Energy Constraints of Localization Techniques in Wireless Sensor Networks (WSN): A Survey <s> ANCHOR FREE AND ANCHOR BASED <s> Wireless sensor network (WSN) are more and more widely used in many different scenarios. The localization information is an important criterion for the capability of WSN. Nowadays, there are many localization algorithms. DV-hop is a classical Range-free localization algorithm, by which unknown nodes can obtain anchors' information within designated hops, and estimate the distances from themselves to anchors. Unknown nodes then use the information to localize themselves. But the estimative distances may incur large error, and will jeopardize the localization precision. In order to solve the problem, we propose a selective anchor node localization algorithm (SANLA) for wireless sensor networks in this paper, and it can make unknown nodes choose three anchors which are the most accurate to execute trilateration. The experiment results illustrate that our algorithm is valid and effective. <s> BIB001 </s> Energy Constraints of Localization Techniques in Wireless Sensor Networks (WSN): A Survey <s> ANCHOR FREE AND ANCHOR BASED <s> Localization is a fundamental problem in wireless sensor networks and its accuracy impacts the efficiency of location-aware protocols and applications, such as routing and storage. Most previous localization algorithms assume that sensors are distributed in regular areas without holes or obstacles, which often does not reflect real-world conditions, especially for outdoor deployment of wireless sensor networks. In this paper, we propose a novel scheme called reliable anchor-based localization (RAL), which can greatly reduce the localization error due to the irregular deployment areas. We first provide theoretical analysis of the minimum hop length for uniformly distributed networks and then show its close approximation to empirical results, which can assist in the construction of a reliable minimal hop-length table offline. Using this table, we are able to tell whether a path is severely detoured and compute a more accurate average hop length as the basis for distance estimation. At runtime, the RAL scheme 1) utilizes the reliable minimal hop length from the table as the threshold to differentiate between reliable anchors and unreliable ones, and 2) allows each sensor to determine its position utilizing only distance constraints obtained from reliable anchors. The simulation results show that RAL can effectively filter out unreliable anchors and therefore improve the localization accuracy. <s> BIB002
Anchor-based localization involves determining the location of an unknown node (fixed or mobile) with respect to a node with known location called Anchor node BIB001 [38] [39] BIB002 , while Anchor free is without reference to an anchor node. Research has established that the presence of anchors in the network for localization does increases the degree of accuracy in estimating the positional coordinates of the unknown node(s) Anchor nodes are those nodes in the network that already know their location by means of direct placement at know coordinates, hard coding the location or acquired by using localizing systems such as a Global Positioning System (GPS) receiver. Deploying anchor nodes is both labour and hardware intensive hence the need for anchor-free localization techniques. Since sensor nodes are deployed randomly via air planes into unfriendly terrains or harsh environments for environmental monitoring and military surveillance, there is the need for algorithms that may not necessarily depend on anchor nodes for localization. There is the need for optimal localization algorithms that will not depend on anchors for the following reasons: a. The use of small, low cost and disposable sensor nodes. VLSI technology and advancement in the manufacturing of Micro-Electro-Mechanical Systems (MEMS) has enabled wide application areas of wireless sensor networks. The small, low cost and disposable nature of these sensor nodes has also informed large scale deployment and wide coverage of these nodes in harsh and unfriendly terrains for various environmental monitoring and military applications. b. Labour and risk in placing anchor nodes Anchor nodes with hard coded location coordinates will have to be placed at the right position in the network. This may not be possible in some harsh environmental monitoring applications and will be at a high cost if it can be done. c. Global Positioning System (GPS) coverage Underground, underwater and some plane area above sea level are unreachable for GPS usage. Hence applications that will depend on GPS to estimate the location of anchors may be prone to errors since the localization results are directly determined by GPS accuracy. 