Underwater Acoustic Communication: Technology Advances for Practical Marine Applications

Due to the constant improvement of point-to-point underwater acoustic communication technology, underwater acoustic communication networks in oceans will become a reality. In recent years, unmanned vehicles such as AUVs and Gliders, have gradually matured and been perfected, and can be used as a novel kind of underwater acoustic node. In this paper, a novel Unmanned Surface Vehicle (USV) integrated underwater acoustic modem is introduced. Due to the complexity of underwater acoustic channels, the harsh marine environment and aspects of the USV's manipulation, underwater acoustic communication that links modeling and estimation is essential for USV communications and applications. Firstly, the underwater acoustic communication distance, USV running condition and the Marine environment are considered as the main factors affecting a USV's acoustic communication. Corresponding to the above factors, communication energy loss model, USV motion model and marine environmental impact model are established respectively. Secondly, analyzing the interactions among the three models, a USV underwater acoustic communication link strength model is constructed. Lastly, USV underwater acoustic communication is tested on our USV carrying an EvoLogics acoustic modem, icListen self-contained hydrophone, CTD, electronic compass and GPS at Jihongtan reservoir in Qingdao in July 2015. In the experiments, the bank station transmitting modem is fixed at a depth of 2m, and the receiving modem is integrated in our USV at a depth of 3m. Underwater acoustic communications for fixed points at 50m intervals, and for mobile reception, are executed in a 1000m line respectively. Verification and inspection of the proposed communication link strength model are carried out by analyzing the signal strength along with the communication distance, signal frequency, roll Angle, pitch Angle, velocity of USV, velocity of sound and noise, From the above experiments, it is concluded that the USV as a mobile relay node is very effective, and its underwater acoustic communication link strength model can justifiably be adopted to describe its communication ability. This will provide a reference for underwater acoustic applications of USV in the future.

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Experimental demonstration of underwater acoustic communication using bionic signals

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Because of the hostile ocean transmission condition, underwater acoustic communication technology is one of the most difficulty communication technology. Frequency-Hopping underwater acoustic communication system is drawing more and more attention for its good performance under serious noise and multi-path interference condition. Synchronization signal processing is a key factor of FH underwater acoustic communication system. In the research of FH shallow underwater acoustic data communication system, a synchronization method employing adaptive line-spectrum enhancing (ALE) method, frequency diversity, pulse width judgment and frame synchronization signal correlation processing to detect synchronization signal under the condition of low SNR and heavy amplitude fluctuation is presented. The sea experiment results are satisfactory.

Direct-sequence spread-spectrum (DSSS) underwater acoustic (UWA) communication can obtain considerable matching processing gain at the receiving end, which is the preferred communication method for high quality underwater acoustic communication. However, under the mobile condition, the DSSS receiving signal will face fast carrier phase fluctuation interference, resulting in a serious degradation of the matching processing gain at the receiving end. Aiming at this problem, this paper proposes a dual differential spread-spectrum (DDSS) UWA communication technology: double differential encoding of the original information sequence at the transmitting end; dual-difference correlation detector for decoding at the receiving end, which can effectively suppress fast carrier phase interference. The simulation results show that the DDSS UWA communication can achieve -8dB SNR robust acoustic communication under the condition of relative motion speed of 8m/s, and the communication error rate is less than 10−3.

With the increasingly fierce competition for marine resources, underwater acoustic communication as the main form of underwater communication, its security has received more and more attention. The traditional underwater acoustic communication technology with fixed frequency and modulation may cause information leakage and location exposure of the communicating platform. The communication based on ocean noise is helpful to improve the communication security. This article proposes a new covert underwater acoustic communication scheme based on ship-radiated noise and chaos signal. Firstly, the characteristics of chaos signal is studied, which is generated by the Chebyshev sequence, and the reconstruction process of ship-radiated noise is analyzed. Secondly, we generate mimic ship-radiated noise containing the secret message based on the time and frequency feature of the chaos signal and ship-radiated noise. Finally, resampling technology is used to equalize the Doppler frequency shift, and time inversion mirror technology is used to perform multipath channel equalization. Simulation and sea experiment verify the effectiveness and feasibility of this scheme. In the sea experiments, the proposed scheme can reliably transimit over 40 per second when the communication distance is about 10km.

Underwater acoustic communication technology is the core technology in the marine field, occupying a very important position, and has an important supporting role for underwater observation operations and military activities. Underwater acoustic technology is currently the most widely used and longest underwater wireless communication technology. With the increasing demand for development, the complexity of underwater acoustic channels, underwater environment and other factors make the communication speed, communication distance and communication stability of underwater acoustic communication greatly restricted. Despite these limitations, underwater acoustic communication is still valued by professionals from various countries and industries, and has been developed and applied to some extent. This article summarizes the basic content of underwater acoustic communication technology, leads to its general structure as a communication system, the current development of underwater acoustic communication technology, and the characteristics of underwater acoustic communication systems due to the characteristics of underwater acoustic channels. This article summarizes the development and some applications of the underwater acoustic communication system and the full text and some prospects for the future underwater acoustic communication.

In November 2012, an experiment demonstrating biological mimicry method for covert underwater acoustic communication (UAC) was conducted at Lianhua Lake in Heilongjiang China. Dolphin whistles were used for synchronization while dolphin clicks were used as information carrier. The time interval between dolphin clicks conveys the information bits. Channel estimates were obtained with matching pursuit (MP) algorithm, which is useful for sparse channel estimation. Adaptive RAKE Equalization was employed at the receiver. Bit error rates were less than 10(-4) with 37 bits per second data rate in the lake trial.

In-Band Full-Duplex (IBFD) Underwater Acoustic (UWA) communication technology can transmit and receive communication signals in the same frequency band at the same time. Theoretically, its frequency utilization efficiency can reach twice of the traditional half-duplex (HD) UWA communication systems. Therefore, it holds great research significance and application value for UWA communication where available spectrum resources are seriously limited. The main challenge in the implementation of IBFD-UWA communication system is to counteract the local self- interference (SI) signal. Generally, SI cancellation (SIC) can be divided into three main aspects, namely, SI suppression in space, SIC in analog domain and digital domain. Digitally Assisted Analog SIC (DAA-SIC) is concerned as it can deal with complexSI propagation channel. However, in the implementation process of the DAA-SIC scheme, its performance will be affected by hardware conditions, such as the effective number of bits of analog to digital converter (ADC) of auxiliary acquisition link, non-linear distortion of power amplifier (PA) and so on. To deal with the influence of hardware parameters on the performance of DAA-SIC scheme, this paper proposes a new analog SIC scheme. The core of this scheme is to reconstruct the output signal of PA and reduce the non-linear distortion of power amplifier and the influence of effective number of bits of auxiliary link through Memory Polynomial (MP) model and Digital Pre- Distortion (DPD) process. The performance of the existing scheme and the new DAA-SIC scheme is verified by numerical simulation. The simulation results show the effectiveness of the proposed scheme.

In-Band Full-Duplex (IBFD) Underwater Acoustic (UWA) communication technology plays a major role in enhancing the performance of Underwater acoustic sensor networks (UWSN). Self-Interference (SI) is one of the main inherent challenges affecting the performance of IBFD UWA communication. To reconstruct the SI signal and counteract the SI effect, this is important to estimate the short range channel through which the SI signal passes. Inaccurate estimation will result in the performance degradation of IBFD UWA communication. From the perspective of engineering implementation, we consider that the UWA communication modem shell has a significant influence on the short-range SI channel, which will limit the efficiency of self-interference cancellation in the analog domain to some degree. Therefore we utilize a simplified model to simulate the influence of the structure of the IBFD UWA communication modem on the receiving end. This paper studies the effect of acoustic-shell coupling on near-end self-interference signal of IBFD UWA communication modem. Some suggestions on the design of shell structure of IBFD UWA communication modem are given.

To achieve covert underwater acoustic communication (UAC) at energies with a low probability of detection (LPD), a system should work at a low signal-to-noise ratio (SNR) with no repeating patterns in the transmitted signal. Not all covert UAC schemes can satisfy both requirements. The present paper studies a truly covert UAC scheme. The transmitted signal consists of both the initial signal (the previously recorded ambient noise of the ocean) and an information-bearing signal generated by multi-step operations on the initial signal (time-domain symmetrical division, phase rotation, time reversal, and counterpart exchange). As the two signals are transmitted together, the receiver does not require pre-knowledge of the initial signal for demodulation. Hence, each symbol can be assigned a different initial signal, thereby avoiding repeating patterns in the signal and improving the covertness of the system. After optimizing the rotation phase condition, the performance of the UAC system was analyzed in numerical simulations. The scheme achieved covert UAC and exhibited good performance at low SNR.