doc.: IEEE 802.15-14-0554-00-0008

Submission

Byung-Jae Kwak et al., ETRI

Sept. 2014

Slide 1

Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

Submission Title: Updates on Fully Distributed Synchronization Scheme for PACDate Submitted: Sept. 14, 2014Source: [Byung-Jae Kwak, Kapseok Chang, Moon-Sik Lee]1, [Junhyuk Kim, Kyounghye Kim, June-Koo Kevin Rhee] 2

Affiliation: [ETRI, Korea]1, [KAIST, Korea]2

Address:E-Mail: bjkwak@etri.re.kr, kschang@etri.re.kr, moonsiklee@etri.re.kr, kim.jh@kaist.ac.kr, khye.kim@kaist.ac.kr, rhee.jk@kaist.edu,Re:

Abstract: Description of the latest enhancement of the fully distributed synchronization scheme for PAC and give heads up on issues related network synchronization.

Purpose: DiscussionNotice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15.

doc.: IEEE 802.15-14-0554-00-0008

Submission

Byung-Jae Kwak et al., ETRI

Updates on Fully Distributed Synchro-nization Scheme for PAC

Sept. 2014Athens, Greece

Sept. 2014

Slide 2

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Byung-Jae Kwak et al., ETRI

Executive Summary (1/2)• Synchrony among PDs is the fundamental

assumption in PAC• No discovery, no data communication without

synchrony• Sync performance has significant implication

for other designs– Ex: frame length, preamble design of RB, length of

guard period, etc.– Synchronization is a prerequisite for other

progress

Sept. 2014

Slide 3

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Byung-Jae Kwak et al., ETRI

Executive Summary (2/2)• What is not change?: The basic concept.

– Structure of Sync. Period– Timing reference signal is transmitted using ran-

dom access– Scalable over large range of # PDs

• What have been changed?– No more collision detection Use inter-arrival time

instead– Better efficiency: smaller timing reference signal– More fairness: new backoff method

Sept. 2014

Slide 4

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Collision Detection vs. Inter-Arrival Time• Objective: to adapt to PD density

– Ex: Too many collision: CW is too small– Ex: Too short inter-arrival time: CW is too small

• Collision Detection– Not reliable if two timing reference signals with a big power

difference collide– Requires frequency sync: could be a problem for “Initial

Synchronization Procedure”– Collision is a good indicator of PD density, but is not fool-

proof• Inter-arrival time

– Does not require special field– More reliable indicator of PD density then collision detection

Sept. 2014

Slide 5

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Firefly• Source

– O. Simeone, U. Spagnolini, Y. Bar-Ness, S. Strogatz, “Distributed Synchronization in Wireless Networks,” IEEE Signal Processing Magazine, vol. 25, no. 5, Sept. 2008, pp. 81-97.

– R. E. Mirollo, S. H. Strogatz, “Synchronization of pulse-coupled bio-logical oscillators,” SIAM J. Appl. Math., vol. 50, no. 6, pp. 1645-1662, Dec. 1990.

• Ideal assumptions: Single hop, no collision, no path-loss, full duplex, etc.

• Interesting math problem that provides valuable insights: You only need to make it work for real problems! ;-)

• Random access based distributed sync: Only plausible scheme for scalable fully distributed D2D like PAC.

Sept. 2014

Slide 6

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The Sync Period• Frame: periodic time resource of fixed duration

• Sync period– Comprises backoff slots– Where timing reference signal is transmitted

Sept. 2014

Slide 7

Syncperiod

Frame

Syncperiod

time

Sync period

Backoff slot

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Timing Reference Signal• Consists of three fields

– Preamble: packet detection, AGC, frequency off-set, frame sync, channel estimation, etc.

– Timing offset indication field: # backoff slots– CW indication field: CW of transmitter

Sept. 2014

Slide 8

Preamble Timing offsetIndication field

CWIndication field

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Transmission of Timing Reference Signal

• Use random access• Transmission must be completed within sync period

Sept. 2014

Slide 9

Timing reference signal

Sync period

Beginning of 9th backoff slot

TimingReference

signal

Sync period

Beginning of transmissionof timing reference signal

End of transmissionof timing reference signal

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Random Access Procedure

Sept. 2014

Slide 10

No

No

Yes

Yes

Start

Initialize CW

Choose random number n;N := WC

n = 0?

CCA for one backoff slot

Backoff slot idle?

N := N – 1; n := n – 1;

Tx timing reference signal

Yes

N = 0?

Update CW

No

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Random Access Procedure: Ex 1• 3 PDs, CW = 16

Sept. 2014

Slide 11

Sync period

Backoff slot

A

B

C

9 8 7

3 2 1

7 6 5 3 2 1 0

3 26

4 3

Timing reference signal

02

n=N=

n=N=

n=N=

12 11 10

7 6 5 3 2 1 0/16

1 0

4

04

9

13 12 11 10 9 8 7 6

5 4 3 27 6 58 4 3

5 4

15

2 1 0/16

3 2 1

14 13 12

6 5

0/167

8 7 6 5 4 3 2 1 011 10 9 8

6 5

4

15 14 13 12 11 10

15 14 13 12111

010

47 6 5

19

9 8

3

* *

* *

* * * * *

* * *

* *

* * *

t1 t2 t3 t4 t5 t6 t7 t9 t10t8 t11 t12 t13 t14 t15

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Random Access Procedure: Ex 2• PD A performing CCA and transmitting timing refer-

ence signal.

Sept. 2014

Slide 12

Syncperiod

Frame

Syncperiod

01234*

7 6 5 412340/8 7 6 512346 5

012301234 ** **

3

*

0/8N=

n=

CCA

Dev.A

t1 t2 t3 t4 t5 t6 t7 t8 t9 t10 t11 t12 t13 t14 t15

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Byung-Jae Kwak et al., ETRI

Update of CW

 

Sept. 2014

Slide 13

MIAT≔𝛽𝐴 ∙MIAT+(1− 𝛽𝐴) ∙ IAT

CWoth≔𝛽𝐵 ∙CWoth+(1−𝛽𝐵) ∙CW

doc.: IEEE 802.15-14-0554-00-0008

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Byung-Jae Kwak et al., ETRI

Phase Update (1/5)• Frame vs. phase (timing)

Sept. 2014

Slide 14

Syncperiod

Frame

Syncperiod

ϕ ϕ

ϕ ϕ

ϕ

0o

90o

180o

270o

0 t

360o

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Phase Update (2/5)

• 2 hop example

Sept. 2014

Slide 15

A B C

A

B

C

t1

t1 t2

t2

Timing reference signal

Timing reference signal0

0

0

ϕ

ϕ

ϕ

t

t

t

doc.: IEEE 802.15-14-0554-00-0008

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Phase Update (3/5)• Phase update using “concave down” function

Sept. 2014

Slide 16

ϕ

x

Dϕ2Dϕ1

ϕ1 ϕ2 ϕ3 ϕ4

e

e

x1

x2

x3

x4

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Phase Update (4/5)• Phase update using “180o rule”

– If a PD receives a timing reference signal and its phase is smaller than 180o at the time of reception, it maintain its phase.

– If a PD receives a timing reference signal and its phase is greater than 180o at the time of reception, it updates its phase to 360o.

• Other rules tried: “360o rule”, averaged timing, etc.

Sept. 2014

Slide 17

doc.: IEEE 802.15-14-0554-00-0008

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Phase Update (5/5) • Phase update in the

presence of timing offset (backoffs)

Sept. 2014

Slide 18

A

B

C

Timing reference signal

0

ϕ

t

D

0

t10 t2 t3

t

t

t

ϕ

ϕ

ϕ

Sync period

(1)

(2)

(3)

(4)

(5)

(6)

(7)

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Byung-Jae Kwak et al., ETRI

Synchronization Stages

Sept. 2014

Slide 19

Initial synchronization

Maintaining synchronization

Re- synchronization

Start

doc.: IEEE 802.15-14-0554-00-0008

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Initial Synchronization Procedure• Scan for timing reference signal• No sync found starts my own timing make transi-

tion to maintaining synchronization procedure• Single timing found follow the sync make transi-

tion to maintaining synchronization procedure • Multiple timing found randomly follow one of the

timings found follow a procedure to achieve net-work with neighboring PDs (Next page)

Sept. 2014

Slide 20

doc.: IEEE 802.15-14-0554-00-0008

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Byung-Jae Kwak et al., ETRI

Multiple Timing• When does this happen?

– Multiple PDs initializing at the same time (more on this later)– Multiple groups of PDs with different timing meet (more

common)• Behavior of PDs

– Upon receiving a timing reference signal, PDs perform MIAT update, update, oscillator phase (timing) update

– Oscillator phase update is performed according to the “180o rule”

– If a PD updates its oscillator phase, it does not transmit tim-ing reference signal in the current frame.

– This behavior is repeated until synchrony is achieved, and make transition to maintaining synchronization procedure

Sept. 2014

Slide 21

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Synchronization Performance• Average of 10 independent runs• 500 x 500 m2 area (multi-hop)• Timing reference signal = 3 backoff slots

Sept. 2014

Slide 22

180o rule Concave down

Single-hop (1000 PDs) 108.1 36.3

Single-hop (83 PDs) 54.2 23.4

Multi-hop (1000 PDs, Clus-tered random drop) 53.8 2 out of 10 runs did

not converge

Multi-hop (1000 PDs,Uniform random drop) 82.3 2 out of 10 runs did

not converge

and 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Phase diff

f(x)

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Multiple PDs Initializing at the Same Time

• When does this happen?• Rarely, if ever!• Ex: Geomagetic storm caused by Solar Eruption1

• We should be more concerned about multiple groups of PDs meeting

Sept. 2014

Slide 23

1: Latest major eruption 2014.09.10. Ejects billions of tons of hydrogen and helium ions, electrons, and protons, and causes Northern lights, but also disturbance in radio communications.

(Source: NASA) (Source: Wikipedia)

doc.: IEEE 802.15-14-0554-00-0008

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Maintaining Synchronization• Fine adjustment of timing error due to drift, in-

terference, etc.• Detection of lost synchrony• Merging two (or more) networks when they

meet– Quite common– We do not want interruption of communica-

tions– Belong to re-sync?

Sept. 2014

Slide 24

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Merging Networks When They Meet

• When does this happen? All the time.

Sept. 2014

Slide 25

When train arrives Busy commercial area

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Merging Networks When They Meet

Sept. 2014

Slide 26

Disruptiveor seamless?

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Re-synchronization• What is it?

– When? Why? How?• Disruptive or seamless?

– Stop communication altogether?– Different frame length?– Do we need a scheme different from initial

sync procedure?

Sept. 2014

Slide 27

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Byung-Jae Kwak et al., ETRI

How Does PAC Handle This?

• How do we gracefully fail if we have to?

Sept. 2014

Slide 28

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Byung-Jae Kwak et al., ETRI

Discussion

Sept. 2014

Slide 29