1. Energy-Efficient Target Coverage in Wireless Sensor Networks
PLLAB 김성민

2. Outline Introduction Proposal Conclusion
Maximum Set Covers(MSC) Problem
MSC Problem is NP-Complete
MSC heuristic
Conclusion

3. Critical Issue: Power Scarcity!!!
Introduction
Characteristics of WSN
Dense
Limited resourse …
Critical Issue: Power Scarcity!!!

4. Target Coverage Problem
Given
m targets
n sensors randomly deployed
Assume
same remaining energy
same range
How to optimize the sensor energy utilization?

5. Proposal C = {s1, s2, s3, s4} R = {r1, r2, r3} Disjoint sets:
Lifetime G = 2
Our Approach:
S1 = {s1, s2} with t1 = .5; S2 = {s2, s3} with t2 = .5
S3 = {s1, s3} with t3 = .5; S4 = {s4} with t4 = 1
Lifetime G = 2.5

6. Proposal

7. Maximum Set Covers (MSC)
Given
C : set of sensors
R : set of targets
Goal
Determine a number of set covers S1, …, Sp and t1,…,tp
Where:
Si completely covers R
Maximize t1 + … + tp

8. Maximum Set Covers (MSC)
Theorem: MSC is NP-Complete
MSC problem belongs to the class NP and is NP-hard, so MSC is NP-Complete
Proof ????
So, this paper presents Two heuristics.

9. MSC Heuristic We first model the MSC problem as an Integer Programming
Given :
A set of n sensor nodes: C = {s1 , s2, …, sn}
A set of m targets: R={r1 , r2, …, rm}
The relationship between sensors and targets:
Ck = { i | sensor si covers target rk}
s r1 C = {s1 , s2, s3};
s r2 R = {r1, r2, r3}
s r3 C1 = {1,3}; C2 = {1,2}; C3 = {2,3}
Variables:
xij = 1 if si ∈ Sj, otherwise xij = 0
tj ∈ [0, 1], represents the time allocated for Sj

10. MSC Heuristic (IP) first constraint : each sensor life time <=1
second constraint : each target is covered by at least one sensor

11. MSC Heuristic (IP) The term xijtj is not linear
Therefore we set yij = xijtj

12. MSC Heuristic (LP)
We are ready to introduce LP-MSC heuristic

13. MSC Heuristic (LP)
O (p3n3)

14. Greedy Heuristic Input parameter C - the set of sensors
R - the set of targets
w – sensor lifetime granularity, 0 < w <= 1

15. Greedy Heuristic
O (im2n)

16. Result

17. Result

18. Result

19. Conclusion
Schedule the sensor node activity to alternate between sleep and active mode
Our contributions:
Propose maximum covers set approach
Prove it is NP-complete
Propose an efficient heuristic

20. 감사합니다