30-04-2012, 03:23 PM
Effects of Multiple Scattering on the Implementation of an Underwater Wireless Optical Communications Link
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INTRODUCTION
Now more than ever, wireless radio frequency (RF)
communications are an essential part of military and civilian
activities. Land-to-land and land-to-air systems are vital for
transmitting data over large distances without the need for a
cable connection. Underwater vehicles and devices may also
benefit from a wireless link. Military platforms like
submarines and unmanned underwater vehicles (UUV’s)
would gain the ability to coordinate missions and pass data
between platforms while at speed and depth. Commercial
oceanographic applications stand to profit as well. Data
logging sensors such as seismometers or temperature sensing
devices obtain the capability to transmit their data without the
need to be recovered or unmoored. It is well known however,
that while radio frequencies have enjoyed relatively large
success in free space, they experience high attenuation in
water, and are typically not used for communication between
subsurface vehicles and devices.
SYSTEM IMPLEMENTATION
The underwater optical communications system (UWOC)
in this study was developed entirely from the same hardware
that is used in the Frequency Agile Modulated Imaging
System (FAMIS). The FAMIS system is an underwater laser
imaging system also being developed at the Naval Air Warfare
Center at Patuxent River [7,8]. Detection and imaging in the
underwater environment is a challenging task. As previously
mentioned, because radio frequencies undergo large amounts
of attenuation in water, traditional RF and radar systems are
not an optimum choice for this task. However optical systems
employing either continuous wave (CW) [9,10] or
pulsed/ranged gated [11] techniques are gaining popularity for
use in underwater scenarios. FAMIS is a novel system that
attempts to marry the transmission characteristics of optical
energy, with the well-established signal processing techniques
of RF and radar systems.
EXPERIMENTAL SETUP AND PROCEDURE
A block diagram for the laboratory BPSK link is shown in
Figure 2. Experiments were conducted in a 1m x 1m x 3.66m
water tank with windows on each end. Maalox antacid was
used as a scattering agent [14] and a transmissometer was used
to directly measure the attenuation coefficient in the testing
tank. The transmitter and receiver are located outside the
tank on opposite sides. At the transmitter end, a frequencydoubled
diode-pumped solid-state laser (532nm) is used as the
optical source. While this laser has a maximum adjustable
output power of 5W, an output setting of 3W is used in this
study. The CW optical signal enters an electro-optic
modulator that has a modulation bandwidth of approximately
10-100Mhz. A carrier frequency of 70Mhz was chosen for
these experiments. A programmable frequency synthesizer
was used to generate the RF carrier signal.
CONCLUSIONS
This work investigated the effects of multiple scattering
in turbid environments on a modulated optical signal for
establishing wireless underwater communication links.
Preliminary experiments showed that recovery of the RF
sub-carrier was largely unaffected by water clarity.
Changes in the absolute phase of the recovered RF
envelope vs. increasing water turbidity confirm that
multiple scattering is occurring, however it appears that
unlike acoustic systems, the multipath effects are static
during the acquisition time. This was supported when short
data bursts were detected error free at a rate of 1Mbps.