Joseph A. Rice

Shallow water acoustic networks

Underwater acoustic networks are generally formed by acoustically connected ocean bottom sensor nodes, autonomous underwater vehicles (AUVs), and surface stations that serve as gateways and provide radio communication links to on-shore stations. The quality of service of such networks is limited by the low bandwidth of acoustic transmission channels, high latency resulting from the slow propagation of sound, and elevated noise levels in some environments. The long-term goal in the design of underwater acoustic networks is to provide for a self-configuring network of distributed nodes with network links that automatically adapt to the environment through selection of the optimum system parameters. This article considers several aspects in the design of shallow water acoustic networks that maximize throughput and reliability while minimizing power consumption.

Spread spectrum underwater acoustic telemetry

Spread spectrum methods are used in communication systems to provide low probability of intercept in hostile environments, to provide multiple access capability in systems shared by many users, and to provide processing gain in channels where the transmitted signal is distorted by multipath propagation. All three of these scenarios are of interest to underwater acoustic communication systems. In this paper we consider spread spectrum modulation and demodulation methods for low rate transmission in underwater acoustic communication channels. Two types of spread spectrum systems are considered: direct-sequence spread spectrum (DS-SS) systems that employ digital phase modulation with coding, and frequency-hopping spread spectrum (FHSS) systems that employ coded MFSK modulation and noncoherent demodulation. These methods are investigated for application in fading multipath channels typical of underwater propagation. System design examples are given and performance estimates are made.

A tri-modal directional transducer

Acoustic transducers made from piezoelectric ceramic cylinders usually exploit the breathing or omnidirectional (omni) mode of vibration. However, with suitable voltage distribution, higher order extensional modes of the cylinder can be excited which produce directional radiation patterns. These modal radiation patterns can then be combined to synthesize desired beam patterns which may be steered by incrementing the excitation. This paper describes a model for the combined acoustic response of the extensional modes of vibration of a piezoelectric ceramic cylinder, a method of synthesizing a desired radiation pattern, and an experimental implementation of a directional transducer that uses these techniques. This tri-modal transducer is broadband and directional with a frequency independent beam pattern yet simple, small, and lightweight.

Passive phase-conjugate signaling using pulse-position modulation

Acoustic communication is an attractive option for networking distributed assets, such as UUVs, autonomous sensors, and other systems that must talk to one another in the ocean. The ocean is often a complex multipath channel and impressive progress has been made in developing equalization algorithms to overcome this. Unfortunately, many of these algorithms are computationally demanding and not as power-efficient as one would like; in many applications it is better to trade bit rate for longer operational life. For example, a wakeup function is intrinsically low rate but must be low power and highly reliable. We discuss a fundamentally different modulation scheme to address these requirements. An added benefit is that it leads to a receiver that is much less complex than an adaptive equalizer.

Passive phase‐conjugate signaling using pulse‐position modulation

Acoustic communications is an attractive option for networking distributed assets, such as UUVs, autonomous sensors, and other systems that must talk to one another in the ocean. Although the ocean is often a complex multipath channel, impressive progress has been made in developing equalization algorithms to overcome this. Unfortunately, many of these algorithms are computationally demanding and not as power efficient as we would like, operating at the bottom of the ocean. At the same time, the networking applications being envisioned often also require low rate, but highly reliable signaling, for purposes such as wakeup. The passive phase‐conjugate concept [J. Acoust. Soc. Am. 95, 1450–1458 (1994)] is a very simple demodulation technique which nevertheless is coherent and can overcome signal distortion due to multipath. To explore its use in low power, lightweight applications, we have extended this approach to use pulse‐position modulation (PPM) and have tested it during several SignalEx experiments us...

Multimode directional telesonar transducer

Directional transducers will improve the performance of undersea acoustic modems. Current designs provide only vertical directionality with omni-directionality in the horizontal plane. Horizontal directionality can yield a higher source level and higher received signal to noise through the increase in the DI or equivalently the narrower beam pattern of the device. Moreover, a horizontal steered directive beam allows selective and more secure communications between the nodes of the system, This paper reports the development of a prototype device that achieves desired directivity by the controlled combining of the three most fundamental extensional modes of vibrational of a piezoelectric cylinder.

A trimodal directional modem transducer

With suitable voltage distribution, higher order extensional modes of a piezoelectric cylinder can be excited which produce directional radiation patterns. These modal radiation patterns can then be combined to synthesize desired beam patterns which may be steered by incrementing the excitation. This paper describes a directional modem communication transducer which uses the combined acoustic response of the first three extensional modes of vibration of a piezoelectric ceramic cylinder, a method of synthesizing a desired radiation pattern and an experimental implementation of a two-ring directional modem transducer that uses these techniques. This trimodal transducer has a smooth response in the band from 15 kHz to 20 kHz, with a frequency-independent 90 degree beam width which may be steered in 45 degree increments from a coded input. The interior of the transducer contains the electronics and the unit may be deployed from a Type A launch tube. (Works supported by a Phase II, SBIR from ONR and SPAWAR).

Channel characterization for high‐frequency acoustic communications

Undersea internets or seawebs are currently being developed and tested for a variety of applications in which some kind of device (e.g., an ADCP, hydrophone, vertical line array, autonomous undersea vehicle) needs to pass information to another (e.g., radio buoy, surface ship). The underlying physical layer is often an acoustic modem. However, the ocean medium, though it can carry acoustic waves with remarkable fidelity to long distances, is also somewhat unreliable. Storms may cause network outages by both driving up the ambient noise and corrupting the clarity of the acoustic mirror formed by the air–sea interface. Meanwhile, as the SONAR community is well aware, the refractive effects of the ocean can lead to sweet spots and dead zones (shadows). The required sound output of a modem varies accordingly. We have been conducting a program (SignalEx) to: (1) better understand propagation in the high‐frequency range (around 10 kHz), currently of greatest interest for modems; (2) understand how the environme...

Acoustic communication channel modeling for the Baltic Sea

A physics-based numerical propagation model which includes the effects of multipath spread and Doppler spread is currently being developed as a prediction and analysis tool for underwater acoustic communication problems. The model uses three-dimensional Gaussian beams and quadrature detection to obtain the channel response for finite-duration constant-wavelength tones. By using a very short pulse, the output of the quadrature detector represents an estimate of the band-limited channel impulse response, which may be used to determine the multipath spread. In addition, the Doppler shifts associated with source/receiver motion can be accumulated from the individual beams, providing Doppler spread. The usefulness of the model for experiment planning is demonstrated for a shallow-water Baltic Sea test site. The fidelity of the model is tested via a comparison of simulated channel responses with those measured in the Baltic Sea test.

Element localization for bottomed arrays without transponders

Beamforming for an array typically requires array‐element localization (AEL) to within one‐tenth of a wavelength. Often it is impractical to install dedicated, large‐bandwidth AEL transponders with an array. This paper presents an approach to AEL for fixed, bottom‐mounted arrays without transponders. High‐SNR signals from multiple near‐field imploding lightbulbs and interpolated correlations allow a statistical reduction in the inherent uncertainty in arrival‐time measurements imposed by the bandwidth of the sensors. The method is illustrated with an application to the SWSS (shallow‐water sensor string) array offshore of San Diego, at depths from 180 to 200 m.