Exploring Localization in wireless sensor network

IJSRD - International Journal for Scientific Research & Development| Vol. 3, Issue 08, 2015 | ISSN (online): 2321-0613

Exploring Localization in Wireless Sensor Networks Hari Narayanan1 Saba Khalid2 Shara Khursheed3 1,2,3 Department of Computer Engineering 1,2,3 Integral University Abstract窶年ode Localization in Wireless Sensor Networks (WSN) refers to determining the location of sensor nodes which are deployed in sensor field that are responsible for monitoring applications like environment perceived applications, tracking, traffic monitoring etc. This paper covers the localization techniques and study which algorithm provides higher accuracy, low expense and better power utilization.

Key words: Localization, Beacon Nodes, Decentralized

The parameters which help in localization are as follows: A. Distance/Angle In this parameter, the distance and angle between two nodes is calculated [6]. The methods of estimation of distance and angle are as follows 1) Time of Arrival (ToA) In this method the distance is estimated using speed of signal and propagation time. Distance is directly proportional to the propagation time and speed of signal.

I. INTRODUCTION A Wireless Sensor Network (WSN) consists of several sensor nodes which are deployed in a sensor field. A Sensor Node consists of a low power processor and is relatively less costly with the ability to sense and communicate and perform computation. They are generally smaller is size and communicate in short distances. With the current advancement in technology the WSNs have found greater importance in the future and it find its applications in habitat monitoring, target tracking traffic monitoring, smart environments etc.[1,2] In these vivid applications the location of sensor node is required and hence we apply localization techniques to determine the location of the sensor nodes in the WSNs. Localization is a technique of finding out the physical co-ordinates of sensor nodes sensor field. These sensor nodes are deployed at random and we need the appropriate technique for the right kind of application with minimum complexities, less cost and better power utilization.

The positioning of sensors node can be fixed or mobile [3,4].

The fixed sensors have known co-ordinates and are called beacon nodes or anchor nodes. The concept of anchor nodes is used in GPS (Global Positioning System) but it consumes way more power and reduces battery life of sensor node and is not practical for large scale deployment. The node positions can be absolute (global) or relative. Absolute position is obtained by beacon nodes and relative position is determined by various techniques like trilateration and triangulation [4,5].

Localization is a cost effective, easy to deploy and power saving method and it is practical in large scale and diverse environments.

Fig. 1: WSN II. LOCALIZATION In Wireless Sensor Networks the location of sensor nodes need to be defined in the sensor field to gather the information available at sensor nodes. This is called Localization.

Fig. 1: Time of Arrival (ToA) 2) Time Difference of Arrival (TDOA)

Fig. 2: Time Difference of Arrival The TDOA is a very popular technique to measure the distance between the nodes [7]. They are of two types: multinode TDoA and multi-signal TDoA. Multi-node TDoA measures the time difference of the transmitted signals between multiple anchor/beacon nodes using ToA (Time of Arrival) technique. Multi-signal TDoA measure time difference of different type of signal such as radio signal and sound (from a speaker) signal as given in the Figure 2.

3) Received Signal Strength Indicator (RSSI) This method is the most used and experimented where distance between two nodes is measured by the signal strength between them. This method is most commonly used because almost all nodes have the capacity to measure the strength of the received signal which eventually reduces or diminishes as with the square root of distance [2,9,10].

The RSS measurements for the nodes i and j in time t is calculated as: Where is constant as the power transmitted; is the attenuation constant; is the distance between two nodes and is uncertainty factor due to multipath and shadowing.

4) Angle of Arrival (AoA) In the Angle of Arrival technique the angle direction of signal to the antennas determine the distance. This is estimated by multiple antennas or directional antennas. It analyses the phase difference between the signals to determine the

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Exploring Localization in Wireless Sensor Networks (IJSRD/Vol. 3/Issue 08/2015/112)

distance between the sensor nodes [2,9]. This method has high complexity and is not cost effective since it also uses RSSI technique to compute the distance between the sensor nodes.

B. Fundamental Position Discovery Techniques Following methods are used to compute the positioning of sensor nodes: 1) Triangulation:

Fig. 3: Triangulation technique between nodes A and B In this direction of nodes is estimated by using the law of cosines in the figure 3, where the position of both A and B nodes are known. Hence angle of arrival method determines the angle between the two nodes[5]. This makes the technique direction based rather than distance based.

2) Trilateration: In this method the position of node is determined by intersection of three circles of the neighboring nodes and this in turn determines location of unknown node. But this method is faulty practically since natural environment has more noise and requires more than three nodes.[2,11] 3) Maximum Likelihood Multi-Lateration Technique: This method is more practical in noisy environments. This uses trilateration but distance measurements are also taken into account by neighboring nodes and it further helps in closing in for the estimation of the required node [2,11]. It also uses multi-node time difference of arrival method.

4) Bounding Box: The bounding box is a technique which uses squares instead of circles to estimate the position of the unknown nodes [13].

This method is not so popular because of more possibility of errors.

which in turns computes the request and generates appropriate response and send the response back to the sender. The methods used in centralized algorithms are SemiDefinite Programming (SDP) (based on Linear Matrix Inequalities) and Multi-Dimensional Scaling (MDS) (based on Law of Cosines in Linear Algebra).

Distributed Algorithms are implemented when each node has its own computational power and performs operation within the wireless sensor networks. The methods in Distributed Algorithms are: 1) Anchor Based/Beacon Based Distributed Algorithms In this an anchor/beacon node is used to calculate the positions of the nodes in the WSNs. The node figure out the location based on the distance measured from the anchor nodes to the sensor nodes whose position is to be determined.

The methods of Anchor Based Algorithms are:  Diffusion: The position of node is assumed to be at the centroid of its neighbor’s position [20].  Bounding Box: Where the intersection of the square sized bounding regions determine the position of the node. This method is used when a lot of computation can’t be performed [14].  Gradient Method with Multi-lateration: In this method a counter is maintained for every hop count of the signal which in turn is used to measure the distance between the nodes to identify the position of sensor nodes [13,14]. Multi-lateration technique is then used to compute the position of the sensor node. This algorithm has high adaptability while nodes are added or deleted in a network.



Fig. 4: Bounding Box III. CLASSIFICATION OF LOCALIZATION ALGORITHMS The localization algorithms in WSNs can be classified into four categories:  Centralized[14] vs Distributed Algorithms[13,16]  Relative[17] vs Absolute Positioning[13]  One hop[9] vs Multi-hop[13,16]  Range Free[2,9] vs Range Based Localization[2,9] A. Centralized vs Distributed In centralized algorithms a single node acts as a sink for all signals [17,18, 19].The Computation is very high in case of centralized algorithms because the signal from many nodes is centralized to one node which must have high computational power. All the nodes send the information to central node

Fig. 5: Multi-Lateration Technique Approximate point in Triangle (APIT) algorithm: In this algorithm instead of anchor nodes anchor triangles are used to triangulate the position of sensor node. The neighboring region is divided in to rectangles [13].The point of intersection of these rectangular regions determines the location. The anchor triangles are selected by the signal strength in each region. The centroid of these triangles gives the estimated position of the sensor node.

Fig. 6: Approximate point in triangle method.

2) Relaxation Based Distributed Algorithms: This method combines the advantages of distributed and centralized algorithms. It uses the distributed algorithms to determine the position of the nodes and estimates the distance

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and then defines temporary anchor nodes in the region which generally are neighbors of the nodes. Then it uses a Spring Model Technique to calculate the distance in multiple iterations until zero force acts on the node [20,21]. The anchor nodes are fully distributed and concurrent.

3) Coordinate System Stitching Algorithms: In this method first a given network is divided into small overlapping sub-regions then a local map is generated for every sub-region. Then a single global coordinate of that node is generated using a registration procedure [22]. This method is applied in cluster based environment or a static computational node network.

4) Hybrid Localization Scheme: This method uses a combination of two or more techniques to get a better performance such as Multi-Dimensional Scaling and Proximity Based Map or MDS and Ad Hoc Positioning System [19].

5) Interferometric Range Based Localization Algorithm: It works when the interference is generated when two transmitters emit radio waves simultaneously at slightly different frequencies. This method is advantageous for obtaining range information about the nodes[17].

6) Error Propagation aware Localization: This method is used after error is calculated in a particular algorithm and is reduced to improve accuracy of an algorithm by removing the noises and interfering elements in the signal[23,24]. This is used to minimize location error and propagation error.

C. Single Hop Vs Multi-Hop : Single Hop technique is used to directly communicate with the beacon nodes for determining the positions [4]. The techniques used are:  Active Badge  Active Office  RADAR  Cricket  SpotOn In Multi-hop technique the sensor nodes communicate with several nodes to determine the position of the sensor node and examples are as follows: 1) DV Hop: This method counts the number nodes travelled in the path and the distance is measured in terms of hop count. Average distance is calculated to estimate a hop range and multilateration technique is used to determine the position of the node [25].

2) DV-Distance: The Euclidean distance is calculated from the far away anchor nodes which depend on the number of nodes in between [25].

If there are more hops i.e. nodes then there is higher accuracy of the estimated distance.

D. Range Based vs Range Free : Range based approach depends on the distance and angle between the nodes and uses methods like Angle of Arrival(AoA), Time of Arrival (ToA), Time Difference of arrival(TDOA) and Received Signal Strength Indicator (RSSI).

Range Free Approach doesn’t need any kind of such information only connectivity and topology for localization and distance estimation. This method has been the field of research and still continues to be.

B. Relative vs Absolute Positioning: Relative Localization is a method in which the relative position of node is determined with respect to a fixed node or a reference node. This reference node is manually determined and is easy to compute[12,21].

Absolute Localization is a method where a GPS node is a reference node which determines the exact position IV. COMPARISON OF VARIOUS LOCALIZATION TECHNIQUES of the node and is less effective due to high overhead[12].

Localization Factors Governing the Technique Technique Power Computational capacity Methods Used Cost and Accuracy Consumption High(Due to Costly But Highly  SDP Centralized Very High increased Accurate  MDS Computation)  Bounding Box Low Cost and Low(In comparison to  Multilateration Low Distributed relatively accurate Centralized)  Diffusion  APIT Relative Cost Effective and Less Less Positioning Relatively Accurate Absolute High(Due the use of Costly and Highly High Positioning GPS) Accurate  Active Badge  Active Office Less(Due To direct Costly and Highly Less Single Hop Communication between the  RADAR Accurate nodes)  Cricket  SpotOn High(Since information of  DV Hop Cost Effective and High Multi Hop several nodes in WSN is Highly Accurate  DV Distance transferred)

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Range Based

Range Free

High(Since It requires all the information about the network) Low(Since it doesn’t require any Information about the network)

High

-

Depends upon the type of WSN

Low

-

Highly Cost effective but not so Accurate

Table 1: Comparison of Various Localization Techniques [7] G. Mao, B. Fidan, and B. D. O. Anderson, “Wireless V. CONCLUSION sensor network localization techniques,” Computer Networks, pp. 2529–2553, 2007.

In this paper we presented a survey on localization of nodes [8] Y.Ohta, M.Sugano, M.Murata "Autonomous in wireless sensor networks. This paper gives a brief idea Localization Method inWireless Sensor Networks" about wireless sensor networks and its different areas of Proceedings of the 3rd Int'I Conf. on Pervasive application. Localization is a hot area of research in wireless Computing and Communications Workshops 2005 sensor networks due its application as discussed in the paper.

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