A Tutorial on IEEE 802.11ax High Efficiency WLANs

Abstract

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 OFDMA-based random access approach as well as novel spatial frequency reuse techniques. In addition, this tutorial will highlight selected significant improvements (including PHY enhancements, MU-MIMO extensions, power saving advances, and so on) which make this standard a very significant step forward with respect to its predecessor 802.11ac.

Full text

IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 21, NO. 1, FIRST QUARTER 2019 197
A Tutorial on IEEE 802.11ax High
Efficiency WLANs
Evgeny Khorov , Anton Kiryanov, Andrey Lyakhov, and Giuseppe Bianchi
Abstract—While celebrating the 21st year since the very first
IEEE 802.11 “legacy” 2 Mbit/s wireless local area network stan-
dard, 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 tuto-
rial 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.
Index Terms—Wireless LAN, quality of service, OFDM, IEEE
802.11ax, high efficiency WLANs, Wi-Fi, dense deployment,
OFDMA, UL MU-MIMO.
I. INTRODUCTION
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 inhab-
ited deployment — homes, offices, cafes, parks, airports, etc.
Moreover, it has been extended with several technical facili-
ties which have permitted its evolution from “just” a low-rate
Manuscript received August 17, 2017; revised March 20, 2018 and July 5,
2018; accepted August 23, 2018. Date of publication September 20, 2018; date
of current version February 22, 2019. The work of E. Khorov, A. Kiryanov,
and A. Lyakhov was supported by the Russian Science Foundation under
Agreement 16-19-10687. (Corresponding author: Evgeny Khorov.)
E. Khorov and A. Lyakhov are with the Institute for Information
Transmission Problems, Russian Academy of Sciences, Moscow 127051,
Russia (e-mail: khorov@iitp.ru; lyakhov@iitp.ru).
A. Kiryanov is with the Institute for Information Transmission Problems,
Russian Academy of Sciences, Moscow 127051, Russia, and also with the
National Research Institute Higher School of Economics, Moscow 101000,
Russia (e-mail: kiryanov@iitp.ru).
G. Bianchi is with the University di Roma Tor Vergata, 00173 Rome, Italy
(e-mail: giuseppe.bianchi@uniroma2.it).
Digital Object Identifier 10.1109/COMST.2018.2871099
cable replacement to a full fledged comprehensive network
infrastructure and a wireless access alternative to cellular
connectivity [1].
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 accom-
plished by means of faster modulation and coding schemes,
wider channels, and the adoption of Multiple Input Multiple
Output (MIMO) technologies [2]. Unfortunately, the analy-
sis 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 (see [3], [4] 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 deploy-
ment. The network operation is in fact further affected by
interference patterns and frequency-selective attenuation, as
well as medium access inefficiencies and network configura-
tion 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 recogni-
tion that, today, WLAN devices are deployed in very diverse
environments, characterized by the presence of a massive num-
ber 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 environ-
ments [5], whose coverage requires a multiplicity of Access
Points (APs) — in principle even up to hundreds [6]—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 [7], [8].
This work is licensed under a Creative Commons Attribution 3.0 License. For more information, see http://creativecommons.org/licenses/by/3.0/

198 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 21, NO. 1, FIRST QUARTER 2019
TAB LE I
LIST OF ACRONYMS
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 [9].
Apart from that, in real scenarios, networking devices rarely
operate in the saturated mode, i.e., the portion of data avail-
able 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 acknowledg-
ment. Thus, for small data payloads the overhead expressed in
percentage of channel time may be huge, significantly degrad-
ing the application-layer throughput ultimately experienced by
the end users [4].
Another challenge comes from the diminishing asymme-
try 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 rea-
sons discussed later on, such as an improved power consump-
tion 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) [8]. This
Task Group has attracted considerable interest by 802.11 stake-
holders, 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 [11]. 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 car-
ried out. The specification framework document (SFD) started
in 2014 [12] and was finalized in May 2016. The first pro-
posal 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.

KHOROV et al.: TUTORIAL ON IEEE 802.11ax HIGH EFFICIENCY WLANs 199
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, our 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 quan-
titative 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 fur-
ther 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 acknowl-
edge that a few earlier overviews have been already written at
the beginning of the development process, including [13]–[15]
as well as our previous 2015 report [16]. However, such 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.1
This tutorial will introduce the reader to the techni-
cal details of the proposed Orthogonal Frequency-Division
Multiple Access (OFDMA) approach (including OFDMA ran-
dom 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 oth-
ers. Moreover, we will give an insight into the novel power
management techniques which have already become a part of
the 802.11ax draft standard.
We will also try to make this tutorial more insightful
by including numerical results obtained by the researchers
from both industrial companies and the academic community.
Besides, we will highlight a number of open issues, some of
which have to be solved in the framework of the development
of the 802.11ax amendment and some of which will be con-
verted into proprietary algorithms designed by each vendor
individually.
The rest of this paper is organized as follows. In Section II,
after a brief review of the state-of-the-art before 802.11ax, we
1It is worth to remark that, while we are writing this paper, IEEE 802.11
is launching a Full Duplex Topic Interest Group, which means that the
standardization process will not likely start before another year or two.
briefly introduce the main characterizing features of the new
technology. In the subsequent sections, we enter into greater
detail 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).
II. 802.11AX AT A GLANCE
Before summarizing in the next Section II-B the distinguish-
ing features currently being proposed by the IEEE 802.11ax
Task Group, we start with a brief overview of the evolution
of the 802.11 standards (Section II-A). So the reader will be
able to better appreciate the next steps taken in the ongoing
standardization activity.
A. Before 802.11ax: State of the Art
In the last 20 years, a number of amendments, and specifi-
cally 802.11a/b/g/n/ac (we restrict to the ones focusing on the
“traditional” ISM 2.4 and 5 GHz bands), have been proposed
to improve the nominal data rate.
The older ones, namely 802.11a/b/g, “simply” introduce
new modulation and coding schemes so as to bring the data
rate from the original 2 Mbit/s of the “legacy” 802.11-1997
up to 54 Mbit/s in both the 2.4 GHz (802.11g) and the 5 GHz
(802.11a) ISM unlicensed bands.
The 802.11n proposal represents a significant step forward
with respect to the above early Wi-Fi standards. Data rates
significantly increased (up to a theoretical maximum of 600
Mbit/s) via a combination of techniques. These include i) the
ability to exploit channels with a width of 40 MHz, which
is twice larger than those used in previous 802.11 PHYs;
ii) the usage of higher 5/6 coding rates opposed to the previous
3/4 coding rates, and — arguably the most notable 802.11n
breakthrough — iii) the transition towards MIMO technology,
i.e., the usage of multiple antennas to transmit up to 4 spa-
tial streams simultaneously between a pair of devices, hence
significantly increasing data rates.
In addition to the raw data rate increase, 802.11n provides
several crucial improvements also at the MAC layer. Its goal
is to reduce overhead in terms of interframe spaces, pream-
bles, and control frames, which otherwise would not permit to
properly take advantage of the performance gains provided by
the newly designed PHY. Indeed, 802.11n introduces a new
Reduced InterFrame Space (RIFS) of 2 µs which can be used
instead of the 10 or 16 µs Short InterFrame Space (SIFS)
to separate transmissions of the same STA, if no response
is expected between these transmissions. Moreover, 802.11n
introduces two aggregation methods, namely the A-MSDU
(Aggregated MAC Service Data Unit) and the A-MPDU
(Aggregated MAC Protocol Data Unit). The first one appends
several aggregated packets with a single MAC header and
check sum. The second one assigns a MAC header and frame
check sum to each aggregated packet. This aggregation per-
mits the improvement of transmission reliability by allowing

200 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 21, NO. 1, FIRST QUARTER 2019
the decoding of at least some packets in case of short noise
bursts, at the expense of slightly increased overhead.
Since contention-based channel access inevitably leads to
collisions, from the very beginning IEEE 802.11 tried to add
various contention-free channel access mechanisms to the stan-
dard. Both the “historical” Point coordinated function (PCF,
obsolete now) and the subsequent Hybrid Controlled Channel
Access (HCCA) allow an AP to access the channel without
contention. Channel access coordination is accomplished by
introducing an Interframe Space called PIFS (PCF InterFrame
Space) which, being shorter than the DIFS (Distributed coor-
dination function InterFrame Space) used by the remaining
STAs, permits the AP to acquire the channel access with-
out any contention, so as to transmit data or poll the STAs
and grant them channel access. In practice, contention-free
access techniques have seen a very marginal deployment,
especially because of their inefficiency in scenarios when sev-
eral APs work in the same area. Indeed, if several APs use
PIFS, their transmissions will start simultaneously and col-
lide. This problem is partially addressed in the HCCA TXOP
Negotiation mechanism introduced in 802.11aa. The mecha-
nism allows various APs to use different time intervals for
transmission. Unfortunately, HCCA TXOP Negotiation can
only avoid collisions between APs which can communicate
with each other. Moreover, it does not reduce the collision
probabilities between an AP and the alien STAs, which still
can use random access.
The IEEE 802.11 Working Group has historically put a sig-
nificant effort to improve the Quality of Service (QoS) in
Wi-Fi networks. Specifically, the 802.11e amendment intro-
duces Enhanced Distributed Channel Access (EDCA) and
HCCA which distinguish voice, video, best effort and back-
ground traffic and serve them differently. While EDCA just
assigns different priorities to these types of traffic, the sophis-
ticated HCCA allows an AP to schedule transmissions taking
into account specific QoS requirements, like the delay bound,
the packet loss ratio, or the required bandwidth. However,
determining exact requirements is a non trivial task, and
arguably another key reason behind the scarce deployment of
the contention-free HCCA.
For many devices which use Wi-Fi (e.g., laptops and smart-
phones) power consumption is an important issue. In 802.11
networks, power management is based on alternating between
two states: awake and doze. In the awake state, a STA can
transmit and receive frames, while in the doze state, its radio
is switched off. An active STA is always awake, while a
power-saving (PS) STA alternates between these states. The
AP buffers data destined for PS STAs until the STA wakes
up and retrieves it. Many amendments introduce new power-
saving features, but most of them are related to switching off
the radio for a rather long time, i.e., for hundreds of mil-
liseconds or even for seconds. Some of them require a PS
STA to contend for the channel if it wants to retrieve data
from the AP. Such methods are inefficient in dense environ-
ments because of collisions, huge overhead and large delays.
Some other methods allow an AP and a PS STA to schedule
a series of times when the STA retrieves data from the AP.
The period of the series depends on the QoS requirements.
The tight dependence of these methods with HCCA function-
ality — specifically with the Traffic Specification (TSPEC)
information element which parametrizes QoS requirements —
prevents their usage in consumer electronics.
Finally, the 802.11ac amendment [17]–[19] was introduced
mainly with the purpose of significantly increasing the data
rate of a 10x factor with respect to 802.11n. Besides increas-
ing the number of spatial streams up to 8, 802.11ac addresses
the problem of how to cope with terminals that, for obvious
manufacturing reasons, could not deploy more than 1 or 2
antennas. To this purpose, the 802.11ac first introduces the
DL MU-MIMO, which allows an AP to assign various DL
spatial streams to different STAs — the UL MU transmis-
sion was postponed to subsequent standards owing to the tight
synchronization requirements which would have required a
significant re-design. Additionally, 802.11ac widens the trans-
mission bands up to 160 MHz (also exploting non-contiguous
80+80Mhz channels) and increases the constellation order to
256-QAM, which raises data rates up to 7 Gbps. To reduce
the header-induced overhead at such high data rates, the
amendment increases the maximal length of a frame from
65 535 (802.11n) to 4 692 480 octets. Nevertheless, for
short packets, such as instant messages, Web requests, TCP
acknowledgments, etc. the channel is still used inefficiently.
B. Main Features of 802.11ax
Similarly to the previous amendments that improve the
nominal bit rates, 802.11ax contains a new PHY protocol
with higher modulation and coding schemes. In contrast to
802.11ac, 802.11ax does not increase the number of the
MIMO spatial streams and does not widen the channel. Thus
the nominal data rates are increased up to 9.6 Gbps, which is
just 37% higher than that of 802.11ac (rather small compared
to the 10x growth of 802.11n or 802.11ac!) [20]. The desired
increase of the user throughput is achieved by more efficient
spectrum usage.
The key feature of 802.11ax is the adoption of an OFDMA
approach, an approach widely used in cellular networks, but
brand new in Wi-Fi. The rationale is that the very wide chan-
nels (80 MHz, 80+80 MHz and 160 MHz) introduced by
802.11ac suffer from frequency selective interference, which
significantly impairs the practically achievable rates. With
OFDMA, adjacent subcarriers (tones) are grouped together
into a resource unit (RU) and a sender can choose the best RU
for each particular receiver, which actually results in higher
Signal-to-Interference-plus-Noise Ratio (SINR), Modulation
and Coding Scheme (MCS) and throughput. Moreover, since
the efficiency of high data rates degrades when a STA has
only few data to transmit, advanced aggregation techniques
aimed to reduce channel access, acknowledgment (ACK) and
preamble-induced overhead become useless. Allocating nar-
row RUs for such STAs is an efficient remedy. According to
the latest TGax investigations, OFDMA provides a 6 times
higher throughput than legacy DCF [21], see Fig. 1.
OFDMA makes Wi-Fi radio access closer to the LTE one.
However in contrast to LTE, OFDMA works on top of the
legacy DCF and is coordinated by the AP. It means that having

KHOROV et al.: TUTORIAL ON IEEE 802.11ax HIGH EFFICIENCY WLANs 201
Fig. 1. OFDMA gain in the overlapped network scenario [21].
Fig. 2. An example of OFDMA transmission in 802.11ax.
accessed the channel, the AP can start a usual DL transmission,
DL MU transmission (using OFDMA, MIMO or both), or
allocate RUs for UL MU transmission.
In LTE, OFDMA is time-based, i.e., various tones corre-
spond to different user equipment during one Transmission
Time Interval (TTI). In 802.11ax, OFDMA is frame-based,
i.e., an MU frame contains data to/from different users and
various tones are assigned to the users for the entire frame
duration, see Fig. 2.
For a DL MU transmission, a PHY preamble specifies the
duration of the frame and the tone mapping between STAs.
Conversely, for an UL MU transmission, such a schedule is
specified in the preceding frame, which can be either a Trigger
frame, a new control frame which allocates the channel for UL
MU transmission, or a data frame, the header of which con-
tains scheduling information. The latter is especially useful for
acknowledging DL MU transmissions. An UL MU transmis-
sion starts exactly one SIFS after the DL frame containing a
schedule. This permits to synchronize the STAs participating
in the UL MU transmission, whatever techniques the STAs
use: OFDMA, MU-MIMO, or both.
Introducing OFDMA in Wi-Fi affects the other MAC and
PHY functionality. First, TGax changes the OFDM parameters
to improve the flexibility and the efficiency of the OFDMA
operations. Second, TGax changes the PHY frame format to
include OFDMA-related information in the PHY preamble.
Moreover, TGax continues moving MAC-layer information
to the PHY preamble, since sometimes the preamble can be
decoded even if the entire frame is corrupted. Third, OFDMA
causes numerous MAC changes related to the MU operation
and the fairness between the devices of different generations.
Apart from OFDMA, many efforts have been put to improve
throughput and to decrease power consumption in overlap-
ping and dense networks. The list of the new features includes
among others:
•BSS coloring: inherited (and extended) from 802.11ac
and 802.11ah, allows to distinguish inter- and intra-
BSS frames based on their preambles even if the frame
payloads are corrupted by collisions;
•several modifications of the legacy virtual carrier sense,
known as Network Allocation Vector (NAV);
•virtualization;
•microsleep operation, which enables a STA to switch off
its radio just for the duration of an alien frame;
•redesigned Target Wake Time, originally introduced in
802.11ah; and
•opportunistic power save.
Apart from that, a considerable volume of work has been
done to improve spatial reuse in a dense deployment by
changing the sensitivity threshold and the transmission power.
Actually, to date this topic is still the most debated one in the
TGax ongoing activities, since it might significantly influence
fairness in the network and degrade the performance of legacy
devices.
Finally, TGax reuses the concept of periodic channel reser-
vations during which only predefined STA(s) can transmit.
Originally introduced in 802.11s (Mesh coordination function
Controlled Channel Access, MCCA) [22] to avoid collisions
in mesh networks, the concept is reused by the aforemen-
tioned HCCA TXOP Negotiation in 802.11aa, the Periodic
Service Periods in millimeter-wave 802.11ad, and the Periodic
Restricted Access Window in 802.11ah [23] designed for the
Internet of Things. In 802.11ax, periodic channel reservations
(namely, the Quiet Time periods) can be used to protect direct
link communications. However, the mechanism can also be
applied for time division between BSSs in dense deployment.
Table II summarizes the main novel features of 802.11ax
which are described in greater detail in the following sections.
III. PHY: MODULATION AND FRAME FORMAT
A. Modulation
The 802.11ax PHY inherits several aspects from its
predecessor 802.11ac. Similarly to 802.11ac, it is based
on Orthogonal Frequency-Division Multiplexing (OFDM)
and supports operations in 20 MHz, 40 MHz, 80 MHz,
80+80 MHz2and 160 MHz channels.
To increase the number of tones, which is favorable for
OFDMA, TGax has quadrupled the duration of the OFDM
symbols used for the PHY payload [24] up to 12.8 µs. Such
long OFDM symbols are more resilient to the inter-user jitter
inherent in outdoor scenarios, which is very important for the
UL MU transmission which may be simultaneously performed
by several users. Moreover, longer symbols permit to reduce
the overhead due to Guard Intervals (GI). Indeed, based on the
channel conditions, an 802.11ax device can separate OFDM
symbols by the GI selected among the values {0.8 µs,1.6µs
2In contrast to continuous 160 MHz channel, an 80+80 MHz channel is
combined from two non-adjacent 80 MHz channels.

202 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 21, NO. 1, FIRST QUARTER 2019
TAB LE I I
MAIN FEATURES OF 802.11AX
and 3.2 µs}, which allows the reduction of overhead down
to 6%, opposed to the 12-25% GI overhead in the 802.11ac
standard.
The 802.11ax amendment also introduces new modulation
techniques in addition to legacy BPSK, 16-QAM, 64-QAM,
and 256-QAM. The first one is an optional 1024-QAM [25],
which may be exploited in indoor scenarios with very good
channel conditions - i.e., a high SINR. Together with forward
error correction codes (convolutional or low-density parity-
check) — which have code rates of 1/2, 2/3, 3/4 and 5/6 —
these modulations generate a palette of data rates with a max-
imum of 9.6 Gbps. Such a high rate is achieved when data is
transmitted at the highest HE-MCS11 with a code rate of 5/6
in a 160 MHz or 80+80 MHz channel with 8 spatial streams
and a GI of 0.8 µs.
Additionally, the 802.11ax amendment describes an optional
Dual Carrier Modulation (DCM) [26]. DCM enhances trans-
mission robustness by allocating the same signal on a pair
of tones, which are separated far apart in the frequency
domain. According to preliminary investigations carried out by
TGax members, such a technique helps to cope with sub-band
interference and provides more than a 2dB gain in the Packet
Error Ratio (PER) performance [26]. It should be also noted
that because of duplicating data, the usage of DCM reduces
the data rate twice, and so DCM is allowed to be used only
with the relatively robust MCS0, MCS1, MCS3 and MCS4.
B. PHY Frame Format
TGax defines 4 types of PHY frames (referred to as PPDU,
PHY Protocol Data Unit, following the amendment): for the
Single User (SU) transmission, for the extended range SU
transmission,3for the DL MU transmission and for the UL
3An extended range PPDU was designed for robust delivery and can only
be transmitted in a 20 MHz channel at one of the three lowest MCSs without
MIMO.
MU transmission. These four different frame types leverage a
baseline frame structure extended with selected fields special-
ized for the different frame types (Fig. 3). The main feature of
the DL MU transmission is that the frame contains a common
preamble describing which tones a particular receiver shall
decode to obtain its part of the Data field. Similarly, for the
UL MU transmission, the preamble is common and it is emit-
ted by all the STAs. Then, each STA sends its own part of the
Data field using a predefined set of tones (see Section IV).
For all the frame types, the preamble is duplicated in
every 20 MHz subchannel within the transmission band and
consists of two parts: the legacy part and the HE one, see
Fig. 4[28]. While the former is included for backward compat-
ibility, the latter one provides signaling for the new 802.11ax
functionality and it can be decoded only by 802.11ax devices.
The legacy part contains training fields, which synchronize
the transmitter and the receiver, and the legacy signal field
(L-SIG), which describes the parameters of the rest of the
frame. Specifically, L-SIG allows the calculation of the frame
duration. Even though the legacy devices decode the rest of
the frame with errors, they consider the channel as busy, even
if the signal strength is too low.
To simplify the 802.11ax frame detection in case of high
interference, the HE part of the preamble starts with a rep-
etition of the L-SIG field [29], which is followed by the
mandatory HE-SIG-A field, an optional HE-SIG-B field and
training fields (HE-STF and HE-LTF) needed for tuning
MIMO.
Let us consider the HE-SIG-A and HE-SIG-B fields in more
detail. HE-SIG-A provides information about MCS, band-
width, a number of spatial streams (NSTS) and some other
parameters that are needed to correctly decode the rest of the
frame. TGax continues moving some MAC signaling to the
PHY preamble, an approach indeed widely exploited starting
from 802.11ah [23]. Since the preamble has a rather rigid
structure and it is transmitted at the lowest MCS, the cost of

KHOROV et al.: TUTORIAL ON IEEE 802.11ax HIGH EFFICIENCY WLANs 203
Fig. 3. 802.11ax PHY frame format [27].
Fig. 4. Legacy preamble and HE-SIG-A are duplicated on each 20 MHz
subchannel.
Fig. 5. Repetition mode for HE-SIG-A, [30].
such additional information is high. However, the inclusion of
part of the MAC-related information in the preamble is advan-
tageous, since i) the preamble is transmitted with the most
robust MCS and ii) it can be decoded before the PHY payload
is fully received and its checksum is calculated. Specifically,
HE-SIG-A also contains information such as network (or Basic
Service Set, BSS, in terms of IEEE 802.11) Color — see
Section V-A, remaining Transmission Opportunity (TXOP)
duration, whether the frame is sent in UL or DL, etc. Apart
from that, HE-SIG-A also contains the spatial reuse parameter
(SRP) which is used to signal the sum of transmission power
and an acceptable level of interference to allow for the spatial
reuse operation as described in Section V.
Since 802.11ax networks are designed for both indoor and
outdoor deployment, transmissions are prone to the Doppler
effect mainly caused by reflections from fast moving objects,
such as cars and trains [31]. To improve the resistance to
high mobility, the amendment proposes to periodically insert
in the PHY packet payload midambles, i.e., copies of the
HE-LTF field. Thanks to midambles, the channel can be
estimated not only during the packet preamble, but also con-
tinuously throughout the packet which is very fruitful for the
high-velocity communications, i.e., when the channel quickly
varies.
In case of a ≥40 MHz channel, the HE-SIG-A field is
duplicated on each 20 MHz subchannel. In an extended range
variant of the SU frames, the content of HE-SIG-A is repeated
after an additional bit interleaving procedure [30].
In case of both UL and DL SU transmission, as well as in
a UL MU transmission, all the necessary information can be
fitted into HE-SIG-A which consists of two legacy OFDM
symbols. However, in case of a DL MU transmission, the
information for various users may differ and shall be spec-
ified for each of them separately. In this case, an additional
HE-SIG-B field of variable length is included in the frame
preamble [33], [34]. Specifically, the field contains two blocks:
one with common and one with per-user information. The
common block describes the OFDMA resource allocation,
while the per-user block consists of several subfields defin-
ing for each resource unit its MCS, the number of spatial
streams, etc.
As mentioned above, the HE-STF and HE-LTF fields are
used for MIMO. Specifically, the main purpose of the HE-STF
field is to improve the automatic gain control estimation in a
MIMO transmission, while the HE-LTF fields provide a tool
for the receiver to estimate the MIMO channel between the
set of constellation mapper outputs and the receive chains.
Similarly to the legacy PPDU, the Data field contains the
SIGNAL subfield needed to initialize the encoder/decoder
scrambler and the encoded MAC frame. The Data field is
transmitted with 4 times longer OFDM symbols.
Quadrupling the symbol duration means 4 times more cal-
culations at the receiver side, while the time available for
the receiver to do such calculations before sending back an
acknowledgment or response is limited by the SIFS. This can
bring problems for low-cost Wi-Fi devices, which will not
be able to generate an acknowledgment in time. A straight-
forward solution — increasing SIFS — was not approved
because of backward compatibility as well because it would
have decreased the channel usage efficiency. Rather, TGax
provides the possibility to extend the tail of a frame with
an extension. To minimize the overhead induced by the
extension, its duration is flexible and depends both on the
intended frame receiver and the payload size. Specifically,
when declaring its capabilities, each STA indicates which
maximal extension (0, 8 µsor16µs) is needed to pro-
cess a frame with a given MCS and a number of spatial
streams. Note that this value can be reduced if the encoded
payload is not divisible by the OFDM symbol size, and,
thus, the last OFDM symbol contains padding. Indeed, the
receiver needs less time to decode the bits obtained from
such a thin OFDM symbol. In particular, the amendment
splits the last OFDM symbol into 4 segments of equal
size. Thus, the extension can be reduced from the requested

204 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 21, NO. 1, FIRST QUARTER 2019
maximum value by the number of empty segments multiplied
by 4 µs[35], [36].
C. Open PHY Issues
In the course of the past two decades, the 802.11 standard-
ization process has focused on the introduction of new (or
improved) functionalities, but it has mostly avoided to deter-
mine how to use them. However, the performance of a network
significantly depends on how these functionalities are used,
and 802.11ax is not an exception. Having extended the set of
possible data rates, the amendment also adds new degrees of
freedom — such as DCM and shorter GIs — which affect
the transmission rate and the reliability. A high number of
options complicates the selection of the best rate defined by
a set of transmission parameters. Specifically, sophisticated
rate control algorithms (e.g., Minstrel [37]) try various MCS,
and, having obtained statistics, select the best ones for trans-
mission. A wide palette of 802.11ax options increases the
time needed to obtain statistics. Moreover, in 802.11ax dense
networks, every 20MHz sub-band may have its own level of
interference. Thus, the best rate may be different for various
sub-bands. Finally, in 802.11ax networks, the AP not only
selects an appropriate rate for its own transmission, but also
for the UL MU transmission. For that, it collects reports on
signal strength from associated devices prior to allocating UL
channel time to them. Although rate control is out of scope
of the standard, this problem is of high importance for the
vendors, and 802.11ax developers need to revisit again this
well-investigated area, owing to the new degrees of freedom
and constraints.
Another issue is that the 802.11ax PHY preamble is longer
than the legacy one. Thus it should be used only for long
transmissions which benefit from the new 802.11ax features.
Moreover, since the 802.11ax frames cannot be decoded by
legacy devices, virtual carrier sense does not work prop-
erly, which can degrade performance in scenarios with hidden
STAs. This issue needs to be addressed both by the standard
developers (see Section V) and by the community of Wi-Fi
researchers, which can design smart algorithms to protect
transmissions.
IV. MU TRANSMISSIONS &CHANNEL ACCESS
A. 802.11ax OFDMA Fundamentals
Since the design of OFDMA for 802.11 networks is a
non-trivial task, it has been investigated in many papers. For
example, [38] proposes a novel OFDMA-based MAC proto-
col called OMAX. Unfortunately, the authors consider only
random access. In contrast, TGax has designed a much more
flexible and powerful framework, which can be used for both
deterministic and random access. Let us describe it in detail.
In 802.11ax, the channel resources are allocated over time
and frequency, but in order to simplify resource management
and device operation, and to retain compatibility with legacy
devices, the OFDMA transmission is organized on a per-frame
basis. This means that a frame can carry information from or
to multiple STAs. In such a frame, various tones are assigned
TABLE III
THE MAXIMUM NUMBER OF RUSFOREACH BANDWIDTH
to different STAs but the duration of all the RUs within such
a frame is the same.
An RU can contain 26, 52, 106, 242, 484, 996 or 2x996
tones (including service ones). The entire 20 MHz band,
40 MHz band, 80 MHz band and 80+80 (160) MHz band
corresponds to a 242-tone RU, 484-tone RUs, 996-tone RUs
and two 996-tone RUs, respectively. Each wide RU can be split
into two approximately twice-narrower RUs. In turn, each of
them can be split again, separately from another one. The
only exception is that a 242-tone RU can be replaced by
two 106-tone RUs and one 26-tone RU. Because of various
problems with binary convolutional codes, see a description
in [39], multiple RU allocations for a STA are forbidden. Even
though MU-MIMO and OFDMA can be used together, both
UL and DL MU-MIMO shall be performed only in ≥106-
tone RUs. The maximum number of RUs for each bandwidth
is indicated in Table III.
Thanks to MU-MIMO, up to eight users can be assigned to
an RU. It is also possible to allocate up to four spatial streams
per user, if the total number of spatial streams does not exceed
eight.
Let us consider how the DL and UL OFDMA transmissions
are organized. In the case of the DL OFDMA transmission,
the HE-SIG-B field of the common preamble contains an RU
allocation map which is followed by per-user content fields
indicating the RUs assigned to an STA and the transmission
parameters to be used by the STA (NSTS, MCS, coding, etc).
Note that an RU can represent either an SU or an MU-MIMO
allocation. In the latter case spatial configuration shall be also
signaled to the STA.
Organizing the UL MU transmission is a more challenging
task. MU transmissions in Wi-Fi shall be synchronized in the
time domain. Since it is difficult to maintain strict time syn-
chronization because of clock drifting, an AP coordinates the
UL MU transmission as follows. The AP transmits a new type
of a control frame — Trigger frame — in which it specifies
the common parameters of the upcoming UL MU transmis-
sion (duration, GI which shall be the same for all the STAs
participating in the UL MU transmission [40]), allocates RUs
for the STAs, and defines transmission parameters for each
particular STA (MCS, coding, etc.). To achieve synchroniza-
tion, the MU transmission is performed immediately, i.e., a
SIFS after the Trigger frame [41], see Fig. 6. Since it may
take more than SIFS to prepare a UL transmission, the AP
can pad the Trigger frame [42].
For UL MU OFDMA transmissions, the AP shall receive
signals from different STAs at almost the same power level.

KHOROV et al.: TUTORIAL ON IEEE 802.11ax HIGH EFFICIENCY WLANs 205
Fig. 6. An example of UL OFDMA transmission.
For that, 802.11ax defines a power pre-correction mechanism,
according to which the AP indicates in the Trigger frame its
current transmit power and the target signal strength that the
AP is expected to receive from a STA in the following UL
transmission. Thus, having known the AP’s transmit power
and the signal strength of the received Trigger frame, the STA
can estimate the path loss to the AP and it can calculate an
appropriate transmit power for the following UL transmission.
Note that since the AP (not an STA!) selects the MCS for the
UL transmissions, each STA also includes information about
its UL power headroom, i.e., the difference between its max-
imum transmit power and its current transmit power for the
assigned MCS.
In order to be efficient, the AP shall allocate RUs only to
STAs which have data to transmit. For that, STAs report to the
AP the amount of buffered data they have. Such reports may
be requested by the AP or sent by STAs on their own [43].
Another challenge arises because the AP does not know
whether the channel is idle from the STA’s point of view.
For each STA, the AP specifies in the Trigger frame whether
the STA shall perform carrier sensing before an OFDMA
transmission or not. If carrier sensing is required, the STA
shall perform both virtual carrier sensing and physical car-
rier sensing in at least the 20 MHz channel(s) that contain(s)
subcarriers allocated for the STA. If physical carrier sensing
indicates busy medium, i.e., the STA detects high energy, it
cancels the UL transmission. The UL transmission is forbid-
den even if some but not all the subcarriers are idle. However,
in some cases the STA can neglect virtual carrier sensing,
i.e., NAV, if it has been set by a frame originating from an
intra-BSS neighbor or the STA is going to transmit ACK or
BlockAck which duration does not exceed some agreed value.
However, the STA always cancels the UL transmission if its
duration exceeds the UL MU transmission duration indicated
in the Trigger frame.
B. Performance Improvements
802.11ax also allows performing a UL MU transmission
just after a DL MU transmission, which can be useful, e.g.,
for sending acknowledgment frames simultaneously. For that,
the DL MU transmission shall also contain the Trigger frame
describing the UL RU allocations. Moreover, there is another
possibility to solicit a UL MU transmission in this case,
namely by including information in the DL PPDU MAC
header. Similarly, the AP can acknowledge a UL MU trans-
mission by sending acknowledgments via a DL MU PPDU.
Following the described ideas, 802.11ax also implements cas-
cading MU transmissions which means that within a TXOP,
DL MU and UL MU transmissions can alternate. Note that
within cascading MU transmissions the AP can exchange
frames in an MU manner with different sets of STAs.
MU transmissions in Wi-Fi shall be aligned in the time
domain. Thus, if a STA has a short frame to transmit, it
either uses padding or tries to aggregate it with another
frame. In case when the remaining space is not enough for
aggregating the whole frame, padding is the only option. To
avoid wasting channel resources, 802.11ax STA is allowed to
fragment frames in order to fill the remaining airtime with
user payload4[44]. To improve the efficiency even more,
the 802.11ax STA can also aggregate frames from different
Access Categories (ACs) [45]. A similar approach is used in
the 802.11ac DL MU MIMO [19, Sec. 9.19.2.3a].
Since the aggregation of several fragments is complicated,
TGax has found a compromise, having defined several optional
levels of HE fragmentation. The first level permits to send only
one fragment without any aggregation. The second level allows
a STA to aggregate not more than one fragment per MSDU
in an A-MPDU. Finally, the third level allows the aggregation
of two or more fragments per MSDU in an A-MPDU [46].
C. Special Trigger Frames
OFDMA permits to cope with frequency selective
interference by assigning the best subcarriers for STAs. Apart
from that, it reduces the overhead caused by backoffs, inter-
frame spaces, preambles and PHY headers, which carry
common information for all the STAs in case of a DL trans-
mission. The overhead is higher for short control frames, for
which OFDMA is especially favorable. Thus, in addition to
the basic Trigger frame for data and management frames,
802.11ax has special Trigger frames which initiate parallel
Request To Send / Clear To Send (RTS/CTS) handshakes,
request block acknowledgments from a group of STAs, and
collect beamforming reports or buffer status reports (BSR).
Let us consider how these frames are used in detail.
To protect a DL MU transmission from hidden nodes, TGax
introduces the MU-RTS/CTS handshake [47]. Thanks to the
UL MU transmissions in 802.11ax, the CTS frames can be
sent simultaneously. The main peculiarity of the MU-RTS/CTS
frames is that a CTS frame is transmitted on the primary
20 MHz, 40 MHz, 80 MHz or the entire 160 MHz or
80+80 MHz channel being duplicated on each 20 MHz sub-
channel using the legacy CTS frame format. The channel
which shall be used by a particular STA to transmit the CTS
is determined in MU-RTS and shall contain all the subcarri-
ers which will be used for the following transmission to the
STA. It is done to set NAV at all legacy STAs which receive
4Note that in legacy Wi-Fi, STAs use fragmentation only when the
frame size exceeds the fragmentation threshold. Moreover, the joint usage
of aggregation and fragmentation is explicitly forbidden.

206 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 21, NO. 1, FIRST QUARTER 2019
Fig. 7. An example of an MU-RTS/CTS exchange.
this CTS, and thus to protect the transmission from colli-
sions. The protocol allows several receivers to transmit CTS
frames simultaneously, however these CTS frames are abso-
lutely equal from a PHY perspective, thus they do not collide,
see Fig. 7. Nevertheless, such an approach has an important
limitation. Having received several equal CTSs in the same
channel, the AP cannot obtain information which receiver(s)
sent the CTS. Such a limitation may force the AP either not to
plan parallel transmissions which occupy subcarriers from the
same 20 MHz channel, or to ignore the fact that some recip-
ients may not answer with CTS. Since both the workarounds
may degrade performance, currently TGax is looking for a
better solution [48].
The 802.11ax amendment proposes an additional way for
acknowledging UL MU transmissions by sending new Multi-
STA Block ACK (BA) frames. Similarly to the existing
Multi-TID BA frame which is used to acknowledge a set of
frames from various ACs, a Multi-STA BA frame is used to
replace ACKs or BAs to several STAs [49], [50]. To shorten
transmission, a Multi-STA BA frame can be sent in a legacy
manner with only a legacy 802.11a preamble. A Multi-STA
BA can act as a BA or as an ACK.
Another new frame defined in 802.11ax is the MU Block
ACK Request (BAR) frame which is a variant of the Trigger
frame. It is used to solicit acknowledgements from multiple
STAs in the UL MU transmission instead of sending individual
BAR frames [51]. Similarly, 11ax defines GCR MU BAR to
poll acknowledgments for groupcast transmission with retries,
a novel groupcast method introduced in 11aa. In addition to the
acknowledgment, a recipient of an MU-BAR frame can trans-
mit another data or management frame if it does not exceed
the indicated UL MU duration [52].
One more variant of the Trigger frame is used to collect
BSRs. In each BSR, each STA informs the AP about the
amount of buffered traffic in a queue of the requested AC
(AC_BE, AC_BK, AC_VI, or AC_VO) or of a subset of ACs.
Finally, 11ax defines special Trigger frames used to poll
beamforming information or to request information about the
channel.
D. UL OFDMA Random Access
Besides the scheduled UL MU access described above,
TGax has designed an optional mechanism which allows
performing random UL OFDMA transmissions [53]. Such a
feature is especially important when the AP does not know
which associated STAs have data to transmit, or when an
unassociated STA wants to transmit an association request.
DCF/EDCA is not efficient for short transmissions because
of the large overhead caused by PHY headers and interframe
spaces.
The designed random access is similar to the multichannel
slotted Aloha. Specifically, a Trigger frame can allocate some
RUs for random access. Specifically if the user identifier for
some RU is 0 or 2045, the corresponding record in the Trigger
frame defines a group of contiguous RUs for random access
which can be used by associated and unassociated STAs cor-
respondingly. The RUs of a group are of the same size and
have the same transmission parameters. Along with the num-
ber of contiguous RUs, the AP can indicate that no other RU
for random access is planed in the series of cascading MU
transmissions till the end of TXOP.
To decide whether to transmit and in which RU, STAs use
the so-called OFDMA Back-off (OBO) procedure [54]. Each
STA chooses a random value from [0, OCW], where OCW is
the OFDMA contention window. If the current OBO value is
less than the number of RUs allocated for random access by
a Trigger frame, the STA randomly selects an RU from those
allocated for random access and transmits a frame in this RU.
Otherwise, the STA decreases OBO by the number of RUs
allocated for random access and waits for the next Trigger
frame containing RUs for random access.
If the transmission attempt fails, the STA doubles its OCW
until it reaches OCWMAX and selects an OBO value from
the new interval. If the transmission attempt is successful, the
STA resets its OCW to the minimum value OCWMIN .Both
OCWMAX and OCWMIN are specified by the AP in beacons
and in the probe response frames.
Since random access is less efficient than scheduled access,
it is worth to use it only for short packet transmissions and for
BSR. In the latter case, a STA having data for transmission can
generate a BSR and send it with random access to ask the AP
for channel resources. It is clear that such a scheme turns out
to be more efficient than pure UL OFDMA Random Access,
as confirmed in [55]. Nevertheless, some results from [55]are
preliminary, so the performance evaluation of such a scheme
is a topic for future research.
E. EDCA Improvements
In 802.11ax networks, OFDMA works on top of the legacy
CSMA/CA (Carrier Sense Multiple Access With Collision
Avoidance) mechanism called EDCA or DCF.5It means that to
transmit a Trigger frame, the AP shall contend for the channel
with other STAs. Consider a network with an AP and several
STAs having UL traffic. Since the number of STAs is usually
much higher than one, the AP rarely wins the contention if
the AP uses the same channel access parameters. However,
when the AP succeeds, it sends a Trigger frame to allocate
resources for the associated STAs. As shown in Section II-B,
5Since both methods are well-known and widely analyzed in the literature
(see [56], [57]), we do not describe them.

KHOROV et al.: TUTORIAL ON IEEE 802.11ax HIGH EFFICIENCY WLANs 207
Fig. 8. Percentage of OFDMA UL MU transmissions and legacy STAs
transmissions vs. the AP CW parameters [58].
OFDMA is much more efficient than EDCA. So, to achieve
higher throughput, the STAs should rarely access the channel
with EDCA but they should almost always use OFDMA. In
other words, the AP shall almost always win the contention.
Fortunately, the AP can change the EDCA parameters for
all the associated STAs by broadcasting them in beacons. So,
by setting high values for CWmin and CWmax , the AP can
almost forbid EDCA transmissions in the network.
A problem arises, if there are some legacy STAs in the
network which cannot use OFDMA transmissions. Since the
EDCA parameters cannot be set individually, setting the same
high values of CWmin and CWmax for both 802.11ax and
legacy STAs will block the legacy STAs. This may lead to a
situation, when the performance of the legacy clients signifi-
cantly degrades. Another problem is related to a misbehaving
AP, which allocates less RUs for a client of a concurrent ven-
dor. To avoid such problems, TGax introduces the second set
of EDCA parameters which is used only by those 802.11ax
STAs which were granted RUs during some preceding time
interval.
Fig. 8presents numerical results for a scenario with a
network with ten legacy STAs and ten 802.11ax STAs. AIFSN
is the same for all devices. Legacy STAs use the default CW
limits: CWmin =16, CWmax =1024. For 802.11ax STAs,
CWmin =128, CWmax =1024, while the CW limits of the
AP vary. Fig. 8shows that by tuning the EDCA parameters,
we can make sharing channel resource both fair and efficient,
i.e., 802.11ax STAs almost always use OFDMA transmis-
sions, while legacy STAs obtain as much channel time as in
a pre-802.11ax network. Such suitable values of CWmin and
CWmax at the AP are marked with red ovals in Fig. 8.The
usage of different EDCA parameter sets is studied in more
detail in [59].
TGax has also improved the RTS/CTS mechanism which
helps to mitigate collisions from hidden nodes and reduces
collision duration. Historically, the use of the RTS/CTS mech-
anism is determined by the length of the transmitted data
frame. If the frame length exceeds the RTS threshold then
the data transmission is preceded by an RTS/CTS handshake.
TGax has proposed an alternative RTS/CTS mechanism which
has two major distinctions. First, the use of the mechanism is
determined by the duration of the transmission rather than by
the length of the frame, explaining the name of the mecha-
nism — duration-based RTS/CTS threshold. It is more natural
to focus on the duration of the transmission rather than on the
packet length, because with a high MCS even a rather long
frame can be transmitted fast enough, which finally yields a
relatively high overhead caused by the RTS/CTS handshake
performed with a slow MCS. Second, the value of the duration-
based RTS/CTS threshold is under the control of the AP which
can have a better view of the network situation and it can sig-
nal the threshold value to associated STAs. In such a way, the
AP can lower the threshold if interference from hidden nodes
is suspected in a dense environment or increase it otherwise
to reduce the transmission overhead and optimize the usage
of network resources.
F. Open MU & Channel Access Issues
Having introduced OFDMA, Wi-Fi developers made Wi-Fi
similar to LTE. Obviously, this means that all the issues rel-
evant to channel resource allocation in LTE became relevant
to Wi-Fi. However, resource allocation in Wi-Fi is much more
difficult than in LTE for the following reasons.
First, traditional LTE networks operate in license bands.
This means that an operator can control interference from
neighboring cells and adjust inter-cell interference to achieve
better performance. In contrast, Wi-Fi networks operate
in license-exempt bands where nobody can guarantee the
interference level in future. This complicates channel quality
estimation and makes Wi-Fi developers design sophisticated
algorithms to reduce interference, see Section V.
Second, in LTE networks the channel is divided into
resource blocks of equal size. For the downlink channel, the
base station can select an arbitrary subset of resource blocks
to transmit some data for a user. For the uplink, the resource
blocks in the subset need to be contiguous. In Wi-Fi, the
restrictions on possible RUs are more sophisticated, which
complicates the development of optimal schedulers, i.e., algo-
rithms which allocate RUs for each STA in order to maximize
some utility function.
Third, for UL transmissions, Wi-Fi allows the increase of
the power spectral density if the STA transmits in a narrow
RU. Specifically, the STA can transmit with the same power
whatever RU it uses. Note that since the STAs are located in
different places, it does not violate the energy emitting con-
straints but brings much benefit. Indeed, the higher the power
spectral density, the higher MCS can be used. On the first
sight, this means that each Trigger frame shall allocate RUs for
all the STAs which have data in the uplink channel. However,
after some investigation it becomes clear that the problem is
much more difficult. The first issue is that according to the
standard the highest MCSs cannot be used with 26-tone RUs.
Thus by splitting the channel into too narrow RUs we may
obtain a lower throughput. The second one is the impossibil-
ity of splitting some channels into a given number of RUs.

208 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 21, NO. 1, FIRST QUARTER 2019
Fig. 9. UL throughput with various RU configuration.
For example, in case of three STAs with UL traffic, the AP
can divide a 40 MHz channel into two RUs (242-tone + 242-
tone) or into four RUs (242-tone + 2x 106-tone + 26-tone), but
not into three RUs. This means that a 26-tone RU is wasted.
Fortunately, such small RUs are favorable to be allocated for
buffer status reports transmitted with random access. Some
studies show that there is no straightforward solution for the
RU allocation and an optimal allocation of RUs depends on
the device location.
Fig. 9shows the UL throughput in an 802.11ax saturated
network operating in a 80MHz channel with ten STAs uni-
formly located in a circle of radius 35 m around the AP. The
resources are allocated in a proportionally fair manner with
the optimal static division of the channel into RUs. The hor-
izontal axis represents all the possible combinations of RUs
in lexicographical order. The left combination, i.e., combina-
tion #1, represents a case when there are zero 996-tone RUs,
zero 484-tone RUs, . . . and 37 26-tone RUs. Combination #2
represents a case with one 52-tone RU and 35 26-tone RUs.
Combination #3 has two 52-tone RUs and 33 26-tone RU.
Finally, the right combination stands for the case with the only
996-tone RU. The results show that the average throughput sig-
nificantly depends on how the channel is divided into resource
units. Although in all these cases the RUs are assigned accord-
ing to the same policy — proportionally fair — the efficiency
of channel resource usage varies more than two times. Thus,
even in the case of a baseline utility function (e.g., the geo-
metric average of the throughputs which gives a proportionally
fair resource allocation), the selection of the best RU allocation
scheme is very sophisticated (see [60]). To a greater extent,
this will be the case with other more complex QoS-aware
schedulers, like M-LDWF [61] and EXP-PF [62].
Fourth, a portion of RUs shall be allocated for the RA.
Obviously, the number of RUs allocated for the RA affects
the latency and the network capacity and shall be selected
based on some estimation of the traffic patterns. Note that in
the case of arrival of packets for uplink transmission, the STA
can use the legacy EDCA to transmit either these packets or
BSR. However, a) such transmissions are less efficient than
the OFDMA ones, and b) because of the EDCA parameters
transmitted by the AP, the time needed to access the channel
with EDCA is much longer than that with OFDMA.
Fifth, a Wi-Fi network consists of devices produced by var-
ious manufacturers. In the legacy Wi-Fi, all the STAs in the
network should use the same channel access parameters broad-
cast by the AP. Thus, all the devices have the same opportunity
to transmit.6In an 802.11ax network, the channel resources
are allocated by the AP. So a misbehaving AP can allocate
more channel time to those STAs which are produced by the
same vendor. The methods of detecting such misbehaving APs
should be a subject of further investigation.
Sixth, an open issue is how to select an appropriate duration
of an MU frame. This may affect the efficiency of the channel
usage as well as the fairness and the QoS. Moreover, an AP
shall find a trade-off between long frames favorable for heavy
data traffic and short frames efficient for random access and
for BSR.
V. OVERLAPPING BSS MANAGEMENT
AND SPATI AL REUSE
Since the dense deployment scenario is the main one for
TGax, there are a lot of debates on how to improve the
performance in case of dense networks. On the one hand,
TGax wants to decrease interference between networks, but,
on the other hand, it wants to allow spatial reuse, i.e., simulta-
neous transmissions in overlapping networks to increase total
throughput. A considerable activity is related to carrier sens-
ing, dynamic sensitivity thresholds and dynamic transmission
power control. Since the launch of TGax, about one hundred
submissions on these topics were proposed, most of which
were rejected. Here we describe the accepted ones briefly.
A. BSS Color
To determine which BSS is the originator of a frame without
decoding the entire frame, 802.11ax uses the non-unique ID
of the BSS, called the BSS color [63], which is transmitted
in the frame preamble. Initially, the BSS color field of 3 bits
length appeared in 802.11ah to reduce power consumption,
because the receiver can stop decoding a frame coming from
an alien BSS. Since the BSS color is selected randomly by
the AP, the colors of two neighboring BSSs may coincide
or collide in terms of 802.11ax. To decrease the BSS color
collision probability, TGax has agreed to increase the length
of the BSS color field to 6 bits [64]. If the collision occurs, the
STAs associated to an AP can notify it about collision and the
AP can start a procedure of changing its BSS color. For that,
it advertises the future BSS color and the moment when the
color will be changed by sending special information element
in beacons. So all the STAs, even dozing ones can obtain
information about the change of BSS color.
The identification of a BSS by the BSS color field is used
for determining channel access rules and for power saving
6Although having been standardized, the centralized channel access meth-
ods, such as PCF or HCCA, which allow the AP to poll the STAs are
not used in out-ot-the-shelf devices because of their extreme implementation
complexity and some flaws in the behavior in dense deployment.

KHOROV et al.: TUTORIAL ON IEEE 802.11ax HIGH EFFICIENCY WLANs 209
mechanisms. To disable the BSS color for a particular frame,
the BSS color field of this frame is set to zero.
B. Two NAVs
The Wi-Fi channel access follows the listen-before-talk
principle, i.e., a STA performs carrier sensing before trans-
mitting a frame. The channel is supposed to be busy in the
following cases.
1) If during carrier sensing a STA detects a frame preamble,
it considers the channel as busy for the frame duration
that is signaled in the preamble.
2) If during carrier sensing a STA detects an unknown sig-
nal at more than 20 dBm above the minimum sensitivity
level.
3) If the channel is indicated to be virtually busy.
The virtual carrier sensing in Wi-Fi, called NAV, is orga-
nized as follows. In the MAC header, a STA indicates the
NAV value, i.e., for how long the following frame exchange
will occupy the channel. Having correctly decoded the frame,
the other STAs set NAV, i.e., they consider the channel to
be busy during the indicated time. If a STA receives a frame
indicating a larger NAV value, it increases its NAV, but the
STA does not decrease NAV even if the indicated NAV value
is smaller. The STA cancels its NAV, if it receives a CF-End
frame.
In the legacy Wi-Fi, STAs do not take into account by which
frame the NAV value was set. However, this may lead to the
following misbehavior. Suppose a frame from the same BSS
sets the NAV value at a STA. After that, the STA receives
a CF-End frame coming from an Overlapping BSS (OBSS).
According to the existing rules, the STA will reset the NAV
and it will not consider the medium to be virtually busy any-
more. Since the STA may not hear an ongoing transmission
that was protected by NAV, it can start its own transmis-
sion which causes a collision. As dense deployment was not
a common scenario earlier, such a situation was not exten-
sively researched. However, this reasoning is no longer true
for 802.11ax networks. Thus, to prevent resetting NAV by CF-
End from an OBSS, 802.11ax STAs will support two NAVs:
one for their own BSS and the other for all the OBSSs, and
they will modify the NAVs separately [65].
C. Quiet Time Period
Ad Hoc and direct links7operation are promising solutions
that reduce the channel busy time. However, such operations
in the proximity of an 802.11ax network can increase the over-
all interference and cause significant performance degradation.
To address this problem, the 802.11ax amendment defines the
Quiet Time Period (QTP) mechanism. QTP allows a STA to
request the AP for a QTP which is a series of periodic time
intervals of equal duration used for ad hoc or direct links oper-
ation. The QTP is described by the offset of the first reserved
interval, the duration and period of the intervals, and the total
number of requested intervals. If the AP satisfies the request, it
7Direct links allow two STAs associated with the same AP to communicate
directly without using the AP as a relay.
disseminates information about the reserved QTP and forbids
the other STAs to access the channel during QTP.
This mechanism has been proposed rather recently, so
its description contains many open issues which should be
addressed in the very near future. Specifically, the standard
describes the only way — which is defined as optional but
without an alternative — to disseminate information about
QTP: at the beginning of a reserved time interval, the AP
broadcasts information about its duration and type of oper-
ation which is allowed during the interval. Such a behavior
has several drawbacks. First, the information is broadcast only
once, so it can be lost. Second, the type of operation does not
identify the set of STAs which can access the channel during
the interval. Finally, there is not any explicitly defined mech-
anism to silence legacy STAs which ignore novel 802.11ax
messages.
D. Adjustment of Sensitivity Threshold and Transmit Power
A possible solution to improve spatial reuse in a dense
deployment environment is by tuning carrier sensing mech-
anisms [66], e.g., by means of using Dynamic Sensitivity
Control (DSC). The idea of DSC is based on the dynamic
adjustment of the carrier sensing threshold referred to as the
DSC threshold, which determines when the STA considers the
medium to be busy. Obviously, to prevent transmissions within
a BSS from being blocked by an OBSS, the DSC threshold
should be increased. Nevertheless, to allow communication
between all devices within a BSS, the DSC threshold shall be
small enough not to miss a transmission within this BSS.
Smith [9] and Afaqui et al. [67] propose to set
the DSC threshold at the STAs to TxPower −
max
i∈BSS
PassLoss(AP ,i)−MRG, where TxPower is the
AP’s transmit power, PassLoss(AP,i) is the signal attenuation
between the AP and STA i, and MRG (margin) is a tunable
parameter with a recommended value in the range (18,
25) dB. Since it may be difficult to obtain the AP’s TxPower
and to estimate attenuation, the authors propose the following
practical implementation. Each STA maintains the average
received signal strength indicator (RSSI) value (AvgRSSI)of
beacons received from the AP and set the DSC threshold to
AvgRSSI−MRG. However, the attenuation may increase so
that the signal strength from the AP’s beacon will be less than
AvgRSSI−MRG, and the STA will start to ignore beacons.
To prevent such an undesirable behavior, it is proposed to
decrement AvgRSSI by RSSIDEC dBs (some constant value)
if several beacons are lost in a row and, thus, to automatically
decrease the DSC threshold. The authors vary the MRG and
the RSSIDEC parameter values to evaluate the efficiency
of the proposed scheme in terms of aggregated throughput,
fairness (calculated according to Jain’s fairness index), the
number of hidden nodes, and PER (Fig. 10). The results show
the increase of these metrics observed with DSC, compared
to the legacy constant carrier sensing threshold. Thus, the
gain in throughput and fairness is achieved at the cost of a
higher number of hidden nodes and, consequently, of a higher
PER. On the one hand, it is natural to think that DSC may
decrease fairness, because close to the AP STAs set a higher

210 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 21, NO. 1, FIRST QUARTER 2019
Fig. 10. Increase of Throughput, PER, and the number of hidden nodes,
with DSC [67].
DSC threshold and have more chance to transmit a packet.
However, DSC reduces the number of exposed nodes which
allows the achievement of a gain in fairness. Having analyzed
the results, the authors recommend setting MRG to 20 and
RSSIDEC to 6.
Having reduced the number of exposed STAs, DSC
increases the number of hidden STAs. To address this issue,
various methods have been proposed. One of them is using
the RTS/CTS mechanism together with DSC. This approach
is evaluated in [68] and it has been proved to be effec-
tive. In [69], the DSC approach is combined with inter-BSS
Fig. 11. Illustration of OBSS_PD and TX_PWR adjustment in a 20MHz
channel [27].
TDMA (Time Division Multiple Access) which makes trans-
missions of OBSSs orthogonal in the time domain if they
severely interfere with each other. Although DSC with TDMA
are opposite approaches and have opposite goals, the authors
show that combining DSC and TDMA demonstrates the
best performance, simultaneously increasing the average and
the worst throughput. Unfortunately, the implementation of
inter-BSS TDMA is too complicated and requires a tight syn-
chronization between the OBSSs. So the approach was not
approved by TGax.
To balance between spatial reuse and collision avoidance,
TGax decides to bind changes in the sensitivity threshold for
the OBSS frames (named as OBSS Preamble Detection thresh-
old, OBSS_PD) and the transmit power (TX) according to a
simple rule: the higher the OBSS_PD, the lower the TX. Such
a rule has a simple explanation. By default, an STA transmits
signals of power TX_PWR and considers the medium to be idle
if the signal strength is less than OBSS_PD =−82 dBm. Let
the STA receive a signal from an OBSS STA XdB stronger
than −82 dBm. This means that the attenuation between the
STA and the OBSS STA is XdB weaker than necessary for
considering the medium idle. If the STA wants to start a new
transmission in this case, it shall first increase its OBSS_PD
by XdBm, and second, it shall decrease its transmit power
also by XdB in order not to produce a huge interference at
the location of the OBSS STA (Fig. 11).
STAs may dynamically change their OBSS_PD and
TXPWR parameters. During backoff, a STA sets up its
OBSS_PD to some value. Every time, it senses the start of
a packet it suspends its backoff. Right after the STA under-
stands that this packet belongs to OBSS, it can resume backoff
even before the end of the packet, if the signal strength is less
than OBSS_PD and no other conditions (e.g., NAV) require
the channel to be considered as busy. When the STA obtains
channel access, it can start transmission with the power not
higher than that corresponding to the used value of OBSS_PD.
Such a power level is used till the end of TXOP.
The AP may specify the colors of the OBSSs for which
the described rule is applied. To achieve the maximum benefit
from spatial reuse, the rule should be applied for such BSSs,
the signal from which is much lower than that from associated
STAs. Obviously, an algorithm on how to make a decision is
beyond the scope of the standard.

KHOROV et al.: TUTORIAL ON IEEE 802.11ax HIGH EFFICIENCY WLANs 211
Fig. 12. Primary and secondary channels in 802.11ac networks.
Another option allowing spatial reuse operation is related to
Trigger frames. Specifically, the AP may allow an alien trans-
mission to overlap with the UL transmission in its own BSS,
if the received signal from such an alien transmission does not
exceed some acceptable level of interference. Such an accept-
able level of interference depends on the current interference
in the channel near the AP and on the used MCS. To allow an
overlapped transmission, in the Trigger frame the AP speci-
fies the spatial reuse power Sas the sum of its transmit power
plus the acceptable level of interference minus some margin.
Having received the Trigger frame at some power R,anOBSS
STA can start a transmission with power S−Rafter the end of
the trigger frame if such a transmission does not exceed the
end of the scheduled UL transmission. Naturally, to access
the channel, the OBSS STA needs to use backoff, resuming it
after the end of the Trigger frame and ignoring the upcoming
UL transmission.
E. Channel Bonding and Preamble Puncturing
In 802.11ac, STAs can adaptively select the bandwidth in
which a particular frame is transmitted. Specifically, the stan-
dard defines a hierarchy of channels shown in Fig. 12.Having
obtained channel access in the primary 20 MHz channel fol-
lowing the EDCA rules, a STA can expend the bandwidth by
step-by-step concatenation of the secondary channels if they
are idle. In other words, if the secondary 20 MHz channel
is idle, the STA can transmit in 40 MHz bandwidth. If both
the secondary 20 MHz and the secondary 40 MHz channels
are idle, 80 MHz bandwidth can be used. In contrast, even
if the secondary 40 MHz channel is idle but the secondary
20 MHz channel is busy, the STA can only transmit in the
primary 20 MHz channel. This limitation is especially crucial
for dense networks, where the secondary 20 MHz channel of
a BSS can be the primary 20 MHz channel of another one.
To improve the efficiency of channel bonding in dense envi-
ronment, 802.11ax introduces a new optional feature called
preamble puncturing. For an MU OFDMA transmission in a
channel greater than or equal to 80 MHz, one or more busy
20 MHz subchannels can be punctured. It means that frame
preamle is not transmitted and RUs are not allocated in these
subchannels. In dense deployment, such a feature allows using
channel resources in a much more flexible way.
F. Virtualization
One of the widespread features in modern APs is the sup-
port for multiple “virtual” APs (VAPs). This means that a
single physical device can create multiple independent BSSs,
reaching up to 32 VAPs in some equipment. This may be use-
ful, when, for example, one wants to separate a guest Wi-Fi
network from an internal corporate network without installing
an additional AP. One of the shortcomings of the existing
VAPs is that a lot of service information for all VAPs may
be the same, but it is transmitted separately by each of them.
To decrease the overhead, the 802.11ax amendment introduces
the Multiple BSSID support, which allows the sending of iden-
tical information for all the BSSs simultaneously [70], e.g., via
a common beacon. All the BSSs in the multiple BSSID use
the same BSS color, and the frames of BSSs from a Multiple
BSSID set are considered as intra-BSS frames [71].
G. Load Balancing
In dense networks, load balancing is an important problem,
since every STA has several candidate APs to associate
with. Although the problem has attracted considerable interest
among the researchers, it is out of scope of the standard, since
the decision on association is done by vendor specific algo-
rithms. In [72], some algorithms are studied in the context
of 802.11ax.
H. Open Issues With Dense Deployment
For several years, the group has been debating about the
methods which could improve performance in scenarios with
overlapping networks. Some solutions have been pushed into
the standard by several so-called special interest groups (SIGs)
which usually come to an agreement outside the IEEE 802
sessions making it difficult to accept other ideas proposed by
independent members not involved in SIGs. At the end of 2016
there was an investigation [73] which revealed a violation of
IEEE rules and ceased the operation of SIGs. Nevertheless, the
question what to do with all the accepted proposals remains
open.
Since the most severe debates were associated with the
solutions that improve performance in scenarios with overlap-
ping networks, there is a strong need now for an independent
study on whether the accepted proposals can indeed improve
performance, and in which scenarios. This can be a fruitful
research area.
This task is complicated by the lack of acceptable tools
to make an accurate performance evaluation of overlapping
networks with mathematical modeling, testbeds or simulation.
Mathematical modeling typically introduces some assump-
tions like so-called protocol interference model [74], which
unpredictably affects the obtained results. The most promis-
ing simulation platform is ns-3 [75]. However apart from
the necessary 802.11ax functionality, capture effects need be
implemented to correctly model collisions. Currently there
are some activities in this direction [76]. As for a testbed,
although some silicon manufactures have already announced
802.11ax chipsets, the first 802.11ax devices will not appear
very soon. Numerous software defined radios, such as the

212 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 21, NO. 1, FIRST QUARTER 2019
Wi-Fi Application Framework of National Instruments [77],
have very simplified MAC operations to be used in the desired
experiments.
Apart from that, even the approved mechanisms raise
many open issues related to their operation, joint usage and
optimization. First, it is not clear, how color collisions of
neighboring BSSs may affect performance in dense and highly
mobile scenarios. Second, how to use QTP (and whether it
can be used) for avoiding collisions in overlapping networks.
Third, whether and how rather heavy QTP signaling can be
optimized. Fourth, how to select an adaptive sensitivity thresh-
old based on the attenuation between the STAs in the same and
neighboring BSSs. Finally, there is no clear algorithm of joint
usage of BSS colors and Adaptive Sensitivity Threshold and
Transmit Power. A possible approach to the latter problem
is some adaptation of the MAPEL [78] algorithm, which
(i) can take protocol limitations into account and (ii) has low
complexity to dynamically reconfigure network on the flight.
VI. POWER MANAGEMENT
A. Legacy Power Management
In 802.11 networks, power management is based on alternat-
ing between two states: awake and doze. In the awake state, an
STA can transmit and receive frames, while in the doze state,
its radio is switched off. An active STA is always awake, while
a PS STA alternates between these states.
Since the AP does not know the current state of a PS STA, it
buffers all the frames (except for some real time ones) destined
to this STA. To notify the PS STAs about the buffered packets,
the AP includes a Traffic Indication Map (TIM) in beacons.
A PS STA may sleep for a long time, however from time to
time it wakes up to receive a beacon with a TIM element. It
may also wake up earlier, if it has a frame to transmit. In this
case before starting channel access, the STA shall wait for a
frame reception (but not longer than the Probe Delay which
is implementation dependent and may be comparable with the
beacon period).
If the beacon indicates that no buffered packets are destined
to the STA, it returns to the doze state. Otherwise, the STA
sends a PS-Poll frame. As a response to the PS-Poll, the AP
sends buffered frames.
Although the described concept is rather simple, it has been
designed for rather low load and tailored for random access.
In typical 802.11ax scenarios with dense networks, the high
traffic load and the large number of power-limited smartphones
and laptops, legacy power-saving mechanisms are inefficient.
First, they may hang, i.e., the PS STAs may stay in the awake
state for a long time, when traffic is delivered to other STAs.
Second, the AP cannot deliver traffic without being polled.
Third, PS-Polls allow only single-user transmission which is
less efficient than the MU one. Finally, the overhead caused
by PS-Polls is relatively huge.
The current standard also contains several methods which
allow the scheduling in advance of service periods when the
PS STAs can transmit or retrieve buffered packets from the
AP. These methods are deeply connected with the HCCA func-
tionality which is not implemented in out-of-the-shelf devices.
Apart from that, these methods are not suitable for OFDMA
transmissions.
The key idea of the improvements introduced by TGax is
that only the currently transmitting/receiving STAs need to be
awake, while all the other STAs may switch off their radio.
This can be done in the following way. First, the 802.11ax
STAs may stay in the so-called microsleep mode, i.e., they
can switch off their radio interface during some transmissions,
when they cannot be involved in the frame exchange pro-
cess. Second, TGax adapts the Target Wakeup Time (TWT),
a lightweight mechanism designed in 802.11ah to schedule
service periods, without using the HCCA functionality.
B. Microsleep
The microsleep approach was introduced in 802.11ac. In
802.11ac, the PHY header contains the Partial AID which indi-
cates the transmitter and the receiver(s) of a frame. Thus, all
the other STAs can go to the doze state for the frame duration.
802.11ax extends this idea by allowing an STA to doze dur-
ing UL transmissions or the TXOP of another STA in the same
BSS. For that the HE-SIG-A field contains such information
as the BSS Color, the remaining TXOP duration, the trans-
mission direction (UL or DL), etc. Specifically, if a frame has
the same color and it is either a UL frame or a DL MU frame
not intended for the STA,8the STA can be sure that no frames
will be transmitted to it till the end of TXOP and it can go to
the doze state.
C. TWT
In order to minimize the contention between STAs and to
reduce power consumption, TGax adapted the TWT mecha-
nism introduced in 802.11ah, a standard which adapts Wi-Fi
for the Internet of Things scenarios and requirements. TWT
allows an STA — called the TWT requesting STA — to nego-
tiate with another STA or AP — called the TWT responding
STA — periodically when the TWT requesting STA wakes up
for some time (called TWT Service Period or TWT SP) and
exchanges frames with the TWT responding STA. Thanks to
this mechanism, the TWT requesting STA can doze always
except during the TWT SP intervals. In particular, having
established TWT SPs with the AP, the STA is not required
to wake up even for beacons, which can significantly reduce
energy consumption.
Note that the synchronization of TWT SPs between STAs
is beyond the scope of the standard. Moreover, an established
TWT SP itself does not forbid other STAs to access the chan-
nel. So, TWT does not provide contention-free channel access
and the STAs transmit frames in TWT SPs using legacy chan-
nel access methods. To protect transmission from collisions,
virtual carrier sense can be used.
In 802.11ah, the TWT operation is tightly connected with
other 802.11ah enhancements such as with the Restricted
Access Window channel access and the modified control
frames, i.e., TACK, BAT, STACK, NDP Paging [23]. Since
8In case of DL MU transmission, the list of intended recipients is also in
the packet preamble.

KHOROV et al.: TUTORIAL ON IEEE 802.11ax HIGH EFFICIENCY WLANs 213
these enhancements are not supported by 802.11ax STAs,
TGax has reimplemented and extended the concept of TWT.
In 802.11ax networks, TWT SPs can be either individually
agreed or broadcast.
Individually agreed TWT SPs are negotiated between a pair
of devices. During negotiations, they transmit to each other a
special information element which contains TWT parameters
and can be interpreted as request, suggestion, demand, alterna-
tion, acceptation, dictation, or rejection. Either the AP or the
STA can teardown the TWT by transmitting a TWT Teardown
frame.
The most important TWT parameters are the start of the
first TWT SP and the Wake Interval, i.e., the interval between
two consecutive TWT SPs. These two parameters determine
the entire series of TWT SPs. Apart from them, the STAs
negotiate on the following list of parameters.
•Minimum Wake Duration indicating the minimum value
of TWT SP, after which the TWT requesting STA may
return back to the doze state even if it has not received
a frame. If needed a particular SP can be truncated even
below this value, e.g., by transmitting a frame with the
EOSP (End of Service Period) flag set up.
•Which types of frames should be transmitted within TWT
SPs.
•Whether the transmission is to be done at the primary
20 MHz channel.
•Whether TWT SP shall be protected with a NAV protec-
tion mechanism, e.g., (MU-) RTS/CTS or CTS-to-self.
•Whether the TWT responding STA can be in the doze
state outside the TWT SP.
•Whether the TWT requesting STA shall poll the TWT
responding STA at the beginning of each TWT SP to
indicate that it is awake, or the TWT responding STA
sends frames to the TWT requesting STA without being
polled.
•Whether the TWT SPs are Trigger-enabled. Trigger-
enabled TWT SPs are favorable for UL MU operation
and are only possible if TWT responding STA is the AP.
Within such TWT SPs, the AP shall send at least one
trigger frame allocating resources for the TWT request-
ing STA. Trigger-enabled TWT SPs are very fruitful for
power-saving STAs. First, they enable all benefits of UL
OFDMA transmission described in Section IV. Second,
according to the standard, having waked up, a STA can-
not immediately start a transmission without waiting for a
frame which can set up its NAV or some timeout expires.
Trigger frames allow the STA to shorten the waiting time.
The TWT requesting STA should not initiate a transmission
of frames to the TWT responding STA inside the Trigger-
enabled TWT SPs and outside any negotiated TWT SPs to
prevent collisions with ongoing hidden transmissions.
The broadcast TWT SPs are similar to the individually
agreed ones, except for small discrepancies. In particular, they
are not negotiated but they are scheduled by the AP which
distributes information about them in beacons. Those STAs
which have received this information but have not established
individual TWT SPs with the AP, should transmit information
only within the announced broadcast TWT SPs. Consequently,
Fig. 13. An example of power save with Uplink OFDMA Random Access.
these STAs may doze always except for the TWT SPs and
some beacons. The broadcast TWT SPs are very fruitful in
conjunction with UL OFDMA Random Access. Specifically,
the AP can schedule a series of Trigger frames. To notify the
STAs about the first Trigger frame target transmission time in
advance, the AP uses Broadcast TWT. To notify STAs about
the following Trigger frames, the AP raises a special flag in
every Trigger frame except for the last one. This flag means
that another Trigger frame for random access follows the cur-
rent UL transmission and DL ACKs, if any, see Fig. 13. Since
a Trigger frame also contains the duration of the following
transmission, the STAs may doze till the next Trigger frame
allocating RUs for random access [79].
Awaking for beacons may be avoided in the following way.
With the TWT signaling, the STA may negotiate with the AP
on the interval during which the STA will not wake up to
receive beacons.
D. Opportunistic Power Save
The Opportunistic Power Save (OPS) mechanism allows
an AP to split a beacon interval into several subintervals —
broadcast TWT SPs — and to provide, at the beginning of
each subinterval, information on which STAs are going to be
served in this subinterval. Based on this information, the non-
AP STAs may opportunistically go to doze state until the next
broadcast TWT SP.
This mechanism is based on the joint usage of TWT and the
legacy TIM element. TIM is used in legacy power management
mechanisms to indicate the set of STAs for which the AP has
buffered data. In OPS, TIM is transmitted by the AP together
with the broadcast TWT SP advertisement at the beginning of
the TWT SP. In this case, TIM indicates a set of STAs that
should be awake during the current TWT SP since the AP is
going to transmit to them or trigger them for UL traffic. If a
STA is not indicated in TIM, it can doze during the current
TWT SP.
The idea of OPS is close to the TIM Segmentation [23]
used in 802.11ah. However, in contrast to TIM Segmentation,
OPS reduces time granularity.
E. Power Management Open Issues
The adopted power management mechanisms provide
an excellent framework for increasing the lifetime of the

214 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 21, NO. 1, FIRST QUARTER 2019
battery-powered devices. At the same time, their usage raises
a number of questions.
Since Microsleep and Opportunistic Power Save allow a
STA to switch off its radio, they should be deliberately used
if the STA receives some QoS-sensitive traffic. When the STA
is always awake, the AP can instantly modify its schedule
decision once a packet destined for this STA arrives in the
queue. In contrast, both Microsleep and Opportunistic Power
Save make this impossible.
As for TWT, the most important issue is how to guarantee
quick and contention-less channel access for a STA during the
negotiated TWT SP in a dense environment. When the TWT
SP begins, the channel may be busy with transmissions from
the neighboring networks. Thus, in spite of obvious advantages
of TWT, its real efficiency is a subject of investigation.
VII. CONCLUSION
The 802.11ax amendment aims at challenging the densifica-
tion of Wi-Fi deployments, by targeting a significant increase
in the throughput density. In other words, it targets a greater
throughput-per-area opposed to “just” the absolute throughput
increase of past amendments via more advanced modulation
and coding schemes. As comprehensively discussed in this
tutorial, the new 802.11ax amendment, which has now reached
a relatively stable version (version 3.0 at the time of writ-
ing), introduces significant novelties and departs from the past
Wi-Fi versions significantly.
Arguably, the most disruptive innovation consists in the
adoption of OFDMA for both directions (DL and UL).
Loosely speaking, this change of channel access paradigm
brings the next Wi-Fi generation somewhat closer to how
cellular networks (in their fourth generation - LTE) oper-
ate. Still, as thoroughly discussed in the section dedicated
to the new OFDMA operation, not only the technical details
but also the deployment scenarios are very different and jus-
tify the novel (and nontrivial) mechanisms introduced by the
802.11ax amendment. Moreover, 802.11ax is not limited to
OFDMA only. In contrast, it introduces several important inno-
vations, including novel PHY functionalities, the extension of
MU MIMO also to the UL direction, new flexible mecha-
nisms mitigating interference from the overlapping networks,
and the introduction of more aggressive power management
approaches, all topics which have been addressed in detail in
this tutorial.
In addition to introducing the reader to the various (and in
some cases quite complex) technical aspects of 802.11ax, we
tried whenever possible to give further hints on open issues
interesting for industry and academia. Being a framework,
the standard provides a list of new features, potentially fruit-
ful for the efficiency of Wi-Fi networks, while the real gain
from these features is determined by vendor-specific algo-
rithms. One of the most crucial challenges is related to 11ax
OFDMA scheduler that shall take into account 11ax peculiar-
ities. Being implemented at the AP, the scheduler to a great
extent determines the overall performance of the whole BSS.
The second challenge is related to the optimal operation in
dense environment. Dynamic adjustment of sensitivity thresh-
old and transmit power is one of the most important and, at
the same time, arguable part of the standard, and its efficiency
raises many questions both at TGax meetings and in academic
papers. The third issue is related to energy saving, since it
requires finding the balance between energy consumption and
throughput. For example, both microsleep operation and TWT
can increase collision probability in dense environment. Thus,
there exist many optimization problems of the joint usage of
various 11ax components.
In the tutorial, we have described in detail these and many
other issues which should be resolved in order to implement
forthcoming standard in a real equipment. Usually, the effi-
cient solutions cannot be found without deep and thorough
investigation made by researches from the top telecommuni-
cations companies and leading universities. We believe that
our tutorial will attract further quantitative and/or foundational
attention to 802.11ax challenges from the research community.
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Evgeny Khorov is the Head of the Wireless
Networks Lab, Institute for Information
Transmission Problems, Russian Academy of
Sciences. In 2015, he was a Visiting Research
Fellow with King’s College London. For break-
through results of the joint project, in 2015 and
2017 Huawei RRC awarded him as the Best
Cooperation Project Leader. Being a voting member
of IEEE 802.11, he has contributed to 802.11ax
standard with many proposals. He has authored
over 70 papers. His main research interests are
related to 5G and beyond wireless systems, next generation Wi-Fi, protocol
design and cross-layer optimization. He has led dozens of national and
international projects sponsored by academia funds and industry. He was
a recipient of the Moscow Prize for Young Scientists in 2013 and Russian
Government Award in Science and Technology for Young Scientists, the Best
Paper Award from IEEE ISWCS in 2012, and the Best Cited Review Paper
Award from Elsevier Computer Communications in 2018. He gives keynotes
and tutorials and participates in panels at large conferences, including IEEE
Globecom in 2017, IEEE PIMRC in 2017, IEEE ICC in 2016, ISWCS in
2014, and NEW2AN in 2018. He chairs TPC of the IEEE Globecom 2018
CA5GS Workshop and IEEE BlackSeaCom 2019. He also serves as an
Editor for Ad Hoc Networks.
Anton Kiryanov received the B.S. and M.S.
degrees from the Moscow Institute of Physics and
Technology (State University) in 2011 and 2013,
respectively, and the Ph.D. degree in telecommunica-
tions from the Institute for Information Transmission
Problems, Russian Academy of Sciences (IITP RAS)
in 2017. His major research interests are connected
with Wi-Fi and cellular networks with the focus on
channel access methods and radio resource manage-
ment for quality of service provisioning. He is cur-
rently a Senior Researcher with Network Protocols
Research Lab, IITP RAS. From 2014 to 2015, he was an IEEE 802.11 vot-
ing member and took part in the development of upcoming IEEE 802.11ax
amendment by contributing a number of proposals which were accepted be
task group AX. He has authored over 15 papers published in international
journals and conference proceedings. He was a recipient of the Best Paper
Award at the International Symposium on Wireless Communication Systems
in 2012. He participates in national and international projects and makes
research within the framework of joint research projects with the leading
telecommunication companies.
Andrey Lyakhov isaFullProfessor,theDeputy
Director, and the Head of the Network Protocols
Research Laboratory, Institute for Information
Transmission Problems, Russian Academy of
Sciences. His main research interests are related
to design and analysis of wireless network proto-
cols, wireless network performance evaluation meth-
ods, and stochastic modeling of wireless networks
based on random multiple access. He has over
20 years of experience in Wi-Fi networks design
and performance evaluation. He has authored three
monographs, about 100 papers cited in Scopus, and has ten patents. He has
authored several courses on analytical modeling of network protocols and he
is a supervisor for master and Ph.D. students. He was a member of technical
and program committees of large IT conferences (ICC, MACOM, MobiHoc,
Networking, and MASS). He was a recipient of number of International
and Russian awards. He led a number of joint research projects with top
telecommunication companies and collaborative projects (e.g., FP7 ICT col-
laborative project FLAVIA “Flexible Architecture for Virtualizable wireless
future Internet Access,” from 2010 to 2012).
Giuseppe Bianchi has been a Full Professor
of networking with the School of Engineering,
University of Roma Tor Vergata since 2007.
His research activity is documented in over 200
peer-reviewed international journal and conference
papers, having received over 15000 Google Scholar
citations. He has carried out pioneering research
work on WLAN performance modeling, and is cur-
rently interested in wireless systems, network pro-
grammability, privacy and security, and performance
evaluation. He has chaired over ten international
conferences (e.g., IEEE Infocom, ACM CoNext, ITC, WoWMoM, and
LANMAN), and has coordinated several large scale European Projects (FP6-
DISCREET, FP7-FLAVIA, FP7-PRISM, FP7-DEMONS, H2020-BEBA, and
H2020-SCISSOR). He has been (or still is) an Editor for top journals in
his field, including the IEEE/ACM TRANSACTIONS ON NETWORKING,the
IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, and the IEEE
TRANSACTIONS ON NETWORK AND SERVICE MANAGEMENT.