Connectivity-Guaranteed and Obstacle-Adaptive Deployment Schemes for Mobile Sensor Networks

1. Connectivity-Guaranteed and Obstacle-Adaptive Deployment Schemes for Mobile Sensor Networks Guang Tan, Stephen A. Jarvis, and Anne-Marie Kermarrec IEEE Transactions on Mobile Computing, VOL. 8, NO.6, JUNE 2009 Yun-Jung Lu

2. Outline • Introduction • Preliminaries • The Connectivity-Preserved Virtual Force (CPVF) Scheme • The Floor-Based Scheme • Performance Evaluation • Conclusion Yun-Jung Lu

3. Introduction • In an mobile sensor network, the sensors are able to relocate and self-organize into a network. • The mobility and self-management of sensors are desirable for many application scenarios, including remote harsh fields, disaster areas or toxic urban regions, where manual operations are unsafe or burdensome. Yun-Jung Lu

4. Self-deployment Problem • Given a target sensing field with an arbitrary initial sensor distribution, how should these sensors self- organize into a connected ad hoc network that has the maximum coverage, at the cost of a minimum moving distance? Yun-Jung Lu

5. A number of proposed scheme to this problem • Potential Fields or Virtual Force • When two electromagnetic particles are too close in proximity, a repulsive force pushes them apart. • Voronoi Diagrams (VDs) • Allow sensors to move to maximize coverage in its own subarea Yun-Jung Lu

6. Several Problems in practice • The communication range of a sensor may not be large enough to cover all Voronoi neighbors. • An incomplete view of the Voronoi neighbors may result in very inaccurate VDs being constructed. Yun-Jung Lu

7. The Rest of Problems • Network Connectivity? Network partition can still occur in a dense network. • Generally, connectivity must be considered in protocol design. • Obstacle-free? • Naturally, the real-world environments have obstacles or holes render such schemes ineffectual. Yun-Jung Lu

8. The goals of this paper • To achieve connectivity for a network with an arbitrary initial distribution, communication/sensing range, or node density • To minimize moving distance, which dominates energy consumption in the deployment process • To be able to work without any knowledge of the field layout, which can be irregular and have obstacles of arbitrary shape Yun-Jung Lu

9. Preliminaries I • System Assumptions • All sensors have the same communication range rc and sensing range rs. • At any given time, a sensor knows its own position and can recognize the boundary of the obstacles within its sensing range. • Sensors move in steps of variable size. • In each step, a sensor moves in a straight line at a uniform speed for a period and denote by T. • There is a reference point O ; all the sensors will try to connect to O generality. Yun-Jung Lu

10. Preliminaries II • Obstacle Avoidance • BUG2: “Path-Planning Strategies for a Point Mobile Automaton Moving amidst Unknown Obstacles of Arbitrary Shape,” Algorithmica, 1987 • Reference Line: the straight line (Start, Target) • H: hitting point • Right-hand rule Yun-Jung Lu

11. Preliminaries III • Lazy Movement (With multiple hop communication, not all disconnected sensors need to move to get connected.) • At the end of each step, a sensor checks its neighbors to see if there are any ahead of it; • If so, then it chooses the nearest neighbor as its candidate path parent. Yun-Jung Lu

12. The Connectivity-Preserved Virtual Force (CPVF) Scheme • Achieving Connectivity • Maximizing Sensing Coverage Yun-Jung Lu

13. Achieving Connectivity • Initially, all sensors are required to decide their states regarding connectivity. • Flooding a message to the network • Sensor receives such a message, becomes aware that they are also connected • After a certain period of time, if a sensor still has not received such a message, it can decide that it is disconnected. • It will allow a small random time period to elapse after which it starts to move using the BUG2 Algorithm(with lazy movement) toward the base station. Yun-Jung Lu

14. Maximizing Sensing Coverage • Virtual Force is used to determine the direction to move. • The obstacles and neighboring sensors exert repulsive forces onto a sensor. • The sum of all forces determines the subsequent direction of that sensor. Yun-Jung Lu

15. Maximizing Sensing Coverage • Connectivity Preserving Conditions • The distance between s and s’ at time t’ is no greater than rc • The distance between s’ ’s position at t’ and s’ ’s position at t + T is no grater than rc • A sensor can approximately determine the maximum valid step size by checking a set of possible values, for example, VT, 0.9＊VT, …, 0.1*VT, 0. t’ A VT A fba fca B s’ C s’ V : the moving speed T : the moving time of one step Yun-Jung Lu

16. CPVF Yun-Jung Lu

17. The Floor-Based Scheme Yun-Jung Lu

18. The Floor-Based Scheme • Achieving Connectivity • Identifying Movable Sensors • Expanding Coverage Yun-Jung Lu

19. A High-level View Yun-Jung Lu

20. Achieving Connectivity Yun-Jung Lu

21. Identifying Movable Sensors • To identify sensors that can move without partitioning the network and whose move is expected to increase network coverage • The Rules to achieve that: • Obtain a list of neighbors within two hops of itself • Try to find for each child a new parent • Loop check for a particular child • If all the children can find parents without crating loops, then it means that the sensor can safely move away. Yun-Jung Lu

22. Expanding Sensing Coverage • With all movable sensors identified, we can now expand the network’s coverage. • Three types of expansion policy • Floor-line-guided expansion • Boundary-line-guided expansion • Interfloor-line-guided expansion Yun-Jung Lu

23. Floor and Boundary • Expansion Point • Expansion Circle is min(rc, rs) Expansion Circle frontier point Yun-Jung Lu

24. Floor and Boundary Yun-Jung Lu

25. Interfloor Frontier Point Yun-Jung Lu

26. Inviting Movable Sensors • If a sensor can not find any expansion points in its expansion circle, it will stop the process. • Else, it will flood a Invitation Message to find some sensors to cover these points. • Invitation Message contains an EP to the network and a TTL value. Yun-Jung Lu

27. A movable sensor • It collects a certain number of invitations, and picks one with the highest priority. • It sends an AcceptInvitation message to the inviter. Yun-Jung Lu

28. In the former case, • The inviting sensor constructs a virtual place-holding fixed node in the tree, and sends a message to the root on behalf of the invited sensor to update the location information maintained by its ancestors. Yun-Jung Lu

29. Floor-based Scheme Yun-Jung Lu

30. Performance Evaluation • An event-based simulator using C++ • 240 sensors are initially randomly distributed in a subarea {(x, y):0≦x ≦500m, 0≦y ≦500m} of a target field {(x, y):0≦x ≦1000m, 0≦y ≦1000m} • The base station is located at (0,0). • The maximum moving speed is 2 m/s. • The period length is 1 second. • The simulation runs for 750 seconds. Yun-Jung Lu

31. Comparison between CPVF, FLOOR, and OPT Yun-Jung Lu

32. Comparison between FLOOR, VOR, and Minimax Yun-Jung Lu

33. Moving Distance in Obstacle-Free Fields Yun-Jung Lu

34. Conclusion • Two sensor deployment schemes are proposed for mobile sensor network in this paper. • The major difference of the proposed schemes with the previous works is their adaptability to arbitrary network densities or communication ranges and to obstacles. Yun-Jung Lu