20-07-2012, 03:46 PM
Designing an Adaptive Acoustic Modem for Underwater Sensor Networks
[attachment=28225]
Abstract
There is a growing interest in using underwater networked
systems for oceanographic applications. These networks
often rely on acoustic communication, which poses a number of
challenges for reliable data transmission. The underwater acoustic
channel is highly variable; each link can experience vastly different
conditions, which change according to environmental factors as
well as the locations of the communicating nodes. This makes it
difficult to ensure reliable communication. Furthermore, due to
the high transmit power, the energy consumed in transmitting data
is substantial, which is exacerbated at lower data rates. The main
challenge that we address in this article is how to build a system
that provides reliable and energy efficient communication in underwater
sensor networks. To this end, we propose an adaptive underwater
acoustic modem which changes its parameters according
to the situation. We present the design of such a modem and provide
supporting results from simulations and experiments.
INTRODUCTION
UNDERWATER sensor networks have a wide range of
oceanographic applications including marine exploration,
environmental monitoring and coastal surveillance. The
preferred mode of wireless communication in these networks
is based on acoustic signals. This is due to the fact that radio
frequencies suffer high attenuation underwater. Optical communication
is possible but only in clear water at relatively
short distances. Unfortunately, acoustic communication is
challenging due to large and variable multipath delay spread,
Doppler shifts, and long propagation delays [1].
ADAPTIVE ACOUSTIC MODEM DESIGN
Underwater acoustic modems consist of three fundamental
components as shown in Fig. 1: a transducer, an analog transceiver
and a digital hardware platform for signal processing and
control. This article focuses on the design of the physical layer
on the digital platform.
In the following, we provide details about the major parts
of the digital hardware platform. In each case, we give a
general description of its function, discuss the parameters that
can be adapted, and describe options that we studied in our
experiments.
SIMULATION AND SEA TEST RESULTS
To evaluate the proposed adaptive modem, we did both simulations
and sea tests.We executed a set of simulations to find the
best data rates for different links in a network and to understand
the potential benefits of modifying the data rates on a per link
basis. We also performed sea tests to evaluate the performance
of the major components of the proposed adaptive modem in a
real environment.
The parameters for the chirp, FSK, and DSSS modulation
schemes used in our simulations and sea tests are given in
Table I.
CONCLUSION
This article makes a case for an adaptive acoustic modem in
underwater sensor networks. We describe the potential benefits
of the adaptive modem and describe a general digital hardware
platform architecture. We perform a set of experiments and sea
tests that quantify the benefit of different modulations, types
of channel estimation, and symbol synchronization. The results
show that rate adaptation can lead to substantial energy savings
while ensuring reliable communication. Furthermore, they
show that a chirp signal is a good candidate for symbol synchronization
and channel estimation. The simulations and sea tests
indicate that DSSS modulation consistently outperforms FSK.
Currently, we have finished the design of the major components
and are building our system using hardware and software
codesign implementation on an field-programmable gate array
(FPGA) platform. Eventually, our proposed adaptive modem
will be able to change its modulation scheme and data rate automatically
according to channel conditions in real-time.
[attachment=28225]
Abstract
There is a growing interest in using underwater networked
systems for oceanographic applications. These networks
often rely on acoustic communication, which poses a number of
challenges for reliable data transmission. The underwater acoustic
channel is highly variable; each link can experience vastly different
conditions, which change according to environmental factors as
well as the locations of the communicating nodes. This makes it
difficult to ensure reliable communication. Furthermore, due to
the high transmit power, the energy consumed in transmitting data
is substantial, which is exacerbated at lower data rates. The main
challenge that we address in this article is how to build a system
that provides reliable and energy efficient communication in underwater
sensor networks. To this end, we propose an adaptive underwater
acoustic modem which changes its parameters according
to the situation. We present the design of such a modem and provide
supporting results from simulations and experiments.
INTRODUCTION
UNDERWATER sensor networks have a wide range of
oceanographic applications including marine exploration,
environmental monitoring and coastal surveillance. The
preferred mode of wireless communication in these networks
is based on acoustic signals. This is due to the fact that radio
frequencies suffer high attenuation underwater. Optical communication
is possible but only in clear water at relatively
short distances. Unfortunately, acoustic communication is
challenging due to large and variable multipath delay spread,
Doppler shifts, and long propagation delays [1].
ADAPTIVE ACOUSTIC MODEM DESIGN
Underwater acoustic modems consist of three fundamental
components as shown in Fig. 1: a transducer, an analog transceiver
and a digital hardware platform for signal processing and
control. This article focuses on the design of the physical layer
on the digital platform.
In the following, we provide details about the major parts
of the digital hardware platform. In each case, we give a
general description of its function, discuss the parameters that
can be adapted, and describe options that we studied in our
experiments.
SIMULATION AND SEA TEST RESULTS
To evaluate the proposed adaptive modem, we did both simulations
and sea tests.We executed a set of simulations to find the
best data rates for different links in a network and to understand
the potential benefits of modifying the data rates on a per link
basis. We also performed sea tests to evaluate the performance
of the major components of the proposed adaptive modem in a
real environment.
The parameters for the chirp, FSK, and DSSS modulation
schemes used in our simulations and sea tests are given in
Table I.
CONCLUSION
This article makes a case for an adaptive acoustic modem in
underwater sensor networks. We describe the potential benefits
of the adaptive modem and describe a general digital hardware
platform architecture. We perform a set of experiments and sea
tests that quantify the benefit of different modulations, types
of channel estimation, and symbol synchronization. The results
show that rate adaptation can lead to substantial energy savings
while ensuring reliable communication. Furthermore, they
show that a chirp signal is a good candidate for symbol synchronization
and channel estimation. The simulations and sea tests
indicate that DSSS modulation consistently outperforms FSK.
Currently, we have finished the design of the major components
and are building our system using hardware and software
codesign implementation on an field-programmable gate array
(FPGA) platform. Eventually, our proposed adaptive modem
will be able to change its modulation scheme and data rate automatically
according to channel conditions in real-time.