Shipborne Underwater Acoustic Communication System and Sea Trials with Submersible Shenhai Yongshi

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 lowcomplexity 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.

Fractional Fourier domain hopped communication method based on chirp modulation for underwater acoustic channels

To improve the data rate of underwater acoustic frequency-hopped communications, frequency hopping is applied to different orders of fractional Fourier domain (FrFD), to enable non-intrusive, bandwidth-limited acoustic communications. An FrFD frequency-hopped communication method based on chirp modulation, namely multiple chirp shift keying-FrFD hopping (MCSK-FrFDH), is proposed for underwater acoustic channels. Validated by both simulations and experimental results, this method can reach a bandwidth efficiency twice more than conventional frequency-hopped methods with the same data rate and anti-multipath capability, suggesting that the proposed method achieves a better performance than the traditional frequency hopped communication in underwater acoustic communication channels. Results also show that in practical scenarios, the MCSK-FrFDH system with longer symbol length performs better at the low signal-to-noise ratio (SNR), while the system with larger frequency sweeping range performs better at a high SNR.

Frequency Diversity for OFDM Mobile Communication via Underwater Acoustic Channels

The major constraint on the performance of orthogonal frequency division multiplexing （OFDM） based underwater acoustic （UWA） communication is to keep subcarriers orthogonal. In this paper, Doppler estimation and the respective compensation technique along with various diversity techniques were deliberated for OFDM-based systems best suited for underwater wireless information exchange. In practice, for mobile communication, adjustment and tuning of transducers in order to get spatial diversity is extremely difficult. Considering the relatively low coherence bandwidth in UWA, the frequency diversity design with the Doppler compensation function was elaborated here. The outfield experiments of mobile underwater acoustic communication （UWAC） based on OFDM were carried out with 0.17 bit/（s-Hz） spectral efficiency. The validity and the dependability of the scheme were also analyzed.

Soft Direct-Adaptation Based Bidirectional Turbo Equalization for MIMO Underwater Acoustic Communications

This paper proposes a soft direct-adaptation based bidirectional turbo equalizer for multiple-input multiple-output underwater acoustic communication systems. Soft, rather than hard, direct-adaptation based equalizer combined with the fast self-optimized least mean square algorithm is employed to achieve a faster convergence rate, and the second-order phase-locked loop is embedded into the equalizer to track the time-varying channel. Meanwhile, by utilizing a weighted linear combining scheme, the conventional soft direct-adaptation based equalizer is combined with the time-reversed soft direct-adaptation based equalizer to exploit bidirectional diversity and mitigate error propagation. Both the simulation and experimental results demonstrate that the soft direct-adaptation based bidirectional turbo equalizer outperforms the single-direction soft direct-adaptation based turbo equalizer, and achieves a faster convergence rate than the hard direct-adaptation based bidirectional turbo equalizer.

Feedback strategies for iterative channel estimation in OFDM underwater acoustic communications

The extremely limited bandwidth in underwater acoustic communication makes channel estimation using fewer pilot symbols more challenging. Iterative channel estimation( ICE) can be used to refine channel estimation with limited number of pilots,by coupling the channel estimator with channel decoder. In this paper,various feedback strategies in ICE are discussed. The performance of a decision feedback based on the cost function is improved by modifying the design and another four feedback strategies are summarized,including hard/soft decision feedback and their threshold-controlled versions. Simulation results show that ICE can achieve impressive gains over the non-iterative receiver and the gains are more significant with fewer pilots. Furthermore,soft decision feedback outperforms hard decision feedback; while the feedback based on the cost function and soft decision feedback have quite close performance.

MIMO Underwater Acoustic Communication in Shallow Water with Ice Cover

Although multiple-input multiple-output(MIMO) underwater acoustic(UWA) communication has been intensively investigated in the past years, existing works mainly focus on open-water environment. There is no work reporting MIMO acoustic communication in under-ice environment. This paper presents results from a recent MIMO acoustic communication experiment which was conducted in Bohai Gulf during winter. In this experiment, high frequency MIMO signals centered at 10 kHz were transmitted from a two-element source array to a four-element vertical receiving array at 1 km range. According to the received signal of different array elements, MIMO acoustic communication in under-ice environment suffers less effect from co-channel interference compared with that in open-water environment. In this paper, time reversal followed by a single channel decision feedback equalizer is used to process the experimental data. It is demonstrated that this simple receiver is capable of realizing robust performance using fewer hydrophones(i.e. 2) without the explicit use of complex co-channel interference cancelation algorithms, such as parallel interference cancelation or serial interference cancelation.

Power optimization algorithm for OFDM underwater acoustic communication using adaptive channel estimation

An adaptive channel estimation algorithm for the channel length is proposed to construct a channel estimation model suitable for orthogonal frequency division multiplexing(OFDM)underwater acoustic communication signals for the dependence of traditional channel estimation algorithms on channel length information.This algorithm can be adopted to evaluate channel estimation quality in real time and to adaptively adjust the channel length of the channel estimation algorithm according to the evaluation result,which satisfies the need of accurate estimation of unknown underwater acoustic channels and communication application;based on the study on the relationship between the OFDM communication bit error rate and the subcarrier signal to noise ratio,a self-adjusting optimization scheme for OFDM subcarrier transmitting power is proposed,which realizes underwater communication with the low bit error rate through higher energy efficiency.The validity of the research content is verified through simulation and field experiments.

LDPC Coded DQPSK Modulation with Iterative Detection Method for Underwater Acoustic Communications

Aimed at the conventional serial communication system fails when the channel was frequency-selective,long delay and multi-paths effect,resulting in inter-symbol-interference( ISI),an iterative system for underwater acoustic communication( UWAC) was proposed,which employed iterative differential detection and spread spectrum technique. The proposed scheme adopts π /4-DQPSK soft demodulation to overcome the difficulty of phase ambiguity. For suppression of ISI in UWAC,joint demodulation and decoding iteration was used. The communication scheme has the characteristic of long communication range at low SNR,and the maximum range is up to 100 km. The theoretic analysis and simulation results show that the proposed scheme outperforms traditional serial one. When SNR is- 6 dB,after four joint iterations,compared with serial scheme,BER can decrease from 10- 2to 10- 5.

Null subcarriers based Doppler scale estimation with polynomial interpolation for multicarrier communication over ultrawideband underwater acoustic channels

This paper addresses the extremal problem of the null subcarriers based Doppler scale estimation in underwater acoustic （UWA） orthogonal frequency division multiplexing （OFDM） communication. The cost function constructed of the total energy of null subcarriers through discrete Fourier transform （DFT） is proposed. The frequencies of null subcarriers are identified from non-uniform Doppler shift at each tentative scaling factor. Then it is proved that the cost function can be fitted as a quadratic polynomial near the global minimum. An accurate Doppler scale estimation is achieved by the location of the global scarifying precision and increasing the computation minimum through polynomial interpolation, without complexity. A shallow water experiment is conducted to demonstrate the performance of the proposed method. Excellent performance results are obtained in ultrawideband UWA channels with a relative bandwidth of 67%, when the transmitter and the receiver are moving at a relative speed of 5 kn, which validates the proposed method.

Novel joint encoding/decoding algorithms of fountain codes for underwater acoustic communication

Fountain codes are considered to be a promising coding technique in underwater acoustic communication（UAC） which is challenged with the unique propagation features of the underwater acoustic channel and the harsh marine environment. And Luby transform（LT） codes are the first codes fully realizing the digital fountain concept. However, in conventional LT encoding/decoding algorithms, due to the imperfect coverage（IC） of input symbols and short cycles in the generator matrix, stopping sets would occur and terminate the decoding. Thus, the recovery probability is reduced,high coding overhead is required and decoding delay is increased.These issues would be disadvantages while applying LT codes in underwater acoustic communication. Aimed at solving those issues, novel encoding/decoding algorithms are proposed. First,a doping and non-uniform selecting（DNS） encoding algorithm is proposed to solve the IC and the generation of short cycles problems. And this can reduce the probability of stopping sets occur during decoding. Second, a hybrid on the fly Gaussian elimination and belief propagation（OFG-BP） decoding algorithm is designed to reduce the decoding delay and efficiently utilize the information of stopping sets. Comparisons via Monte Carlo simulation confirm that the proposed schemes could achieve better overall decoding performances in comparison with conventional schemes.

Modeling and Channel Estimation for Piezo-Acoustic Backscatter Assisted Underwater Acoustic Communications

Relying on direct and converse piezoelectric effects,piezo-acoustic backscatter(PAB)technology reflects ambient acoustic signals to enable underwater backscatter communications at near-zero power,which was first realized through a prototype.In this paper,we propose a mathematical model of the PAB assisted underwater acoustic(UWA)communication,and address the sparse channel estimation problem.First,we present a five-stage backscatter process to derive the backscatter coefficient,and propose the channel model for the shallow-water communications.Then,we formulate the shallow-water acoustic channel estimation problem as a sparse vector recovery one according to the compressed sensing theory,and leverage the orthogonal matching pursuit(OMP)algorithm to obtain the channel estimator.Finally,simulation results are provided to corroborate our proposed studies.

Collision Classification MAC Protocol for Underwater Acoustic Communication Networks Using Directional Antennas

Traditional underwater acoustic communication networks(UACNs)generally use omnidirectional transmission technology that causes a large number of data-packet collisions,thus resulting in low network throughput and high end-to-end delays.Compared with omnidirectional transmission technology,directional technology only sends and receives data packets in a specified direction.This can significantly reduce the probability of collisions and improve network performance.However,it also causes a deafness problem,which occurs when the sending node sends a data packet to the receiving node but the receiving node is unable to reply to the sender,because its antenna beam is closed.To resolve this issue,this study proposes a collision classification media access control(CC-MAC)protocol for UACNs.With this protocol,the underwater acoustic channel is divided into two subchannels,and the nodes transmit corresponding data types on them.The sending node can estimate the current status of the receiving node(i.e.,no collision,normal collision,deafness)according to the type of the data packet received and the sub-channel it arrived on,and it can choose correct options to improve network efficiency.Finally,we verify the performance of CC-MAC via simulations,showing that the protocol achieved higher network throughput and lower end-toend delays.

Space-division multiple access for CDMA multiuser underwater acoustic communications

Time reversal mirror （TRM） can use the physical characteristics of the underwater acoustic （UWA） channel to focus on the desired user in multi-user UWA communication. The active average sound intensity （AASI） detector can estimate all azimuths of users with the same frequency band at the same time in order to achieve directional communication by vector combination. Space-division multiple access （SDMA） based on TRM combined with the AASI detector is proposed in this paper, which can make the capacity of the code division multiple access （CDMA） UWA system significantly increase. The simulation and lake test results show that the 7-user UWA multi-user system can achieve low bit error communication.

Low Complexity Adaptive Equalizers for Underwater Acoustic Communications

Interference signals due to scattering from surface and reflecting from bottom is one of the most important problems of reliable communications in shallow water channels. To solve this problem, one of the best suggested ways is to use adaptive equalizers. Convergence rate and misadjustment error in adaptive algorithms play important roles in adaptive equalizer performance. In this paper, affine projection algorithm （APA）, selective regressor APA（SR-APA）, family of selective partial update （SPU） algorithms, family of set-membership （SM） algorithms and selective partial update selective regressor APA （SPU-SR-APA） are compared with conventional algorithms such as the least mean square （LMS） in underwater acoustic communications. We apply experimental data from the Strait of Hormuz for demonstrating the efficiency of the proposed methods over shallow water channel. We observe that the values of the steady-state mean square error （MSE） of SR-APA, SPU-APA0 SPU-normalized least mean square （SPU-NLMS）, SPU-SR-APA0 SM-APA and SM-NLMS algorithms decrease in comparison with the LMS algorithm. Also these algorithms have better convergence rates than LMS type algorithm.

Design of an Acoustic Communication System Based on FHMA for Multiple Underwater Vehicles

This paper designs an underwater acoustic communication system based on tamed spread spectrum and Frequency Hopping Multiple Access (FHMA) for multiple underwater vehicles. In this system, multiple underwater vehicles can communicate with the console on the water surface simultaneously successfully. The communication system is com-posed of tamed spread spectrum modulation and demodulation, frequency hopping modulation and demodulation with synchronization function, 4FSK modulation and demodulation and Rake Receiver. In order to make the system more feasible, underwater channel and the effect of user number on Bit Error Ratio (BER) are also taken into account. The simulation results indicate that it is feasible to use this system to accomplish underwater communication reliably for multiple users due to the low BER.

Application of OFDM-CDMA in Multi-user Underwater Acoustic Communication Based on Time Reversal Mirror

In the paper, a novel method, which exploits both the Orthogonal Frequency Division Multiplexing (OFDM)-Code Division Multiple Access (CDMA) technique and the time reversal mirror technique, is investigated to realize multi-user underwater acoustic communication. By taking advantages of these two techniques, the proposed method has the capability of effectively suppressing multi-path interference, reducing the error rate, enhancing the processing gain and improving user capacity. Simulations verify the correctness and effectiveness of the proposed method.

Pilot-Free End-to-End Underwater Acoustic Communication System Based on Autoencoder

The long delay spreads and significant Doppler effects of underwater acoustic(UWA)channels make the design of the UWA communication system more challenging.In this paper,we propose a learning-based end-to-end framework for UWA communications,leveraging a double feature extraction network(DFEN)for data preprocessing.The DFEN consists of an attentionbased module and a mixer-based module for channel feature extraction and data feature extraction,respectively.Considering the diverse nature of UWA channels,we propose a stack-network with a two-step training strategy to enhance generalization.By avoiding the use of pilot information,the proposed network can learn data mapping that is robust to UWA channels.Evaluation results show that our proposed algorithm outperforms the baselines by at least 2 dB under bit error rate(BER)10^(−2)on the simulation channel,and surpasses the compared neural network by at least 5 dB under BER 5×10^(−2)on the experiment channels.

Self-adjusting decision feedback equalizer for variational underwater acoustic channel environments

Aimed at the abominable influences to blind equaliza-tion algorithms caused by complex time-space variability existing in underwater acoustic channels, a new self-adjusting decision feedback equalization （DFE） algorithm adapting to different under-water acoustic channel environments is proposed by changing its central tap position. Besides, this new algorithm behaves faster convergence speed based on the analysis of equalizers’ working rules, which is more suitable to implement communications in dif-ferent unknown channels. Corresponding results and conclusions are validated by simulations and spot experiments.

Immune particle swarm optimization of linear frequency modulation in acoustic communication

With the exploration of the ocean, underwater acoustic communication has attracted more and more attention in recent years. The underwater acoustic channel is considered to be one of the most complicated channels because it suffers from more serious multipath effect, fewer available bandwidths and quite complex noise. Since the signals experience a serious distortion after being transmitted through the underwater acoustic channel, the underwater acoustic communication experiences a high bit error rate （BER）. To solve this problem, carrier waveform inter- displacement （CWlD） modulation is proposed. It has been proved that CWlD modulation is an effective method to decrease BER. The linear frequency modulation （LFM） carrier-waves are used in CWlD modulation. The performance of the communication using CWID modulation is sensitive to the change of the frequency band of LFM carrier-waves. The immune particle swarm optimization （IPSO） is introduced to search for the optimal frequency band of the LFM carrier-waves, due to its excellent performance in solving complicated optimization problems. The multi-objective and multi- peak optimization nature of the IPSO gives a suitable description of the relationship between the upper band and the lower band of the LFM carrier-waves. Simulations verify the improved perfor- mance and effectiveness of the optimization method.

A Survey on Underwater Acoustic Sensor Networks: Perspectives on Protocol Design for Signaling, MAC and Routing

Underwater acoustic sensor networks (UASNs) are often used for environmental and industrial sensing in undersea/ocean space, therefore, these networks are also named underwater wireless sensor networks (UWSNs). Underwater sensor networks are different from other sensor networks due to the acoustic channel used in their physical layer, thus we should discuss about the specific features of these underwater networks such as acoustic channel modeling and protocol design for different layers of open system interconnection (OSI) model. Each node of these networks as a sensor needs to exchange data with other nodes;however, complexity of the acoustic channel makes some challenges in practice, especially when we are designing the network protocols. Therefore based on the mentioned cases, we are going to review general issues of the design of an UASN in this paper. In this regard, we firstly describe the network architecture for a typical 3D UASN, then we review the characteristics of the acoustic channel and the corresponding challenges of it and finally, we discuss about the different layers e.g. MAC protocols, routing protocols, and signal processing for the application layer of UASNs.