Vector sensors for underwater : acoustic communications

Acoustic vector sensors measure acoustic pressure and directional components separately. A claimed advantage of vector sensors over pressure-only arrays is the directional information in a collocated device, making it an attractive option for size-restricted applications. The employment of vector sensors as a receiver for underwater communications is relatively new, where the inherent directionality, usually related to particle velocity, is used for signal-to-noise gain and intersymbol interference mitigation. The fundamental question is how to use vector sensor directional components to bene t communications, which this work seeks to answer and to which it contributes by performing: analysis of acoustic pressure and particle velocity components; comparison of vector sensor receiver structures exploring beamforming and diversity; quanti cation of adapted receiver structures in distinct acoustic scenarios and using di erent types of vector sensors. Analytic expressions are shown for pressure and particle velocity channels, revealing extreme cases of correlation between vector sensors' components. Based on the correlation hypothesis, receiver structures are tested with simulated and experimental data. In a rst approach, called vector sensor passive time-reversal, we take advantage of the channel diversity provided by the inherent directivity of vector sensors' components. In a second approach named vector sensor beam steering, pressure and particle velocity components are combined, resulting in a steered beam for a speci c direction. At last, a joint beam steering and passive time-reversal is proposed, adapted for vector sensors. Tested with two distinct experimental datasets, where vector sensors are either positioned on the bottom or tied to a vessel, a broad performance comparison shows the potential of each receiver structure. Analysis of results suggests that the beam steering structure is preferable for shorter source-receiver ranges, whereas the passive time-reversal is preferable for longer ranges. Results show that the joint beam steering and passive time-reversal is the best option to reduce communication error with robustness along the range.Sensores vetoriais acústicos (em inglês, acoustic vector sensors) são dispositivos que medem, alem da pressão acústica, a velocidade de partícula. Esta ultima, é uma medida que se refere a um eixo, portando, esta associada a uma direção. Ao combinar pressão acústica com componentes de velocidade de partícula pode-se estimar a direção de uma fonte sonora utilizando apenas um sensor vetorial. Na realidade, \um" sensor vetorial é composto de um sensor de pressão (hidrofone) e um ou mais sensores que medem componentes da velocidade de partícula. Como podemos notar, o aspecto inovador está na medição da velocidade de partícula, dado que os hidrofones já são conhecidos.(...)This PhD thesis was supported by the Brazilian Navy Postgraduate Study Abroad Program Port. 227/MB-14/08/2019

Low probability of detection underwater acoustic communications

Low probability of detection (LPD) underwater acoustic communications are an essential requirement for command and control of autonomous underwater vehicles (AUV) or submarines performing covert missions, avoiding their detection while communicating. Based on low power signals, these covert communications may also extend the autonomy of battery-operated AUVs, and contribute to reducing the impacts of the environmental noise level on marine life. The present thesis aims to develop LPD communications based on modeled and real data from three shallow water experiments. Thus, a superimposed training method has been proposed. A bitstream is created superimposing a long probe to the message before transmission. Computationally simple, the algorithm explores temporal diversity to increase the processing gain and uses a Wiener filter for equalization. Experimental results presented bit-error rates (BER) < 10−2 for signal-tonoise ratios (SNR) < −8 dB. To understand the effects of coastal upwelling phenomena over low SNR communications, a study compares the acoustic propagation for different sound speed profiles using a propagation model and analyzes data from the BioCom’19 experiment, performed off Cabo Frio Island (Brazil). Temporal and spatial coherence of low power signals propagating in this harsh environment are estimated, and both a criterion for multichannel combining and a double Wiener filter to improve equalization are presented. Passive time-reversal (pTR) techniques have been widely employed for communications. To address the pTR channel mismatch due to the environmental variability between the probe and the data transmissions, this work proposes a superimposed training pTR (STpTR) approach for single and multichannel systems. Despite the high noise levels, varying from -5 to +6 dB, the STpTR combined with a Wiener filter achieved BER < 10−2, for bit rates up to 220 bps. To improve covert communications for AUVs, this work also presents a study about vector sensor multichannel combining. Using the STpTR approach, results from an experiment on the coast of Algarve/Portugal indicate that combining the pressure and particle velocity channels of a vector sensor may provide an average SNR and mean squared-error gain of up to 9.4 and 3.1 dB, respectively, compared to the pressure channel. Therefore, a better understanding of the environment combined with the superimposed training pTR using a vector sensor may improve the LPD communication system’s performance and robustness while keeping covertness.No ´últimos anos, os trabalhos de investigação sobre comunicações acústicas submarinas com baixa probabilidade de deteção (BPD) tem sido incentivados pela indústria, pelos governos, e pela própria academia em razão de suas m´múltiplas aplicações. Na ´área militar, as comunicações BPD permitem que submarinos e veículos autónomos possam se comunicar sem serem detectados. Na ´área civil, permitem a economia de energia de sensores alimentados por baterias, aumentando o tempo de funcionamento, bem como contribui para reduzir os impactos sobre a vida marinha causados pelos altos n´ıveis de ru´ıdo submarino, entre outras aplica¸c˜oes. Neste contexto, esta tese pretende desenvolver comunica¸c˜oes BPD utilizando um modelo de propaga¸c˜ao ac´ustica e dados obtidos a partir de trˆes experimentos em ´aguas rasas. Este trabalho apresenta um m´etodo de treinamento superposto para comunica¸c˜oes submarinas em um ambiente com baixa rela¸c˜ao sinal/ru´ıdo, e demonstra sua aplica¸c˜ao para comunica¸c˜oes BPD. Computacionalmente simples, o m´etodo foi desenvolvido para funcionar com um ´único projetor acústico, transmitindo com baixa potência, e um hidrofone, sem o ganho de um arranjo de sensores distribuídos no espaço. Antes da transmissão, uma longa sequência de comprimento m´máximo ´e somada `a mensagem para efeitos de equalização e sincronismo. Os dois sinais são binários, modulados em fase e possuem 2047 bits. Porém, possuem amplitudes diferentes. Na realidade, a amplitude do sinal de treinamento ´e ligeiramente superior `a da mensagem. Em um ambiente com baixa rela¸c˜ao sinal ru´ıdo, um sinal de treinamento mais forte permite ocultar a mensagem a ser transmitida, bem como melhorar o ganho para a estima¸c˜ao da resposta impulsiva e para a sincroniza¸c˜ao do sistema. A mensagem ´e composta por 3 bits nulos e 4 sequˆencias de 511 bits. Delimitados por uma curta sequˆencia de comprimento m´aximo de 31 bits, para dupla sincroniza¸c˜ao, os pacotes de dados possuem 480 bits e transportam o seguinte pangrama: (The Quick Brown Fox Jumps Over the Lazy Dog 0123456789!@#$). O m´etodo explora a diversidade temporal do canal, transmitindo a mesma sequˆencia diversas vezes para aumentar o ganho de processamento do sinal e implementar a corre¸c˜ao de erros atrav´es da soma coerente dos sinais. A resposta impulsiva do canal ´e estimada pela transformada r´apida de Hadamard, e a equaliza¸c˜ao do sinal ´e feita por um filtro de Wiener. A remo¸c˜ao da interferˆencia causada pelo sinal de treinamento ´e realizada pelo m´etodo “hyperslice cancellation by coordinate zeroing (HCC0)”, e a seguir a mensagem ´e decodificada. Resultados obtidos a partir de um experimento em ´aguas rasas, utilizando uma fonte e um ´único hidrofone, apresentaram taxas de erro de bit menores que 10−2, para relações sinal/ruído inferiores a −8 dB. A ressurgência costeira ´e um fenómeno oceanográfico dinâmico que modifica, profundamente, a estratificação de temperatura do oceano, influenciando diretamente na propagação acústica. Por outro lado, os crescentes n´níveis de ruído antropogénico não apenas reduzem o desempenho dos sistema de comunicação, corrompendo a informação transmitida, mas tamb´em afetam a vida marinha. Para compreender os efeitos da ressurgˆencia costeira sobre as comunica¸c˜oes com baixa rela¸c˜ao sinal/ru´ıdo, um estudo analisa os dados do experimento BioCom’19, realizado nas proximidades da Ilha do Cabo Frio, Rio de Janeiro (Brasil). As respostas impulsivas do canal e a propaga¸c˜ao ac´ustica, para diferentes perfis de velocidade do som, foram estimadas usando o modelo de propaga¸c˜ao ac´ustica “Monterey-Miami Parabolic Equation model (MMPE)”. Al´em disso, o desempenho do sistema de comunica¸c˜oes foi correlacionado com os perfis de temperatura durante o experimento. Os resultados obtidos indicam uma significativa redu¸c˜ao da energia ac´ustica nos receptores durante a ressurgˆencia, degradando o desempenho do sistema. A coerˆencia temporal e espacial dos sinais de baixa potˆencia transmitidos no experimento BioCom’19 foi estimada, e um crit´erio para combina¸c˜ao dos sinais, provenientes dos m´ultiplos hidrofones, foi proposto. Utilizando dados de um arranjo piramidal e um arranjo vertical linear, de 4 hidrofones cada, a coerˆencia foi estimada antes e depois do filtro de Wiener para mostrar o impacto do multicaminhamento sobre a taxa de erro de bit. Os resultados mostram que a coˆerencia temporal pode ser utilizada como crit´erio para combinar sinais consecutivos em um mesmo canal, enquanto a diversidade espacial permite a combina¸c˜ao de m´ultiplos canais do arranjo de sensores. Sequˆencias cuja coerˆencia temporal esteja acima de um limite pr´e-definido s˜ao somadas. A coerˆencia espacial entre canais foi estimada e comparada em termos da taxa de erro de bit. Para diferentes taxas de transmiss˜ao, as taxas de erro de bit est˜ao em concordˆancia com a evoluc˜ao da coerˆencia espacial. Quanto mais elevada a coerˆencia, melhor o desempenho e menor a taxa de erro de bit. Um duplo filtro de Wiener para melhorar a equaliza¸c˜ao dos sinais de baixa potˆencia, durante a ressurgˆencia, tambem foi proposto. Utilizando dados dos 4 hidrofones de um arranjo piramidal, as respostas impulsivas foram estimadas para observar a variabilidade das condi¸c˜oes de propaga¸c˜ao. Em uma condi¸c˜ao de perfil de temperatura isot´ermico, as respostas impulsivas apresentaram multicaminhamento curto, com chegadas mais fortes nos receptores. `A medida que a ressurgˆencia ocorria, foram observadas quedas abruptas de temperatura superiores a 10◦C, na posi¸c˜ao dos hidrofones, acarretando uma forte refra¸c˜ao da onda sonora para o fundo marinho. Em consequˆencia, sinais mais fracos foram observados nos hidrofones. Os resultados obtidos com dados do BioCom’19 mostram que, para uma rela¸c˜ao sinal/ru´ıdo variando entre −3.9 e 7.3 dB, o duplo filtro de Wiener forneceu um ganho do erro m´edio quadr´atico de at´e 2.8 dB, comparado com o filtro de Wiener simples. As t´ecnicas de tempo reverso passivo (TRP) tem sido amplamente empregadas nas comunica¸c˜oes submarinas. Por´em, as r´apidas altera¸c˜oes das condi¸c˜oes de propaga¸c˜ao em canais submarinos variantes no tempo, durante as transmiss˜oes da sequˆencia de treinamento e da mensagem, degradam o desempenho das t´ecnicas TRP de equaliza¸c˜ao. Assim, esse trabalho prop˜oe um m´etodo de TRP utilizando treinamento superposto, para sistemas com um ou m´ultiplos sensores. O m´etodo proposto utiliza uma sequˆencia de treinamento, com o efeito Doppler corrigido, para estimar o canal e realizar o TRP. O m´etodo compara 3 differentes estrat´egias para melhorar a performance do sistema de comunica¸c˜ao: a diversidade temporal devido aos sinais idˆenticos transmitidos continuamente, a diversidade espacial devido aos 2 arranjos de hidrofones, piramidal e linear, com 4 hidrofones cada um, al´em da combina¸c˜ao dos 2 arranjos (8 hidrofones). Neste m´etodo, a t´ecnica de TRP com treinamento superposto minimiza o multicaminhamento e realiza a corre¸c˜ao de erros atrav´es da soma coerente dos diferentes canais. A interferˆencia intersimb´olica residual ´e removida pelo filtro de Wiener. Resultados obtidos com dados do experimento BioCom’19 mostram que o m´etodo proposto pode fornecer um ganho do erro médio quadrático de até 1.62 dB para canais independentes, e 3.13 dB, para canais combinados, comparativamente ao m´etodo sem o TRP, usando apenas o filtro de Wiener. Neste contexto, o m´etodo de TRP utilizando treinamento superposto alcan¸cou taxas de erro de bit < 10−2 para uma rela¸c˜ao sinal ru´ıdo, na banda de transmiss˜ao, variando entre −5 a +6 dB. Focado em comunica¸c˜oes com baixa probabilidade de detec¸c˜ao para pequenos ve´ıculos submarinos, este trabalho tamb´em apresenta um estudo sobre a combina¸c˜ao dos canais de press˜ao e velocidade de part´ıcula dos sensores vetoriais. Os sensores vetoriais possuem pequenas dimens˜oes, adequadas `a utilizac˜ao em ve´ıculos autˆonomos, e permitem obter um ganho de diversidade para as comunica¸c˜oes BPD. Para testar o m´etodo de treinamento superposto com o tempo reverso passivo utilizando sensores vetoriais, um experimento foi realizado, em ´aguas rasas, na costa do Algarve/Portugal. Para reduzir a rela¸c˜ao sinal/ru´ıdo para uma faixa de 0 a −10 dB, foi adicionado ru´ıdo gravado no experimento. Os resultados experimentais indicam que a combina¸c˜ao dos canais podem fornecer um ganho da rela¸c˜ao sinal/ru´ıdo e do erro m´edio quadr´atico de at´e 9.4 e 3.1 dB, respectivamente, comparados com os resultados do sensor de press˜ao. Portanto, as principais contribui¸c˜oes dessa tese s˜ao (i) a proposta do m´etodo de treinamento superposto para comunica¸c˜oes com baixa probabilidade de detec¸c˜ao, (ii) a compreens ˜ao dos efeitos da ressurgˆencia costeira sobre as comunica¸c˜oes, e seus impactos sobre a coerˆencia temporal e espacial, (iii) o m´etodo de treinamento superposto em conjunto com o tempo reverso passivo para lidar com a varia¸c˜ao do canal entre o tempo de recep¸c˜ao do sinal de treinamento e da mensagem, e (iv) a combina¸c˜ao dos m´ultiplos canais dos sensores vetoriais para comunica¸c˜oes BPD. Dessa forma, uma melhor compreens˜ao do canal submarino e a utiliza¸c˜ao dos m´etodos propostos combinados com sensores do estado da arte, como os sensores vetoriais, se configura como um avan¸co neste campo do conhecimento, permitindo aumentar a robustez do sistema BPD, bem como reduzir a probabilidade de detec¸c˜ao, mantendo a ocultação das comunicações.I am also grateful for the sponsorship provided by the Brazilian Navy through the Postgraduate Study Abroad Program, Grant No. Port.227/MB/2019

Cost-Effective and Energy-Efficient Techniques for Underwater Acoustic Communication Modems

Finally, the modem developed has been tested experimentally in laboratory (aquatic environment) showing that can communicates at different data rates (100..1200 bps) compared to state-of-the-art research modems. The software used include LabVIEW, MATLAB, Simulink, and Multisim (to test the electronic circuit built) has been employed.Underwater wireless sensor networks (UWSNs) are widely used in many applications related to ecosystem monitoring, and many more fields. Due to the absorption of electromagnetic waves in water and line-of-sight communication of optical waves, acoustic waves are the most suitable medium of communication in underwater environments. Underwater acoustic modem (UAM) is responsible for the transmission and reception of acoustic signals in an aquatic channel. Commercial modems may communicate at longer distances with reliability, but they are expensive and less power efficient. Research modems are designed by using a digital-signal-processor (DSP is expensive) and field-programmable-gate-array (FPGA is high power consuming device). In addition to, the use of a microcontroller is also a common practice (which is less expensive) but provides limited computational power. Hence, there is a need for a cost-effective and energy-efficient UAM to be used in budget limited applications. In this thesis different objectives are proposed. First, to identify the limitations of state-of-the-art commercial and research UAMs through a comprehensive survey. The second contribution has been the design of a low-cost acoustic modem for short-range underwater communications by using a single board computer (Raspberry-Pi), and a microcontroller (Atmega328P). The modulator, demodulator and amplifiers are designed with discrete components to reduce the overall cost. The third contribution is to design a web based underwater acoustic communication testbed along with a simulation platform (with underwater channel and sound propagation models), for testing modems. The fourth contribution is to integrate in a single module two important modules present in UAMs: the PSK modulator and the power amplifier

Characterisation of long-range horizontal performance of underwater acoustic communication

Underwater acoustic communication is a rapidly progressing field of technology, largely due to recent advances in low cost and power efficient digital signal processors. Unfortunately, the unpredictable and time varying physical properties of the underwater acoustic channel reduce communication reliability over long ranges. This study sought to characterise the performance of horizontal underwater acoustic data communication in various scenarios with particular application to subsea monitoring and control systems.To fulfil the experimental needs, two custom-built high frequency ambient noise recorder and modem control units were developed to operate with commercial underwater acoustic modems. Additionally, an underwater acoustic communication simulator based on the Bellhop propagation model was developed for Matlab, capable of producing performance predictions in both spatial and temporal studies. A series of short-term trials were conducted to determine the limitations of modem performance over different ranges. These trials included shallow water studies off the coast of Perth, Western Australia (D < 30 m), and a French deep water trial (D ≤ 1000 m) which used stand-alone modems. Experimental findings were compared to predictions obtained using two-dimensional range-depth performance simulations.A long-term investigation of the environmental influences on modem reliability was carried out off the coast of Perth in approximately 100 m of water. This involved simultaneously collecting environmental and modem performance data for over 16 days. The signal to noise ratio remained high for the duration of the trial so modem performance fluctuations could be attributed to changes in channel propagation. Using multiple linear regression, the measured environmental parameters were correlated with the observed modem performance and their contributions to an overall fitting curve were calculated. It was determined that the sound speed profile, in addition to the sea surface roughness, contributed strongly to the fitting curve, with a weaker contribution from the measured signal to noise ratio. This result was confirmed by performing temporal simulations which incorporated more detailed time-dependant environmental parameters. By progressively adding more parameters to the simulator including ambient noise, wave height and the sound speed profile, simulations provided more accurate predictions of the observed performanceOverall, the horizontal performance of underwater acoustic communication was characterised in several scenarios from a series of experimental and numerical investigations. Additionally, the developed simulator was shown to be an effective and flexible tool for predicting the performance of an underwater acoustic communication system. The results and tools discussed in this thesis provide an extensive investigation into the factors influencing horizontal underwater acoustic communication. The analysis demonstrates that whilst underwater acoustic communication can be effective, it is not yet a viable alternative to cabled telemetry for long-range subsea monitoring and control applications, where reliability is crucial. Underwater acoustic communication would best be suited as a non-critical or backup method for continuous monitoring systems until channel prediction and equalisation techniques are further refined

Acoustic Oceanographic Buoy Test during the MREA’03 Sea Trial

Rep 04/03 - SiPLAB 10/Nov/2003Environmental inversion of acoustic signals for bottom and water column properties is being proposed in the literature as an interesting concept for complementing direct hydrographic and oceanographic measurements for Rapid Environmental Assessment (REA). The acoustic contribution to REA can be cast as the result of the inversion of ocean acoustic properties to be assimilated into ocean circulation models specifically tailored and calibrated to the scale of the area under observation. Traditional ocean tomography systems and methods for their requirements of long and well populated receiving arrays and precise knowledge of the source/receiver geometries are not well adapted to operational Acoustic REA (AREA). An innovative concept that responds to the operational requirements of AREA is being proposed under a Saclantcen JRP jointly submitted by the the Universit´e Libre de Bruxelles (ULB), SiPLAB/CINTAL at University of Algarve, the Instituto Hidrogr´afico (IH) and the Royal Netherlands Naval College (RNLNC) and approved by Saclantcen in 2003 under the 2004 SPOW. That concept includes the development of water column and geo-acoustic inversion methods being able to retrieve environmental true properties from signals received on a drifting network of Acoustic-Oceanographic Buoys (AOB). A prototype of an AOB and a preliminary version of the inversion code, was tested at sea during the Maritime Rapid Environment Assessment’2003 sea trial (MREA’03) and is described in this report together with the results obtained.This report presents the AOB system and the results obtained during its testing in the MREA’03 sea trial. The MREA’03 sea trial took place off the Italian coast, near Elba I. in the period 26 May - 27 June 2003. The authors of this report would like to thank: the SACLANT Undersea Research Centre for the opportunity for participating in the sea trial • the scientist in charge Dr. Emanuel Coelho; the collaboration of Saclantcen personnel; the master and crew of the R/V Alliance; the contribution of Prof. J.-P. Hermand from ULB for the discussions and pictures shown in this report

Effects of errorless learning on the acquisition of velopharyngeal movement control

Session 1pSC - Speech Communication: Cross-Linguistic Studies of Speech Sound Learning of the Languages of Hong Kong (Poster Session)The implicit motor learning literature suggests a benefit for learning if errors are minimized during practice. This study investigated whether the same principle holds for learning velopharyngeal movement control. Normal speaking participants learned to produce hypernasal speech in either an errorless learning condition (in which the possibility for errors was limited) or an errorful learning condition (in which the possibility for errors was not limited). Nasality level of the participants’ speech was measured by nasometer and reflected by nasalance scores (in %). Errorless learners practiced producing hypernasal speech with a threshold nasalance score of 10% at the beginning, which gradually increased to a threshold of 50% at the end. The same set of threshold targets were presented to errorful learners but in a reversed order. Errors were defined by the proportion of speech with a nasalance score below the threshold. The results showed that, relative to errorful learners, errorless learners displayed fewer errors (50.7% vs. 17.7%) and a higher mean nasalance score (31.3% vs. 46.7%) during the acquisition phase. Furthermore, errorless learners outperformed errorful learners in both retention and novel transfer tests. Acknowledgment: Supported by The University of Hong Kong Strategic Research Theme for Sciences of Learning © 2012 Acoustical Society of Americapublished_or_final_versio