Keenan Ball

The WHOI micro-modem: an acoustic communications and navigation system for multiple platforms

The micro-modem is a compact, low-power, underwater acoustic communications and navigation subsystem. It has the capability to perform low-rate frequency-hopping frequency-shift keying (FH-FSK), variable rate phase-coherent keying (PSK), and two different types of long base line navigation, narrow-band and broadband. The system can be configured to transmit in four different bands from 3 to 30 kHz, with a larger board required for the lowest frequency. The user interface is based on the NMEA standard, which is a serial port specification. The modem also includes a simple built-in networking capability which supports up to 16 units in a polled or random-access mode and has an acknowledgement capability which supports guaranteed delivery transactions. The paper contains a detailed system description and results from several tests are also presented

MIMO-OFDM for High-Rate Underwater Acoustic Communications

Multiple-input-multiple-output (MIMO) techniques have been actively pursued recently in underwater acoustic communications to increase the data rate over the bandwidth-limited channels. In this communication, we present a MIMO system design, where spatial multiplexing is applied with orthogonal-frequency-division-multiplexing (OFDM) signals. The proposed receiver works on a block-by-block basis, where null subcarriers are used for Doppler compensation, pilot subcarriers are used for channel estimation, and a MIMO detector consisting of a hybrid use of successive interference cancellation and soft minimum mean square error (MMSE) equalization is coupled with low-density parity-check (LDPC) channel decoding for iterative detection on each subcarrier. The proposed design has been tested using data recorded from three different experiments. A spectral efficiency of 3.5 b/s/Hz was approached in one experiment, while a data rate of 125.7 kb/s over a bandwidth of 62.5 kHz was achieved in another. These results suggest that MIMO-OFDM is an appealing solution for high-data-rate transmissions over underwater acoustic channels.

An OFDM Design for Underwater Acoustic Channels with Doppler Spread

In this paper, we study the performance of orthogonal frequency division multiplexing (OFDM) over underwater acoustic multipath channels with different Doppler scales on different paths. We first derive an exact inter-carrier-interference (ICI) expression after incorporating the compensation of nonuniform Doppler shifts across OFDM sub-carriers. Based on the assumption that the residual ICI is dominantly from immediate neighbors, we suggest a practical design that divides subcarriers into groups, where each group of eight subcarriers consists of three contiguous data subcarriers, one pilot subcarrier, and five carefully spaced null subcarriers. We use the orthogonal matching pursuit (OMP) algorithm for sparse channel estimation that identifies distinct physical paths with different Doppler scales. System performance is evaluated using data recorded from the GLINT08 and SPACE08 experiments. Relative to the receiver that ignores the residual ICI, we observe that explicitly suppressing the residual ICI induced by Doppler spread leads to improved performance for the SPACE08 data, while not for the GLINT08 data.

Multi-band acoustic modem for the communications and navigation aid AUV

An acoustic communications system with the capability to operate at multiple data rates in two frequency bands has been designed and developed for use in 21-inch AUVs. The system is specifically designed around the 21-inch diameter Bluefin Robotics AUV, though it could be adapted to smaller vehicles (12-inch), or similar free-flooded vehicles. The system includes both high (25 kHz) and mid-frequency (3 kHz) modems and supports data rates from 80 bps to more than 5000 bps. Both of the modems utilize four-channel arrays to increase reliability. The high-frequency modem is also used to support multi-vehicle navigation via one-way travel time measurements using synchronized clocks on all of the vehicles in a work group

An underwater network testbed: design, implementation and measurement

This paper presents the design, implementation and measurement of Aqua-Lab, an underwater acoustic sensor network lab testbed. Aqua-Lab consists of a water tank, a set of acoustic communication hardware, and a set of software. One important component of the software is an emulator that we developed to provide user-friendly programming interfaces and emulate realistic network settings. Using Aqua-Lab, we explore basic characteristics of data transmission using Micro-Modems and conduct a set of experiments in both field and lab environments. Our results from lab testbed are consistent with those from the field experiments, and thus demonstrate that Aqua-Lab can be used to experimentally evaluate algorithms and protocols designed for underwater sensor networks.

Further results on high-rate MIMO-OFDM underwater acoustic communications

MIMO-OFDM is one viable solution for high data rate transmissions over underwater acoustic channels. Our previous work has reported experimental results for MIMO-OFDM with two transmitters and QPSK modulation. In this paper, we deal with MIMO-OFDM with more transmitters and higher order modulation. With data collected from the RACE experiment at Narragansett Bay, Rhode Island, March 2008, we report MIMO-OFDM performance results of QPSK/8-QAM/16-QAM/64-QAM modulations with two transmitters, QPSK/8-QAM/16-QAM modulations with three transmitters, and QPSK/8-QAM modulations with four transmitters. With large constellations and MIMO spatial multiplexing, very high data rates can be achieved, e.g., a spectral efficiency of 3.5 bits/sec/Hz is approached in three different configurations.

The PLUSNet Underwater Communications System: Acoustic Telemetry for Undersea Surveillance

The Persistent Littoral Undersea Surveillance Network (PLUSNet) demonstrates multi-sensor and multi-vehicle anti-submarine warfare (ASW) by means of an underwater acoustic communications network. This ONR-funded multi-institution effort is part of a larger research and development framework which aims to provide autonomous detection and tracking of quiet submarines in support of the Navy Sea Power 21 concept. This paper describes the acoustic network implementation using the WHOI Micro-modem, and highlights the physical and messaging communication layers

Receiver comparisons on an OFDM design for Doppler spread channels

Underwater acoustic channels induce large Doppler drifts that render intercarrier interference (ICI) for OFDM transmissions. Assuming that after proper Doppler compensation the residual ICI is limited to only direct neighbors, we propose an OFDM signal design that decouples channel estimation and data demodulation. We investigate eight receivers that are categorized into three groups: (i) three receivers that ignore the residual ICI, (ii) three receivers that are based on a basis expansion model (BEM) and pursue channel estimation independently along each basis, and (iii) two receivers that are based on discrete-path modeling. The receiver performance is compared based on data from the SPACE experiment conducted off the coast of Marthas Vineyard, Massachusetts, October 2008. The receiver based on the discrete-path modeling and a basis pursuit algorithm achieves the best performance while the receiver based on BEM and least-squares channel estimation performs the worst. The performance differences among different receivers drastically increase as the channels Doppler spread and the signal constellation size increase. Interestingly, the BEM based receivers are often inferior to the ICI-ignorant counterparts, implying that the ability of ICI compensation could be limited by the estimation accuracy of the much increased number of model parameters.

Passive and active acoustics using an autonomous wave glider

The recently developed wave glider has the potential to be an effective unmanned platform for acoustic applications. We present the results of a variety of experiments that quantify this potential. The radiated self-noise of the autonomous platform is evaluated using an integrated passive acoustic recorder during a set of field trials off the coast of Hawaii. We present the radiated noise spectra from these trials to illustrate the dependence on hydrophone location and sea state. Using the same instrumentation, we demonstrate the ability of a modified wave glider to detect marine mammals using passive acoustic monitoring techniques. We also evaluate the performance of the wave glider operating as an active acoustic gateway, highlighting the potential of this platform to serve as a navigation reference and communications relay for scientific, industrial, and military subsea assets. To demonstrate the potential of the wave glider platform to support acoustic navigation, we assess the performance of time-of-flight range estimation and seafloor transponder localization. These tests were performed using commercial off-the-shelf acoustic positioning hardware integrated with the wave glider to illustrate that the low self-noise of the wave glider makes it possible to achieve acoustic positioning performance similar to previously reported results. Finally, we show that the glider can operate as a station-keeping surface communications gateway and provide recommendations for its use.

Tracking Large Marine Predators in Three Dimensions: The Real-Time Acoustic Tracking System

Large marine predators like sharks and whales can have a substantial influence on oceanic ecosystems, and characterizing their interactions with the physical and biological environment is an important goal in marine ecology. Studies of foraging ecology are of particular importance, but sampling prey aggregations encountered by these predators is extremely difficult because of the small spatial scales over which prey aggregations often occur (meters to hundreds of meters). We developed the real-time acoustic tracking system (RATS) to allow large marine predators to be accurately tracked over these small spatial scales to facilitate proximate environmental sampling. The system consists of an array of four free-floating buoys capable of detecting 36-kHz pings emitted by an animal-borne acoustic transmitter. Upon detection, the buoys transmit their position and the arrival time of the ping via a radio modem to a computer on board a nearby ship, and a software program uses differences in arrival times from all of the buoys to estimate the location of the tagged animal. The positions of the tagged animal, buoys, ship, and support boats can be monitored via a graphical user interface to allow proximate environmental sampling and maintenance of the array around the tagged animal. In situ tests indicate that average positional accuracies for a transmitter inside either a four- or three-buoy array (buoys spaced 1-1.75 km apart) are less than 10 m, and that accuracies remain near 10 m for transmitters located up to 500 m away from the edge of the array. The buoys can consistently detect the transmitter up to 1000 m away, but detection rates decrease between 1000 and 2000 m; no detections were obtained beyond 2300 m. Field deployments of the system have demonstrated an unprecedented ability to monitor the movements of baleen whales in real time, allowing a suite of prey and oceanographic observations to be collected within meters to tens of meters of a tagged animal.