1. Coverage Protocol for Wireless Sensor Networks Using Distance Estimates
Mingze Zhang, Mun Choon Chan and A. L. Ananda
School of Computing
National University of Singapore, IEEE SECON 2007
26 OCT 07 Gilsoo Kim

2. Contents Introduction Neighbor node distance estimation
Problem formulation
Maximum likelihood distance estimation
Evaluation
Configurable Coverage Protocol (CCP)
Vacancy inside triangle
Node selection constraints
Protocol description
Conclusion

3. Introduction Coverage problem in WSNs Motivations
Failure-prone sensor devices are deploy in higher density to meet various design specifications
Number of active sensors needed to cover the area of interests is to be minimized
Motivations
Global localization is expensive, error-prone and not required for coverage algorithm
By requiring complete coverage, excessive overlap may occur

4. Contents Introduction Neighbor node distance estimation
Problem formulation
Maximum likelihood distance estimation
Evaluation
Configurable Coverage Protocol (CCP)
Vacancy inside triangle
Node selection constraints
Protocol description
Conclusion

5. Problem formulation
Estimating the distance b/w two nodes can be easier and less error-prone than global localization
(na-nx), (nb-nx), and nx are correlated
By taking into account values of them, d can be estimated na nb nx

6. Maximum likelihood distance estimation
Maximum likelihood estimation is used to estimate the size of X and thus the distance d
Node distribution can be estimated as a Poisson (random) point process
Probability of having certain number of nodes inside an area given the value of the area
where,

7. Maximum likelihood distance estimation
Additional information helps to improve the estimation accuracy
Multiple transmission power levels
Example
Two power level sensor nodes
Final estimates can be calculated as the average of four possible combinations

8. Evaluation (1/2)
Compare the performance of using one and two transmission power levels
Power levels are 0.5 and 1 (normalized values)
Multiple Tx power gives significant improvements
Performance improves with the increasing node density
Distance estimates can provide enough accuracy for coverage problem

9. Evaluation (2/2) Adaption of realistic propagation model
Upper and lower bound
Degree of irregularity (DOI)
Maximum radio range variation per
unit degree change in the direction of
radio propagation
DOI=0.05
DOI=0.2
Estimation error increases almost linearly with DOI
Average error can still be tolerable for coverage applications (about 15% of rc)
Two power level: First=0.25~0.5, Second=0.5~1

10. Contents Introduction Neighbor node distance estimation
Problem formulation
Maximum likelihood distance estimation
Evaluation
Configurable Coverage Protocol (CCP)
Vacancy inside triangle
Node selection constraints
Protocol description
Conclusion

11. Configurable Coverage Protocol (CCP)
CCP only makes use of the distance information among the neighboring nodes
Can be built on any other distance estimation scheme
CCP allows the users to specify the coverage objective α
The ratio of the size of the vacancy inside the triangle to the area of triangle should be less than or equal to 1- α
By ensuring that coverage objective is met
locally, the global coverage will be satisfied too

12. Vacancy inside triangle (1/2)
Triangle vacancy calculation
Area of triangle (Heron’s formula)
Overlapping area b/w two nodes

13. Vacancy inside triangle (2/2)
Exceptional cases of vacancy calculation
CCP tries to avoid exceptional cases during selection of active nodes
Hard to calculate
Potentially increases the number of active nodes
For a sufficient high node density, these cases are not likely to happen
Even when these cases are included and no vacancy is assumed the error is small
Exceptional cases of triangle vacancy calculation
Inefficiency caused by exceptional cases

14. Node selection constraints (1/2)
Connectivity constraints
Set of active sensor nodes must be connected at all times
A node should only volunteer itself if it is able to communicated with both end vertices of the edge
Angle constraints
Exceptional cases analyzed in previous section shall be avoided as much as possible
Exceptional cases occur when there are small (or large) angles inside the triangle
CCP tries to select the triangle that maximizes the minimum angle

15. Node selection constraints (2/2)
Angle constraints (cont’d)
Any node that has an angle smaller than ß1 will just ignore the new triangle and edge message
It is essential for every node that is trying to compete for new vertex to hear power on message
Node that can form an angle larger than max(ß1, ß2) meet the angle constraints and are preferred

16. Protocol description (1/3)
Selection of starting node
At the beginning, all nodes are in “UNDECIDED” state
A node should volunteer to be starting node with probability p (normally p=1/N)
It first waits for a random time ts=[0, tsmax]
If it doesn’t hear any messages within ts, it will change its state “ON” and broadcast the power on message
If it receives any power on message, it will simply cancel the timer

17. Protocol description (2/3)
First edge and first triangle formation
First edge
All neighbors around starting node will set a timer t1
If the timer is fires, the node will change its state to “ON”
When a node turns “ON”, it broadcasts power on message including edge information (IDs, length of edge)
First triangle
Upon receiving the edge information, the neighboring node will set a timer t2
If the timer fires, the node turns “ON” and form the first triangle
The node will broadcast the power on message including the triangle information (IDs, length of three edges)

18. Protocol description (3/3)
Node selection process
Each node will first examine whether it has any triangle associated w/ itself and share a common vertex
If there is only one triangle associated with the edge, and it satisfies the vacancy requirements, it will announce an creation of a new triangle
If there are already two triangles associated with this edge, it will take no action
When a node notices that it is within one of the triangles formed, it turns itself “OFF”
The protocol terminates when all nodes are either in the “ON” or “OFF” states

19. Evaluation (1/3) Performance metrics Parameter settings
Average vacancy
Number of active nodes
Parameter settings
Parameters
value
Sensing radius rs
1 (normalized value)
Communication range rc 3
Area size
30 x 30
Two power level
rc=1, rc=2
a, b in time t2
0.5

20. Evaluation (2/3) Comparison between CCP and ODGC (α =1)
ODGC has a slightly better performance
However, performance degradation is small
The vacancy is a result of the distance estimation and location error
Only coverage objective of 0.98 or below can be achieved

21. Evaluation (3/3) Performance of CCP with α <1
CCP is able to meet the coverage objectives most of the time
Reasons why the objective can not be met
Network density is too low
Distance estimation error
Biggest savings comes from moving from 95% to 90% coverage (12%)

22. Conclusion Simple distance estimation scheme
Based on number of node
Configurable coverage protocol
Needs only distances among the neighboring nodes
Vacancies are estimated distributively
Global coverage objective α can be maintained
CCP performs very similar to OGDC for complete coverage
Less active sensor nodes are needed than necessary amount

23. Q & A
Thank you