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Physical modelling of acoustic shallow-water communication channels
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [sv]

Akustiska kanaler för undervattenskommunikation är flervägskanaler där ljudet reflekteras från havets yta och botten och bryts vid ljudhastighetsförändringar. I grunt vatten är impulssvaret långt jämfört med symbolernas tidslängd i ett utskickat meddelande. Detta orsakar intersymbolinterferens, vilket gör det svårt att återskapa meddelandet.

Denna avhandling behandlar fysikalisk modellering av kommunikationskanalen. Sådan modellering kan öka insikten om de svårigheter som finns vid design av kommunikationssystem och kan vara till hjälp vid utveckling av lämpliga modulationstekniker och avkodningsalgoritmer. Ljudutbredningen simuleras med en strålgångsmetod med reflektionskoefficienter beräknade för plana vågor. I artikel 4 och 5 utvidgas modellen till en algoritm för gaussisk strålsummation.

De viktigaste vetenskapliga bidragen är följande.

Artikel 1:

Aktuell kunskap om impulssvaret hos kommunikationskanalen gör det betydligt lättare att tolka det mottagna meddelandet. I denna artikel studeras tidsvariabiliteten hos impulssvaret i termer av bitfelssannolikhet när ett gammalt impulssvar används för kanalutjämning. Tidsvariabiliteten visar sig variera avsevärt med mottagarpositionen, inte bara när det gäller avståndet till sändaren, utan även när det gäller placeringen i djupled.

Artikel 2:

En hybridmetod presenteras, där strålgång i ett avståndsberoende medium kombineras med lokal fullfältsmodellering av interaktionen med havsbottnen. Metoden används för simuleringar av akustisk kommunikation i grunt vatten.

Artikel 3:

För att kunna göra tillförlitliga simuleringar av ljudutbredning behöver man god kunskap om mediets ljudhastighetsprofil --- information som inte alltid är tillgänglig. I denna artikel används den hybrida strålgångsmetoden från artikel 2 för att skatta ljudhastighetsprofilen från kommunikationsdata. Miljöparametrarna som beräknats genom inversionen minskar avvikelsen mellan simulerade och observerade skattningar av impulssvaret jämfört med avvikelsen då en uppmätt, två dagar gammal ljudhastighetsprofil används vid simuleringen. Miljömodellen används också för en genomgång av alternativa käll- och mottagarpositioner.

Artikel 4:

Gaussisk strålsummation är en vågutbredningsmodell som liknar strålgång men kan ge korrekta resultat i strålgångens singulära områden, som skuggzoner och kaustikor. I denna artikel diskuteras hur några olika val av den komplexa strålparametern $\epsilon$ fungerar i grunda vågledare. Bäst resultat erhålls om man väljer $\epsilon$ så att strålen blir smal i punkten närmast mottagaren och får en plan vågfront där.

Artikel 5:

En adaptiv metod för gaussisk strålsummation i grunda vågledare presenteras. Algoritmen ger en noggrannhet som är bättre än eller minst lika bra som strålgångens, även i grunda vågledare med en starkt djupberoende ljudhastighet.

Abstract [en]

Acoustic underwater communication channels are multipath channels where sound is reflected from the surface and the bottom of the sea and refracted by sound speed variations. In shallow water, the impulse response is typically long compared to the time length of the symbols in a transmitted message. This causes inter-symbol interference, which makes the message difficult to decode.

This thesis deals with physical modelling of the communication channel. Such modelling can provide insight into the difficulties of communication system design and may serve as an aid in the development of appropriate modulation techniques and decoding algorithms. The sound propagation is simulated by a ray tracing method with plane-wave reflection coefficients, in papers 4 and 5 expanded to a Gaussian beam summation algorithm.

The main scientific contributions are the following.

Paper 1:

Up-to-date knowledge of the impulse response of the communication channel considerably simplifies the extraction of information from a detected signal. In this paper the time variability of the impulse response is studied in terms of the bit-error rate, when an old impulse response is used for channel equalisation. The time variability is found to vary significantly with the receiver position, not only in range, but also in depth.

Paper 2:

A hybrid raytrace method is presented, combining ray tracing in a range-dependent water column with local full-field modelling of the seabed interaction. The method is applied to simulations of acoustic communication in shallow water.

Paper 3:

To be able to make reliable simulations of sound propagation, one needs to know, quite accurately, the sound speed profile of the medium --- information which is not always available. In this paper the hybrid raytrace method from paper 2 is used to estimate the sound speed profile from communication data. The environmental model obtained by the inversion reduces the mismatch between the modelled and the observed impulse response estimates, compared to the mismatch with a two-days-old sound speed profile. The model is also used to investigate alternative source-receiver configurations.

Paper 4:

Gaussian beam summation is a wave propagation model similar to ray tracing, which can yield correct results in singular regions like shadow zones and caustic points. In this paper some different choices of the complex beam parameter $\epsilon$ are discussed for shallow waveguides. Best results are observed when $\epsilon$ is chosen so that each beam is narrow at the point where it is closest to the receiver and has a plane wavefront there.

Paper 5:

An adaptive method for Gaussian beam summation in shallow waveguides is presented. The algorithm yields better or at least as good accuracy as ray tracing, even in shallow waveguides with a strongly depth-dependent sound speed profile.

Place, publisher, year, edition, pages
Stockholm: KTH , 2007. , 34 p.
Series
Trita-AVE, ISSN 1651-7660 ; 2007:72
Keyword [en]
underwater acoustics, hydroacoustics, time variability, ray tracing, Gaussian beam summation, sound propagation models, shallow water, waveguides, caustics, underwater communication
Keyword [sv]
undervattensakustik, hydroakustik, tidsvariabilitet, strålgång, gaussisk strålsummation, ljudutbredningsmodeller, grunt vatten, vågledare, kaustikor, undervattenskommunikation
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-4572ISBN: 978-91-7178-805-4 (print)OAI: oai:DiVA.org:kth-4572DiVA: diva2:12923
Public defence
2007-12-19, F3, KTH, Lindstedtsvägen 26, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100819Available from: 2007-12-10 Created: 2007-12-10 Last updated: 2010-08-19Bibliographically approved
List of papers
1. Time variability of an underwater acoustic channel
Open this publication in new window or tab >>Time variability of an underwater acoustic channel
2003 (English)In: Proceedings of the Tenth International Congress on Sound and Vibration: Code 62825 / [ed] Nilson A., Boden H., 2003, 2633--2641 p.Conference paper, Published paper (Refereed)
Abstract [en]

Underwater acoustic communication systems deal with the problem of fading multipath propagation as well as Doppler shifts. Knowledge of the channel impulse response considerably simplifies the extraction of information from a transmitted signal. Supposed that the impulse response is known at one time instant, the bit-error rate is here studied using that impulse response for the equalisation at later instants, to get a picture of the time variability of the channel. The study is performed by computer simulations with different modulation schemes using channel impulse responses determined from measurements in the Mediterranean during Yellow Shark 94.

Series
Acoustical Physics, ISSN 1063-7710 ; 48:3
Keyword
Acoustic wave interference; Acoustic wave propagation; Bit error rate; Communication systems; Computer simulation; Data communication systems; Doppler effect; Hydrophones; Waveform analysis; Channel impulse response; Signal destortion; Time dispersion; Vertical reciever arrays; Underwater acoustics
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-7790 (URN)
Conference
The Tenth International Congress on Sound and Vibration; Stockholm; 7 July 2003 through 10 July 2003
Note
QC 20100819Available from: 2007-12-10 Created: 2007-12-10 Last updated: 2010-08-19Bibliographically approved
2. Hybrid raytrace modelling of an underwater acoustics communication channel
Open this publication in new window or tab >>Hybrid raytrace modelling of an underwater acoustics communication channel
2004 (English)In: Proceedings of the Seventh European Conference on Underwater Acoustics, 2004, 1211--1216 p.Conference paper, Published paper (Refereed)
Abstract [en]

A hybrid method is described for modelling sound propagation in range-dependent shallow water environments. The method combines ray tracing in a range-dependent water column with local full-field modelling of interactions with a seabed composed of multiple range-dependent layers of fluid or solid materials. The method is assessed by simulations of the Yellow Shark 94 experiments in the Mediterranean, using environmental parameters available from previous acoustic full-field inversions of the experimental acoustic data, and from an oceanographic survey with a towed-oscillating CTD profiler. Results from applying the method to simulation of transmission and decoding of communication signals in the shallow-water waveguide are presented.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-7791 (URN)
Conference
The Seventh European Conference on Underwater Acoustics, ECUA 2004 Delft, The Netherlands 5-8 July, 2004
Note
QC 20100819Available from: 2007-12-10 Created: 2007-12-10 Last updated: 2010-08-19Bibliographically approved
3. Inverting acoustic communication signals for the sound speed profile
Open this publication in new window or tab >>Inverting acoustic communication signals for the sound speed profile
2006 (English)In: Journal of the Acoustical Society of America, ISSN 1520-8524, Vol. 120, no 3, 1347--1355 p.Article in journal (Refereed) Published
Abstract [en]

Given data from a shallow-water acoustic communication experiment, the acoustic environmental parameters influencing the sound propagation conditions are estimated, particularly the sound speed profile. This inverse problem is solved using a differential evolution algorithm with an objective function measuring the mismatch between observed and modeled impulse response estimates. The model wave field is computed by a hybrid ray-trace plane-wave method, and its accuracy is assessed in a related range-independent case using solutions obtained with a full-field transform integral method. The environmental parameters obtained by inversion reduce the mismatch between the modeled and the observed impulse response estimates, compared to the mismatch using a sound speed profile measured two days before the experiment. The resulting environmental model is used to investigate alternative source-receiver configurations and to predict the optimal transmitter depth.

Keyword
Acoustic signal processing; Computer simulation; Environmental impact; Evolutionary algorithms; Inverse problems; Parameter estimation; Problem solving; Wave propagation; Acoustic communication signals; Field transform integral method; Sound propagation; Sound speed profile; Acoustic fields; water; accuracy; acoustics; algorithm; article; environmental parameters; evolution; priority journal; sound; sound detection; velocity
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-7792 (URN)10.1121/1.2234851 (DOI)000240474100021 ()2-s2.0-33748527517 (Scopus ID)
Note
QC 20100819Available from: 2007-12-10 Created: 2007-12-10 Last updated: 2010-08-19Bibliographically approved
4. Gaussian beam summation in shallow waveguides
Open this publication in new window or tab >>Gaussian beam summation in shallow waveguides
2008 (English)In: Wave motion, ISSN 0165-2125, E-ISSN 1878-433X, Vol. 45, no 4, 445-456 p.Article in journal (Refereed) Published
Abstract [en]

The complex beam parameter a controls the beamwidth and the curvature of the wavefront of a Gaussian beam. Here some strategies for choosing a are analysed for a shallow cylindrically symmetric waveguide and examplified by three choices of a used in the calculation of synthetic impulse responses by Gaussian beam summation.

The geometry of the shallow waveguide puts a restriction on the feasible width of the beams, since beams that are too wide cause problems with end effects in the summation. Best results in the numerical examples are observed when choosing epsilon to give plane wavefronts and narrow beams at the points contributing to the field, so that the end effects are negligible.

The expansion of a point source into beams for launch-angle-dependent epsilon is shown to agree with the commonly used expansion, corresponding to launch-angle-independent epsilon.

Keyword
Acoustics; Caustics; Gaussian beam; Shadow zone; Waveguide; Acoustic waves; Circular waveguides; Computational methods; Numerical methods; Wavefronts; Beamwidth; Gaussian beam summation; Shallow waveguide; Gaussian beams; Acoustic waves; Circular waveguides; Computational methods; Gaussian beams; Numerical methods; Wavefronts; acoustic wave; electromagnetic wave; Gaussian method; numerical model; point source; wave propagation
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-7793 (URN)10.1016/j.wavemoti.2007.09.003 (DOI)000255418300006 ()2-s2.0-40649124533 (Scopus ID)
Note
QC 20100819. Uppdaterad från In press till Published 20100819.Available from: 2007-12-10 Created: 2007-12-10 Last updated: 2017-12-14Bibliographically approved
5. An adaptive algorithm for Gaussian beam summation in shallow waveguides
Open this publication in new window or tab >>An adaptive algorithm for Gaussian beam summation in shallow waveguides
(English)Article in journal (Refereed) Submitted
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-7794 (URN)
Note
QC 20100819Available from: 2007-12-10 Created: 2007-12-10 Last updated: 2010-08-19Bibliographically approved

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