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Localization is a process by which sensor nodes in Wireless Sensor Networks (WSN) determine spatial relationships between themselves (Srinivasan, & Wu, 2007; Rasmussen, Capkun, & Cagalj, 2007; Mi, Stankovic, & Stoleru, 2012). The determination of spatial relationship signifies that the networks are able to assign location coordinates to each other either with reference to a coordinate system localized to a particular network or with reference to a global coordinate system such as GPS (Global Positioning System). Broadly, the approaches used toward localization consist of two types of nodes:
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Beacon node / Anchor node: Node, who knows its physical location (either via GPS receiver or by manual configuration);
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Regular node / Non-Beacon node: Node, who does not know its position and no special hardware requires to acquire its information.
There are many advantages of knowing the location information of sensor nodes in WSN, which include: (i) network coverage checking and location-based information querying; (ii) selective forwarding instead of broadcasting information; (iii) neighbour discovery; and (iv) data aggregation after finding neighboring node:
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Beacon-based approach: Beacon-based (BB) approach uses Beacon node(s) as the basis for computing the location coordinates of other non-Beacon nodes. The idea is to implement the Beacon node with directional antennas and the non-Beacon nodes localizing themselves based on the range of whichever antenna they belong to. Naturally, the security is an important concern in BB approach when an adversary attempts to compromise some Beacon nodes (Liu, Ning, Liu, Wang, & Du, 2008). There are some algorithms based on BB approach, such as ROPE (Robust Position Estimation) (Lazos, Poovendran, & Capkun, 2005), SeRLoc (Secure Localization) (Lazos, Poovendran, 2004) and HirLoc (High Resolution Localization) (Lazos & Poovendran, 2006);
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Without Beacon-based approach: Without Beacon-based (WBB) approach is inspired by the notion of automating localization process without using Beacon nodes. The idea is to develop a localized coordinate system, where the coordinates/position of nodes is assigned with reference to other nodes in the network. However, when communication is established with other networks, the local coordinate system can be converted to a global coordinate system using Beacon nodes. One of the main advantages of BB approach is scalability. Generally, the WBB approach involves with following three steps to establish a localized coordinate system in the network.
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Discovery of one-hop neighbors and sharing a pairwise key with other node;
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Calculation of distance to every neighboring node;
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Assigning coordinates to neighboring nodes.
In the context of node localization, the WBB approach seems to be found more suitable for WSN, because (i) in BB approach, a large number of beacon nodes required to localize all non-beacon nodes; (ii) BB approach is not scalable well in comparison to WBB approach; (iii) in BB approach, localization is completely dependent on Beacon nodes: