Resumo-Este artigo apresenta um modelo empírico para o ruído do canal acústico submarino em águas rasas a partir da análise de dados provenientes de medições realizadas em campo. Uma função de densidade de probabilidade para a distribuição da amplitude do ruído é proposta e funções de verossimilhança são obtidas. Em decorrência, uma expressão para a probabilidade de erro de símbolo para sinalização binária no canal é apresentada. Além disso, são fornecidos os resultados de simulação realizada com as amostras reais de ruído coletadas em campo, de modo a verificar o efeito do ruído no desempenho de sistemas acústicos de comunicação submarina com sinalização binária. Palavras-Chave-Comunicação Submarina, Ruído Acústico Submarino, Distribuição de Ruído.Abstract-This article presents an empirical model for the noise of the acoustic underwater channel in shallow water from the analysis of field data measurements. A probability density function for the noise amplitude distribution is proposed and the associated likelihood functions are derived. As a result, an expression to the probability of symbol error for binary signaling is presented for the channel. Additionally, the results of simulations conducted using the field collected noise samples are presented, in order to verify the noise effect on the performance of underwater acoustic communication binary signaling systems.

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The number of sensors present in an array imply in cost, length, weight, and computational complexity. So, it is desired that an array satisfies its project purpose using the minimum of elements. In this study, we analyze the characteristics of a uniform circular array (UCA). Previous studies show the ambiguity problem when working with only few sensors. The grating lobes and potentials ambiguities in uniform linear array are generally avoided limiting the space between sensors in half of the wavelength. In UCA, these problems are solved also by limiting the space between sensors using an adequate number of sensors for a given radius or changing the radius for a given number of sensors. This study shows that it is possible to avoid the grating lobes controlling the sensor's directivity. The directional gain model is used to represent a realistic baffled vertical stave, three hydrophone each, and the Multiple Signal Classification (MUSIC) is applied to detect sources. Simulation results show the accuracy when using directives sensors. This array is analyzed here, working with experimental data, acquired in an acoustic tank.

The delay and sum beamforming is the most simple technique in direction of arrival (DOA) Estimation. Although its performance on spatial discrimination is poor, compared to other beamforming, delay and sum still is used in large operating sound navigation and ranging (SONAR) because of its low computational cost. A circular hydrophone array (CHA), commonly used in SONAR system, is an attractive alternative to provide a more uniform directive response over all azimuth angle. This array is analyzed here, working with experimental data, acquired in a acoustic tank and in the sea. Maximum likelihood estimation (MLE) is applied to denoising noisy channels, summing up them after in delay-and-sum. First of all, a noise in an acoustic tank is considered to represent the hydrophones, cables, and acquisition system noises. Then, an environmental noise is collected in the sea. The MLE use both of them to calculate the weights in the beamforming. A boat is used to running around the array, and the DOA of the uniformly weight and MLE in delay-and-sum shows the performance improvement.