View
216
Download
0
Category
Tags:
Preview:
Citation preview
Mobi-Sync: Efficient Time Synchronization forMobile Underwater Sensor NetworksJun Liu, Zhong Zhou, Zheng Peng and Jun-Hong CuiComputer Science & Engineering Department, University of ConnecticutIEEE Globecom 2010
Outline
• Introduction
• Challenge
• Goals
• Networks architecture
• Mobi-sync
• Simulation
• Conclusion
Introduction
• The world's oceans cover over 70 % of its surface• Underwater Wireless Sensor Networks (UWSNs)
Introduction
• Transmission rate• WSN: 3 x 108 m/s
• UWSN: 1500 m/s
• Propagation delay
• Time synchronization
A3:00
B3:00
3:10
Introduction
• Time synchronization
• Clock divergence• Clock drift
• Clock offset
iii btatC )(
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
Tim
e
T1
T4
T2
T3
T5
T6
Ordinary node Super node ”A”
tr1
tr2
SR
RS1
RS2
One run message exchange
T1,T3,T5 : sending time of SR,RS1,RS2
T2,T4,T6 : receiving time of SR,RS1,RS2
tr1,tr2 : the first ,second response time
Mobi-sync
• Phase 1
• Delay calculation
Ordinary nodeSuper node ”A”
d1
d2
d3
T1
T2
T3
T4
T5
T6
L2
L1
β
θ
d1 : are propagation distance of SRd2 : are propagation distance of RS1
d3 : are propagation distance of RS2
L1,L2 : straight-line distance super node “A” move relatively to the ordinary node during tr1,tr
Mobi-sync
Vp : propagation speed
Ordinary nodeSuper node ”A”
d1
d2
d3
T1
T2
T3
T4
T5
T6
L2
L1
β
θ
T1 T2 T3 T4 T5 T6
1ms 5ms 7ms 11ms13ms 17ms time
tr1
tr2
iii btatC )(
Mobi-sync
Super node ”A”d1
d2
d3
T1
T2
T3
T4
T5
T6
L2
L1
β
θ
Ordinary nodeSuper node ”A”
d1
d2
d3
T1
T2
T3
T4
T5
T6
L2
L1
θ
Ordinary node
assumption
β=0
α
Mobi-sync
Super node ”A”d1
d2
d3
T1
T2
T3
T4
T5
T6
L2
L1
θ
Ordinary node
α
L1x =Vx*ti L2x =Vx*ti
L1y =Vy*ti L2y =Vy*ti
Vx=0.3m/msVy=0.4m/ms
L1tr1=2 L1=1mL2tr1+tr2=8 L2=4m
T1 T2 T3 T4 T5 T6
1ms 5ms 7ms 11ms13ms 17ms time
tr1
tr2
Mobi-sync
Super node ”A”d1
d2
d3
T1
T2
T3
T4
T5
T6
L2
L1
θ
Ordinary node
α
Combine Cosine theorem for common angle α
L1=1 , L2=4 , h1=18 , h2=36τ1= fd(L1,L2,h1,h2)=1
Mobi-sync
• Phase 2
• Linear regression
Tim
e
T1
T4
T2
T3
T5
T6
Ordinary node Super node ”A”
SR
RS1
RS2
One run message exchange
i is the index of the messages exchange round
Mobi-sync
• Phase 3
• Calibration
• Due to nodes mobility, the distance d1might be different from the distance d2, 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.
r : initial distance between an ordinary node and a super node
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.
Recommended