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Wireless Sensor Networks: nodes localization issue

Wireless Sensor Networks: nodes localization issue,Jean-Philippe Montillet,Carlos Pomalaza-Raez

Wireless Sensor Networks: nodes localization issue  
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In the near future, advances in processor, memory and radio technology will enable small and cheap nodes capable of wireless communication and signicant computation capability. The avai- lability of micro-sensors and low power wireless communications will enable the deployment of very dense, fully distributed sen- sor/actuator networks for a wide range of environmental monitoring applications(in-home, re detection in forests, outdoors, atmospheric applications). Moreover these systems will eventually incorporate actuation, as well as sensing allowing these systems to inuence and interact with their environment. These ad-hoc sensor networks will consist of nodes located arbitrarily and will be largely unattended. These unattended networks must self-congure and recongure to adapt to their environment and the availability of other nodes within the system. One of the major challenges for researchers is to localize the sensor nodes with relatively high accuracy. For military, police radio net- works, knowing the precise location of each person with a radio can be critical. In ofces and in warehouses, object location and tracking applications are possible with large-scale ad-hoc networks of wireless tags. Thus, Global Positioning System (GPS) has been suggested as a means to obtain location information in ad-hoc networks (1). But the straight forward of adding GPS to all nodes is not a viable solution due to the issues enumerated below. GPS cannot work indoors or in the presence of dense vegetation, foliage or other obstacles that block the line-of-sight from the GPS satellites. The size of GPS and its antenna increases the sensor node form factor. Sensor nodes are required to be small and unobtrusive. Finally, due to practical considerations as low-power consump- tion and cost preclude the use of GPS on all nodes. Instead of using GPS technology, distributed algorithms are proposed to locate small devices in wireless ad-hoc sensor network. These algorithms use either the cooperative ranging method based on Time-Of-Arrival(TOA), Time-Difference-O-Arrival(TDOA), Angle- Of-Arrival(AOA), Received-Signal-Strength(RSSI)techniques, or hop count method. The basic idea behind these algorithms is the same in GPS technology, performing a triangulation between the nodes does not know its position(unknown node) and neighbour nodes knowing their positions(beacon nodes) to solve the set of geometric constraints. Beside, some studies (2) (3) show beacon placement and density in the transmission range of a given unknown node, affect the quality of spatial localization. Fixed beacon placement approaches such as uniform and very dense placement are not always viable and will be inadequate in very noisy environments in which sensor networks may be expected to operate. This paper focus on the problem of nodes localization in wireless sensor ad-hoc networks. It is organized as follows : In the next section, the nodes localization problem is explained in term of error due to localization techniques (TOA, AOA, RSSI, and TDOA). In section 3, beacon nodes solution is studied to decrease the error. Two approaches are used in order to analyze the problem :arithmetic and via the direct method algorithm. Section 4, simulation results are explained. Section 5 is the conclusion. II. NODES LOCALIZATION In many proposed applications for wireless peer-to-peer and ad- hoc networks, knowing the location of the devices in the network is a key issue. One of the way to localize tiny sensor nodes is by local positioning. Devices communicates among them and perform a triangulation to determine their locations based on received signal strength (RSSI), time difference of arrival (TDOA), or time of arrival (TOA) techno- logies. The range estimate will be degrade due to multipath and noise in the channel and the inaccuracies of devices reference clocks. Error due to multipath can be reduced using very wide bandwidth like in the UWB technology (communication systems) or radar-like technology. The error due to inaccuracies of devices clock can be brought down using very accurate clocks with low parts-per-million (PPM) and low phase noise oscillators. In the RSSI technique, the pathloss model is frequently used for the fading channel.pi;j is the power transmitted by node i and received by node j.
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