1. A Coverage-Preserving Node Scheduling Scheme for Large Wireless Sensor Networks Di Tian, and Nicolas D. Georanas ACM WSNA ‘ 02

2. Outline  Introduction  Node self-scheduling Algorithm Coverage-based Off-duty Eligibility Rule Node scheduling Scheme Based On Eligibility Rule  Performance Evaluation  Simulation Results  conclusion

3. Introduction  Neighboring nodes can cover the current node ’ s sensing range, this node can be turned off  If we can schedule sensors to work, the system lifetime can be prolonged

4. Node self-scheduling Algorithm  Coverage-based Off-duty Eligibility Rule  Node scheduling Scheme Based On Eligibility Rule

5. Coverage-based Off-duty Eligibility Rule  Each node has its neighbors ’ location information Each node broadcast a Position Advertisement Message (PAM)  Assume all nodes ’ sensing range r  Denote node i ’ s sensing area as S(i)

6. Coverage-based Off-duty Eligibility Rule  Definition 1: Neighbor is node set in the deployment region d(i,j):distance between node i and node j

7. Coverage-based Off-duty Eligibility Rule  The off-duty eligibility rule can be expressed as  The expression is equivalent to

8. Coverage-based Off-duty Eligibility Rule

9.  Definition 2: Sponsored sector S j → i : the sponsored sector by node j to node i Θ j → i : The central angle of the sector

10. Coverage-based Off-duty Eligibility Rule  Lemma 1 : Ensure whether the neighbors can cover the current node ’ s sensing area  The union of sponsored sector is equivalent to calculation whether the union of central angles can cover the whole 360 o

11. Coverage-based Off-duty Eligibility Rule  Sponsored Coverage Calculation – extension model Different sensing range  Case 1: Happen whenever  Node i can be turned off

12. Coverage-based Off-duty Eligibility Rule  Case 2: Happen whenever both

13. Coverage-based Off-duty Eligibility Rule  In summary, a node ’ s neighbor set definition is modified as

14. Node Scheduling Scheme Based on Eligibility Rule  The operation is divided into rounds the self-scheduling phase  Obtain neighboring nodes ’ position information  Each node decides whether it is eligible for off-duty or not the sensing phase  Longer than the self-scheduling phase

15. Node Scheduling Scheme Based on Eligibility Rule  Neighbor Information Obtaining Step Each node broadcast a Position Advertisement Message (PAM)  Node ID, Location information  Each node transmit PAM with the minimize power as long as its sensing range (TO minimize energy consumption)

16. Node Scheduling Scheme Based on Eligibility Rule Blind points may appear

17. Node Scheduling Scheme Based on Eligibility Rule

18.  Back-off Based Self-scheduling Step Each node start its determination after a random back-off time period Td if the node is eligible for turning off  It broadcast a Status Advertisement Message (SAM) to announce its status Neighboring nodes delete the sender from their neighbor lists There is a chance that nodes could select the same Td To avoid a blind point, each node needs to wait for a short period time Tw after sending the SAM out, instead of turning off its communication unit immediately

19. Node Scheduling Scheme Based on Eligibility Rule  If one SAM is received during the “ ready-to- off-period ” and the transmitter is one of its off-duty sponsors, the node will reinvestigate its off-duty eligibility  Otherwise, the node will turn itself off after Tw

20. Node Scheduling Scheme Based on Eligibility Rule

21. Performance Evaluation  Deploy 100 nodes in a 50m×50m square space  Each node has a sensing range of 10 meter  To calculate sensing coverage, divide the square space into 1m×1m cells Assume an event occurs in each cell

22. Performance Evaluation (cont.)

27. Simulation Results  To implement the scheme as an extension of LEACH  Simulator NS-2

28. Simulation Results

29.  The initial energy of all nodes is 2J  The time duration of each round is 10 seconds

30. Simulation Results

34. Conclusion  Propose a coverage-preserving node- scheduling scheme Turning off some redundant nodes Reduce energy consumption Increase system lifetime