ieee 802.11ax: wireless networking in high-density...

IEEE 802.11ax: Wireless Networking in High-density WLANs

Boris [email protected]

March 8, 2017Wireless NetworkingGroup Webinarhttp://wnrg.upf.edu/

2

About this Webinar

● About IEEE 802.11ax details, there are many excellent white papers / tutorials / slides that can be found just browsing the web

● In this Webinar, we will focus only in the following features:

– Dynamic Spectrum Access / Channel bonding

– MU transmissions

– Dynamic Sensitivity Control / BSS coloring / SR Opportunities

● These slides can be found at http://wnrg.upf.edu/

Will IEEE 802.11ax improve the user experience in dense WLAN deployments?

http://wnrg.upf.edu/

3

About this Webinar

High-density WLANs

Dynamic Channel Bonding

MU transmissions

Dynamic Sensitivity

Control & BSS coloring

Introduction

Final remarks

4

WLANs

● Broadband Internet access (few Gbit/s)

● ISM bands (2.4, 5 GHz)

● Decentralized Management

● Chaotic & Dense deployments

● Interference limited performance

● CSMA/CA

5

Internet

DATA

t

AP

A

B

SUCC. TX

ACK

DATA

ACK

0

SUCC. TX COLLISION

DATA

0

0

07

77

remaining backoff

14 11 8

slotted backoff countdown

DCF = CSMA/CA (slotted backoff) + Stop & Wait ARQ

6

High-density WLAN deployments

Bellalta, Boris. "IEEE 802.11 ax: High-efficiency WLANs." IEEE Wireless Communications 23.1 (2016): 38-46.

7

More traffic, and more heterogeneous

● Previsions show a clear increase in mobile / wireless traffic

– High-definition video traffic will become dominant

● The network must offer high and constant transmission rates● Traffic / content aware mechanisms, including traffic differentiation

– IoT traffic

● Many small and miscellaneous devices● High aggregate traffic

Flexible Resource Allocation

8

Next PHY/MAC amendment: IEEE 802.11ax-2019

Next steps in the WLAN evolution, < 6GHz

● Improve performance & spectrum efficiency

– High-order modulations, channel coding, spatial multiplexing

– Wider channels ~ channel bonding

– Multi-user transmissions (MU-MIMO, OFDMA)

– Interference cancellation (including full duplex)*

● Improve spatial reuse

– Dynamic channel selection

– Transmit power control*

– Dynamic sensitivity control*, BSS coloring + Spatial Reuse Opportunities

* Not likely to be included in 11ax

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IEEE 802.11ax

Feature IEEE 802.11ac IEEE 802.11ax

Frequency bands [GHz] 5 GHz 2.4 and 5 GHz

Channel widths [MHz] 20, 40, 80, 160, 80+80 The same

OFDM 64 tones / 20 MHz 256 tones / 20 MHz

OFDM symbol duration 4 μs (GI = 0.8 μs) 16 μs (GI = 3.2 μs)

Modulations BPSK, QPSK, 16-QAM, 64-QAM, 256-QAM

Adds 1024-QAM

Forward Error Coding Convolutional, LDPC optional LDPC mandatory

MU transmissions DL MU-MIMO DL & UL MU-MIMO (in RUs > 106 subcarriers)DL & UL OFDMA

OFDMA (RU sizes, subcarriers)

- 26,52,106,242,484,996,2x996

MU-MIMO stations Up to 4 Up to 8

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Comments

● OFDMA, and UL-MU-MIMO are the most disruptive features included in 11ax compared to previous high-throughput amendments (IEEE 802.11n/ac)

– From single, or few packets transmitted at each channel access, to many

11

About this Webinar

High-density WLANs


MU transmissions

Dynamic Sensitivity


Introduction

Final remarks

12

Dense WLAN deployments

● AP density (APs/m2)

– 1 AP every 25 m2

● STA density

– 1 STA every m2

– In the figure, 1 STA every 6.25 m2

– Higher if we talk about IoT devices

● Observations

– Each color represents a different channel

– Presence of walls, objects, people, and other “obstacles” should be taken into account

– The figure is optimistic, as the radius of the carrier sense range can be several times larger

5 m

5 m

carrier sense range

What are the most common patterns in terms of overlapping

WLANs?

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Collisions

● Collisions with nodes inside the carrier sense range

– Two or more nodes finish their backoff at the same time

● Unnecessary channel contention pauses due exposed nodes

● Collisions with nodes outside the carrier sense range (i.e., hidden nodes)

5 m

5 m

A

14

Exposed Nodes



– Transmission A prevents

transmissions B, C, D

– Area throughput reduction


5 m

5 m

A

B

C

D

15

Hidden Nodes




– Transmission A suffers interference from transmissions B and C

– SNR for A seems high enough to not be much affected, though…

– Does the RTS/CTS mechanism work? 5 m

5 m

A

C

B

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Comments

● OFDMA, and UL-MU-MIMO are the most disruptive features included in 11ax compared to previous high-throughput amendments (IEEE 802.11n/ac)


● High-density WLANs stress some coexistence issues, but not everything is negative, and there is room to find innovative solutions

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Improving performance / efficiency

Activity periods

Contention periods

Improving the efficiency means lower temporal spectrum occupancy, so, we also improve coexistence with other

networks, reduce collisions, exposed and hidden node problems

t

t

WLAN 1

WLAN 2

WLAN 1

WLAN 2

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About this Webinar

High-density WLANs


MU transmissions

Dynamic Sensitivity


Introduction

Final remarks

19

Wider Channels

● Allows for higher transmission rates

● Motivation: Wider channel, more data subcarriers

● Drawbacks: Increases contention with neighboring WLANs

RU Width11ax Data

Subcarriers

Rate [Mbps](64-QAM, ¾, 16us,

1 SU-SS)

Gain(vs 20 MHz)

20 234 65.812 1

40 468 131.625 2

80 980 275.625 4.1880

160 1960 551.250 8.3761

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Wider Channels

5 m

5 m

P

● Dynamic Channel bonding➔ Maximize channel width➔ Minimize contention

160 MHz

20 MHz

40 MHz

80 MHz

160 MHz

f

21


22

Wider Channels

Faridi, Azadeh, Boris Bellalta, and Alessandro Checco. "Analysis of Dynamic Channel Bonding in Dense Networks of WLANs." IEEE Transactions on Mobile Computing (2016).

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Comments

● OFDMA, and UL-MU-MIMO are the most disruptive changes in 11ax from previous high-throughput amendments


● High-density WLANs stress some coexistence issues, but not everything is negative, and there is room to find innovative solutions

● The use of wider channels only improves the spectrum utilization and allows for higher transmission rates in low-density WLAN scenarios

– Channel width adaptation

24

About this Webinar

High-density WLANs


MU transmissions

Dynamic Sensitivity


Introduction

Final remarks

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MU transmissions

STA A

STA B

STA CSTA D

STA A

STA B

STA C STA D

Downlink MU Uplink MU

● Downlink and Uplink

● OFDMA and MU-MIMO

● Centralized and Decentralized

● New frames

– Trigger, MU-RTS, MU-ACK

● Required info at the AP:

– Channel Reports

– Buffer State Reports

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MU transmissions

t

fIndep. transmissions

a) MU uplink b) MU downlink c) MU downlink

OFDMARU

MU-MIMO(RU > 106 subcarriers)

SU-MIMO

● RUs = {26,52,106,242,484,996,2x996} subcarriers● Up to 8 MU-MIMO beams● Up to min(Map,Msta) SU-MIMO streams

W

d) SU uplink

P

Scheduling?

padding

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DL MU transmission

W

RU1

RU2

RU3

STA A

STA BSTA C STA DSS

t

MU-RTS

CTS BACK

STA A

STA B

STA C STA D

DL-MU-PPDU

AIFS

TDL-MU

AP

STA A

STA B

STA C

STA D

SIFS

UL MU transmission

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t

RTS

CTS BACK

PPDU

AIFS

TDL-SU

AP

STA A

STA B

STA C

STA D

RTS

CTS BACK

PPDU

TDL-SU

4 TDL-SU > TDL-MU ?

W

RU1

STA ASS

W

RU1

STA DSS

… vs 4 SU transmissions

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MU

SU

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4 TDL-SU > TDL-MU ?

TDL-SU = 0.9415 ms, → 4 TDL-SU = 3.7660 ms

TDL-MU = 0.97350 ms

What about throughput?

MU-DL: 49.307 Mbps

SU-DL: 12.746 Mbps

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Map

= 8 antennas

Msta

= 4 antennas

W = 160 MHz

Bellalta, Boris, and Katarzyna Kosek-Szott. "AP-initiated Multi-User Transmissions in IEEE 802.11 ax WLANs." arXiv preprint arXiv:1702.05397 (2017).

32

Channel Sounding

~Number of stations

t

Trigger

AIFS

TCS

AP

STA A

STA B

STA C

STA D

1/λcs

CS reports

Trigger

CS reports

NDPA NDP

t

33

Channel Sounding

~Number of stations

t

Trigger

AIFS

TCS

AP

STA A

STA B

STA C

STA D

1/λcs

CS reports

Trigger

CS reports

NDPA NDP

t


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UL MU transmission, AP-initiated

W

RU1

RU2

RU3

STA A

STA BSTA C STA DSS

t

MU-RTS

CTS

MU-ACK

STA A

STA B

STA CSTA D

UL-MU-PPDU

AIFS

TUL-MU

AP

STA A

STA B

STA C

STA D

Trigger

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UL MU transmission

● We may have collisions between UL MU transmissions and UL SU transmissions

– STAs included in UL MU transmissions may also start a transmission at the same time

● Prioritize AP transmissions (for instance using an special EDCA Access Category)


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Buffer State Reports

● To schedule UL MU transmissions, the AP requires to know the buffer state of the stations

● 11ax supports two approaches:

– Solicited BSR: each station explicitly delivers its BSRs in any frame sent to the AP as a response to a BSR Poll send by the AP.

– Unsolicited BSR: user stations implicitly report its BSRs in the QoS Control field of any frame sent to the AP.

~Number of STAs

1/λbs

Tbs t

37

MU transmissions in 11ax, vs 11ac?


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Comments



● High-density WLANs require some new considerations for coexistence, but not everything is negative, and there is room for innovation

● The use of wider channels only improves the spectrum utilization and allows for higher transmission rates in low-density WLAN scenarios

– Channel width adaptation

● MU transmissions allow for reducing overheads, though new ones are required: Is there any optimal tradeoff?

● 802.11ax may only outperform 802.11ac in scenarios with many STAs assoc. to a single AP

39

About this Webinar

High-density WLANs


MU transmissions

Dynamic Sensitivity


Introduction

Final remarks

40

Exposed Nodes


● Unnecessary channel contention pauses due to exposed nodes

– Transmission A prevents

transmissions B, C, D

– Area throughput reduction


5 m

5 m

A

B

C

D

41Bellalta, Boris. "Throughput Analysis in High Density WLANs." IEEE Communications Letters (2016).

42

Dynamic Sensitivity Control

● Goal: reduce the number of exposed nodes

– STAs will transmit more often → Higher throughput

● How: Increasing the CSth

● Drawback: the number of hidden nodes may also increase

5 m

5 m

A

B

C

D

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● STA A is transmitting to AP A

● STA B detects the channel as busy as it has CSth = -82 dBm, and the received power is -67 dBm

● STA B defers any transmission attempt for the duration of the on-going transmission

-35 dBm

- 75 dBm

AP A

STA A

AP B

STA B

- 67 dBm

CSth = -82 dBm

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-35 dBm

- 67 dBm

- 85 dBm

- 91 dBm

- 58 dBm

- 43 dBm

SNR=56 dB

SNR=42 dB

AP A

STA A

AP B

STA B

Adjusting CSth

we are

effectivelydoubling the throughput

CSth = -62 dBm

● If STA B CSth = -62 dBm, it would have detected the channel as idle, and it would have initiated a transmissions to AP B

● SNR at both sides seems high enough

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t

t

Beacon (Ptx = 20 dBm)

-45 dBm -38 dBm -51 dBm -47 dBm

Initial measuring period

1) The STA calculates E[RSSI], and2) Updates CS

th = max(CS

th,min, min(CS

th,max,E[RSSI] + Margin))

Continuous CSth adaptation Initial measuring period

window

CSth

update CSth

update CSth

update

Avoid HN from stations in the same WLAN

Guarantees an extra exclusion area

20 dBm

E[RSSI]

CSth,max

CSth

Margin(dB)

CSth,min

46


Afaqui, M. Shahwaiz, Eduard Garcia-Villegas, Elena Lopez-Aguilera, Graham Smith, and Daniel Camps. "Evaluation of dynamic sensitivity control algorithm for IEEE 802.11 ax." In Wireless Communications and Networking Conference (WCNC), 2015 IEEE, pp. 1060-1065. IEEE, 2015.

● Using only DSC, gains of 10 % in throughput are obtained

● Gains of 20 % when combined with channel allocation and rate control

● Higher number of hidden nodes may prevent further gains

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BSS coloring

t

ACK

ACK

DATA

DATA

Tx detected

Frommy

BSS?

yes no

Defer

Ignore

RSSI>CS-

OBSS’

yes no

Defer

● The CCA’ can be set following the previous DSC algorithm● We avoid Hidden Node problems between STAs of the same WLAN

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Spatial Reuse Opportunities

● Identify Spatial Reuse (SR) opportunities by acquiring knowledge from OBSSs

– By using the BSS color

● Instead of CSth, adapt the transmission power of the “secondary” user to minimize any harmful interference to the “primary” user

● The duration of the SR opportunity is lower than the duration of the transmission from the primary user

t

t

OBSS A

BSS

OBSS A Tx

Duration of the “primary” transmission

tOBSS B

OBSS B Tx



49

Comments




● The use of wider channels improves the spectrum utilization and allows for higher transmission rates in low-density WLAN scenarios

– In high-density WLAN scenarios, the extra channel contention is harmful



● DSC, BSS coloring and Opportunistic SR seem very promising approaches to reduce the exposed node problem in high-density WLANs, and hence, improve the area throughput

– First results show performance gains far below (my) expectations. Where is the problem?

– DSC may not be included in 11ax, but maybe in pre-11ax solutions

50

About this Webinar

High-density WLANs


MU transmissions

Dynamic Sensitivity


Introduction

Final remarks

51

Upcoming 802.11 amendments

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11ad/11ay

11ax

5G

● IEEE 802.11aq: Pre-association Service Discovery

– Providing more information about the services offered by that WLAN

● IEEE 802.11ai: Fast and lightweight hand-off

– Hand-off delay < 100 ms

– Reduction of control & management frames

● IEEE 802.11ad/ay: Short-range, very high-throughput communications at 60 GHz (mmW)

– Highly directional links

– Multi Gbits / second

Multiple radio interfaces● Software Defined Radios● 5G, 11ax, 11ad/ay, others● Multi-interface networking (MPTCP)● Dynamic network selection based on QoE requirements

53

11ad/11ay

11ax

5G

LTE

Coexistence between Random-access and Scheduled-access technologies in ISM bands

Is time for WLANs to move to a (more) scheduled operation?

54

Comments




● The use of wider channels improves the spectrum utilization and allows for higher transmission rates in low-density WLAN scenarios

– In high-density WLAN scenarios, the extra channel contention is harmful



● DSC, BSS coloring and Opportunistic SR seem very promising approaches to reduce the exposed node problem in high-density WLANs, and hence, improve the area throughput

– First results show performance gains far below (my) expectations. Where is the problem?

– DSC may not be included in 11ax, but maybe in pre-11ax solutions

Future WLAN scenarios will integrate 11ax & 11ad/ay, and many other 802.11 features

55

Will IEEE 802.11ax improve the user experience in dense WLAN deployments?

56

Our research

● Understanding the dynamics and fundamental performance limits of adaptive systems

– an ultimate goal is to understand and characterize formally the implications and effects of including self-adaptive capabilities in the performance of wireless networks

● In order to operate as close as possible to optimal conditions in highly-dynamic scenarios, decisions about next actions/configurations can be made at the network edge or in the cloud

– Combining efficiently "short-term" local decisions based on partial information with much more accurate but "mid/long term" decisions based on a global view of the network is still an unsolved challenge

– Machine Learning and Information Extraction techniques from collected datasets

● Strategies, policies and protocols for maximizing the use of the spectrum resources in all dimensions (space, frequency and time)

– interference cancellation/full-duplex communications, NOMA, dynamic spectrum access, transmit power control, sensitivity adaptation and directional transmissions

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Thank you very much!!

Boris [email protected]

http://www.dtic.upf.edu/~bbellalt/

mailto:[email protected]

http://www.dtic.upf.edu/~bbellalt/

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