Mobi-Sync: Efficient Time Synchronization for Mobile Underwater Sensor Networks 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 The world's oceans cover over 70 % of its surface Underwater Wireless Sensor Networks (UWSNs)

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

Introduction Time synchronization Clock divergence Clock drift Clock offset

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 Time T1T1 T4T4 T2T2 T3T3 T5T5 T6T6 Ordinary nodeSuper node ”A” t r1 t r2 SR RS 1 RS 2 One run message exchange T 1,T 3,T 5 : sending time of SR,RS 1,RS 2 T 2,T 4,T 6 : receiving time of SR,RS 1,RS 2 t r1,t r2 : the first,second response time

Mobi-sync Phase 1 Delay calculation Ordinary node Super node ”A” d1d1 d2d2 d3d3 T1T1 T2T2 T3T3 T4T4 T5T5 T6T6 L2L2 L1L1 β θ d 1 : are propagation distance of SR d 2 : are propagation distance of RS 1 d 3 : 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

Mobi-sync V p : propagation speed Ordinary node Super node ”A” d1d1 d2d2 d3d3 T1T1 T2T2 T3T3 T4T4 T5T5 T6T6 L2L2 L1L1 β θ T1T1 T2T2 T3T3 T4T4 T5T5 T6T6 1ms 5ms 7ms 11ms13ms17ms time t r1 t r2

Mobi-sync Super node ”A” d1d1 d2d2 d3d3 T1T1 T2T2 T3T3 T4T4 T5T5 T6T6 L2L2 L1L1 β θ Ordinary node Super node ”A” d1d1 d2d2 d3d3 T1T1 T2T2 T3T3 T4T4 T5T5 T6T6 L2L2 L1L1 θ Ordinary node assumption β =0 α

Mobi-sync Super node ”A” d1d1 d2d2 d3d3 T1T1 T2T2 T3T3 T4T4 T5T5 T6T6 L2L2 L1L1 θ Ordinary node α L 1x =V x *t i L 2x =V x *t i L 1y =V y *t i L 2y =V y *t i Vx=0.3m/ms Vy=0.4m/ms L1  tr1=2 L1=1m L2  tr1+tr2=8 L2=4m T1T1 T2T2 T3T3 T4T4 T5T5 T6T6 1ms 5ms 7ms 11ms13ms17ms time t r1 t r2

Mobi-sync Super node ”A” d1d1 d2d2 d3d3 T1T1 T2T2 T3T3 T4T4 T5T5 T6T6 L2L2 L1L1 θ Ordinary node α Combine Cosine theorem for common angle α L1=1, L2=4, h1=18, h2=36 τ 1 = fd(L1,L2,h1,h2)=1

Mobi-sync Phase 2 Linear regression Time T1T1 T4T4 T2T2 T3T3 T5T5 T6T6 Ordinary nodeSuper node ”A” SR RS 1 RS 2 One run message exchange i is the index of the messages exchange round

Mobi-sync 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. 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

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.