Joint vector sensor beam steering and passive time reversal for underwater acoustic communications

This paper investigates how to advantageously combine acoustic vector sensor field components for underwater communications. The joint vector sensor beam steering and passive time-reversal receiver structure is proposed and compared against beam steering and standard passive time-reversal separately. The beam steering method takes into account proper directions in order to benefit from highly correlated channels. On the other hand, passive time-reversal was weighted to avoid combinations of possible noisy channels. Performance of receiver structures are quantified using simulation and recorded data from a shallow-water field experiment. In this experiment, a four-element three-dimensional vector sensor array was tied to a drifting ship receiving coherent communication signals from a bottom-moored sound source. Analytical expressions and a numerical simulation based on the experimental acoustic scenario indicate a relationship between source-receiver ranges and the vector sensor channels correlation, providing an initial understanding of the suitability of each receiver structure. Then, using individual or combined vector sensors, such structures were tested with experimental data, where the range relationship hypothesis from the simulation was nearly confirmed. Error analysis shows that shorter ranges favor the beam steering, whereas channel diversity is mostly explored in longer ranges. Furthermore, the proposed joint method, designed for vector sensors, has achieved up to ten times less error than individual approaches, also showing the benefit of exploring beamforming and diversity together.info:eu-repo/semantics/publishedVersio

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

Vector sensor steering-dependent performance in an underwater acoustic communication field experiment

This paper shows the performance resulting from combining vector sensor directional components in an underwater acoustic communication experiment. The objective is to relate performance with transmission direction and range. Receiver structures based on beamforming and passive time-reversal are tested in order to quantify and compare the steerability impact of vector sensor directional components. A shallow water experiment is carried out with a bottom fixed two-axis pressure-gradient vector sensor. A ship suspended acoustic source transmits coherent modulated communication signals at various ranges and from several directions. Results show that one vector sensor can provide an up to 10 times smaller error bit rate than a pressure sensor, favoring communication robustness without size penalty.info:eu-repo/semantics/publishedVersio

Análise da geração de energia elétrica a partir de ondas do mar e máquina de indução

Este trabalho se baseia no sistema de geração de energia elétrica a partir das ondas do mar desenvolvido pelo Laboratório de Tecnologia Submarina (LTS), COPPE/UFRJ. O princípio de funcionamento deste sistema consiste na movimentação dos flutuadores pela ação das ondas do mar, que através de braços mecânicos em forma de alavanca, acionam bombas hidráulicas. Estas bombas estão ligadas a uma base fixa e injetam água num sistema hidráulico a alta pressão, armazenando a energia intermitente extraída das ondas. Uma válvula fixa ao sistema hidráulico controla um jato d„água com pressão e vazão suficiente para acionar uma turbina Pelton e um gerador elétrico.Neste trabalho, a partir da modelagem analítica detalhada do sistema de armazenamento (acumulador hidropneumático e câmara hiperbárica) e da turbina, analisou-se o comportamento da tensão e potência gerada por uma máquina de indução rotor gaiola de esquilo conectada diretamente à rede elétrica. É apresentado um estudo prévio da máquina de indução para auxiliara análise das oscilações de tensão e potência que surgem devido ao torque variável no eixo do rotor e para compreender o comportamento da máquina atuando como gerador (análise da região de operação).A capacidade da rede à qual a máquina é conectada também foi um ponto de análise. Um modelo equivalente do sistema elétrico foi desenvolvido onde a relação de curto-circuito foi alterada de forma a avaliar os efeitos quanto à qualidade da energia.A partir da modelagem do sistema hidráulico, mecânico e elétrico,as simulações foram implementadas utilizando-se o programa PSCAD-EMTDC. Os resultados são apresentados para diferentes parâmetros hidráulicos e elétricos. O comportamento da flutuação da tensão de acordo com a relação de curto-circuito foi analisado verificando pontos críticos da geração e a variação da potência relacionada com o dimensionamento do sistema hidráulico. As influências nos limites de geração e na qualidade da energia são analisadas

Experimental demonstration of a single acoustic vector sensor for JANUS performance enhancement

This study shows the underwater communication performance using an acoustic pressure-gradient vector sensor. Combining the estimated particle velocity channels with the acoustic pressure results in a cardioid-like beam steered output, which is used to improve the signal-to-noise ratio. A shallow-water field experiment was carried out using a single vector sensor as a receiver and a ship-suspended sound source, transmitting the frequency-hopped JANUS modulated signal at several ranges and directions. Bit error rate analysis demonstrates how performance can be enhanced through vector sensor channel combining. Firstly, by relating the error with beam pattern varying the azimuth steering angle. Second, by relating the error with transmitting stations, where individual channels of the vector sensor can be compared. Besides such findings, this study also presents tools for better understanding the directional characteristic, such as the design factor to combine the particle velocity to the pressure sensor and azigrams. Finally, results show that the JANUS bit error rate can be reduced up to five percent by combining the vector sensor components compared to the pressure sensor.info:eu-repo/semantics/publishedVersio

Vector hydrophone passive time reversal for underwater acoustic communications

The use of vector hydrophones as a receiver for underwater communications has been the subject of research since such device is a compact option to pressure-only arrays. A vector hydrophone, usually called acoustic vector sensor, is a device that measures pressure and particle velocity components. This paper investigates a method to combine those channels based on passive time-reversal (PTR). Simulation and experimental data are used to quantify communication performance, comparing vector hydrophones to pressure-only arrays. The analyzed acoustic scenario consists of a shallow-water area (about 100 m), where a vector hydrophone array receives communication signals from a bottom moored source. Simulations help in the understanding of diversity by analyzing spectral characteristics of vector hydrophone channels and the PTR q-function. While in simulation, the benefits of PTR using particle velocity channels are perceptible seen by exploring diversity, communication performance with experimental data is degraded due to time-varying. Finally, the achieved performance using a single or a small array of vector hydrophones enforces its benefits for communication enhancement.info:eu-repo/semantics/publishedVersio