Connectivity-Guaranteed and Obstacle-Adaptive Deployment Schemes for MSN

1. The 28th International Conference on Distributed Computing Systems Connectivity-Guaranteed and Obstacle-Adaptive Deployment Schemes for MSN Writer : Guang Tan et al. Speaker : Heon-Jong Lee Laboratory of Intelligent Networks

2. Outline • Introduction • Preliminaries • CPVF(connectivity-preserved virtual force) scheme • Floor-based scheme • Performance evaluation Mobile Sensor Robot

3. Introduction • Self-deployment problem • Given a target sensing field with an arbitrary initial sensor distribution, how to guide sensors to self-organize into a connected network that has the maximum coverage at the cost of the minimum moving distance? • Previously proposed methods • potential field[5], virtual forces[13] • Voronoi Diagram(VD)[2, 4] • combination[9] Mobile Sensor Robot

4. Introduction • Previous methods have several problems • 1. They assume that a sensor can easily detect all(or most) of its Voronoi neighbors through local communication • Significant sensing overlaps or voids among sensors may be ignored, leading to poor network coverage Mobile Sensor Robot

5. Introduction • 2. While concentrating on the motion planning of sensors, tend to assume that the network remains connected throughout the process of sensor relocation • It can be guaranteed by a high node density, or a large Rc • 3. Sensing field is obstacle-free[8, 12] • However, many real-world environments have buildings, plants, water etc. Mobile Sensor Robot

6. Introduction • In this paper aim to achieve three goals: • 1.To achieve connectivity for a network with arbitrary initial distribution, communication/sensing range, or node densities • 2. To minimize moving distance, which dominates energy consumption in the deployment process • 3. To be able to work without any knowledge of the field layout • The CPVF scheme and the Floor scheme Mobile Sensor Robot

7. Preliminaries • System Assumptions • rs, rc • sensor’s neighbors : sensors within rc • a sensor knows its own position and can determine neighbor’s location by communication • a sensor can recognize the boundary of the obstacles within its rs • step : a sensor moves in a straight line at a uniform speed for a fixed amount of time(period, T) • maximum moving speed is V • the filed is on 2-D coordinate plane • a reference point O(base station) is at (0,0) Mobile Sensor Robot

8. Preliminaries • Obstacle avoidance algorithm • Using BUG2[7] • Help a sensor move from a starting point Start to a destination point Target • Moves along the straight line(Start, Target)(reference line),until it encounters an obstacleat some hitting point H • The sensor follows the boundaryusing the right-hand rule untilit gets back to the reference line Mobile Sensor Robot

9. Preliminaries • Lazy movement • To reduce the unnecessary movement • A sensor checks its neighbors to see if there are any ahead of it(closer to its current destination) • It choose the nearest neighbor as its candidate path parent. • To avoid deadlock, it sends a PathParentInquirymessage once a periodto the path parent. S’’ The Nearest Neighbor S S’ stop PathParentInquiry disregards path parent and suspend walk PathParentInquiry Mobile Sensor Robot

10. CPVF scheme • The Connectivity-preserved virtual force • Achieving connectivity • Sensors around the BSflood a message • After a certain period oftime, if a sensor still hasnot received such amessage, it can decidethat it is disconnected Disconnected! Move using BUG2 with lazy movement toward the BS BS Connected! Mobile Sensor Robot

11. CPVF scheme • Maximizing sensing coverage using VF • VF method(like [13]) is used in our scheme only for determining moving directions • Step size needs special care • too long : cause network partition • too short : network have poor coverage Mobile Sensor Robot

12. Maximizing sensing coverage using VF • Connectivity preserving condition • 1. The distance between s and s’ at time t’ is no greater than rc; and • guarantee the connection throughout [t, t’] • 2. The distance between s’’s position at t’ and s’ position at t+T is no greater than rc. • s cannot control s’’s mothion during [t’, t+T] • A sensor can determine the valid step size(ex: VT, 0.9×VT,…, 0.1×VT, 0) (VT is the maximum step size) • Allowing sensors to changeparent connections provides more freedom for sensors t t+T s t’ s’ Mobile Sensor Robot

13. Maximizing sensing coverage using VF • Changing parent • To allow a sensor to connect to a new parent, care needs to be taken not to create loops in the tree p p’ joining s success LockTree the tree has been successfully locked LockTree UnLockTree LockTree I cannot change parent unless it receives an UnLockTree message Mobile Sensor Robot

14. Coverage performance of CPVF • Simulation • using C++ • 240 sensors are initially randomly distributed in a sub-area {(x, y) : 0 ≤ x ≤ 500m, 0 ≤ y ≤ 500m} of a target field {(x, y) : 0 ≤ x ≤ 1000m, 0 ≤ y ≤ 1000m} • Base station location : (0, 0) • rs : 40m, rc : 40m, 60m • Maximum moving speed : 2m/s • Period length : 1 sec. • Simulation running time : 750 sec. Mobile Sensor Robot

15. (b), (c) significant overlap of sensing disk: every sensor makesmovement decisionsbased only on the information of itsneighbors coverage = Mobile Sensor Robot

16. The Floor-based scheme • In (a), they are unaware of overlap because of their short communication ranges. Mobile Sensor Robot

17. The Floor-based scheme • In (b), key idea is to divide the field into floors of common height 2rs, and make sensors try to stay in the central lines, called floor lines, of the floors. • Three phases • Achieving connectivity • Identifying movable sensors • Expanding coverage Mobile Sensor Robot

18. Achieving connectivity • It need two intermediate destinations • initial location (x, y) • Dest1 = (x, FloorLine(y)) • Dest2 = (0, FloorLine(y)) • y coordinate of the sensor’s nearest floor line • Dest3 = (0, 0) • using BUG2 andlazy movement Mobile Sensor Robot

19. Identifying movable sensors • The purpose of the second phase is • to find out sensors that (1) can move without partitioning the network and (2) whose move is expected to bring a gain in coverage • (1) : If all the children can find another parents to join without creating loops, than it means that the sensor can safely move away • (2) : It calculates the area currently covered exclusively by itself; if an area is beyond λAmax, then it does not move(λ is a parameter (say 60%), Amax is the maximum area that can be exclusively covered by itself) Rs Mobile Sensor Robot

20. Determining the coverage status of a point • Decide whether a point needs to invite some movable sensor to fill in that uncovered area • when rc/rs is small, we need non-local communication • Floor header node • the smallest x-coordinate in a floor • maintains the location of the nodes in its floor • Procedure • check its neighbors • calculate floors and send query messageto the floor header nodes • The floor header node send back responses 3rd floor Header 2nd floor Header S 1st floor Header Mobile Sensor Robot

21. Expanding sensing coverage • Two types • floor/boundary lineguided expansion • inter-floor lineguided expansion • Procedure • 1. Find an expansionpoint(EP) on its expansioncircle of radius min(rc, rs) • 2. If EP isn’t covered, then invite some movablesensor to relocate tothe point left-hand rule Mobile Sensor Robot

22. Inviting movable sensors • Procedure • The sensor which has EP periodically(once a period) sends an Invitation message to the network • A movable sensor picks the message according to the priority order: • Floor-line > Boundary-line > Inter-floor-line • Nearest source(with Euclidean distance) • The movable sensor sends an AcceptInvitation message to that inviter and move • The inviting sensor send the location information to the root on behalf of the invited sensor Mobile Sensor Robot

23. CPVF : 74.5% CPVF : 26.4% CPVF : 37.1% Mobile Sensor Robot

24. Performance Evaluation • Coverage(Obstacle-free) Optimal is a centralized scheme [1](only suitable for a non-obstacle environment) 110% higher Mobile Sensor Robot

25. Performance Evaluation • Moving distance(Obstacle-free) Oscillation happens very often Optimal is a centralized scheme [1]they know the destination already Mobile Sensor Robot

26. Performance Evaluation • Oscillation avoidance techniques in CPVF • 1. one-step oscillation avoidance • If the next step size is smaller then VT/δ, than a sensor cancels its movement for the next step (VT is the maximum step size) • A similar strategy has been used [5] • 2. two-step oscillation avoidance • Calculate its future location at the end of next step with its past location at the end of last step • If the distance between them is smaller than VT/δ, than a sensor cancels its movement for the next step • A similar strategy has been used [9] Mobile Sensor Robot

27. Performance Evaluation • Oscillation avoidance techniques in CPVF Mobile Sensor Robot

28. Performance Evaluation • Fields with random obstacle • 4 obstacles are randomly drawn • 160 sensor and 300 runs 587.2 1293.7 Mobile Sensor Robot