$ $\mu$ s
SIFS: 16 $\mu$ s
Data (100 bytes at MCS 3 on 26-tone RU): $800 / (24 \times 4 \times 0.5) = 16.7$ symbols $\to$ 17 OFDM symbols $= 17 \times 13.6 = 231$ $\mu$ s
SIFS + multi-STA ACK: $16 + 60 = 76$ $\mu$ s

Total: $106 + 40 + 16 + 231 + 76 = 469$ $\mu$ s. Efficiency: $8{,}000 / (469 \times 10^{-6}) = 17.1$ Mbps.

OFDMA is $4.9\times$ more efficient for small packets because it eliminates per-user contention overhead. $\blacksquare$

Quick Check

What is the primary advantage of 802.11ax OFDMA compared to 802.11ac MU-MIMO for serving many users?

OFDMA achieves higher peak throughput per user

OFDMA eliminates per-user contention overhead by scheduling multiple users in a single OFDM symbol

OFDMA uses less transmit power than MU-MIMO beamforming

OFDMA does not require channel state information at the AP

Correction:

OFDMA eliminates per-user contention overhead by scheduling multiple users in a single OFDM symbol

OFDMA allows the AP to serve multiple users simultaneously within the same OFDM symbol by assigning each user a different set of subcarriers (resource unit). This eliminates the per-user overhead of CSMA/CA contention, DIFS, backoff, and preambles that would be required if users transmitted sequentially. MU-MIMO also serves multiple users simultaneously but on the same subcarriers using spatial separation, requiring more antennas and CSI. OFDMA is particularly efficient for small packets (IoT, ACKs) where the overhead would dominate.

⚠️Engineering Note

OFDMA Scheduling Challenges in Practice

While OFDMA transforms Wi-Fi into a scheduled system, several practical challenges limit its effectiveness:

Buffer status reporting: The AP needs to know each STA's queue length to make efficient RU assignments. Buffer Status Reports (BSR) consume uplink resources and may be stale by the time the AP schedules the next transmission.
Channel quality feedback: Unlike cellular OFDMA, 802.11ax does not have per-RU CQI reporting. The AP relies on the received signal strength of uplink frames to estimate per-STA channel quality, which is imprecise.
Legacy coexistence: In a mixed 802.11ax/ac/n network, legacy STAs cannot participate in OFDMA and use CSMA/CA instead. The AP must alternate between OFDMA MU transmissions and legacy single-user transmissions, reducing efficiency.
Small RU limitations: 26-tone RUs ( $\sim$ 2 MHz) support only MCS 0--7 (no 256-QAM or higher) and have limited frequency diversity, resulting in lower spectral efficiency than larger RUs.

In practice, 802.11ax APs achieve 2--3 $\times$ per-user throughput improvement in dense scenarios (vs. the theoretical $4\times$ target), primarily due to these scheduling imperfections.

Practical Constraints

•
26-tone RUs limited to MCS 0-7 (no 256-QAM)
•
No per-RU CQI feedback in 802.11ax (unlike cellular LTE/NR)
•
Legacy STA coexistence reduces OFDMA scheduling efficiency

📋 Ref: IEEE 802.11ax-2021, §26

Why This Matters: Wi-Fi OFDM and Chapter 14

The OFDM fundamentals from Chapter 14 — FFT-based modulation, cyclic prefix design, and the time-frequency resource grid — directly underpin the 802.11 PHY. The Wi-Fi OFDMA resource unit structure is the 802.11 analogue of the LTE/NR resource block discussed in Chapter 24. The key difference is that Wi-Fi OFDMA operates within a contention-based MAC framework: the AP must first win CSMA/CA access before scheduling OFDMA users, whereas cellular OFDMA is fully scheduled from the start.

See full treatment in Principle of OFDM

Common Mistake: OFDMA Is Not Always More Efficient Than CSMA/CA

Mistake:

Assuming that 802.11ax OFDMA is always superior to legacy CSMA/CA for all traffic patterns.

Correction:

OFDMA excels for many users with small packets (IoT sensors, VoIP, ACKs) where per-user contention overhead dominates. For a single user with large files (e.g., downloading a movie), CSMA/CA with A-MPDU aggregation on a full 160 MHz channel is equally efficient — OFDMA adds scheduling overhead without multi-user benefit. The AP must dynamically choose between SU and MU modes based on traffic patterns.

Resource Unit (RU)

The basic frequency-domain allocation unit in 802.11ax/be OFDMA. RU sizes range from 26 tones ( $\sim$ 2 MHz) to 2 $\times$ 996 tones (160 MHz). Each user is assigned one or more RUs within the channel bandwidth.

Related: OFDMA (Wi-Fi)

OFDMA (Wi-Fi)

Orthogonal Frequency Division Multiple Access in 802.11ax/be. The AP divides the channel into resource units and assigns them to different users, enabling simultaneous multi-user transmission within a single OFDM symbol period.

Target Wake Time (TWT)

An 802.11ax power-saving feature where the AP and STA negotiate specific times for communication. Between TWT service periods, the STA sleeps deeply, reducing power consumption by up to $10\times$ .

Related: OFDMA (Wi-Fi), BSS Coloring

BSS Coloring

A 6-bit identifier in the 802.11ax preamble that distinguishes frames from different BSSs. Combined with the OBSS/PD threshold, it enables spatial reuse by allowing stations to ignore weak inter-BSS frames and transmit simultaneously.

Related: Target Wake Time (TWT)

CSMA/CA MAC Protocol 802.11be (Wi-Fi 7)