Jun Liu, Zhong Zhou, Zheng Peng and Jun-Hong Cui
Computer Science & Engineering Department, University of
Connecticut
IEEE Globecom 2010

Outline
•
Introduction
•
Challenge
•
Goals
•
Networks architecture
•
Mobi-sync
•
Simulation
•
Conclusion

Introduction
•
Underwater Wireless Sensor Networks (UWSNs)

Introduction
•
Transmission rate
• WSN: 3 x 10 8 m/s
•
UWSN: 1500 m/s
•
Propagation delay
•
Time synchronization
3:00
A
3:00
B
3:10

Introduction
•
Time synchronization
•
Clock divergence
•
Clock drift
• Clock offset
C i
( t )
 a i t
 b i

Challenge
•
Long delays significantly affect the synchronization accuracy
•
Propagation delays are changing continuously in mobile
UWSNs
• synchronization even more difficult
•
Energy consumption is heavy of acoustic transmissions, so energy efficiency is very important

Goals
•
A time synchronization scheme for mobile UWSNs
• high accuracy
• energy efficiency

Networks architecture
•
Surface Buoys :
•
GPS receivers to obtain global time references
•
Super Nodes :
• communicate with the surface buoys and get synchronized
• offer time and speed information to ordinary nodes
•
Ordinary Nodes :
• synchronized with neighbors

Mobi-sync
•
Mobi-Sync consists of three phases
•
Phase 1: propagation delay estimation
•
Message exchange and delay calculation
•
Phase 2: linear regression
•
Phase 3: calibration

Mobi-sync
•
Phase 1
•
Message exchange
Ordinary node
Super node ”A”
T
1
T
4
T
6
SR
RS
1
T
2
T
3 t r1 t r2
RS
2
T
5
One run message exchange t
T
1
, T
3
T
2 r1
, T
, t r2
4
,
, T
T
5
6
: sending time of SR,RS
1
,RS
2
: receiving time of SR,RS
1
,RS
2
: the first ,second response time

Mobi-sync
•
Phase 1
•
Delay calculation d
1 d
2 d
3
: are propagation distance of SR
: are propagation distance of RS
1
: are propagation distance of RS
2
L
1
,L
2
: straight-line distance super node “A” move relatively to the ordinary node during t r1
, t r d
3
T
6
T
4
T
1
Ordinary node d
2 d
1
T
5
θ
L
2
β
T
3
L
1
T
2
Super node ”A”

Mobi-sync
C i
( t )
 a i t
 b i d
3
T
6
T
4
T
1
Ordinary node d
2 d
1
T
1
T
2 t r1
T
3
T
4
T
5
T
6
1ms 5ms 7ms 11ms13ms 17ms time t r2
T
5
θ
L
2
β
T
3
L
1
T
2
Super node ”A”
V p
: propagation speed

Mobi-sync d
3
T
6
T
4
T
1
Ordinary node d
2 d
1
T
5
θ
L
2
β =0 assumption
β
T
3
L
1
T
Super node ”A”
T
6
T
4
T
1
Ordinary node d
3 d
2 d
1
T
5
θ
L
2
T
3
α L
1
T
Super node ”A”

Mobi-sync d
3
T
6
T
4
T
1
Ordinary node d
2 d
1
T
1
T
2 t r1
T
3
T
4
T
5
T
6
1ms 5ms 7ms 11ms13ms 17ms time t r2
T
5
L
1x
L
1y
=V x
*t i
=V y
*t i
L
2x
L
2y
=V x
*t i
=V y
*t i
θ
L
2
T
3
α L
1
T
Super node ”A”
Vx=0.3m/ms
Vy=0.4m/ms
L1
 tr1=2 L1=1m
L2
 tr1+tr2=8 L2=4m

Mobi-sync d
3
T
6
T
4
T
1
Ordinary node d
2 d
1
Combine Cosine theorem for common angle α
T
5
θ
L
2
T
3
α L
1
T
Super node ”A”
L1=1 , L2=4 , h1=18 , h2=36
τ
1
= fd(L1,L2,h1,h2)=1

Mobi-sync
•
Phase 2
•
Linear regression i is the index of the messages exchange round
Ordinary node Super node ”A”
T
1
SR
RS
1
T
2
T
3
T
4
RS
2
T
5
T
6
One run message exchange

Mobi-sync r : initial distance between an ordinary node and a super node
•
Phase 3
•
Calibration
•
Due to nodes mobility, the distance d
1 might be different from the distance d
2
, so the initial distance
• Update initial distance “r” and re-calculation the speed vectors
• We assign the initial skew as “1”. And since the first estimated skew has been obtained, we can update it and re-calculation.

•
Simulation

Simulation
•
Related works
•
TSHL
•
TSHL combines one-way and two way MAC-layer message delivery. One-way communication allows TSHL to estimate the clock skew, and Two-way is to compute the clock offset.
•
MU-Sync
• The first linear regression allows the cluster head to estimate the draft skew by totally ignoring the variance of propagation delays. And the second one is used to correct the estimated skew and calculate the offset.

Simulation

Simulation

Conclusion
• we presented Mobi-Sync, a novel time synchronization scheme for mobile UWSNs.
•
Mobi-Sync objects to improve the synchronization accuracy as well as the energy efficiency.