A SURVEY OF UNDERWATER WIRELESS SENSOR NETWORKS
Bo Dong
Faculty Advisor: Dr. Ahmed Mahdy
Department of Computing Sciences, Texas A&M University–Corpus Christi
6300 Ocean Drive, Corpus Christi, Texas 78412
bdong@islander.tamucc.edu
ABSTRACT
Wireless Sensor Networks (WSN) have recently commanded the attention of
many researchers. However, when compared to terrestrial WSN, Underwater Sensor
Networks (UWSN) present a novel-networking paradigm. The deployment of UWSN is
not straightforward, basic challenges need to be addressed due to the type of environment
found underwater. A comparison of the two types of sensor networks is presented. In
addition, this paper discusses major challenges on different aspects of UWSN including
communication protocols and architectures, and proposes research directions. In
addition, a comparison also compares and contrasts the two sensor networks and alludes
to future advances in research in this field.
INTRODUCTION
Most of our planet is coved by water. As more research is being done on
underwater systems, data collection and environment monitoring become apparently
major players. This raises the need for an effective way to collect data and monitor the
environment. Underwater wireless sensor networking offers an unmatched option. The
characteristics of the underwater environment present researchers with many challenges
especially effective communication and sensor localization techniques. In terrestrial
wireless sensor networks, the nodes use radio frequency (RF) to build up the
communication. In underwater environments, due to water absorption, radio does not
work well. Compared to radio waves, sound has superior propagation characteristics in
water, making it the preferred technology for underwater communications. “In fact, radio
waves propagate at long distances through conductive seawater only at extra low
frequencies (30—300 Hz), which require large antennae and high transmission power.
Optical waves do not suffer from such high attenuation but are affected by scattering.
Moreover, transmission of optical signals requires high precision in pointing theatrics
narrow laser beams.”[1] Thus, most of underwater networks use acoustic signals to
communicate to each other. The remainder of this paper describes those challenges and
proposes possible ways to solve corresponding challenges.
ACOUSTIC UNDERWATER WIRELESS SENSOR NETWORKS
Underwater acoustic propagation depends on many factors that make designing an
underwater wireless sensor network challenging. In the following, we show how different
factors may affect the design process.
a. Bandwidth: The acoustic band under water is limited due to absorption; most acoustic
systems operate below 30kHz. According to [2], no research or commercial system can
exceed 40km  kb/s as the maximum attainable range  rate product.
b. Propagation delay: The speed of RF is 3  10 8 m/s while the acoustic signal
propagation speed in an underwater acoustic channel is about 1.5  10 3 m/s. The


propagation delay in underwater is five orders of magnitude higher than in RF. the low
speed of sound causes multi-path propagation
to stretch over time delay. It greatly

effects the real-time application of UWSN.

c. Shadow zones: It can be defined as the area with high bit error rates and temporary
losses of connectivity due to the extreme characteristics of the underwater channel.
Salinity, density and temperature variations of the water can influence acoustic
communication, such as temporary losses of connectivity. This is evident in the sound
speed formula. Sound speed under water is given by empirical formula [3]:
C 1449.2  4.6T  0.055T 2  0.00029T 3  (1.34  0.01T)(S  35)  0.016Z (1)
where, C speed of sound (m/s)
T temperature (deg C)
S salinity (practical salinity units “psu” equivalent to parts per
thousand)
Z depth (m)
d. Energy: Battery power is limited because underwater batteries are extremely difficult
to recharge. Unlike terrestrial WSN, UWSN cannot use solar energy to regenerate the
power of the batteries.
e. Failure: Underwater sensors are prone to failure because of fouling and corrosion.
f. Attenuation: attenuation is the reduction in amplitude and intensity of a signal.
Attenuation at distance x is given as [4]
A(x)  x k ax
Where, k is spreading factor
a is frequency dependent term obtained as a  10(. ( f ))
where, ( f ) is absorption coefficient given by Thorp’s expression.

The formula illustrate attenuation is dependent on frequency as well as distance. It is
very important in determining signal strength asa function of distance.

DIFFERENCES
WITH TERRESTRIAL WIRELESS SENSOR NETWORKS
Although WSN and UWSN are different, mainly due to the unique characteristics
of water, certain aspects of WSN research can be applied to UWSN. The main
differences between terrestrial and underwater sensor networks are as follows:
a. Communication method: UWSN uses acoustic signal while WSN uses radio waves.
b. Cost: While terrestrial sensor nodes are expected to become increasingly inexpensive,
underwater sensors are expensive devices. It is due to the UWSN’s transceivers
complexity and the increased protection required by the hardware.
c. Power: UWSN needs more power because it uses acoustic signal and covers a longer
distance. Compared to acoustic signal, RF needs less power, since the processing at
receivers is not that complex.
d. Memory: The connection of an acoustic signal can be disabled by special underwater
situations, like shadow zones. Due to this fact, underwater sensors need to acquire more
data to prevent the loss of data. However, this is not an issue for terrestrial sensors.
e. Density: In terrestrial sensor application, like tracking system, sensors can be deployed
densely. While an underwater sensor is more expensive than terrestrial sensor, it will cost
more money to deploy densely. Even if money is not an issue, it is still not easy to deploy
them.
As a matter of fact, those differences are the clues to develop new generation
UWSN. First, we should generate some new kind of sensors to reduce cost. For example,
we can use nano-technology to develop nano-sensors. Also, it is necessary to devise
periodical cleaning mechanisms against corrosion and fouling, which may impact the
lifetime of underwater devices. Moreover, the deployed network ought to be highly
reliable, so as to avoid failure of monitoring missions due to failure of single or multiple
sensors. Second, we need to do a new power control algorithm for UWSN. “Many
complex power control algorithm using RTS-CTS-ACK have been proposed in past for
wireless terrestrial networks”[5][6]; however, these algorithms cannot fit into UWSN due
to the underwater channel characteristics and significant propagation delays. Third,
network protocol is a vitally important factor in saving power and providing reliable
connection using sparse underwater sensors. Nowadays, many different protocols for
terrestrial WSN have been developed. However, they cannot fit UWSN. Not only do the
architectures of UWSN impact the development of a new protocol, but also the
characteristics of underwater. It is another different place with terrestrial sensor network.
Therefore, we may develop different kinds of protocol according to the architectures of
UWSN. The following will discuss the idea that protocols should be designed according
to the type of architecture.
USWN ARCHITECTURES
According to [7], “UWSN can be roughly classified into two broad categories:

long-term non-time-critical aquatic monitoring

short-time time-critical aquatic exploration.
In [1], UWSN are classified into three types:

Static two-dimensional underwater acoustic sensor networks (UW-ASNs) which are
for ocean bottom monitoring.

Static three-dimensional UW-ASNs which are for ocean-column monitoring.

Three dimensional networks of autonomous underwater vehicles (AUVs)
a. Long-Term Non-Time-Critical Aquatic Monitoring
This kind of UWSN can be work for a long time and the data collected by the
sensors are not real-time data. For long-term monitoring, energy saving is a central issue
to consider in the protocol design.
b. Short-Term Time-Critical Aquatic Exploration
Compare to Long-term non-time-critical UWSN, this kind of UWSN focus on
real-time data. Therefore, how to make data transfer efficiently need to be more concern
when designing network protocol. Also, this kind UWSN just work for a short term that
means energy saving is not as important as long-term one.
c. Comparison Of The Two Classifications
The difference between the two classifications is static and mobile. In [7], longterm non-time-critical and short-term time-critical UWSN are base on mobile ability.
That’s why they concern the location aware in either way. Moreover, long-term and
short-term did not distinguish 2D or 3D. Obviously, there are some differences in
protocol design.
In static two-dimensional underwater, all the sensors are anchored to the bottom
of ocean. The underwater sensor nodes are interconnected to one or more underwater
sinks (uw-sinks) by acoustic signal. Since, all the sensors are fixed at bottom, therefore,
we don’t need to concern underwater movement, which make protocol design easily. 2D
UW-ASNs always are used to environmental monitoring, it ought to be kind of long-term
non-time-critical UWSN. Therefore, the challenges of long-term non-time critical UWSN
also fit it.
Static three-dimensional underwater sensor network, compared to twodimensional one, this kind UWSN also tell the depth in order to cover 3D area. The
protocol for three-dimensional UWSN is hard to design. The speed and propagation delay
of acoustic signal is different at different depth which will make some sensors at certain
depth use more energy to send and receive data. Also, this issue makes it hard to build up
an efficient routing. Another challenge in this architecture is how to make the sensor stay
at fix position.
The last one, three-dimensional networks of autonomous underwater vehicles
(AUVs), can be regarded as long-term or short-term. “And one vital important design
objective is to make them rely on local intelligence and less dependent on
communications from online shores.” [1]
Although, we need to do the protocol according to the different application, we
still need focus on some common requirements, such as security, reliable as well as
resilient.
UWSN DESIGN: RESEARCH CHALLENGES
Power Consumption
Unlike the sensors of terrestrial WSN, UWSN sensor cannot use solar energy to
recharge the battery. And it is more difficult to replace the sensors. The direct way to
resolve this problem is to generate energy by the sensors themselves. The probable
method may be using current movement or chemistry method to generate power to
recharge battery. Also, efficient routing protocol and communication method can
contribute to this issue.
Communication Link
Nowadays, most of UWSN use acoustic signals to communicate. Acoustic signals
bring lots of challenges to the research arena, especially propagation delay and high error
rates. Obviously, it needs improvement. Therefore, trying to use another kind of signal
may be a new direction of research. According to [8], optical signals have been used to
communicate sensors in their two applications. It may be an alternative way. However, it
needs to consider all the factors, especially power issue, to determine whether optical
signal is better than acoustic one.
Distributed Localization and Time Synchronization
Location-aware is vitally important for any aquatic application. Since the data
without location information is useless. Among most of large-scale terrestrial WSN
application, GPS can be used to give the location and synchronize time. In the GPS-free
terrestrial application, Time-Differenc-of-Arrival (TDoA) is used to calculate the distance
according to the different speed of the two signals, such as cricket sensors. Then the
position information can be calculated using those distance data by some algorithms like
SemiDefinite Programming (SDP) [9]. In UWSN, the position information can be
calculated using same way. However, the challenge is that it is very hard to get the
distance between two sensors. GPS cannot be used, since the satellite signal cannot work
in underwater. Although, some methods are used to get the distance between the sensors,
like Angle-of-Arrival (AoA), Time-of-Arrival (ToA), the accuracy is greatly affected by
many factors of underwater environment. Therefore, how to get accurate distance
between two sensors should get more concerned in order to get underwater sensors’
position.
Routing Protocols
In UWSN protocol design, saving energy is a major concern, especially for the
long-term aquatic monitoring applications. Actually, there are numerous terrestrial WSN
energy-efficient protocols being produced in this area. However, due to the node mobility
of UWSN, most of them are not feasible in UWSN, since architecture of UWSN gives
more impacts compare to terrestrial one. Therefore, architecture should give more
attention when design the routing protocol.
CONCLUSION
Underwater wireless sensor networks (UWNS) will become more and more
important on the research of underwater world. This paper describes the unique
characteristics of the underwater environment and its effects on the design of UWSN. In
addition, the differences between terrestrial WSN and UWSN are presented. Even though
they are different, terrestrial WSN is still valuable on UWSN. Major challenges including
power consumption, communication techniques, and routing protocols are discussed.
These aspects are vital important to do future research on UWSN, especially developing
new generation UWSN. It is evident that the need for novel location-aware
communication protocols cannot be underestimated. Even in some cases, location-aware
ability is the most important factor compared to others in UWSN application.
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