05-05-2011, 11:56 AM
Abstract
In this paper, we study the localization problem in large-scale Underwater Wireless
Sensor Networks (UWSNs). Unlike in the terrestrial positioning, the global positioning system
(GPS) can not work efficiently underwater. The limited bandwidth, the severely impaired
channel and the cost of underwater equipment all makes the localization problem very
challenging. Most current localization schemes are not well suitable for deep underwater
environment. We propose a hierarchical localization scheme to address the challenging
problems. The new scheme mainly consists of four types of nodes, which are surface buoys,
Detachable Elevator Transceivers (DETs), anchor nodes and ordinary nodes. Surface buoy is
assumed to be equipped with GPS on the water surface. A DET is attached to a surface buoy
and can rise and down to broadcast its position. The anchor nodes can compute their
positions based on the position information from the DETs and the measurements of distance
to the DETs. The hierarchical localization scheme is scalable, and can be used to make
balances on the cost and localization accuracy. Initial simulation results show the advantages
of our proposed scheme.
1. Introduction
In recent several years, there has been a rapidly growing interest in Underwater Wireless
Sensor Networks (UWSNs). UWSNs can be used for a broad range scientific exploration,
including ocean sampling, environmental monitoring, undersea Explorations, disaster
prevention, assisted navigation, distributed Tactical Surveillance and mine reconnaissance
[1]. There are still many issues unsolved for the large scale UWSNs, such as reliable
transport, routing, MAC and localization [2] [3] [4] [5] [6] because of the limited bandwidth,
and high and variable propagation delays, severely impaired underwater channel, limited
battery power.
In those general wireless sensor networks and applications, as well as the source detection
and tracking applications of our interest, location estimation is a vital component. Without the
node location information, the data received in the sink node can not be identified where it
comes from, and becomes meaningless to the applications such as source tracking. In
addition,
location information can be used to design efficient networking and management protocols.
With regard to the mechanisms used for location estimation, localization algorithms can be
divided into two major categories, range-based and range-free. In the range-based location
International Journal of Advanced Science and Technology
Vol. 4, March, 2009
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algorithms, distance or angle estimates with neighbors will be used for calculating node
locations. Typical range-based algorithms include the sum-distances based algorithm [14]
[15]. In the range-free location algorithms, the neighbor distance/angle information is
assumed to be unavailable for positioning due to the cost and hardware limitation. DV-hop
algorithm is a typical range-free location algorithm [12] [13]. However, as mentioned above,
the acoustic channel is severely impaired and the GPS signal can’t propagate far through
water, the existing positioning schemes for terrestrial WSNs can not be used directly for
UWSNs. There are several localization schemes proposed for UWSNs [7] [8] [11]. In [7], a
hierarchical localization scheme for large scale UWSNs was proposed. The system mainly
consists of three types of nodes: surface buoys, anchor nodes and ordinary sensor nodes. The
buoys are equipped with GPSs. The first localization step is anchor node localization, for
which it is assumed all the anchor nodes can estimate their positions by contacting directly
with surface buoys. The second step is ordinary node localization through anchor node’s
position information. The key of the scheme is that anchor nodes are localized through buoys.
However, it can be very difficult for buoys to communicate directly with anchor nodes under
deep water. On the other hand, a very large number of anchor nodes are used in [7], which
results in very high cost and the localization performance is not satisfying. In [11], an
interesting idea of Dive and Rise (DNR) positioning is presented. Mobile DNR beacons are
used to replace static anchor nodes. Each DNR beacon is equipped with GPS. When DNR
beacon move to water surface, they acquire x-y coordinate through GPS, and move down to
broadcast their position to help localize ordinary sensor nodes. The major drawback of the
DNR scheme is the high expense of the DNR beacons. There are 25 DNR beacons for only
1km x 1km x 1km underwater area, so 25 GPS and 25 moving equipments will be needed,
which is very expensive. And the ordinary sensor nodes can only use the position information
of DNR beacons to calculate their positions, which will degrade the localization
performances.
In this paper, we are motivated to propose a hierarchical localization scheme for large scale
UWSNs. The new scheme mainly consists of four types of nodes, which are surface buoys,
DETs, anchor nodes and ordinary nodes. Surface buoy is assumed to be equipped with GPS
on the water surface. A DET is attached to a surface buoy and can rise and down to broadcast
its position. The anchor nodes can compute their positions based on the position information
from the DETs and the measurements of distance to the DETs. Through the hierarchical
design, we can achieve scalability, and make balances on the cost and localization accuracy.
Initial simulation results show the advantages of our proposed scheme. In the rest of the
paper, we will introduce our proposed localization in Section II. Simulation setting and result
analysis are given in Section III. Finally Section IV concludes the paper.
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