A Tutorial on IEEE 802.11ax High Efficiency WLANs

While celebrating the 21st year since the very first IEEE 802.11 “legacy” 2 Mbit/s wireless local area network standard, the latest Wi-Fi newborn is today reaching the finish line, topping the remarkable speed of 10 Gbit/s. IEEE 802.11ax was launched in May 2014 with the goal of enhancing throughput-per-area in high-density scenarios. The first 802.11ax draft versions, namely, D1.0 and D2.0, were released at the end of 2016 and 2017. Focusing on a more mature version D3.0, in this tutorial paper, we help the reader to smoothly enter into the several major 802.11ax breakthroughs, including a brand new orthogonal frequency-division multiple access-based random access approach as well as novel spatial frequency reuse techniques. In addition, this tutorial will highlight selected significant improvements (including physical layer enhancements, multi-user multiple input multiple output extensions, power saving advances, and so on) which make this standard a very significant step forward with respect to its predecessor 802.11ac.

When, in September 1990, the very first meeting of the 802.11 project was held, hardly anyone could imagine the extent to which that early initiative, devised to - verbatim quoting the original 802.11 Project Authorization Request — “develop a Medium Access Control (MAC) and Physical Layer (PHY) specification for wireless connectivity for fixed, portable and moving stations within a local area”, would have changed our connectivity habits.

Indeed, in these last 28 years, Wi-Fi — specified by the family of the IEEE 802.11 standards — has widely spread across virtually any user’s device, as well as any inhabited deployment — homes, offices, cafes, parks, airports, etc. Moreover, it has been extended with several technical facilities which have permitted its evolution from “just” a low-rate cable replacement to a full fledged comprehensive network infrastructure and a wireless access alternative to cellular connectivity.

Nevertheless, the impressive deployment success of the Wi-Fi technology is also threatening its future growth. Users are more and more demanding; networks’ and clients’ density is ever increasing, and soon the current state-of-the-art of the Wi-Fi technology might fail short in efficiently serving the foreseen customers’ base.

The evolution of the standards shows a significant increase in nominal data rates: from the “legacy” 2 Mbit/s IEEE 802.11-1997, to the 11 Mbit/s of 802.11b, the 54 Mbit/s of 802.11a/g, the 600 Mbit/s of 802.11n, and the above Gbit/s rates of the latest 802.11ac. These Wi-Fi rates have been accomplished by means of faster modulation and coding schemes, wider channels, and the adoption of Multiple Input Multiple Output (MIMO) technologies [2]. Unfortunately, the analysis of the latest 802.11ac networks shows that the further increase of Wi-Fi throughput in a legacy spectrum needs new channel access approaches rather than just widening the band or increasing the number of spatial streams and other documents of the former IEEE 802.11 High Efficiency Wireless LAN Study Group (HEW WLAN SG). Moreover, albeit being a key asset, a high nominal data rate is not fully representative for the performance of a Wi-Fi deployment. The network operation is in fact further affected by interference patterns and frequency-selective attenuation, as well as medium access inefficiencies and network configuration scenarios. And sheer capacity might not even be the main requirement for several applications and services.

A. The 802.11ax Challenge: Dense Networks

The most notable 802.11ax’s design driver is the recognition that, today, WLAN devices are deployed in very diverse environments, characterized by the presence of a massive number of terminals concentrated in localized geographic areas. Corporate offices, mass events, outdoor hotspots, shopping malls, airports, exhibition halls, dense residential apartments, stadiums, and so on, are all examples of dense environments, whose coverage requires a multiplicity of Access Points (APs) — in principle even up to hundreds — which may therefore require to be operated on (partially) overlapping channels. In such environments, the aggregate throughput is not anymore the main performance metric of interest; rather, the target should be an increase of the throughput density, i.e., the throughput-per-area which is defined as the ratio of the total network throughput to the network area.

Obviously, in such environments, the primary source of performance degradation is the massive interference. While previous efforts aimed at avoiding hidden stations (STAs) by forbidding transmissions that may potentially collide, 11ax focuses at improving spatial reuse by avoiding exposed STAs.

Apart from that, in real scenarios, networking devices rarely operate in the saturated mode, i.e., the portion of data available for transmission may be rather small. Irrespectively of the size held by an aggregated packet (within the standardized limits), there is a fixed toll to pay, in terms of time to access the channel, to separate frames and to send an acknowledgment. Thus, for small data payloads the overhead expressed in percentage of channel time may be huge, significantly degrading the application-layer throughput ultimately experienced by the end users.

Another challenge comes from the diminishing asymmetry in traffic patterns. The widespread deployment of social networks characterized by a significant amount of user-generated multimedia content, as well as applications which continuously interact with centralized cloud storage systems, pose a significant burden not only on the downlink (DL) transmission, as it was the case for traditional server-based information retrieval applications, but also on the uplink (UL). For DL the problem was partially solved in 802.11ac with DL Multi-user (MU) MIMO. For uplink, such a technique requires tight synchronization going well beyond what has been so far standardized in previous 802.11 amendments.

For these reasons, as well as for other more technical reasons discussed later on, such as an improved power consumption for battery-operated devices and support for better Quality of user Experience (QoE), in May 2013 the IEEE LAN/MAN Standards Committee launched a HEW Study Group, which was later converted into Task Group AX (TGax). This Task Group has attracted considerable interest by 802.11 stakeholders, as for instance witnessed by the relevant attendance statistics: during the Atlanta meeting in January 2016, as much as half of the IEEE 802.11 attendance credits were accumulated by this Task Group [10], with the remaining half of the crowd distributed among many additional ongoing IEEE 802.11 activities. Even though the new 802.11ax amendment is planned for finalization by 2019, in the last three years a significant amount of work has been already carried out. The specification framework document (SFD) started in 2014 and was finalized in May 2016. The first proposal for the draft 1.0 802.11ax amendment was released on December 1, 2016, while the second one appeared a year later.

B. Contribution and Organization

It is worth to remark that a final consensus on the 802.11ax specification has not been reached yet. Indeed, the initial 802.11ax 1.0 draft standard was balloted in January 2017 and received just 58% of positive votes opposed to the 75% required threshold, and as many as 7334 comments officially filed. The second draft standard obtained only 63% affirmative votes. Only the third version passed the ballot with over 85% of positive votes and 2154 comments.

Still, even if the development process has clearly not yet finished and many open issues need to be addressed before finalization, some firm landmarks have now been set. Therefore, we believe this may be the right time to report about the current status of the 802.11ax proposal and discuss the major solutions and approaches therein under consideration, in a format accessible to the wireless networking community at large.

In this tutorial paper, also leveraging our direct participation to the 802.11ax activities, the goal is threefold:

- providing a snapshot of the major solution and approaches included so far in the standardization work;

- complementing such an information with selected quantitative results which suggest the extent to which the emerging standard is able to maintain its promises of throughput quadruplication stated in the 802.11ax Project Authorization Request (PAR) document [7], and

- identifying the issues or caveats which may require further support from the research community, e.g., in terms of further ideas and/or simulation results.

This work is not the first tutorial on 802.11ax. We acknowledge that a few earlier overviews have been already written at the beginning of the development process. However, earlier tutorial papers were based on very initial ideas being discussed at that times in the 802.11ax task group, and as such are not anymore fully representative of the evolution of the 802.11ax standard. In fact, part of the initially proposed features and technical approaches have been further detailed, improved, or even superseded by the hectic standardization work carried out in the last period. In a few cases some proposals have been rejected and left to future standards. Most notably, the support for full-duplex operation, albeit popular and considered very interesting by the community, was ultimately considered out of the scope of the 802.11ax technology.

This tutorial will introduce the reader to the technical details of the proposed Orthogonal Frequency-Division Multiple Access (OFDMA) approach (including OFDMA random access). It will clearly describe the already adopted frame structure, and will give a comprehensive overview of the new features which enable overlapping Basic Service Set (BSS) management and spatial reuse — BSS coloring, usage of Quiet Time Periods and two Network Allocation Vectors, adjustment of the sensitivity threshold and the transmit power, and others. Moreover, we will give an insight into the novel power management techniques which have already become a part of the 802.11ax draft standard.

This tutorial will also try to  include numerical results obtained by the researchers from both industrial companies and the academic community. Besides, a number of open issues will be higlighted; some of which have to be solved in the framework of the development of the 802.11ax amendment and some of which will be converted into proprietary algorithms designed by each vendor individually.

The rest of the tutorial paper is organized as follows. In Section II, after a brief review of the state-of-the-art before 802.11ax,   the main characterizing features of the new technology are briefly introduced. In the subsequent sections, details on the specific enhancements suggested for the PHY layer (Section III), the major breakthroughs in the channel access operation brought about by the adoption of OFDMA and of the MU-MIMO uplink operation and the corresponding channel access modifications (Section IV), the improvements that enable spatial reuse (Section V) and the new power management solutions proposed (Section VI) are presented.

Continue to read the full tutorial at

A Tutorial on IEEE 802.11ax High Efficiency WLANs (pdf) or

A Tutorial on IEEE 802.11ax High Efficiency WLANs | IEEE Journals & Magazine | IEEE Xplore

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