Pierre-Philippe J. Beaujean

Location-Aware Routing Protocol for Underwater Acoustic Networks

Autonomous underwater vehicles (AUVs) are being used in ever larger numbers, and the rise in the simultaneous use of multiple AUVs is introducing challenging communication problems best addressed in the realm of mobile, ad hoc networking (MANET). This paper describes location-aware source routing (LASR), our modification of the dynamic source routing (DSR) protocol to add location awareness and link quality metrics. A new protocol is needed because of the unique difficulties of underwater networking: radio links do not work through water, and the acoustic links that are used instead have much lower data-rates and much higher latency. Specifically designed for use in underwater acoustic networks, LASR is explained, and initial simulation results are presented to show that the new protocol performs better than two existing techniques

A Robust Fuzzy Autonomous Underwater Vehicle (AUV) Docking Approach for Unknown Current Disturbances

In this paper, a robust autonomous underwater vehicle (AUV) docking approach that can handle unknown water currents is presented. Several assumptions and design considerations are made in our work. First, there is no onboard velocity sensor, and thus water currents will need to be estimated and compensated online. Second, the vehicle uses its ultrashort baseline (USBL) system to home itself toward an acoustic source situated at a rigidly bottom-mounted underwater docking station. Third, the dock is oriented at a preknown heading, and its absolute position is a priori known. Our docking approach incorporates a Tagaki-Sugeno-Kang (TSK) fuzzy inference system (FIS) that assists the vehicle with high level guidance maneuvers in the form of fuzzified commanded heading and speed vector fields. A current compensator is designed and applied to the fuzzy docking guidance to allow the vehicle to maintain its course in the presence of current disturbance. An extended Kalman Filter (EKF) is formulated to estimate the current and vehicle states. Extensive Monte Carlo simulations were performed to evaluate the current estimator and vehicle docking performance under various docking conditions. The simulation results demonstrated the inherent robustness of this designed fuzzy docking approach against unknown current disturbances, without any real-time velocity measurements.

Adaptive array processing for high-speed acoustic communication in shallow water

A novel multichannel signal-processing technique, capable of achieving stable high-speed underwater acoustic communication with a fairly low complexity of implementation, is presented. The approach is to split the space and time processing into two separate suboptimal processes to reduce computational complexity and instabilities associated with large tap vectors at large spread factors or BL products. The first suboptimal process starts with the eigen-spatial decomposition of the acoustic channel into multiple coherent orthogonal paths. Only the most energetic coherent paths are retained. For each selected path, individual synchronization is performed, the mean Doppler shift is estimated, and the message is decoded. The message binary information obtained for each individual path is recombined using maximal-ratio combining (MRC) to generate a decision binary sequence along with an array of quality factors, or metrics, associated with each bit. Each metric corresponds to the signal-to-noise-and-interference ratio (SNIR) associated with each bit. The second suboptimal process is a linear multichannel equalizer (LME). The mean Doppler estimate of the most energetic coherent path is used to resample each input signal of the array that go into the LME, to reduce the rate of fluctuation of the resulting effective channel. These resampled input signals appear as if they have passed through a channel with a lower B (Doppler spread), which allows the final space-time processor (the LME) to operate in a stable manner. The decision binary sequence and the array of quality metrics obtained at the output of the MRC are used as inputs in the decision process of the LME. If the metric associated with a decision bit exceeds a computed threshold, this bit, obtained at the output of the first suboptimal process, is used as a decision input to the LME. This technique is applied to achieve high-speed acoustic communication in very shallow water using coherent modulation techniques. As compared with coherent beamforming alone, the proposed technique causes a significant increase in SNIR, a significant decrease of the bit-error rate, and provides an estimate of the BL product. Experimental results demonstrate that stable acoustic communication can be achieved at 16000 coded bits per second (b/s) over 3.2 km in 40 ft of water and in sea-state 2 conditions. The fading properties of the channel are measured. Experimental results show that the BL product can vary by a decade in 116 ms and by two decades within minutes, from 0.001 to 0.1. The real-time analysis shows a strong correlation between the communication performance, the spread factor, and the spatial coherence of the channel.

Spatio-temporal processing of coherent acoustic communication data in shallow water

Achieving reliable underwater communication in shallow water is a difficult task because of the random time-varying nature of multipath propagation. When the product of Doppler-related signal bandwidth spread and multipath-related time spread of the channel is larger than one, some types of adaptive signal processing may not work very well. In this paper, various methods of coherent space-time processing are compared for a condition of a marginally overspread channel operating at 50 kHz. Various combinations of suboptimal spatially adaptive and time adaptive methods are considered. The coherent path beamformer (CPB) and recursive least squares (RLS) adaptive beamformer, both in combination with RLS time filtering, are analyzed. Also considered in the analysis is the combined RLS space-time optimal adaptive processor. Many experiments using broad-band phase-shift-keyed transmissions in shallow water have been conducted to provide data for testing these various processing methods. Because of the rapid time variation of the multipath, the product of bandwidth spread and time spread at this test site approached unity. In this environment, a suboptimal approach consisting of the adaptive beamformer followed by RLS equalization reduced reverberation and transmission errors.

A performance study of the high-speed, high-frequency acoustic uplink of the HERMES underwater acoustic modem

HERMES is an asymmetrical underwater acoustic modem operating in two separate frequency bands. The high data rate uplink operates between 262 kHz and 375 kHz. The downlink is designed for command-and-control and uses a lower frequency band (62–76 kHz) operated at the same source level but at a lower data rate. The acoustic uplink is capable of achieving data rates of 87768 bits-per-second (bps) in challenging environment such as ports, in the presence of heavy boat traffic. The experimental results acquired over the past year indicate that the high bit-rate uplink of HERMES can operate at this peak data rate at a range of 180 meters, a practical range for many port operations. A unique aspect of this acoustic communication system is the equalization technique, which combines multiple estimations of the underwater acoustic channels into a soft decision error correction process. This technique leads to a significant drop in the bit error rate at the sole expense of processing power.

HERMES - A high-speed acoustic modem for real-time transmission of uncompressed image and status transmission in port environment and very shallow water

A new generation of high-speed, high-frequency acoustic modem, named HERMES, has been developed. The acoustic communications link consists of a topside component and an underwater (wet-side) component. Together these components implement two underwater acoustic communication channels: a very high bit rate broadband data uplink and a low bit rate command downlink. The experimental results, obtained with the high-speed high-frequency acoustic uplink, demonstrate the systems ability to transmit high-resolution, uncompressed acoustic images with sufficient quality for any practical purposes. At the fastest data rate (mode 5), HERMES can transmit an uncompressed, high-resolution 400000-bit sonar image in 4.6 seconds. The results presented in this paper demonstrate that these data can be transmitted at a range of 120 m.

Acoustic positioning using a tetrahedral ultrashort baseline array of an acoustic modem source transmitting frequency-hopped sequences

Acoustic communications and positioning are vital aspects of unmanned underwater vehicle operations. The usage of separate units on each vehicle has become an issue in terms of frequency bandwidth, space, power, and cost. Most vehicles rely on acoustic modems transmitting frequency-hopped multiple frequency-shift keyed sequences for command-and-control operations, which can be used to locate the vehicle with a good level of accuracy without requiring extra signal transmission. In this paper, an ultrashort baseline acoustic positioning technique has been designed, simulated, and tested to locate an acoustic modem source in three dimensions using a tetrahedral, half-wavelength acoustic antenna. The position estimation is performed using the detection sequence contained in each message, which is a series of frequency-hopped pulses. Maximum likelihood estimation of azimuth and elevation estimation is performed using a varying number of pulse and various signal-to-noise ratios. Simulated and measured position estimation error match closely, and indicate that the accuracy of this system improves dramatically as the number of pulses processed increases, given a fixed signal-to-noise ratio.

Least Square and Trended Doppler Estimation in Fading Channel for High-Frequency Underwater Acoustic Communications

Hermes is an asymmetrical point-to-point underwater acoustic modem designed for short-range operations at very high bit rates in ports and shallow waters using broadband acoustic signaling (262-375 kHz). In exploring the possible conversion of Hermes into a multiple-input-multiple-output (MIMO) device, single-carrier phase-modulated spread-spectrum sequences were used for channel estimation and deconvolution purposes. It clearly appeared that the channel estimation and deconvolution routines were quite sensitive to rapid time changes in the acoustic channel impulse response (CIR), which usually reflects the presence of Doppler spread produced for the most part by moving boundaries and oscillating sensors. In this paper, the authors study the least square (LS) channel estimation routine ability to track the time-varying nature of the impulse response using broadband, single-carrier pseudonoise (PN) sequences transmitted by a single source and collected by a single receiver. In addition, the authors evaluate a trend estimation technique, based on the empirical modal decomposition (EMD) method applied to the LS estimate of the CIR. Simulated data produced with a Rayleigh channel model and experimental data collected in a marina are used. This paper shows that the channel estimation method can estimate the time-varying impulse response of the acoustic channel with a high resolution of both time and delay (down to 7 μs) at the expense of high computational requirements. In analyzing the time variation of the main and secondary echoes for a signal-to-noise ratio (SNR) of 32 dB, simulated results indicate that the root mean square error (RMSE) between theoretical and LS estimated response is 7.8% for the main path with an equivalent Doppler spread of 10 Hz and 15.7% for the second path with an equivalent Doppler spread of 15 Hz. Applying the trend estimation technique to the LS CIR greatly reduces this error, down to 2.9% for the main path and to 4.7% for the second path. The experimental data clearly show that the routine can closely track the time variations of the main echo and provide a meaningful estimate of the Doppler spread.

OCEAN TURBINES — A RELIABILITY ASSESSMENT

This paper identifies factors that impact reliability and safety of ocean turbines. We describe how physical and environmental factors will impact the design of its machine condition monitoring (MCM) system. Environmental factors like fouling, corrosion, and inaccessibility of equipment sets this MCM problem apart from those encountered by wind turbines, hydroelectric plants, or even ship hulls and propellers. Fouling constitutes the primary and most persistent source of failure. In addition to compromising turbine efficiency and reliability, fouling reduces sensor data quality — masking faults that will ultimately lead to failure. Unmitigated fouling triggers a form of biological succession known as flocculation that may eventually attract threatened species of tortoises and cetaceans to this rotating machinery. We review and suggest refinements to a class of non-toxic biologically-inspired anti-fouling techniques known as engineered topographies. Advances in this area will enable turbines to operate in portions of the water column that maximize momentum flux while minimizing retrieval cost.

High-Speed High-Frequency Acoustic Modem for Image Transmission in Very Shallow Waters

A one-way, high-speed, high-frequency acoustic modem (HS-HFAM) operating between 260 kHz and 380 kHz has been developed to transmit compressed underwater images and status information in real-time. High data rates are made possible using a high-resolution decision feedback equalizer with parallel algorithm for tracking and compensating large Doppler. Experimental results show that this modem achieves 13298.2 information bits per joule at 87768 information bits per second and at a range of 88 m in ports and shallow waters.