Guang Tan, Stephen A. Jarvis, and Anne-Marie Kermarrec IEEE Transactions on Mobile Computing, VOL. 8, NO.6, JUNE Yun-Jung Lu

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

 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. 3Yun-Jung Lu

 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? 4Yun-Jung Lu

 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 5Yun-Jung Lu

 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. 6Yun-Jung Lu

 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. 7Yun-Jung Lu

 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 8Yun-Jung Lu

 System Assumptions ◦ All sensors have the same communication range r c and sensing range r s. ◦ 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. 9Yun-Jung Lu

 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 10Yun-Jung Lu

 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. 11Yun-Jung Lu

 Achieving Connectivity  Maximizing Sensing Coverage 12Yun-Jung Lu

 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. 13Yun-Jung Lu

 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. 14Yun-Jung Lu

 Connectivity Preserving Conditions ◦ The distance between s and s’ at time t’ is no greater than r c ◦ The distance between s’ ’s position at t’ and s’ ’s position at t + T is no grater than r c  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. A A B B C C f ba f ca V : the moving speed T : the moving time of one step VT A A t’ s’ 15Yun-Jung Lu

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 Achieving Connectivity  Identifying Movable Sensors  Expanding Coverage 18Yun-Jung Lu

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 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. 21Yun-Jung Lu

 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 22Yun-Jung Lu

 Expansion Point  Expansion Circle is min(r c, r s ) frontier point Expansion Circle 23Yun-Jung Lu

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 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. 26Yun-Jung Lu

 It collects a certain number of invitations, and picks one with the highest priority.  It sends an AcceptInvitation message to the inviter. Yun-Jung Lu27

 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 Lu28

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 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. 30Yun-Jung Lu

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 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. 34Yun-Jung Lu