Hefeng Dong

Modeling Underwater Communication Links

Design and operation of underwater acoustic communications systems require a capacity to model the acoustic preparation conditions under various environmental conditions. For the frequencies of interest to most applications in connection to ocean sensor networks ray tracing technique is normally the preferred method. In this study we use a newly developed acoustic propagation model, called PlaneRay. The model is based on classical ray theory but with a novel routine for the determination of all eigenrays connecting a source with receivers. For each eigenray the model calculates the trajectories, travel times and amplitudes and constructs the complete frequency response by coherent addition of all the multiple arrivals. The paper describes the model and its capabilities are illustrated with examples for bottom-to-surface and bottom-to-bottom communication links in Barents and Norwegian Sea at different seasons.

Sound propagation over layered poro-elastic ground using a finite-difference model

This article presents an axisymmetric pressure-velocity finite-difference formulation (PV-FD) based on Biots poro-elastic theory for modeling sound propagation in a homogeneous atmosphere over layered poro-elastic ground. The formulation is coded in a computer program and a simulation of actual measurements from airblast tests is carried out. The article presents typical results of simulation comprising synthetic time histories of overpressure in the atmosphere and ground vibration as well as snapshots of the response of the atmosphere-ground system at selected times. Comparisons with the measurements during airblast tests performed in Haslemoen, Norway, as well as the simulations by a frequency-wave number FFP formulation are presented to confirm the soundness of the proposed model. In particular, the generation of Mach surfaces in the ground motion, which is the result of the sound speed being greater than the Rayleigh wave velocity in the ground, is demonstrated with the help of snapshot plots.

Experimental Assessment of Different Receiver Structures for Underwater Acoustic Communications over Multipath Channels

Underwater communication channels are often complicated, and in particular multipath propagation may cause intersymbol interference (ISI). This paper addresses how to remove ISI, and evaluates the performance of three different receiver structures and their implementations. Using real data collected in a high-frequency (10–14 kHz) field experiment, the receiver structures are evaluated by off-line data processing. The three structures are multichannel decision feedback equalizer (DFE), passive time reversal receiver (passive-phase conjugation (PPC) with a single channel DFE), and the joint PPC with multichannel DFE. In sparse channels, dominant arrivals represent the channel information, and the matching pursuit (MP) algorithm which exploits the channel sparseness has been investigated for PPC processing. In the assessment, it is found that: (1) it is advantageous to obtain spatial gain using the adaptive multichannel combining scheme; and (2) the MP algorithm improves the performance of communications using PPC processing.

A novel probe processing method for underwater communication by passive-phase conjugation

Underwater acoustic communication by passive-phase conjugation uses a channel probe signal transmitted prior to the data signal in order to estimate the channel response. At the receiver, the received probe signal can be truncated to correlate with the late arrived data signal. In a multipath channel, there are paths that undergo incoherent scattering by the sea surface, and they decrease the coherence between the estimated channel response and the channel response for data signal. For a linear frequency modulation probe signal, a novel processing method is proposed to select time delayed arrivals from the received probe, and this method can also suppress the noise level for the truncated probe signal. Applying this method to the data collected in a sea trial, the communication performance is improved.

PlaneRay: An underwater acoustic propagation model using ray tracing and plane wave reflection coefficients

Recently there has been a renewed interest in using ray tracing techniques for propagation modeling in underwater acoustics and this paper describes a newly developed model. A special and essential feature of this model is a unique sorting and interpolation routine for efficient determination of a large number of eigenrays and also for range‐dependent environments. No rays are traced into the bottom and the bottom interaction is modeled by plane wave reflection coefficients. In the current version of the model the bottom structure consists of a fluid sediment layer over a solid half‐space. Important considerations are the accuracy of the ray model and, in particular, how accurate the plane wave reflection coefficients can represent the effects of a layered bottom. This problem is analyzed by comparing the time and frequency domains solution of the ray model with models based on the wave number integration technique. There are two conflicting requirements since ray tracing is valid for high frequencies and...

Experimental research on adaptive multichannel equalization for underwater communications

An experiment was conducted in Trondheim harbor, Norway on 30th June 2010 to test coherent underwater acoustic communication using an adaptive multichannel equalization scheme. Signals of different phase shift keying modulation were transmitted at distances of 2 km and 4 km. The acoustic source suspended by the NTNU research vessel R/V Gunnerus transmitted each signal every 152.548 s for about 45 minutes in a depth fluctuated acoustic channel. In the direction of communication, the water depth changes from 10 m to 380 m. At the receiver, a cross receiving array of 10 hydrophones was used in a water depth of 10 m. Based on passive phase conjugation, the equalization scheme jointly updates the tap weights along with multichannel carrier-phase tracking by digital phase locked loops. The results using independent carrier-phase tracking and common carrier-phase tracking were analyzed and compared. Presented results demonstrate that the equalization scheme along with the common carrier-phase tracking is feasible for a multichannel receiver located at the shallow water.

Experimental Studies of Underwater Acoustic Communications over Multipath Channels

Underwater acoustic experiments were conducted in the Trondheim fjord between fixed transmitters and receivers separated by 2.3 km, 4.3km and 8.0 km. Direct-sequence spread spectrum signals were used and binary information was modulated between two consecutive m-sequences, and recovered by cross-correlating between the two consecutive m-sequences. With the spreading gain, this scheme is robust to SNR fluctuations. Time-updated channel impulse responses were estimated by a periodically transmitted measuring sequence, and fast variations of multipath channels are shown also in the stationary communication scenarios. The results show that the performance is limited by time delay spread caused by multipath.

A forward model for geoacoustic inversion based on ray tracing and plane-wave reflection coefficients

For geoacoustic inversion a fast and accurate forward model is essential, and in recent years a number of such models have been developed. In this paper, as a possible alternative, we describe a newly developed acoustic propagation model based on ray tracing named PlaneRay. A special and essential feature of this model is a unique sorting and interpolation routine for efficient determination of a large number of eigenrays, also for range dependent environments. The bottom is modeled with plane wave reflection coefficient and in principle any number of acoustic or elastic layers can be included. However, in the current version only a simple model with a fluid sediment layer over a solid half space is implemented.

A model for predicting propagation of anthropogenic acoustic noise in the sea

There is a growing environmental concern about the effect high level anthropogenic acoustic signals and noise can have on the life in sea in general, also noise in the form of transients signal generated by air guns used in marine seismic exploration. The signals from such air guns have the potential to travel to far distances and cause annoyance, discomfort or damage to, for instance, sea mammals. Effects could range from mild irritation through impairment of foraging or disruption of social interactions to hearing loss and in extreme cases physical injury or death. It is therefore of considerable interest to have tools for the predictions of long range sound propagation of air gun signals to use in the planning and execution of surveys. In this paper we will describe a new environmental impact analysis model for the prediction of the level and other characteristics of emitted man made acoustic noise in the sea. The main element of the model is an acoustic ray-tracing propagation program, called PlaneRay, which can treat range dependent bathymetry and take into account reflection loss from a layered sea floor with fluid and elastic layers. The sound source in the propagation model can be any transient acoustic waveform, for instance a recorded acoustic signature of an air gun. The model will then propagate the source signal out to a range of several kilometers and calculate the sound level as function of frequency for a large frequency band, typically 1000 Hz. The model will also calculate the complete broad-band time response of the received signal and thereby calculate important parameters, such as the peak-to-peak acoustic pressure amplitude, rms pressure level and time duration of the received signal. The main features of the model are described and examples of predicting long range propagation of an air gun signature under different oceanic conditions are shown.

Experimental assessment of sparse channel estimations for passive-phase conjugation communications

This paper investigates sparse channel estimations to improve the performance of passive-phase conjugation (PPC) communications. PPC processing achieves pulse compression for time delayed arrivals at the receiver. This property is used for underwater communications with a reduced computational load. The channel estimation is required by PPC. In multipath channels, the sparse channel estimations can be used to improve the communication performance. The dominant arrivals are estimated by matching pursuit (MP) processing for PPC processing and the remained arrivals are treated as zeros. Using experimental data collected in a range dependent acoustic channel, MP processing is assessed in improving the performance of PPC communications.