Improving the Coverage of Randomized Scheduling in Wireless Sensor Networks

1. Improving the Coverage of Randomized Scheduling in Wireless Sensor Networks 林振緯 jwlin@csie.fju.edu.tw Department of Computer Science and Information Engineering Fu Jen Catholic University

2. Outline Introduction Background Proposed Approach Simulation Evaluations Conclusions

3. Introduction • Wireless Sensor Network (WSN) Applications • 軍事應用 • 監控戰場上的狀態 • 環境應用 • 監測污染或災害防治 • 健康應用 • 偵測人體健康數據與行為 • 家庭應用 • 將含有起動器(actuator)的sensor network佈署於家中，可以讓人們在遠方或在家裡經由網際網路作許多家事。 • 工業應用 • 偵測產品線上的不良品

4. Introduction • An Application Case • 用於廠區防災 • 工研院在其光電所晶圓廠務區利用溫度、煙霧感測器偵測工廠環境。當災難發生時，後端伺服器將自動通知管理人員及消防隊，還可透過WSN逃難指示板引導逃生方向。

5. Introduction 處理單位(Processing Unit) Storage：將蒐集到的環境資訊儲存在儲存元件中Processor：負責執行事先儲存好的程式碼，以協調並控制感測節點之間不同的單位元件。 感測單位(Sensing Unit) 負責偵測環境，將感測元件感測到的類比訊號轉換成數位訊號，並將資料送到處理單位加以處理。 傳輸單位 (Transceiver Unit) 負責感測元件間的溝通，或是將感測器的資料傳送給無線資料蒐集器。 電力供應單位(Power Unit) 通常感測節點的電力是由電池所支援，因此在軟硬體的設計上，有效的分配電力是很重要的。 • Sensor Node Architecture 應用上： 感測器可能包含定位裝置、移動器和能源產生器

6. Introduction WSN Network Architecture Internet, Satellite or other Communication System. Base station (BS) or Sink Sensor Environment (Field) Wireless Communication link Sensor node Sensor Deployment with Random and Redundant Manners

7. A Well-Known Randomized Scheduling for WSN Introduction Time Slot Subset 1 Subset 2 Subset 3 coverage holes coverage holes coverage holes coverage holes coverage holes coverage holes coverage holes 1 2 3 …

8. Introduction • Motivation • Coverage improvement for the randomized scheduling • Connectivity improvement • Reference • C. Liu, K. Wu, Y. Xiao, and B. Sun, “Random Coverage with Guaranteed Connectivity: Joint Scheduling for Wireless Sensor Networks,” IEEE Trans. Parallel and Distributed Systems, vol. 17, no. 6, pp. 562-575, 2006. • Coverage with connectivity guarantees • No coverage holes • Double range property • Mica1 mote, Mica2 mote, Sensoria SGate, etc. • Reference • G. Xing, X. Wang, Y. Zhang, C. Lu, R. Pless, and C. Gill, “Integrated Coverage and Connectivity Configuration for Energy Conservation in Sensor Networks,” ACM Trans. Sensor Networks, vol. 1, no. 1, pp. 36-72, 2005.

9. Outline Introduction Background Proposed Approach Simulation Evaluations Conclusions References

10. Background Network Model A static sensor network in a two-dimensional field Circle model used for the sensing and communication ranges Double range property Location awareness

11. Background Related Work Random Coverage with Guaranteed Connectivity: Joint Scheduling for Wireless Sensor Networks,” IEEE Trans. Parallel and Distributed Systems q k ≤ ln(1 – t) 1 - n e • Connectivity guaranteed • A given coverage requirement to determine the number of subsets q = r / a r : the size of sensing area of each sensor a : thee size of the whole field n : the total number of deployed sensor nodes t : at least network coverage intensity

12. Background • Related Work • Optimal Geographical Density Control (OGDC) • H. Zhang and J. C. Hou, “Maintaining Sensing Coverage and Connectivity in Large Sensor Networks,” in Journal on Wireless Ad Hoc and Sensor Networks, vol. 1, pp. 89-123, Jan 2005. • Scheduling • Round basis • Coverage and connectivity guarantees • Double range property to guarantee the connectivity • Each round with two phases • node selection and steady state • Tree states for a sensor node • undecided, on , and off

13. One of the neighbors with an (approximate) distance of r (node B) will be selected to be a working node. r X Background • OGDC (Cont.) Based on the above step, node D will be chosen Initially each node is at undecided state A B Node A is a starting node To cover the crossing point of circle A and B the node whose position is closest to the optimal position X, node C will then be selected. E Because of node E’s neighbors can completely cover its own coverage, so node E turns state to off C r D undecided state on state off state

14. Outline Introduction Background Proposed Approach Simulation Evaluations Conclusions References

15. Proposed Approach Goal Eliminating the blind points (coverage holes) in the randomized scheduling Improving the coverage quality of the randomized scheduling Basic idea Adding extra sensor nodes in each subset Activating more sensor nodes at each time slot.

16. Proposed Approach • Basic idea (Cont.) coverage holes coverage holes extra nodes extra nodes extra nodes

17. Proposed Approach Problem How to select appropriate sensor nodes as extra sensor nodes How to minimize the number of extra sensor nodes Solution A distributed manner based on the four-phase execution • The first phase • Determining the belonging subset of each sensor node (the time slot) • The second phase • Classify the neighbors into two parts for the third phase • The third phase • Calculating the responsible sensing range • The fourth phase • Eliminating the coverage holes in each responsible sensing range

18. Proposed Approach The first phase: Following the randomized scheduling algorithm to divide the sensor nodes into multiple subsets. Collecting the information about its neighboring sensor nodes. The second phase: Using the belonging subset number to classify its neighbors into two parts: The neighbors with the same subset. The neighbors with the different subset.

19. Proposed Approach • The second phase (Cont.) 2 1 2 2 3 1 1 1 3 3 2 The neighbors with the different subsets The neighbors with the same subset

20. Proposed Approach The third phase Partitioning the sensor field using the distribution manner Constructing the responsible sensing range Voronoi polygon

21. Proposed Approach The third phase Construing the responsible sensing range (Cont.) The number of the neighbors with the same subset is not enough Not precisely calculating its responsible sensing region Introducing the partition-assistant nodes for assisting the calculation of the responsible sensing region Additionally work at the time slot of its assisted sensor node

22. Proposed Approach No the neighbors with the same subset in the quadrant Asking the farthest neighbor as the partition-assistant node • The third Phase • Partition-assistant nodes (One kind of the extra sensor nodes) The new Voronoi polygon (Its responsible sensing region) Sensor node i and neighbors with same subset number The neighbors without same subset number

23. Proposed Approach The fourth phase: Determining whether it has the capability to independently cover its responsible sensing region. Eliminating the coverage holes. Introducing the coverage-assistant nodes to collectively cover the responsible sensing region Using the optimal circle deployment (circle covering) as the selection template to select the coverage-assistant nodes Additionally work at the time slot of its assisted sensor node

24. Proposed Approach The sensing region of sensor node i Ideal sensor node Sensor Node i Neighbors without the same subset The voronoi polygon of sensor node i (its responsible sensing region) Coverage-Assistant Node • The third Phase • Coverage-assistant nodes (The other kind of the extra sensor nodes) Circle Covering

25. Proposed Approach Si : The set of the sensor nodes that are the neighbors of sensor node i and the sensor node i itself. VSi : The set of the sensor nodes that are the neighbors of sensor node i and have the same working time slot. Rc : The communication radius of a sensor node. Rs : The sensing radius of a sensor node. HSi : The set of sensor nodes that are the neighbors of sensor node i but their working time slots are different • Polynomial Time complexity • The first phase • O( 1 ) • The second phase • O() • The third phase • O() • The fourth phase • O()

26. Outline Introduction Background Proposed Approach Simulation Evaluations Conclusions References

27. Simulation Evaluations Simulation Setup Sensor field: 200 meters * 200 meters Total number of sensor nodes: 500, 1000, 1500, 2000, and 2500 Number of subsets: 2, 3, 4, 5, and 6 The communication / sensing radius ratio: 2 Performance Metrics Coverage intensity Average ratio of the area covered by a subset over the whole area of the sensor field. Ratio of additional sensor nodes Average ratio of the partition-assistant and coverage-assistant nodes in a subset over the total number of sensor nodes Average number of control messages How many messages issued for improving the coverage Energy consumption

28. Simulation Results Coverage Intensity Number of sensor nodes = 1000

29. Simulation Results Coverage Intensity (cont.) Number of subsets = 5

30. Ratio of Additional Sensor Nodes Number of sensor nodes = 1000 Simulation Results

31. Ratio of Additional Sensor Nodes (Cont.) Simulation Results

32. Simulation Results Ratio of Additional Sensor Nodes (cont.) / ratio = 2, Number of subset = 5

33. Simulation Results Average Number of Control Messages

34. Outline Introduction Background Proposed Approach Simulation Evaluations Conclusions

35. Conclusions A distributed approach to improving the coverage performance of the randomized scheduling algorithm Partition-assistant and Coverage-Assistant nodes introduced in the subset Modifying the Voronoi polygon construction and Applying the circle covering Coverage intensity achieved nearly same as the centralized approach without any subsets Low energy consumption with the less than 3 control messages issued the polynomial time complexity

36. Thank you for paying attention jwlin@csie.fju.edu.tw

37. Voronoi Diagram S2 S1 Responsible sensing range S3 S0 S4 S5

38. Circle Covering • What is the minimum number of circles required to completely cover a given polygon? • Y. Guo and Z. Qu, “Coverage Control for A Mobile Robot Patrolling A Dynamic and Uncertain Environment,” Proc. World Congress on Intelligent Control and Automation