As WLAN access point and device vendors work toward product launches at the end of 2018, we will publish a series of blogs covering all aspects of 802.11ax technology. This first one introduces the motivation and main features of 802.11ax.
How to Improve Today’s Wi-Fi?
When deciding how to improve Wi-Fi beyond the current release, 802.11ac, the IEEE and Wi-Fi Alliance surveyed Wi-Fi deployments and behavior in order to identify obstacles to wider use and causes of dissatisfaction among user communities. The conclusion was to focus more on performance under “typical” field conditions, a departure from previous upgrades that advanced peak data rates under “excellent” field conditions. With 802.11ax, peak performance and average and worst-case performance under real-world conditions will see improvements.
Real-world conditions have changed over the years, due in no small part to Wi-Fi’s extraordinary success. Access points are everywhere, indoors and outdoors. In many areas, congestion has become a serious problem.
Some of the most congested areas are busy airports and train stations, multi-dwelling apartment buildings, and school and university settings. All these areas are characterized by overlapping coverage from access points, whether managed in the same network or uncoordinated, serving many data-hungry client devices.
To advance Wi-Fi and meet user expectations, IEEE and Wi-Fi Alliance set out to improve performance for everyone, especially in areas of overlapping coverage.
Internet service for cell phones and PCs is not the only use for Wi-Fi. A growing number of Internet of Things (IoT) sensors are using Wi-Fi for connectivity in many places, but a few limitations have restricted its adoption. To help move the market forward, new features in 802.11ax allow efficient allocation of low data-rate connections, improve IoT sensor battery life, and extend the range of Wi-Fi signals.
Wireless Internet service providers (WISPs) also use Wi-Fi for outdoor point-to-point links, and new features in 802.11ax will extend range, increase data-rates, and reduce the effects of interference.
Rolling Out Wave 1 and Wave 2
We are seeing some parallel development of 802.11ax, similar to the development of 802.11n and 802.11ac. The Wi-Fi Alliance started work on certification tests before the IEEE had finished the underlying specification. Faced with commercial pressures, access point and device vendors are expected to release “pre-standard” equipment ahead of the Wi-Fi Alliance certification, but the industry has managed this before, so we don’t expect any interoperability issues. Another consequence of time-to-market pressures is that 802.11ax will be split: the initial wave 1 release will be followed 1-2 years later by wave 2.
802.11ax Key Features
More than 50 features are in the IEEE 802.11ax standard, but not all will be certified and make it to market. Here is a high-level summary of anticipated commercial features (including both wave 1 and wave 2 features).
- Downlink and uplink OFDMA: Orthogonal Frequency-Division Multiple Access (OFDMA) increases user data rates and reduces latency, especially for large numbers of devices with short frames or low data rate requirements such as IoT devices. It is a multi-user capability, where a transmission can be divided in the frequency domain, with different groups of sub-carriers carrying frames for different destinations. OFDMA is particularly effective in raising network capacity where there are many devices, short frames, or low data-rate streams. Both uplink and downlink OFDMA is mandatory for the Wi-Fi Alliance 11ax certification.
- Downlink and uplink multi-user MIMO: Multi-user Multiple-Input, Multiple-Output (MU MIMO) is a second multi-user capability, originally introduced in 802.11ac for downlink traffic. It improves network capacity by allowing multiple devices to transmit simultaneously, making use of multipath spatial channels. 802.11ax improves on 802.11ac by increasing the size of downlink multi-user MIMO groups, allowing more efficient operation. Uplink multi-user MIMO will be added during wave 2 of the 11ax certification.
- Transmit beamforming: In this existing feature, an access point uses several transmit antennas to land a local maximum signal on a receiver’s antennas. It improves data rates and extends range. While 802.11ac access points incorporated up to 4 antennas (although the specification allowed more), 8-antenna 802.11ax access points are expected to ship in 2019.
- Higher-order modulation: The highest modulation level is extended from 256-QAM to 1024-QAM. This increases data rates under good conditions (high SNR) and results in a better user experience. Peak rates improve by 25% over 802.11ac.
- OFDM symbols: Orthogonal Frequency-Division Multiplexing (OFDM) symbols are the RF building-blocks of 802.11. For technical reasons, subcarrier spacing, symbol duration, and FFT size are all changed in 802.11ax to allow efficient operation of small OFDMA sub-channels. This is necessary for OFDMA operation, and it reduces the per-symbol overhead and increasing network capacity.
- Outdoor operation: A number of features improve outdoor performance. The most important is a new packet format where the most sensitive field is now repeated for robustness. Other features include longer guard intervals and modes that introduce redundancy to allow for error recovery.
- Reduced power consumption: Existing power-save modes are supplemented with new mechanisms, allowing longer sleep intervals and scheduled wake times to reduce the power consumption of client devices. Also, for IoT devices, a 20MHz-channel-only mode allows for simpler, less power-hungry chips that support only that mode.
- Spatial re-use: When contending for a transmit opportunity, a device is allowed to transmit over the top of a distant transmission, which would previously have forced it to wait. This increases network capacity by allowing more simultaneous transmissions in a given geographic area.
The coming years hold much promise for Wi-Fi—and challenges. Wi-Fi must continue to improve performance, overcome the unlicensed band congestion caused by its own success, and become as attractive as possible to IoT developers. The industry is responding to these demands with 802.11ax. The new standard will drive Wi-Fi until 2024 and the next 802.11 PHY amendment.