John R. Potter

Performance of coded OFDM in very shallow water channels and snapping shrimp noise

Although acoustic energy has been used effectively for point-to-point communications in deep-water channels, it has had limited success for horizontal transmissions in shallow water. Time-varying multipath propagation and non-Gaussian snapping shrimp noise are two of the major factors that limit acoustic communication performance in shallow water. Rapid time variation in the channel can limit the use of equalizers to compensate for frequency selective fading introduced due to multipath propagation. OFDM (orthogonal frequency division multiplexing), a communication technique widely used in wired and wireless systems, divides the available bandwidth across a number of smaller carriers, each of which experiences flat fading. This simplifies the equalizer structure and provides robustness against time-varying frequency-selective fading. Another source of signal degradation is impulsive noise from snapping shrimp, which affects several OFDM carriers at the same time. OFDM, when coupled with coding, can provide robustness against impulsive noise by distributing the energy for each bit over a longer period of time. We tested coded OFDM in a very shallow water channel in Singapore waters. The results show that it is a promising technique for use in very shallow, warm water channels

Underwater acoustic channel characterisation for medium-range shallow water communications

The ability to effectively communicate underwater has numerous applications for researchers, marine commercial operators and defence organizations. As electromagnetic waves cannot propagate over long distances in seawater, acoustics provides the most obvious choice of channel. Although acoustics has been used effectively for point-to-point communications in deep-water channels, acoustics has had limited success for horizontal transmissions in shallow water. Time-varying multipath propagation and nonGaussian noise are two of the major factors that limit acoustic communication performance in shallow water. Although it is known that medium-range shallow water propagation is dominated by time-varying multipath arrivals, very few measurements of the variability of the multipath structure are available. In this paper, we present channel measurements made in a shallow water channel (depth 15-20 m) up to a range of 1 km. An analysis of the temporal variability of the arrival structure is presented. Most communication systems make the assumption that the noise is additive and Gaussian. Snapping shrimp dominate the ambient noise spectrum above a few kHz in warm shallow waters. It is known that snapping shrimp noise is impulsive and highly nonGaussian. These noise characteristics need to be taken into account when designing communication systems if robust and near-optimal performance is desired. An analysis of the ambient noise characteristics from some warm shallow water channels is also presented.

Imaging in the ocean with ambient noise: the ORB experiments

Acoustic daylight imaging is a new technique that has been proposed for creating pictorial images of objects in the ocean from the ensonification provided by the incident ambient noise field. To investigate the feasibility of the technique, a series of experiments was performed from the research platform ORB, moored in San Diego Bay, Southern California. Central to these experiments was an acoustic receiver known as ADONIS (acoustic daylight ocean noise imaging system), which consists of a spherical reflector, 3 m in diameter, with an elliptical array of 130 hydrophones at the focal surface. This system, which is broadband, operating between 8 and 80 kHz, forms a total of 126 receive-only beams spanning the vertical and horizontal. The ambient noise power in each beam is mapped into a pixel on a VDU. Various types of targets were used in the experiments, including planar panels and cylindrical, polyethylene drums containing wet sand, seawater or syntactic foam (essentially air), and most of the experiment...

The SUNSET framework for simulation, emulation and at-sea testing of underwater wireless sensor networks

The Sapienza University Networking framework for underwater Simulation Emulation and real-life Testing (SUNSET) is a toolkit for the implementation and testing of protocols for underwater sensor networks. SUNSET enables a radical new way of performing experimental research on underwater communications. It allows protocol designers and implementors to easily realize their solutions and to evaluate their performance through simulation, in-lab emulation and trials at sea in a direct and transparent way, and independently of specific underwater hardware platforms. SUNSET provides a complete toolchain of pre-deployment and deployment time tools able to identify risks, malfunctioning and under-performing solutions before incurring the expense of going to sea. Novel underwater systems can therefore be rapidly and easily investigated. Heterogeneous underwater communication technologies from different vendors can be used, allowing the evaluation of the impact of different combinations of hardware and software on the overall system performance. Using SUNSET, underwater devices can be reconfigured and controlled remotely in real time, using acoustic links. This allows the performance investigation of underwater systems under different settings and configurations and significantly reduces the cost and complexity of at-sea trials. This paper describes the architectural concept of SUNSET and presents some exemplary results of its use in the field. The SUNSET framework has been extensively validated during more than fifteen at-sea experimental campaigns in the past four years. Several of these have been conducted jointly with the NATO STO Centre for Maritime Research and Experimentation (CMRE) under a collaboration between the University of Rome and CMRE.

Viterbi Decoding of Convolutional Codes in Symmetric α -Stable Noise

Algorithms developed with a Gaussian noise assumption perform poorly in impulsive noise, such as that described by the symmetric alpha-stable (SalphaS) distribution. We investigate the performance of antipodal signaling and Viterbi decoding of convolutional codes in SalphaS noise. We demonstrate that the p-norm branch metric is robust in SalphaS noise.

Visual and Passive Acoustic Marine Mammal Observations and High-Frequency Seismic Source Characteristics Recorded During a Seismic Survey

In this paper, we present marine mammal observation statistics, high-frequency seismic source characteristics, and example denoising of marine mammal acoustical recordings using data collected during the mitigation and monitoring program for a 3-D seismic survey by EnCana Corporation, Calgary, AB, Canada, in the Northwest Atlantic during 2003. Marine mammals were observed both visually and acoustically. No marine mammal incidents or adverse reactions were observed during the survey. Acoustical observations were made by the Sea map Passive Acoustic Cetacean Monitoring System (SPACMS), consisting of two hydrophones placed 50 m apart, towed ahead of and to one side of the seismic source. Visual and acoustical detections were uncorrelated, indicating the complementary nature of the two observational techniques. Visual detections were more common per hour of effort than acoustical detections. Acoustical detection rates showed no significant day-night difference. Marine mammals appeared to have avoided very close ranges (100 m) from the seismic array during seismic acquisition, but the overall number of marine mammals in the observable radius (1-2 km) did not change significantly when the seismic source was ldquoonrdquo compared to ldquooff.rdquo Marine mammals were observed in larger groups and appeared to have become less vocal when the seismic source was active. It should be noted however, that the results from this data gathering effort may be affected by potential sources of bias (such as the combination of data from toothed and baleen whales). Signal processing of seismic source signatures indicated some high-frequency energy content consistent with expectations from earlier work. This analysis confirmed that most of the seismic energy was concentrated at lower frequencies (500 Hz). No low-frequency comparisons with near-field data could be made due to the geometry of the SPACMS recording hydrophones and seismic source, which resulted in the Lloyds mirror effect obliterating low-frequency components in the SPACMS records. A wavelet-based denoising method was applied to improve the visibility of marine mammal vocalizations on a spectrogram display.

Acoustic imaging using ambient noise: Some theory and simulation results

This article is concerned with the theory and simulation of passive imaging of quiet objects using the ambient noise field as illumination, a method known as ‘‘Acoustic Daylight.TM’’ A great body of work exists on active and passive acoustic systems, but the principle of using ambient noise as the sole illumination for acoustic imaging is almost unexplored. The possible performance envelope of an acoustic daylight system as a function of object shape, composition, degree, and orientation of anisotropy in the illuminating field is discussed. With such a large number of controlling variables a simple scattering model is required in the first instance for the problem to remain tractable. The theoretical development employs the Helmholtz–Kirchhoff integral with far‐field approximations, evaluated by the stationary phase technique, which results in an analytical approximation for the energy density scattered near the specular angle by an object for a single source–object–receiver geometry. The expression is ap...

An Underwater Convergence Layer for Disruption Tolerant Networking

Delay and Disruption tolerant networking (DTN) embraces the concepts of occasionally-connected networks that may suffer from frequent partitions and that may be comprised of more than one divergent set of protocols or protocol families. Since DTN was conceived to relax most of the assumptions associated with Internet protocols, the most important of which is the availability of an end-to-end path between source and destination for the duration of a communication session, it is possible to envision the application of DTN concepts to under-water acoustic networks, where the bandwidth-limited acoustic channel suffers frequent disruptions due to environmental factors. This paper presents the design and implementation of an Underwater Convergence Layer (UCL) for the DTN2 Reference Implementation, which provides DTN support for the WHOI Micro-Modem. UCL was tested in the field during the Acommsnet10 sea trial, in a heterogeneous context comprising underwater nodes communicating acoustically and surface nodes communicating with radio frequency. Initial results and lessons learned from field testing are also presented. The principal value of this development resides in the delivery of new capabilities towards the application of DTN concepts to the maritime domain, to enable functionalities like multi-hop acoustic communication, data mule using autonomous vehicles and gateways between different transmission media (e.g. between underwater acoustic communications and above water radio/satellite).

An Architecture for Underwater Networks

As electromagnetic waves do not propagate well underwater, acoustics plays a key role in underwater communication. Due to significant differences in the characteristics of electromagnetic and acoustic channels, networking protocols for underwater systems differ from those developed for wired and wireless radio networks. Various schemes have been proposed for one or many aspects of underwater networks. However, no widely accepted common framework exists for underwater acoustics to unify these proposed schemes into a functional underwater network. The availability of such a framework will enable easy integration of independently developed techniques and thus accelerate the pace of research in underwater acoustic networking. In order for a common framework to be successful, it needs to have a wide acceptance. To gain such an acceptance, a framework needs to take into account a wide variety of different constraints and requirements for various underwater applications. This requires inputs from various research groups and end users. To help define the use cases and a common framework for underwater networking, a joint effort has been initiated between acoustic communication research groups at the Acoustic Research Laboratory (National University of Singapore), Woods Hole Oceanographic Institution and the Massachusetts Institute of Technology. In this paper, we discuss the first draft of the framework specifications from this effort. We welcome feedback from the underwater acoustic research community and potential end users of underwater networking systems.

Ambient noise imaging in warm shallow seas; second-order moment and model-based imaging algorithms

Ambient noise can be used to produce images of submerged objects using the mean intensity of the backscattered energy, a technique coined “acoustic daylight” because of its direct analogy to vision. It is suggested that there may be substantial additional information in higher moments of the data. At high frequencies (>10 kHz), absorption suppresses long-range propagation so that a received signal is largely dependent on the local geometry, source characteristics, and the scattering properties of interceding objects. It is shown that for snapping shrimp (Cragnon, Alpheus, and Synalpheus) illumination (the primary sources in warm shallow water above a few kHz), significant information is embodied in the second temporal moments of intensity. There is no visual analog to this concept, which suggests a broader imaging approach which may be termed ambient noise imaging (ANI). Another ANI technique explored is the use of spatial cross correlation, which works well and also has no visual analogy. A model-based p...