In this paper we propose a novel polar code construction scheme, suitable for the underwater acoustic (UWA) multi-user communication system using orthogonal frequency division multiplexing (OFDM). We use the polar codes because the code rate of a polar code can be arbitrarily designed. Moreover, the polar codes can also be used to create a nested code structure, divided into several subsets (codebook). We exploit this feature of the polar codes and place the information bits in the codebook to avoid superimposition of all the users. This helps to eliminate the influence of the power allocation coefficient and make full use of the power of the transducer at the transmitted node. Furthermore, the proposed scheme maximizes the utilization efficiency of the OFDM subcarrier resource for each user by avoiding null pilot tones that are usually required for channel equalization in the traditional schemes. The proposed scheme also does not depend on the channel state information (CSI) as prior information as each user can decode their own data independently by using their own codebook without successive interference cancellation (SIC). Simulation and tank experiment results demonstrate that the proposed scheme can significantly guarantee the reliability of all the users, and the proposed scheme reduces the complexity of the receiver, thus making it most suitable for downlink UWA multi-user communication system.

In recent decades, imaging sonar has been the most widely employed remote sensing instruments in the field of underwater detection. The multibeam echo sounder (MBES) plays an important role in obtaining high-accuracy seabed topography. However, the resolution of the MBES substantially decreases with the increasing distance. Synthetic aperture sonar (SAS) achieves constant resolution on the along-track, improving the fineness of the image. However, conventional side-scan SAS usually only achieves 2D images, and gaps always exist. In this modeling and experimental research paper, we propose a novel underwater acoustic imaging scheme to improve the imaging performance of MBES, based on the complementarity of MBES and SAS systems. We design a 2D transducer array to increase the detection efficiency and obtain spatial gain. Moreover, the processing scheme is analyzed to design the working parameters in actual engineering applications. We exploit a target echo simulation approach to establish the research basics of the imaging algorithms, which also reflects the shapes and shadows of targets to match actual situations as realistically as possible. The proposed imaging algorithm synthesizes a virtual aperture receiving array on the along-track and reserves the multi-element manifold on the across-track. This helps to improve the imaging quality of the MBES and achieves high-resolution 3D detection with no gaps. Simulation and tank experimental results demonstrate that the proposed scheme can significantly improve the detection ability of the MBES, especially for small 3D target detection, thus making it suitable for 3D high-resolution underwater detection applications.

In this paper, we propose a blind side information detection scheme for Partial Transmitted Sequence (PTS) Peak-to-Average Power Ratio reduction (PAPR) method in Underwater Acoustic (UWA) Orthogonal frequency division multiplexing (OFDM) Communication System. The proposed scheme employs a corresponding table between the distributions of pilot tones and the phase rotation candidates to obtain a pseudo-optimum PTS PAPR reduction performance and a dramatic reduction of the total calculation cost. Meanwhile, predefined distributions of pilot tones are used to identify the phase rotation factor combination that has been selected and used at the transmitter. The spectral efficiency and data rate remains unaffected as the proposed PTS technique requires no additional pilot tones except for the tones used for channel estimation. Due to the sparse characteristic of the underwater acoustic channel, compressed sensing is adopted to complete the channel estimation and the phase rotation detection. The Basis Pursuit De-Noising algorithm employed here reduces the number of pilot tones as compared to linear channel estimation method. Simulation results show that the proposed scheme has better PAPR reduction performance as compared to the Conventional PTS (C-PTS) scheme and the performance gap increases with the number of subblocks. Simulation and field experimental results demonstrate that the proposed scheme can differentiate the phase rotation factor, therefore the quality of the UWA OFDM communication system is significantly enhanced.

The detection accuracy of the multibeam echo sounder (MBES) can be damaged severely and inevitably due to the mismachining tolerance of the transducer array. Therefore, it is necessary to demonstrate the performance of the multi-element transducer array to guarantee the accuracy of the processing algorithms. In this paper, a kind of multi-element MBES receiving transducer array self-calibrating method is proposed to manage the serious defocusing problem of the conventional methods. We analyze the error source and their effect on the MBES detection. The near-field beamforming and the least residual error criterion methods are employed to calibrate the average element spacing. The integrated self-calibrating method is managed to calibrate the transducer array, considering the coupling between the circuits and the element with a channel uniformity problem. An experiment is carried in a small-size tank to evaluate the method proposed. The outfield experimental data are compensated by the average element spacing and the beam response error estimated in the tank experiment. The simulations and the field data processing results indicate that the novel method proposed can effectively demonstrate the performance of the receiving transducer array and guarantee the coverage of the MBES system.

High-accuracy level underwater acoustical surveying plays an important role in ocean engineering applications, such as subaqueous tunnel construction, oil and gas exploration, and resources prospecting. This novel imaging method is eager to break through the existing theory to achieve a higher accuracy level of surveying. Multibeam Synthetic Aperture Sonar (MBSAS) is a kind of underwater acoustical imaging theory that can achieve 3D high-resolution detecting and overcome the disadvantages of traditional imaging methods, such as Multibeam Echo Sounder (MBES) and Synthetic Aperture Sonar (SAS). However, the resolution in the across-track direction inevitably decreases with increasing range, limited by the beamwidth of the transducer array of MBES. Furthermore, the sidelobe problem is also a significant interference of imaging sonar that introduces image noise and false peaks, which reduces the accuracy of the underwater images. Therefore, we proposed an accelerated deconvolved MBSAS beamforming method that introduces exponential acceleration and vector extrapolation to improve the convergence velocity of the classical Richardson-Lucy (R-L) iteration. The method proposed achieves a narrow beamwidth with a high sidelobe ratio in a few iterations. It can be applied to actual engineering applications, which breaks through the limitation of the actual transducer array scale. Simulations, tank, and field experiments also demonstrate the feasibility and advantages of the method proposed. 3D high-accuracy level underwater acoustical surveying can be achieved through this 2D MBES transducer array system, which can be widely promoted in the field of underwater acoustical remote sensing.