High-Efficiency IEEE 802.11ax

High-Efficiency IEEE 802.11ax2018-07-27T07:35:26+00:00

Project Description

Introduction

This Technology Brief outlines the new IEEE 802.11ax Wi-Fi standard that pushes wireless data speeds to almost 10 Gbps. Although sometimes called “10G Wi-Fi,” the direction of 802.11ax has been to move away from increases in raw data speed and focus more on real-world Wi-Fi performance. As Wi-Fi deployments have multiplied over recent years, the problem of congestion and interference has started to have a major impact on the average user experience. With the introduction of the Internet-of-Things (IoT) about to add even more pressure on Wi-Fi networks, it was important that a new standard address the critical issues of today’s networks; capacity and efficiency!

More than just the modest increase in data speed, the 802.11ax standard promises much improved performance in high-density environments where a large number of users struggle with throughput. As well as maintaining a flexible backward compatibility with previous Wi-Fi standards, the new 802.11ax carries forward many of the features from the 802.11ac standard. In particular, the developing 802.11ax standard greatly expands the multi-user features and includes new power saving mechanisms that can prolong the battery life of many client devices.

The Evolution of Wi-Fi

From the first IEEE 802.11b networks back in 1999 to today’s extensive 802.11ac deployments, Wi-Fi networks have grown and developed to cope with the increased demands for wireless devices and services. The speed of Wi-Fi networks has progressed from 11 Mbps to 54 Mbps, and then to 150 Mbps for single-stream connections. Since 2013 the 802.11ac standard has again increased speeds as well as spatial streams up to an impressive 6.97 Gbps of bandwidth for 8 streams.

The continued explosion in the number and diversity of wireless devices has still placed great stress on 802.11ac networks. In particular, the rapid adoption of devices with a heavy reliance on voice, video and other bandwidth-intensive applications, are driving the need for more capacity. The issue with the shared medium of wireless is that it is not just raw data speed that is the problem. Despite the impressive speed of current 802.11ac networks, they become inefficient with a large number of users in high-density deployments. In locations such as airports and sports stadiums, the throughput for many Wi-Fi users can slow to a frustrating trickle. To solve this problem, the new developing IEEE 802.11ax standard promises not only to increase Wi-Fi connectivity speeds, but also boost multi-user performance, provide better spectrum reuse, as well as improve device power management for longer battery life.

The 802.11ax standard has therefore been called High-Efficiency Wireless (HEW), with the target to increase average throughput per user more than four times in high-density environments, and generally provide a much improved experience for the average Wi-Fi user.

The 802.11ax Enhancements

The best way to appreciate the 802.11ax enhancements is to compare the standard’s features directly to those of 802.11ac Wave 2. One of the main changes has been to cover wireless operation in both the 5 GHz and 2.4 GHz bands, where 802.11ac only covered the 5 GHz band. As with all previous IEEE standards, there remains backward support for 802.11a/b/g/n/ac devices so that 802.11ax access points and clients can all coexist in the same network.

The following table outlines the main feature differences between 802.11ac Wave 2 and 802.11ax.

Feature 802.11ac Wave 2 802.11ax
Radio Bands 5 GHz 2.4 GHz and 5 GHz
Multi-User Operation Downlink MU-MIMO Downlink MU-MIMOUplink MU-MIMOMU-OFDMA
Max. Spatial Streams 8 8
Beamforming Explicit Sounding Explicit Sounding
Channel Widths 20, 40, 80, 80+80, 160 MHz 20, 40, 80, 80+80, 160 MHz
Subcarrier Spacing 312.5 kHz 78.125 kHz
OFDM FFT Sizes 64, 128, 256, 512 256, 512, 1024, 2048
OFDM Symbol Duration 3.2 µs 12.8 µs
OFDM Cyclic Prefix(Guard Interval) 0.8, 0.4 µs 0.8, 1.6, 3.2 µs
Dynamic Bandwidth Allocation Yes Yes
Non-Adjacent Channel Bonding Yes Yes
Max. Modulation 256 QAM 1024 QAM
Max. Data Rate 6.933 Gbps 9.607 Gbps

The greater speed of 802.11ax is provided by the 1024 QAM modulation and the support for up to eight spatial streams. Whereas 802.11ac also included support for up to eight spatial streams, no more than four has ever been implemented, but it is expected that eight spatial streams will be common for 802.11ax access points. The increase in the number of OFDM subcarriers and the reduced subcarrier spacing, together with the greater OFDM FFT sizes and longer symbol time, allow for improved robustness and efficiency in multipath-fading environments.

The expanded multi-user support is an area that is critical for the improvement in network efficiency. The 802.11ax standard not only supports beamforming and downlink MU-MIMO, it adds uplink MU-MIMO and a new multi-user Orthogonal Frequency Division Multiple Access (OFDMA) mode. The introduction of OFDMA enables 802.11ax networks to maintain a much better performance in high-density environments and is a mechanism that is familiar to those knowledgeable about LTE (cellular) radio networks. Essentially, OFDMA removes the need for carrier sense multiple access (CSMA) protocols to avoid transmit collisions by providing contention-free access to multiple clients for both uplink and downlink. The 802.11 CSMA protocols are known to be a major cause of inefficiency when a large number of access points and clients exist in a high-density deployment. Therefore, the use of OFDMA in 802.11ax networks offers an immediate gain in efficiency without the need for CSMA protocols.

The 802.11ax implementation of OFDMA divides channels into smaller “Resource Units” (RUs) of a predefined number of subcarriers, and then assigns the RUs to multiple client users. This enables an 802.11ax access point to have complete control of uplink and downlink transmissions to multiple clients simultaneously.

As well as the multi-user OFDMA support, 802.11ax introduces another feature that increases the efficiency of high-density network deployments where a large number of access points are operating in a limited area. In this situation, access point Basic Service Sets (BSSs) can overlap when using the same channel, leading to wireless contention and interference problems. The 802.11ax standard implements a “color code” for each BSS that is transmitted in the signal preamble, enabling clients to detect when transmissions are from an overlapping BSS. In an enterprise network, the ability to detect each BSS color code enables clients and access points to set specific signal-detection thresholds and transmit power levels, which provides better management of contention and interference. The result is an improvement in overall network performance and a more efficient use of spectrum resources.

Finally, to address device power management issues the 802.11ax standard includes a mechanism for extended sleep states that reduce power consumption. An 802.11ax access point allows clients to request a specific Target-Wakeup Time (TWT) to transmit or receive frames, rather than rely on periodic beacons. This enables client devices to have much longer sleep states without having to wake up to receive beacons, resulting in significant power savings. Also, the client TWTs can be scheduled and controlled by the access point to both manage contention in the network as well as accommodate delay-sensitive traffic.

Conclusion

The new 802.11ax standard does not promise a complete revolution, but it does represent another significant evolution for Wi-Fi networks. Instead of huge leaps in data transmission speeds, the focus has been on improved user experience and better management of network resources. For high-density networks, there are major improvements in multi-user support with the expectation of more than four times the throughput for individual clients. For client devices, new power management features will help extend battery life.

As 802.11ax access points become available over the next year or two, current-standard client devices will immediately be able to take advantage of many new features. The improvement in network efficiency, better range and coverage, and extended support for indoor and outdoor deployments will provide a huge boost for Wi-Fi service providers and enterprise networks.

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