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: [email protected], [email protected], [email protected], [email protected], [email protected], [email protected],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
doc.: IEEE 802.15-14-0554-00-0008
Submission
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
doc.: IEEE 802.15-14-0554-00-0008
Submission
Byung-Jae Kwak et al., ETRI
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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Byung-Jae Kwak et al., ETRI
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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Byung-Jae Kwak et al., ETRI
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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Byung-Jae Kwak et al., ETRI
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
doc.: IEEE 802.15-14-0554-00-0008
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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
doc.: IEEE 802.15-14-0554-00-0008
Submission
Byung-Jae Kwak et al., ETRI
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
Submission
Byung-Jae Kwak et al., ETRI
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)
doc.: IEEE 802.15-14-0554-00-0008
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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
Submission
Byung-Jae Kwak et al., ETRI
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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Byung-Jae Kwak et al., ETRI
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
doc.: IEEE 802.15-14-0554-00-0008
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Byung-Jae Kwak et al., ETRI
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
doc.: IEEE 802.15-14-0554-00-0008
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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
doc.: IEEE 802.15-14-0554-00-0008
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Byung-Jae Kwak et al., ETRI
Discussion
Sept. 2014
Slide 29
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