Introduction to Sensor Networks

1. Introduction to Sensor Networks Rabie A. Ramadan, PhD Cairo University http://rabieramadan.org rabie@rabieramadan.org 3

2. Localization Techniques in WSNs

3. Why do I need localization ? • In sensor networks, nodes are deployed without priori knowledge about their locations. • Estimating spatial-coordinates of the node is referred to as localization.

4. LocalizationGPS • Global Positioning System (GPS) is an immediate solution. • There some factors against the usage of GPS: • GPS can work efficiently outdoors. • GPS receivers are too expensive to unsuitable for wide-range deployment. • It cannot work in the presence of obstructions.

5. Classifications of Localization Methods • Centralized vs Distributed • Anchor-free vs Anchor-based • Range-free vs Range-based • Mobile vs Stationary

6. Centralized versus Distributed Localization Algorithms • In centralized algorithms, • nodes send data to a central location where computation is performed and the location of each node is determined and sent back to the nodes. • Drawbacks • high communication costs • intrinsic delay

7. Centralized versus Distributed Localization Algorithms • In distributed algorithms, • each node determines its location by communication with its neighboring nodes • robust and energy efficient • Drawback • Can be more complex to implement • At times may not be possible due to the limited computational capabilities of sensor nodes

8. Anchor-Free vs Anchor-Based • Anchor Nodes: • Nodes that know their coordinates a priori • By use of GPS or manual placement • For 2D three and 3D four anchor nodes are needed • Anchor-free • Relative coordinates • Anchor-based • Use anchor nodes to calculate global coordinates

9. Range-Free vs Range-Based • Range-Free • Range-free techniques use connectivity information between neighboring nodes to estimate the nodes‟ position • Local Techniques • Hop-Counting Techniques • Range-Based • Received Signal Strength Indicator (RSSI) • Attenuation • RF signal • Time of Arrival (ToA) • time of flight • Time Difference of Arrival (TDoA) • requires time synchronization • electromagnetic (light, RF, microwave) • sound (acoustic, ultrasound) • Angle of Arrival (AoA) • RF signal

10. Range-Based Techniques • Time of Arrival • All sensors transmit a signal with a predefined velocity to their neighbors. • Then, the nodes, each send a signal back to their neighbors • by using the transmission and received times each node estimates its distance to its neighbor

11. Range-Based Techniques • Received Signal Strength Indicator (RSSI) • The amount of power present in a received radio signal. • Due to radio-propagation pathloss, received signal strength (RSS) decreases as the distance of the radio propagation increases. • The distance between two sensor nodes can be compared using the RSS value at the receiver, assuming that the transmission power at the sender is fixed

12. Range-Based Techniques • TDOA (Time Difference of Arrival) • Transmit both radio and ultrasonic signals at the same time to observe the arrival time difference. • Extra hardware, i.e., ultrasonic channel, is required • Not only radio but also sound signals have multipath effects affected by humidity, temperature, …

13. Range-Based Techniques • Angle of Arrival (AoA) • Gather data using either radio or microphone arrays. • Allow a receiving node determines the direction of a transmitting node. • A single transmitted signal is heard by several spatially separated microphones. • The phase or time difference between the signal‟s arrival at different microphones is calculated and thus the AoA of the signal is found. • Requires directional antennae

14. Proximity base localization • Trilateration / Multilateration technique • Proximity based localization: • Some nodes which can know their position through some technique (ex. GPS) broadcast their position information. • Other nodes listen to these broadcast messages and calculate their own position. • A simple method would be to calculate its position as the centroid of all the positions it has obtained. • This method leads to accumulation of localization error.

15. A 5.LocalizationTrilateration Example • Trilateration • A is 5m from B • A is 10m from C • A is 8m from D C B D

16. Range-Free Localization • DV-HOP • Similar to classical distance vector routing. • An anchor broadcasts a beacon to be flooded in the area.

17. DV-Hop propagation method • Each node maintains a table {Xi ,Yi ,hi} • Updates only with its neighbors. • Each landmark {Xi ,Yi} • Computes a correction • And floods it into the network • Each node • Uses the correction from the closest landmark • Multiply its hop distance by the correction

18. L1L2: 2 hopL1L3: 6 hopL2L3: 5 hop 2 Hop 3 Hop 3 Hop • Corrections computed by the landmarksc1 c2 c3 • Assume A gets its correction from L2 • Its estimate distances to the three landmarks • To L1: 3×16.42 • To L2: 2×16.42 • To L3: 3×16.42

19. Range-Free Localization • DV-hop • Advantages • Simplicity • Dose not depend on measurement error • Disadvantage • Only work for isotropic networks

20. APIT Overview • Anchors • Nodes equipped with high-powered transmitter • Location information obtained from GPS or other mechanism • Location estimation by isolating the environment into triangular regions between anchors