Wireline Quality Underwater Wireless Communication
Using High Speed Acoustic Modems

Abstract--The paper introduces LinkQuest Inc.'s cutting-edge
high speed underwater acoustic modems. LinkQuest Inc.
combined recent advances in high speed digital communication
with broadband underwater acoustics in the development of the
high speed modems. Advanced Broadband Spread Spectrum
Technology results in dramatically increased data rate and
robustness and decrease in power consumption. The modems are
proven to perform at high data rate with virtually no
communication errors under ambient noise, ship noise from small
powerboats and large DP vessels and offshore drilling noise.
The paper describes the working principles and architectures of
the advanced modems and their interface to underwater
instruments such as Acoustic Doppler Current Profilers.
Performance results from various customer deployments such as
real-time monitoring of current profiles from Shell's Ocean
Worker Platform are presented.
I. INTRODUCTION
LinkQuest Inc. of San Diego, California manufactures an
extensive line of cutting-edge high-speed underwater
acoustic modems. These new-generation modems are fast,
consume very little power and are highly robust in field
deployments.
LinkQuest Inc. combined recent advances in high-speed
digital communication such as Digital Subscriber Line [2]
and cellular communication with broadband underwater
acoustics in the development of the high speed modems.
The proprietary “Broadband Spread Spectrum
Technology” is implemented in a state-of-the-art DSP.
Tailored for various ocean environments, these modems
achieved dramatically increased data rate, robustness and
decrease in power consumption compared with existing
systems [1]. A bit error rate of less than 10 −7 is
consistently observed in the field. This is unprecedented
for underwater acoustic communication and is equivalent
to wireline quality.
This paper introduces various models of the advanced
underwater acoustic modems and their typical application
environments. It discusses the architecture, working
principle and performance analysis of the modems. It also
Fig. 1: Model UWM1000
presents the application scenarios and various trial and
deployment results of these modems.
II. MODELS
LinkQuest designs and manufactures four different models
of modems for shallow water and deepwater environments.
All the modems operate at a bit error rate of less than 10 −7
in standard mode.
Model UWM1000, as shown in Fig. 1, operates up to 200
meters in depth and 300 meters in range. The data rate of
the modem is 9600 bits/second in standard mode and
19200 bits/second in optional mode. The system operates
within a frequency band centered around 35 kHz. It
utilizes a directional transducer and has a very low transmit
power consumption of 1 watt. This model weighs about
4.2kg out of water and 2.3kg in water and is very compact
in size. Typical applications for this modem include realtime
monitoring of sensor data in shallow water of lake,
surf zone and continental shelf. Because of its ultra low
power consumption, the modem can easily transfer
hundreds of megabytes of data using a small battery pack.
Model UWM2000 is developed to handle longer and
deeper deployments as compared to model UWM1000.
Although they are very similar, model UWM2000 can
operate up to 1000 meters in depth and 1500 meters in
range. Its transmit power consumption is also increased to
4 watts to reach longer range. The pressure housing is
marginally increased in weight and size. These modems
can be used in medium water depth from a buoy or surface
vessel to communicate with a moored instrument. It can
also be used from most of the oil rigs to monitor and
control underwater sensors. For some of the shallow to
medium water AUVs, the modem provides an excellent
way to transfer the images to the surface vessel.
Model UWM3000, as shown in Fig. 2, is a versatile omnidirectional
modem. It operates at 2500 bits/second in
standard mode and at 5000 bits/second in optional mode.
With selectable transmit power up to 20 watts, it could
reach 3000 meters in range. The 180-degree hemi-sphere
transducer provides extra flexibility in deployment. This
system can be used to command, control and communicate
with underwater instrument and AUV when the range and
the depth required by the application cannot be
accomplished with directional transducer. The system
operates at a frequency band centered around 10 kHz to
minimize the impact from signal attenuation due to water
absorption.
UWM4000, as shown in Fig. 3, is a powerful deep water
modem. It utilizes a directional transducer to achieve a
high acoustic source level and reduce the impact from
acoustic interference and noise. The modem reaches 4000
meters in range and rates up to 6000 meters in depth. This
model offers data rate of 4800 bits/second in standard
mode and 9600 bits/second in optional mode. It operates at
a frequency band centered around 17 kHz. Typical
applications
Fig. 2: Model UWM3000
Fig. 3: Model UWM4000
for the modem include image data transfer from AUV,
collecting data from underwater instruments towed from
surface vessels, monitoring and control sensors from deep
water oil rigs and DP vessels.
III. WORKING PRINCIPLE
LinkQuest’s modems are based on Broadband Spread
Spectrum Technology [3]. The modems employ advanced
modulation scheme and channel equalization to combat
multipaths for improved signal to noise ratio [4]. A high
performance error detection and correction coding scheme
is also implemented. These advanced techniques result in a
highly robust system which guarantees a bit error rate of
less than 10 −7 . The transmit power consumption is also
significantly decreased as compared to existing modems.
The modem’s electronics consists of 3 parts, a DSP board,
an Analog Front End (AFE) board and a DC/DC converter
board (fig. 4). The DSP board serves as both a signal
processing module and a microcontroller. System timing
recovery, modulation, equalization, coding/decoding,
framing and serial interfacing are all conducted in the DSP.
The AFE board interfaced to the DSP board on one side
and the transducer on the other side. It performs signal
filtering and amplification functions. The DC/DC converter
board converts a wide range of input voltage to the
operating voltage of the system and the transducer. The
modem has a 7-pin underwater connector which connects
the modem to an external power supply and serial port.
The serial interface supports RS-232 or RS-422.
LinkQuest’s acoustic modems are two-way, half duplex
modems that have a page link layer communication protocol.
When no data is being transmitted the modem stays in
Sleep Mode whereby it periodically wakes up to monitor
possible data being transmitted by the far-end modem.
Fig. 4: Hardware structure
When the modem is sleeping (i.e. prior to waking up in the
sleep mode) the power consumption is very low (i.e.
approximately 8 mW). When no data is being transmitted,
the modem will spend the majority of the time sleeping in
the Sleep Mode to conserve power. The percentage of
sleep time vs wake-up time in the Sleep Mode is
configurable. The more frequently the modem wakes up
the quicker it is for the modem to acquire signals from the
other modem; however, this increases the power
consumption of the modem. In order to explain the rest of
the working principles of the modem, we will use the
example whereby the bottom modem tries to send data to
the surface modem.
If the bottom modem receives data from its RS-232 link
connected to the bottom instrument while it is in the Sleep
Mode, the bottom modem will switch from the Sleep Mode
to the Transmit Mode immediately and begins to transmit
data. As soon as the surface modem wakes up and detects
the signal from the bottom modem, the surface modem will
switch from the Sleep Mode to the Receive Mode and start
to receive data. After the data transmission is complete,
both the surface modem and the bottom modem will return
to the Sleep Mode until more data are available on the RS-
232 line.
Each of the modems comes with 900k bytes of input data
buffer. This is to ensure that data from the RS-232 line do
not get lost in the acoustic modem in case the
communication channel between the two modems is
temporarily degraded significantly. Once the channel
restores to sufficient operating conditions, the buffered
data will be transmitted to the surface modem in the order
it was received, without any human intervention.
In many applications, the power consumption of the
surface modem is not as critical as the power consumption
of the bottom modem. If data are often uploaded from the
bottom modem to the surface modem, the surface modem
may be configured to wake up frequently to ensure quick
receipt of data sent from the bottom modem, while the
bottom modem may be configured to wake up much less
frequently to conserve power. In applications such as
AUV/UUV, both the surface modem and the bottom
modem can be configured to wake up all the time to ensure
the minimum possible delay.
The beamwidth of the directional transducers used in
models UWM1000, 2000 and 4000 is between 60 to 80
degrees. It provides a good margin for relative movement
between surface and bottom modems. In case the surface
and the bottom modems are in violent relative motion the
surface modem may move out of the beamwidth of the
bottom modem and lose communication temporarily. When
this condition occurs, the advanced algorithms
incorporated in these modems will ensure the completeness
of the data transmission when the surface modem moves
back into the bottom modem’s beamwidth coverage. No
data will be lost when the maximum specified beamwidth
angle between two modems is temporarily exceeded. This
function is particularly useful in cases where a surface ship
is used to collect data from moored instruments or
AUV/UUV.
The modem also employs an automatic rate adaptation
function. While the modem operates at standard high data
rate, it will automatically switch to a lower data rate under
extremely harsh environment. When the noise condition
improves, the modem will switch back to the high data
rate. The automatic rate adaptation scheme provides an
added insurance for extreme/unusual situation while
avoiding difficult and sometimes untimely human
intervention on adjusting the data rate. The quality of the
communication (i.e. bit error rate) is not affected by rate
adaptation.
LinkQuest’s modems provide a completely transparent
wireless RS-232 connection between two end equipment as
if they are directly connected through an RS-232 cable.
LinkQuest’s RS-232 is configured at 9600 baud, 1 start bit,
1 stop bit, 8 data bit, no parity bit and no flow control. This
is the default configuration most commonly used in many
popular Acoustic Doppler Current Profilers and CTD’s. No
special modem set commands are needed to operate the
modem.
LinkQuest’s modems have been seamlessly integrated with
some of the most popular underwater instruments. For
example, the user can use LinkQuest’s modems seamlessly
with various models of Acoustic Doppler Current Profilers
from leading manufacturers. The user can continue to use
DC/DC Converter
DSP
Analog Front End
Transducer
RS-232/RS-422
the manufacturer’s original software to configure the
profiler, monitor currents and upload large amount of data
from the instrument recorders.