Sensor Networks- Part- IV

1. Introduction to Coverage in Sensor Networks

  • Definition: Coverage refers to the deployment of sensor nodes in a specific area (terrain) to ensure that every point within that area is within the sensing range of at least one sensor.

  • Objective: Ensure no area is left unsensed, i.e., every point in the region of interest is monitored.

  • Types of Coverage:

    • Point Coverage: Ensures specific points in the area are covered.

    • Area Coverage: Ensures the entire area is covered.

    • Barrier Coverage: Ensures a barrier (e.g., border) is monitored to detect intrusions.

2. Key Concepts in Coverage

  • Sensing Range vs. Transmission Range:

    • Sensing Range: The radius within which a sensor can detect events or phenomena.

    • Transmission Range: The radius within which a sensor can communicate with other nodes.

    • Typically, the transmission range is larger than the sensing range.

    • In many studies, the two ranges are assumed to be equal for simplicity.

  • Coverage and Connectivity:

    • Connectivity: Ensures that every node in the network can communicate with the sink node (base station).

    • Relationship: If the transmission range is at least twice the sensing range, ensuring coverage automatically ensures connectivity.

3. Types of Coverage Problems

  • Static Sensors:

    • Focus on sensor placement and activation to ensure full coverage.

    • The goal is to deploy sensors such that every point in the area is within the sensing range of at least one sensor.

  • Mobile Sensors:

    • More complex than static sensors.

    • Requires planning the trajectory of sensors to ensure spatiotemporal coverage (coverage over both space and time).

4. Variants of Coverage Problems

  • Sensor Placement Problem:

    • How to deploy sensors to ensure full coverage.

  • Density Control Problem:

    • How to minimize the number of sensors while maintaining adequate coverage.

5. Types of Coverage

  1. Area Coverage:

    • Most common form of coverage.

    • Ensures every point in the area is within the sensing range of at least one sensor.

    • Zhang and Hou’s Theorem: If the communication range (Rc) is at least twice the sensing range (Rs), coverage implies connectivity.

  2. Point Coverage:

    • Ensures a specific set of points in the area are covered.

    • Two variants:

      • Random Point Coverage: Sensors are randomly deployed to cover points.

      • Deterministic Point Coverage: Sensors are placed in a pre-determined manner to cover points.

  3. Barrier Coverage:

    • Ensures a barrier (e.g., border) is monitored to detect intrusions.

    • Three variants:

      • One Barrier Coverage: At least one sensor detects the intruder.

      • Two Barrier Coverage: At least two sensors detect the intruder.

      • K-Barrier Coverage: At least K sensors detect the intruder.

6. Weak vs. Strong Coverage

  • Weak Coverage: Intruders can find paths to avoid detection by sensors.

  • Strong Coverage: No path exists for intruders to avoid detection.

7. Network Lifetime and Coverage

  • Objective: Use the minimum number of sensors while maximizing network lifetime.

  • Network Lifetime Definitions:

    • Time until the first sensor dies.

    • Time until the last sensor dies.

    • Time until a certain percentage (e.g., P%) of sensors die.

8. Deployment Strategies

  • Deterministic Deployment:

    • Sensors are placed at pre-determined locations.

  • Random Deployment:

    • Sensors are randomly scattered (e.g., dropped from an aircraft).

9. Algorithms for Coverage

  • Centralized Algorithms:

    • Global map is computed at a central point.

  • Distributed Algorithms:

    • Nodes compute their positions by communicating with neighbors.

  • Localized Algorithms:

    • A subset of nodes participate in sensing, communication, and computation.

10. Optimal Geometric Density Control (OGDC) Algorithm

  • Objective: Minimize overlap while ensuring coverage of crossings (intersection points of sensing ranges).

  • Key Steps:

    1. A node volunteers as a starting node and broadcasts its position and preferred direction.

    2. Other nodes calculate their deviation from the desired position and choose the optimal location.

    3. The process continues until all crossings are covered.

    4. Nodes go to sleep if their coverage area is fully covered by other nodes.

  • Optimal Distance: For nodes with the same sensing range (Rs), the optimal distance between nodes is √3 × Rs.

11. Applications of Coverage

  • Event-Driven Applications:

    • Examples: Forest fire monitoring, building fire monitoring.

    • Sensors report data when an event occurs.

  • On-Demand Applications:

    • Examples: Inventory control systems.

    • Sensors respond to queries for specific information.

12. Challenges in Coverage

  • Minimizing Overlap: Ensuring that sensor coverage areas do not overlap excessively.

  • Maximizing Network Lifetime: Balancing coverage with energy efficiency to prolong network operation.

  • Dynamic Environments: Adapting coverage in environments where conditions change over time.

13. Summary

  • Coverage is a fundamental problem in sensor networks, ensuring that every point in a region of interest is monitored.

  • Different types of coverage (area, point, barrier) address different monitoring needs.

  • The OGDC algorithm is a key method for achieving area coverage with minimal overlap.

  • The relationship between coverage and connectivity is crucial, especially when the transmission range is at least twice the sensing range.

  • Coverage problems can be addressed using centralized, distributed, or localized algorithms, depending on the network's requirements.