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

2. Localization Techniques in WSNs 2

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. 3

4. Localization GPS 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 6

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 7

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 10

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 11

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, … 12

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 13

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. 5.Localization Trilateration Example Trilateration A is 5m from B A is 10m from C A is 8m from D B C D A

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 {X i,Y i,h i } Updates only with its neighbors. Each landmark {X i,Y i } 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. Corrections computed by the landmarks c 1  c 2  c 3  Assume A gets its correction from L 2 Its estimate distances to the three landmarks To L 1 : 3×16.42 To L 2 : 2×16.42 To L 3 : 3×16.42 3 Hop 2 Hop L 1 L 2 : 2 hop L 1 L 3 : 6 hop L 2 L 3 : 5 hop

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