1. Tom Chao Zhou, CUHK 1 Wireless Sensor Network Speaker: Tom Chao Zhou Feb, 23rd@3DLBS Study Group Subtopic: Sensor Technology

2. Tom Chao Zhou, CUHK 2 Outline Definition Overview Related Research –Applications –Network Services Case Study: Multi-hop Relative Location Estimation (A Googler’s paper)

3. Tom Chao Zhou, CUHK 3 Definition Wireless sensor networks (WSNs) – Sensor nodes: sense, measure and gather information; transmit the data to the user E.g. biological, chemical, optical, and magnetic sensors – Potential applications Military target tracking, natural disaster relief, etc – Resource constraints Energy, short communication range, low bandwidth, limited processing and storage Personal opinion: Energy is especially important if we consider each node is a mobile phone.

4. Tom Chao Zhou, CUHK 4 Overview

5. Tom Chao Zhou, CUHK 5 Applications Connection-less sensor-based tracking system using witness (CenWits) – Mobile sensors worn by people – Use the GPS receivers and location points to determine current location – Use of witnesses to convey a subject’s movement and location information to the outside world – Huang et al. 2005, Sensys

6. Tom Chao Zhou, CUHK 6 Application ZebraNet – Mobile wireless sensor network used to track animal migrations – Positional readings are taking using the GPS – Sent multi-hop across zebras to the base station – Zhang et al. 2004, Sensys

7. Tom Chao Zhou, CUHK 7 Network Services Localization – Problem of determining the node’s location Synchronization – Time synchronization in a wireless sensor network is important for routing and power conservation – Apply time synchronization methods in data synchronization? E.g. p2p data

8. Tom Chao Zhou, CUHK 8 Localization Existing methods – Global positioning system (GPS) GPS receiver Pros: simple Cons: not work if there are obstructions – Beacon (or anchor) nodes Beacon nodes, which know their own position Cons: does not scale well in large networks (may not be true in 3DLBS, wifi spots) – Proximity-based localization Use neighbor nodes to determine their location; act as beacons

9. Tom Chao Zhou, CUHK 9 Localization Moore’s algorithm (Moore et al., 2004, Sensys) – Distributed localization algorithm, use of a robust quadrilateral. Secure localization (Srinivasan et al., 2007) – Securing the localization process, rely on beacon information.

10. Tom Chao Zhou, CUHK 10 Synchronization Energy is conserved when there are less collisions and re-transmissions Energy is saved when nodes are duty-cycled – Periodically turn its radio off to save energy and on to participate in network communication

11. Tom Chao Zhou, CUHK 11 Synchronization Uncertainty-driven approach (Ganeriwal et al. 2005) – Modeling long-term clock drifts between nodes to minimize duty-cycling overheads Lucarelli’s algorithm (Lucarelli et al. 2004) – Bi-directional nearest-neighbor coupling Clock-sampling mutual network synchronization – Rentel et al. 2005 – Distributed and autonomous network synchronization – Non-hierarchical

12. Tom Chao Zhou, CUHK 12 Case Study On the Accuracy of Multi-hop Relative Location Estimation in Wireless Sensor Networks Youssef et al. 2007, IWCMC Goal –Analyze the accuracy of multi-hop relative location estimation and study the different sources of errors –Present the Multi-hop Relative Location Estimation (MRLE) algorithm which avoids reflection error

13. Tom Chao Zhou, CUHK 13 Relative Coordinate System (RCS) Estimate nodes’ positions relative to a coordinate system established by a reference group of nodes –R0,R1,R2 A RCS can be transformed to absolute coordinate system by using only three anchor nodes in 2D (or four anchors in 3D) Note: we only need a proximity measure between nodes

14. Tom Chao Zhou, CUHK 14 Multi-hop Relative Position Estimation A node u knows distances to three non-colinear identified neighbors. u2 is the reflection of u1 across the base line R2 is the reflection resolver

15. Tom Chao Zhou, CUHK 15 Reflection Error Reflection propagation error Measured distances contain error.

16. Tom Chao Zhou, CUHK 16 Reflection Error Skinny triangles Have the RCS near the center of the network

17. Tom Chao Zhou, CUHK 17 MRLE Adding R0, R1, R2 to the set of identified nodes. Iteratively estimate an unidentified node u – Select base line as far as possible from u – Resolver node, the neighbor to u with the highest altitude from the base line

18. Tom Chao Zhou, CUHK 18 Open Research Issues (Personal) This approach assumes wireless sensor nodes are static, how to handle dynamic cases if we consider people who have mobiles moving around? This approach assumes there is only one gateway, how about if there are multiple gateways? (In a shopping mall, we have multiple wi-fi routers (gateways))

19. Tom Chao Zhou, CUHK 19 References Wireless sensor network survey On the Accuracy of Multi-hop Relative Location Estimation in Wireless Sensor Networks Localization and Tracking in Sensor Systems

20. Tom Chao Zhou, CUHK 20 Thanks! Q & A