A Tutorial on IEEE 802.11ax High.pptx

A Tutorial on IEEE 802.11ax High
Efficiency WLANs
Evgeny Khorov, Anton Kiryanov, and Andrey Lyakhov
Institute for Information Transmission Problems,
Russian Academy of Sciences.
Giuseppe Bianchi
Professor
School of Engineering,
University of Roma.
Presented by
Sujan Chandra Roy
IEEE Communications Surveys & Tutorials, 2019 1

Outline
• Background and Contribution
• Main Feature of 802.11ax
• Phy: Modulation and Frame Format
• MU Transmissions and Channel Access
• Overlapping BSS Management and Spatial Reuse
• Power Management Solution
• Conclusion
2

Outline
• Conclusion
3

Background
IEEE802.11n (Wi-Fi 4) and IEEE802.11ac (Wi-Fi 5) protocol is used
for WLAN.
 IEEE802.11n- interference and less number of channel.
 IEEE802.11ac- can’t provide high throughput for long distance.
Due to the rising demand, a significant number of access points
(APs) is often employed to provide coverage and high performance.
To improve the throughput in high-density scenarios (dense WLAN)
IEEE 802.11ax standard is , officially marketed by the Wi-Fi Alliance
as Wi-Fi 6 (2.4 GHz and 5 GHz).
It is also known as High Efficiency Wi-Fi.
4

Challenge of 802.11ax in Dense Networks
The main problem of dense network is the performance
degradation due to massive interference.
Another challenge is the diminishing asymmetry in traffic
patterns for both downlink (DL) and uplink (UL) transmission.
 downlink MU-MIMO: An AP may transmit concurrently to
multiple stations
 uplink MU-MIMO: An AP may simultaneously receive from
multiple stations
The problem of DL was partially solved in 802.11ac with DL
Multi-user multiple input multiple output (MU MIMO).
Downlink (DL) and uplink (UL) transmission problem was
solved in 802.11ax
To improve throughput in dense network by reducing the
interference.
Dense network (Corporate offices, mass events, outdoor hotspots, shopping malls, airports, exhibition halls, dense
residential apartments, stadiums)
1
1
5

Contribution
This paper presented an overview of the IEEE802.11ax
technology for WLAN researcher.
The details overview of this paper is-
 Orthogonal Frequency-Division Multiple Access
(OFDMA) approach
 Frame structure
 BSS coloring
 Usage of Quiet Time
 Adjustment of the sensitivity threshold and the
transmit power Periods
6

Outline
• Conclusion
7

Before 802.11ax
IEEE 802.11n
 Data rates significantly increased than other standard (up to a
theoretical maximum of 600 Mbit/s)
 Channels with a width of 40 MHz
 The transition towards MIMO technology (4 spatial streams)
IEEE 802.11ac
 Increasing the data rate of a 10x factor with respect to
802.11n.
 Channels with a width upto 160 MHz
 The transition towards DL MU-MIMO (spatial streams up to 8 )
 Increases the maximal length of a frame than 11n
8

Main Feature of 802.11ax
The main feature of 802.11ax is
 A new PHY protocol with higher modulation and coding schemes
 Orthogonal frequency-division multiple access (OFDMA)
approach that is used in cellular networks (LTE).
 LTE- OFDMA is time-based
 11ax- OFDMA is frame-based
With OFDMA, adjacent subcarriers (tones) are grouped together into
a resource unit (RU).
As a result sender can choose best RU which results in
 Higher SINR (Signal-to-Interference-plus-Noise Ratio).
 Higher MCS (Modulation and Coding Scheme).
 Higher throughput.
The desired increase of the user throughput in 11ax is achieved by
more efficient spectrum usage.
9
There are maximum of 9 RUs for 20 MHz bandwidth, 18 in case of 40 MHz and more in case of 80 or 160 MHz bandwidth
2
2

Main Feature of 802.11ax
OFDMA provides a 6 times higher throughput than legacy DCF.
OFDMA works on top of the legacy DCF and is coordinated by the AP.
10
Fig. 1. OFDMA gain in the overlapped network
scenario
 In 11ax, AP can start a usual DL transmission using OFDMA, MIMO
or both
 And UL MU transmission allocate the resource unit (RUs).

OFDMA in 11ax
OFDMA is a multi-user version of OFDM enabling concurrent AP
communication (uplink & downlink) with multiple clients by assigning
subsets of subcarriers, called Resource Units (RUs) to the individual clients.
The main benefit of OFDMA is that
 It allows an AP to allocate the whole channel to a single user at a time or
It may partition a channel to serve multiple users simultaneously.
OFDMA is ideal for low bandwidth applications and results in better
frequency reuse, reduced latency, and increased efficiency.
11

OFDMA Transmission in 11ax
12
Fig. 2. An example of OFDMA transmission in
802.11ax
For a DL MU transmission, a PHY preamble specifies the duration of the
frame and the tone mapping between STAs.
An UL MU transmission starts exactly one SIFS (Short InterFrame Space)
after the DL frame containing a schedule.
This permits to synchronize the STAs participating in the UL MU
transmission
UL MU transmission, such a schedule is specified in the preceding frame

OFDMA Effects in Wi-Fi
13
 OFDMA in Wi-Fi affects the other MAC and PHY functionality.
 First, TGax (Task Group AX)changes the OFDM parameters
 Second, TGax changes the PHY frame format to include
OFDMA-related information in the PHY preamble.
 Third, OFDMA causes numerous MAC changes related to the
MU operation and the fairness between the devices of
different generations.
 TGax reuses the concept of periodic channel reservations during
which only predefined STA(s) can transmit to protect direct link
communications.
 Apart OFDMA, many efforts have been put to decrease power
consumption in overlapping and dense networks.

Outline
• Conclusion
14

Modulation
802.11ax PHY inherits several aspects from 802.11ac. Such as
 Orthogonal Frequency-Division Multiplexing (OFDM)
 Supports operations upto 160 MHz channels.
 Used longer OFDM Symbol duration (upto 12.8 µs)
 Based on the channel conditions, an 802.11ax device can
separate OFDM symbols by the GI (Guard Intervals )
selected among the values {0.8 µs, 1.6 µs and 3.2 µs}.
 Frequency Modulation is 1024 QAM with MCS 10, 11.
 11ax describes an optional Dual Carrier Modulation (DCM).
15
 High rate (9.6 Gbps) 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.

PHY Frame Format
TGax defines 4 types of PHY frames
 Single User (SU) transmission-used for a single user.
 Extended range SU transmission -used a single user, but further
away from the Access Point (AP) such as in an outdoor scenario.
 DL MU transmission
 UL MU transmission
16
Fig. 3. 802.11ax PHY frame format
 contains training
fields which
synchronize the
transmitter and
the receive
 L-SIG allows the
calculation of the frame
duration.
 If the signal strength is
too low, they consider the
channel as busy.
HE part of the
preamble starts with
a repetition of the L-
SIG field in case of
high interference.
mandatory
needed for tuning MIMO.
MCS, bandwidth, a
number of spatial
streams (NSTS) and some
other parameters
Optional

Open PHY Issues
The performance of a network significantly depends on how phy
frame functionalities are used.
For 11ax
 DCM and shorter GIs — which affect the transmission rate
and the reliability
DCM enhances transmission robustness by allocating the same
signal on a pair of tones.
Remember that 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.
Another issue is that the 802.11ax PHY preamble is longer than
the legacy one.
17

Outline
• Conclusion
18

802.11ax OFDMA Fundamentals
OFDMA enable 11ax access point to communicate with multiple
devices simultaneously.
This will work by dividing each Wi-Fi channel into smaller sub-channels
known as Resource Units (RU) .
An RU can contain 26, 52, 106, 242, 484, 996 or 2x996 tones.
The OFDMA transmission is organized on a per-frame basis.
In 11ax, up to eight users can be assigned to an RU due to MU-MIMO.
19

RU Locations with Channel Widths
OFDMA allows sub-carriers in a channel bandwidth to be
grouped into smaller portions called “Resource Units” (RU).
These individual RU’s are assigned to different stations
In Wi-Fi 6, subcarrier spacing is 78.125 KHz, which is four times
narrower than 802.11ac (312.5 KHz).
Number of tones = (BW in MHz) ÷ (0.078125 MHz).
The above formula gives us total tones of 256, 512 and 1024 for
20MHz, 40MHz and 80MHz respectively.
To condense, a single RU consists of minimum 26 tones and
maximum of 996 tones.
20

21
RU Locations with Channel Widths
 Each 26 tone RU corresponds to approximately ~2MHz, 52 tones
to ~4Mhz, 106 tones to ~8Mhz and so on.
2 MHz and 26 tones

Resource Unit Map
22
 Correlation between RU’s and Channel bandwidth
 The number of OFDMA users for a particular tone at any given
bandwidth
 Fields with user value as 1 is a SU
(single user) case, where whole
spectrum is allocated to one user.
maximum of 9 users are
supported with 26 tone
 These RU allotment decisions are dynamically made by the AP based
on the client’s traffic type and its available amount for transmission.

OFDMA Downlink (DL) Transmissions
• DL OFDMA - The AP transmits packets to multiple STAs
simultaneously using a different RU for each STA.
23
RU size and frequency allocation, the modulation
MCS and the number of spatial streams

OFDMA Uplink (UL) Transmissions
UL OFDMA - Multiple STAs transmit packets to an AP
simultaneously, with each STA using a different RU.
UL are more complex because the traffic must be transmitted
simultaneously from multiple stations to the AP.
In UL, AP collects the data transmission intention from the STAs and
allocates RUs to them via its own scheduling policy.
This process is performed in a random-access manner.
24
Figure 4. Example of an UL OFDMA transmission in 802.11ax
1. AP broadcasts
2. STAs ask the AP to allocate RUs
1. When sending a buffer state request (BSR), each STA selects one RU
and transmits the BSR to the AP using the selected RU.
2. STAs compete for RUs in the frequency domain.
Sending BSR without collision
1. Assigned a subset of the RU
The actual UL OFDMA
transmission begins with
a (Trigger frame) TF sent
by the AP.
Finally, the AP sends a
block ACK (BA) to the
STAs

Open MU & Channel Access Issues
Channel resource allocation in Wi-Fi is much more difficult than in LTE.
25
LTE Wi-Fi
licensed frequency bands Unlicensed frequency bands
control interference Can’t control interference
channel is divided into resource blocks of
equal size.
DL-arbitrary subset of resource blocks
UL-the resource blocks in the subset need
to be contiguous
algorithms which allocate RUs for each
STA in order to maximize the utility
function.
DL-same as 11ac
UL-in a narrow RU the increase of the
power spectral density to transmits the
data.
 The higher the power spectral density, the higher MCS can be used for UL
transmission.
 But there is some problem for the higher MCS and narrow RUs
1. Highest MCSs cannot be used with 26-tone Rus
2. It is the impossibility of splitting some channels into a given number of RUs.
a. 40 MHz channel into two RUs (242-tone + 242- tone)
b. four RUs (242-tone + 2x 106-tone + 26-tone), but not into three RUs

Open MU & Channel Access Issues
Wi-Fi network consists of devices produced by various
manufacturers.
Previous standard- All the STAs in the network should use the
same channel access parameters broadcast by the AP
11ax- the channel resources are allocated by the AP
 The efficiency of the channel usage as well as the fairness and
the QoS depend on the how to select an appropriate duration of
an MU frame
TGax has also improved the RTS/CTS mechanism which helps to
mitigate collisions from hidden nodes and reduces collision
duration.
26

UL Throughput in an 802.11ax
80MHz channel and ten STAs uniformly located in a circle of radius
35 m around the AP
The resources are allocated in a proportionally fair manner
27
The results show that the average throughput significantly depends on how the
channel is divided into resource units.
Finally, concluded that the selection of the best RU allocation scheme is very
sophisticated

Outline
• Conclusion
28

Overlapping BSS Management and Spatial Reuse
The main problem in Overlapping BSS is interference.
 In 11ax, TGax wants to decrease interference in overlapping
networks to increase total throughput by allowing spatial reuse
(simultaneous transmissions)
11ax AP deployed in dense device environments will support large
number of user, increase throughput, and reduce power
consumption simultaneously. These include
 BSS Color
 Two NAVs
 Quiet Time Period
 Adjustment of Sensitivity Threshold and Transmit Power
 Channel Bonding and Preamble Puncturing
 Virtualization
 Load Balancing
29

BSS Color
To determine which BSS is the originator of a frame without decoding the
entire frame, 802.11ax uses the BSS color.
11ax radios are able to differentiate between BSSs using a BSS color
identifier when other radios transmit on the same channel.
An AP can inform all of its associated clients about a BSS color change in an
Action frame called the BSS color change announcement frame.
An 802.11ax access point has the ability to change its BSS color if the color
is same (color collision).
30
Additionally, an associated 802.11ax
client may send a color collision report to its
associated access point if the client detects
a color collision
Intra-BSS frame
transmission
Inter-BSS frame transmission

Two NAVs
In the previous standard of Wi-Fi, STAs do not take into account by
which frame the NAV value was set which causes a collision.
To avoid collision in dense environment, 802.11ax STAs will support
two NAVs:
one for their own BSS and
the other for all the OBSSs
31

Quiet Time Period (QTP)
Ad Hoc and direct links operation are promising solutions that
reduce the channel busy time.
In 11ax, this operation can increase the overall interference and
cause significant performance degradation.
To address this problem,11ax proposed QTP.
QTP is a mechanism for enhancing STA-to-STA transmission
performance and it could be enabled by setting NAV for other STAs
32

Adjustment of Sensitivity Threshold and
Transmit Power
The spatial reuse operation relies on Dynamic Sensitivity Control
(DSC) .
The idea of DSC is based on the dynamic adjustment of the carrier
sensing threshold which determines the STA position.
To balance between spatial reuse and collision avoidance, TGax
decides to bind changes in the sensitivity threshold for the OBSS
frames and the transmit power (TX).
33
 Sensitivity threshold can be increased
only if the TX power is decreased by
the same value
OBSS
Preambl
e
Detectio
n
threshold
 OBSS STA X is stronger than -82 dBm
 attenuation is weaker than necessary f
considering the medium idle.
 To produce less interference
increase its OBSS_PD by X dBm
decrease its transmit power also
by X dB
 If the signal strength is less than OBSS_PD =
-82 dBm. Then considers the medium to be
idle.

Attenuation Estimation
In 11ax, 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 (margin).
MRG (margin) is a tunable parameter with a recommended value
in the range (18, 25) dB.
If the attenuation increase, then the STA will start to ignore
beacons
To prevent this problem, proposed a decrement AvgRSSI by
RSSIDEC dBs (some constant value) to automatically decrease the
DSC threshold.
34

Results for DSC Threshold at the STA
35
Gain in throughput and fairness is achieved at the cost of a
higher number of hidden nodes
Finally, the authors recommend setting MRG to 20 and
RSSIDEC to 6.

Channel Bonding and Preamble Puncturing
36
Fig. 5. Primary and secondary channels in 802.11ac
networks
 In 11ac, a STA can expend the
bandwidth by step-by-step
concatenation of the
secondary channels if they
are idle.
 In 40MHz channel, if
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.
 To improve the efficiency of channel bonding in dense
environment, 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.

Virtualization
Modern APs is the support for multiple “virtual” APs (VAPs).
To decrease the overhead, the 802.11ax introduces the Multiple BSSID
support.
37
 In 11ax, the load balancing decision on association is done
by vendor specific algorithms.
Load Balancing

Outline
• Conclusion
38

Legacy Power Management
In typical 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.
Hang
Cannot deliver
Less efficient
Overhead caused
The key idea of the improvements in legacy power management is
1. Currently transmitting/receiving STAs need to be awake
2. In the awake state, an STA can transmit and receive frames.
The 11ax STAs
 Stay microsleep mode
Adopts Target Wakeup Time (TWT)
39

Microsleep Mode
 Microsleep approach was introduced in 802.11a
In microsleep mode, STAs can switch off their radio interface
during some transmissions, when they cannot be involved in the
frame exchange process.
802.11ax extends this idea by allowing an STA to doze (its radio is
switched off) during UL transmissions or the TXOP of another STA
in the same BSS.
40

Target Wakeup Time (TWT)
In order to minimize the contention between STAs and to reduce
power consumption, TGax adapted the TWT mechanism.
In 802.11ax networks, TWT Service Period (SPs) can be either
individually agreed or broadcast.
Individually agreed TWT SPs are negotiated between a pair of
devices
The broadcast TWT SPs are similar to the individually agreed ones,
except for small discrepancies. In particular, they are not
negotiated
As for TWT, the most important issue of is how to guarantee quick
and contention-less channel access for a STA during the negotiated
TWT SP.
41

Outline
• Conclusion
42

Conclusion
This paper presented a quantitative and/or foundational
attention to 802.11ax challenges from research
community.
802.11ax promises to improve the average data
throughput per user in dense environments.
43

44
Table: Main novel features of 802.11ax

A Tutorial on IEEE 802.11ax High.pptx

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