The Shipborne acoustic communication system of the submersible Shenhai Yongshi works in vertical, horizontal and slant channels according to the relative positions. For ease of use, an array combined by a vertical-cone directional transducer and a horizontal-toroid one is installed on the mothership. Improved techniques are proposed to combat adverse channel conditions, such as frequency selectivity, non-stationary ship noise, and Doppler effects of the platform’s nonlinear movement. For coherent modulation, a turbo-coded single-carrier scheme is used. In the receiver, the sparse decision-directed Normalized Least-Mean-Square soft equalizer automatically adjusts the tap pattern and weights according to the multipath structure, the two receivers’ asymmetry, the signal’s frequency selectivity and the noise’s spectrum fluctuation. The use of turbo code in turbo equalization significantly suppresses the error floor and decreases the equalizer’s iteration times, which is verified by both the extrinsic information transfer charts and bit-error-rate performance. For noncoherent modulation, a concatenated error correction scheme of nonbinary convolutional code and Hadamard code is adopted to utilize full frequency diversity. Robust and low-complexity synchronization techniques in the time and Doppler domains are proposed. Sea trials with the submersible to a maximum depth of over 4500 m show that the shipborne communication system performs robustly during the adverse conditions. From the ten-thousand communication records in the 28 dives in 2017, the failure rate of the coherent frames and that of the noncoherent packets are both below 10%, where both synchronization errors and decoding errors are taken into account.

In this article, two ultrashort baseline (USBL) location methods for underwater broadband signal are proposed. In order to improve the positioning accuracy of acoustic positioning, the coarse time delay is measured by parabolic interpolation of cross correlation results. According to the coarse time delay, the ambiguity number of the phase at the cross correlation peak is solved and the ambiguity is solved twice, so as to obtain the high-precision time delay difference and improve the positioning accuracy. This method is suitable for the case without near body multipath. Aiming at the near body multipath problem, its influence on ambiguity resolution and positioning accuracy is analyzed. Based on the coarse time delay, adjusting time delay and the Fourier transform of cross-power spectrum phase (CSP) joint method is proposed to further improve the accuracy of coarse time delay, so as to improve the accuracy of ambiguity resolution. On this basis, the location information feedback and real-time attitude information are used to estimate the coarse time delay difference, and the ambiguity number is further constrained to further improve the accuracy of ambiguity resolution. In order to reduce the influence of multipath on the phase and improve the positioning accuracy, a Kaiser window is added to the source and the phase difference is changed to the phase difference at the first point of threshold. Numerical simulation and field test demonstrate that the first algorithm proposed in this article improves the positioning accuracy and the accuracy of ambiguity resolution. The second algorithm improves the accuracy of ambiguity resolution and positioning accuracy in the presence of near body multipath.

In the cableless seafloor observation networks (SONs), the links among network nodes rely on underwater acoustic communication (UAC). Due to the energy constraint and the high-reliability requirement of the cableless SONs, the noncoherent UAC has been a preferred choice, even though a noncoherent UAC scheme generally suffers from low spectral efficiency. In this paper, we propose a high-spectral-efficiency noncoherent UAC transmission scheme which is implemented as an orthogonal frequency-division multiplexing (OFDM) system adopting the on-off keying (OOK) modulation. To simultaneously achieve high performance at a low energy consumption, an irregular recursive convolutional code (IrCC) is employed and an accumulator (ACC) is introduced to achieve a modulation with memory at the transmitter side. The ACC enables a turbo iteration between the soft demapper called the ACC-OOK demapper and the soft decoder on the receiver side, and also reduces the decoding error floor. To account for the unknown signal-to-noise ratio (SNR), an iterative threshold estimation (ITE) algorithm is proposed to determine a proper decision threshold for the ACC-OOK demapper. The IrCC is designed to match the extrinsic information transfer (EXIT) curve of the ACC-OOK demapper, lowering the SNR threshold of the aforementioned turbo iteration. Simulations and experimental results verify the superiority of the proposed noncoherent UAC scheme over conventional ones.

A uniform linear array (ULA) or a multi-element planar array composed of uniform linear arrays is the main applied research array for direction-of-arrival (DOA) estimation in array signal processing. In recent years, the uniform circular array (UCA) is getting more attention because of its consistent azimuthal beam resolution, but the DOA estimation algorithm of a UCA is complex and difficult to implement. To solve this problem, a UCA positioning method based on virtual interpolation and subarray rotation (VISAR) is proposed. This method effectively reduces the computational complexity of DOA estimation for a UCA, and the simulation analysis of the algorithm shows that in terms of positioning performance, the proposed algorithm is better than the existing virtual interpolation algorithms in the case of a low signal-to-noise ratio (SNR).

Underwater wireless sensor networks (UWSNs) are significantly different from terrestrial sensor networks in the following aspects: low bandwidth, high latency, variable topology, limited battery, low processing power and so on. These new features pose many challenges to the design of self-organizing routing protocol for UWSNs. This paper focuses on the application of Ad Hoc On-demand Distance Vector (AODV) routing protocol in UWSNs. In order to solve the problems of packet collision and excessive energy consumption associated with the flooding-based routing discovery method and the periodic hello packet routing maintenance mechanism of AODV, a routing discovery and maintenance method based on location and energy information is proposed, and it is referred to as the route-focusing AODV (RFAODV) routing protocol. In the RFAODV protocol, the routing discovery process is focused on a few nodes through forwarding area control and dynamic delay adjustment. In addition, feedback from a media access control layer and residual energy control are used for routing maintenance. We implement the RFAODV and evaluate its performance according to the sea trial data as parameters in the NS-2. The simulation results show that compared with the other protocols, RFAODV improves the routing discovery success ratio by at least 18%, increases the packet transmission ratio by at least 4%, reduces the protocol overhead by at least 15% and reduces the energy consumption by at least 5% under various simulation scenarios. RFAODV is suitable for large-scale, high-load and dynamic networks underwater wireless sensor networks.