Covert Underwater Research Articles

High covertness camouflage covert underwater acoustic communication based on masking technique

In marine military operations, it is very important to ensure the safe transmission of underwater information, so it is urgent to improve the covertness of underwater communication. In order to meet the requirement of high covertness in underwater acoustic communication, according to the natural characteristics of superimposed interference of marine ambient noise, a high covertness camouflage covert underwater acoustic communication (CUWAC) method based on masking technique using ship-radiated noise as communication carrier is proposed in this paper. This method leverages innovative audio digital technology to embed information into the DCT domain of ship-radiated noise, creating a disguised signal that is hard to detect, thereby significantly enhancing stealth without compromising communication effectiveness. In order to improve the demodulation performance of this camouflage CUWAC, this paper proposes a variable step size p-norm-like LMP sparse adaptive filtering algorithm to offset the interference of the masking signal at the receiving end. This adaptive filtering algorithm is based on the improved modified Gaussian function to construct step size. Furthermore, it introduces the error signal autocorrelation function to adjust the step size and zero-attracting term dynamically. The proposed adaptive filtering algorithm obtains excellent anti-noise performance. Through a series of simulation experiments, this paper not only validates the feasibility of the CUWAC method but also demonstrates its stability and reliability under various marine environmental conditions, providing robust theoretical support for practical applications, proving its efficiency and reliability in complex marine environments.

Recognition method for the bionic camouflage cetacean whistle modulated by CPMFSK signals

Bionic covert camouflage underwater acoustic communication has drawn great interests in recent years as a promising way for covert underwater acoustic communication. However, there is little relevant literature to research the recognition of bionic camouflage communication signals (BCCS). Concentrating on the countermeasures for the BCCS, this paper proposes a recognition method for a bionic cetacean whistle signal modulated by continuous phase multiple frequency shift keying (CPMFSK) signal. Based on the features that CPMFSK modulated whistle signals carry the information by imitating the tremble phenomenon of the real whistles, a method is designed to extract a tremble signal which vibrates among the time–frequency contour (TFC) of cetacean whistles. As the tremble signal of the real cetacean whistle and bionic whistle has obvious time, and frequency distinction, focusing on these distinctions, characteristics like normalized power spectrum mean, wiener entropy, etc. are extracted as the characteristic vectors for SVM classification to achieve recognition. Simulations demonstrate the accuracy of the recognition method in different conditions, including modulation parameters, SNR, and underwater acoustic channels.

Recognition method for the bionic camouflage click communication trains modulated by time delay difference.

Bionic camouflage covert underwater acoustic communication has recently attracted great attention. However, we have not found relevant methods or literature to recognize these bionic camouflage communication signals (BCCSs) in the area of anti-reconnaissance. Focused on recognizing the BCCSs, this article proposes a recognition method based on the statistics of inter-click intervals to recognize the camouflaged click communication train (CCCT), which is modulated by time delay difference (TDD). We first analyze the characteristics of TDD distributions of CCCT and real click train (RCT). According to the coding principle, the TDDs of CCCTs present a ladder-like distribution with a fixed time step, and the TDDs are equal to the integral multiple of the fixed time step. On the contrary, the TDDs of RCTs are approximately random distribution within a certain time range. Therefore, based on the different TDD distributions, this article classifies CCCTs and RCTs by utilizing the statistical property of TDD distributions. To measure the TDDs of diverse cetacean clicks accurately, a new click location scheme based on the dynamic window energy ratio is proposed. Next, based on the statistics of TDD distribution, the influences of the TDDs that are caused by multipath interferences are eliminated by iteration. Simulations demonstrate the accuracy of the recognition method under different conditions.

Bionic Covert Underwater Acoustic Communication Based on Time-Frequency Contour of Bottlenose Dolphin Whistle.

In order to meet the requirements of communication security and concealment, as well as to protect marine life, bionic covert communication has become a hot research topic for underwater acoustic communication (UAC). In this paper, we propose a bionic covert UAC (BC-UAC) method based on the time–frequency contour (TFC) of the bottlenose dolphin whistle, which can overcome the safety problem of traditional low signal–noise ratio (SNR) covert communication and make the detected communication signal be excluded as marine biological noise. In the proposed BC-UAC method, the TFC of the bottlenose dolphin whistle is segmented to improve the transmission rate. Two BC-UAC schemes based on the segmented TFC of the whistle, the BC-UAC scheme using the whistle signal with time-delay (BC-UAC-TD) and the BC-UAC scheme using the whistle signal with frequency-shift (BC-UAC-FS), are addressed. The original whistle signal is used as a synchronization signal. Moreover, the virtual time reversal mirror (VTRM) technique is adopted to equalize the channel for mitigating the multipath effect. The performance of the proposed BC-UAC method, in terms of the Pearson correlation coefficient (PCC) and bit error rate (BER), is evaluated under simulated and measured underwater channels. Numerical results show that the proposed BC-UAC method performs well on covertness and reliability. Furthermore, the covertness of the bionic modulated signal in BC-UAC-TD is better than that of BC-UAC-FS, although the reliability of BC-UAC-FS is better than that of BC-UAC-TD.

Non-coherent turbo coded frequency shift keying for reliable covert underwater acoustic communications

This paper presented efficient receiver structure of non-coherent turbo coded FSK signals for maintaining covertness and performance in underwater communication. In aspect to covertness, direct sequence spread spectrum method, which used to hide the transmitted signal by transmitting it at low power and, thus, making it difficult for an unintended listener to detect the signal in the presence of background noise. In aspect to performance improvement in the underwater acoustic environment with multipath and Doppler spreading, non-coherent FSK which is not required phase information, weighted multiband technology, and turbo equalization model with rate of 1/3 were suggested. In order to analyze the performance by applying four bands and spreading chips with 8 and 32, the experiment was conducted on a lake by moving in the range of 300m to 500m. Through the experimental results, we confirmed the performance was improved as increasing number of bands, turbo iterations, and spreading chips. Furthermore, weighted multiband technology which allocates lower weight to inferior bands based on preamble error rates were applied for some failed packets, all packets were decoded successfully. [This research was funded by Agency for Defense Development, South Korea, Grant No. UD200010DD.]

Bionic covert underwater communication focusing on the overlapping of whistles and clicks generated by different cetacean individuals

Bionic camouflage covert underwater acoustic communication technology is playing an increasingly important role in the covert transmission of underwater information. Most cetaceans are group-living animals, and multiple cetaceans often appear in a specific ocean area at the same time. As a result, different cetacean individuals may generate whistles and clicks simultaneously, and the overlapping of whistles and clicks often appears in the received sound signals. Focusing on the overlapping characteristic of cetacean calls, this paper designs a bionic camouflage covert underwater acoustic communication method to improve the covertness and communication rate. The combined encoding through whistles and clicks overlapping of different cetacean individuals makes the constructed communication signals more in line with the characteristics of original cetacean calls, which improves the communication covertness. The pulse-to-pulse correlation and time delay difference encoding based on whistles and clicks are combined to improve the communication rate. Furthermore, the whistle signal processing algorithm based on time–frequency spectrogram image and morphology processing is utilized to separate the whistles and clicks in the received signals. And the whistle and click sequence are decoded, respectively. Finally, the bit error rates of the combined communication sequence, whistle sequence and click sequence are compared by conducting simulations. And the effectiveness, reliability and covertness of the proposed covert communication method are demonstrated through the simulations.

A frequency hopping pattern inspired bionic underwater acoustic communication

The covert underwater acoustic communication realized from the perspective of bionics draws an increasing interest in recent researches. Inspired by the frequency hopping spectrogram pattern, a bionic underwater acoustic communication method is proposed by using cetacean whistle as communication carrier. The basic idea is to generate synthetic whistle with different spectrogram to transmit the information. The whistle is first modeled after extracting the whistle contour from the noise-polluted recordings. Then a synthetic whistle with a special spectrogram pattern which mimics the whistle spectral characteristic is generated to modulate the information. The bionic communication frame is the combination of the original whistle and the synthetic whistles which are used for synchronization and information modulation respectively. After synchronization, the each received symbol is extracted and correlated with the locally generated signals. The information demodulation is conducted by the correlation peak decision. Finally the bionic effect of communication signals is analyzed through time-frequency correlation coefficient and Mel frequency cepstrum coefficient. Simulation and sea trial verified the feasibility of the proposed method. Less than 10−3 uncoded bit error rate is achieved with the correlation coefficient over 0.9 at a distance of 8.5km in the sea trial.

Optimizing the number of relays for energy efficient multi-hop covert underwater acoustic cooperative networks

Multi-hop underwater acoustic (UWA) cooperative networks can effectively reduce the energy consumption of the system, yet the covertness of communication is also greatly reduced since a spatial diversity gain can be obtained by both the intended receiver and the intercept detector. In this paper, we investigate how to select the optimal number of relays for multi-hop UWA cooperative networks, by considering both the low probability of detection (PD) and energy consumption. We derive the relationship between PD and the number of relays and then analyze the relationship between energy consumption and the number of relays for the system. To address the joint PD and energy consumption optimal problem, we define the energy consumption rate for multi-hop UWA systems by normalizing the energy consumed in single-hop UWA systems at the same source-to-destination distance. Then, we derive the range number of relays under the given PD and energy consumption rate. Moreover, we construct the objective function and derive the optimal number of relays by jointly addressing PD and the energy consumption rate. Finally, we simulate and analyze the impact of several parameters on PD and energy consumption, including the signal-to-noise ratio, working distance, position angle of interceptor and data length. The simulation results show that the number of relays for multi-hop covert UWA cooperative networks can take the value between 2 and 6 under a given condition, which can meet the covert communication requirement with a PD < 0.5, while the system energy consumption is relatively low.

Covert underwater communication based on combined encoding of diverse time-frequency characteristics of sperm whale clicks

Bio-inspired camouflage covert underwater acoustic communication has been studied widely recently. In order to improve the camouflage ability and communication rate, a bio-inspired covert underwater acoustic communication method based on combined encoding of diverse time–frequency characteristics of sperm whale clicks is proposed. The characteristics of time–frequency contour shapes and frequency-band distributions are classified and counted, and the communication codes are constructed based on the two characteristics. According to the inter-click interval characteristic of sperm whale click pulse trains, the bionic communication sequences are constructed through differential time delay difference encoding between different communication codes. Based on the proposed combined encoding method of diverse time–frequency characteristics, the constructed communication sequence contains a large number of different modulated clicks. It conforms to the characteristic that there exists a large number of and a variety of clicks in the true click pulse train, which improves the camouflage ability. The true sperm whale click pulses are inserted into the communication sequences as camouflage pulses to further improve the camouflage ability. Furthermore, the communication rate is improved due to using multiple characteristics to represent different encoding information in the proposed method. Simulations are implemented to demonstrate the effectiveness, reliability and covertness of the proposed covert communication method.

Time-frequency modulation based mimicking dolphin whistle for covert underwater acoustic communication

For covert underwater communications, conventional bio-mimicking methods have been developed. Signal shapes of these methods are similar to real whistles, but not the same since they divide a dolphin whistle into symbols, which are modulated by conventional digital modulations. This paper proposes a dolphin whistle mimicking modulation method that transmits different consecutive whistles with the same frequency contour of the real dolphin whistles and different time-frequency locations. This paper also develops a demodulator for decoding the proposed mimicking whistles. The mimicking performance of the proposed time-frequency shift keying was compared with those of the conventional methods by using correlation coefficients, and the proposed scheme showed the better mimicking performance than the conventional chirp spread spectrum and continuously varying-carrier frequency modulation. Using computer simulations and practical ocean experiments, the bit error rate performance of the proposed demodulator was better than that of the cross-correlation method.

Mimicking Ship-Radiated Noise With Chaos Signal for Covert Underwater Acoustic Communication

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.

Bionic Morse Coding Mimicking Humpback Whale Song for Covert Underwater Communication

A novel method of bionic Morse coding mimicking humpback whale vocal is presented for covert underwater acoustic communication. The complex humpback whale song is translated as bionic Morse codes based on information entropy. The communication signal is made akin to the natural singing of male humpback whales. The intruder can detect the signal but will not be able to recognize the communication signal due to unified resemblance with the natural sound. This novel technique gives an excellent low probability of recognition characteristics. A flawless stealthy underwater acoustic communication has been established which has negligible chances of deciphered with high imperceptibility. Standard mimicry Morse codes have been developed for the characters of the English language and compared with Morse coding. Covert information of one character per second can be watermarked with perfect stealth and clandestine communication. This novel concept has been verified at transmission distance of five km and less than 10−3 Bit Error Rate (BER) is achieved at Signal to Noise Ratio (SNR) down to negative seven dB. Zero BER is attained by estimating the channel by a matching pursuit algorithm and equalizing the errors by virtual time reversal mirror technique.

Mimicking dolphin whistles with continuously varying carrier frequency modulation for covert underwater acoustic communication

This paper proposes a covert underwater acoustic communication scheme to mimic dolphin whistles by continuously varying the carrier frequency and by using antipodal symbol modulation. The continuously varying carrier frequency precisely mimics dolphin whistles, while the conventional method utilizes discrete carrier-frequencies. The antipodal symbol modulation increases bit error rate (BER) performance by a larger Euclidean distance between symbols than conventional chirp spread spectrum (CSS). BER performance was tested by computer simulations and practical ocean experiments, and the degree of mimic (DoM) was evaluated by correlation coefficients and the mean opinion score (MOS) test. BER of the proposed method attains about a 3 dB SNR gain compared with that of conventional CSS, and the DoM of the proposed scheme demonstrates a larger correlation coefficient and a 39% greater MOS score than that of conventional CSS.

Symmetry Oriented Covert Acoustic Communication by Mimicking Humpback Whale Song

To meet the increasing demand of covert underwater acoustic communication, biologically inspired mimicry communication watermarking the data in symmetrical humpback whale song is presented. Mimicry is an entirely different approach from traditional covert communication where data are transmitted by spreading the waveform at a low signal to noise ratio. In this innovative technique, the carrier signal is imitated symmetrical to the ocean background noise, which can be shipping noise, anthropological noise, or the vocals emitted by sea animals. The eavesdropper can detect the communication signal, but will assume it to be real ocean noise due to its symmetry. It excludes the mimicked signal from recognition, which makes the communication covert. In this research, we watermarked the covert information in humpback whale song using discrete cosine transform in the frequency domain. The mimicked symmetrical signal provided excellent imperceptibility with the real song and an outstanding camouflage effect was calculated. We validated the novel concept by simulation and underwater tank experiment. 10−4 BER was achieved in the underwater tank experiment, which was diminished to zero error by using matching pursuit estimation and virtual time reversal equalization. This novel bionic covert communication technique is feasible for clandestine underwater acoustic communication in the presence of an eavesdropper with better imperceptibility.

Covert Underwater Acoustic Communication System Using Parametric Array

AbstractA novel and efficient covert underwater acoustic (UWA) communication scheme using an acoustic parametric array and orthogonal frequency division multiplexing (OFDM) system is presented. The proposed system is robust and can easily be implemented in the ocean environment. The system is also very useful in military applications where the secrecy of transmission signal and location of the transmitter are extremely important. The paper exploits the difference frequency generated by the acoustic parametric array due to the nonlinear behavior of an underwater medium. Besides the lightness and compactness, the parametric array also possesses the advantage of being low-frequency, broadband, highly directive, and narrow beam with no side lobes. The narrow beam width also helps to secure the data from a spatial point of view. Experiments have been performed in a water tank, and the results are presented to show the effectiveness of the proposed scheme.

2P6-1 Time-Delay based Mimicking Dolphin Whistle for Covert Underwater Communication

2P6-1 Time-Delay based Mimicking Dolphin Whistle for Covert Underwater Communication

Chirp-Based FHSS Receiver with Recursive Symbol Synchronization for Underwater Acoustic Communication.

In this paper, we propose a covert underwater acoustic communication method that is robust to fading using a chirp signal combined with a frequency-hopping spread spectrum scheme. A fractional Fourier transform, which estimates the slope of the signal frequency variation, is applied to the receiver to enable a robust and reliable symbol estimation with respect to the frequency and irregular phase variations. In addition, since the recursive symbol synchronization can be implemented using a chirp signal, compression and expansion effects due to the Doppler shift can be mitigated. Simulation and lake trials were performed to verify the performance of the proposed method. The simulation was performed by two different methods.

Study on the structure of an efficient receiver for multi-sensors using direct sequence spread spectrum

In underwater acoustic communication, there has been a lot of interest in detection and acquisition of information about multiple sensors as well as detection of information on a single sensor. However, research on multiple sensors access is lacking. In order to acquire information from each sensor in multiple sensors access underwater communication, we mainly use the spreading method which can obtain information of each sensor individually at the receiving side by multiplying the code with different orthogonal components in the same frequency band. In the case of multiple sensors, successive interference cancellation is used to remove interference from other sensor and apply the RAKE method. However, in a multiple sensor communication environment, if the channel information is not perfect, the performance is reduced. In this paper, we propose a transceiver structure for multiple sensor covert acoustic communication using the spread spectrum method and RAKE method, and propose and effective receiver structure to overcome drawbacks. An actual underwater experiment was conducted to analyze the performance of the covert underwater acoustic communication. The result of experiment indicated that the transmission and receive structure proposed through the actual underwater experiment was well-suited to the communication model for covert multiple accesses. In underwater acoustic communication, there has been a lot of interest in detection and acquisition of information about multiple sensors as well as detection of information on a single sensor. However, research on multiple sensors access is lacking. In order to acquire information from each sensor in multiple sensors access underwater communication, we mainly use the spreading method which can obtain information of each sensor individually at the receiving side by multiplying the code with different orthogonal components in the same frequency band. In the case of multiple sensors, successive interference cancellation is used to remove interference from other sensor and apply the RAKE method. However, in a multiple sensor communication environment, if the channel information is not perfect, the performance is reduced. In this paper, we propose a transceiver structure for multiple sensor covert acoustic communication using the spread spectrum method and RAKE method, and propose and effective receiver struc...

Covert underwater communication by camouflaging sea piling sounds

To achieve covert underwater acoustic communication, a unique technique of mimicking genuine sea-piling sounds as carrier to convey information is presented. It differs from traditional low signal to noise ratio covert underwater acoustic communication. The communication signal can be detected, but it could not be recognized from the perspective of camouflage to realize covert acoustic communication in the sea. According to the features of sea-piling sounds, a quasi-constant period based pulse position modulation scheme is proposed. First we put each piling sound with identical time interval in time domain and it is moved off the original position with a dedicated time delay which is calculated by modulated information. To make the communication signal akin to the piling sounds, the synchronization header is designed by time hopping pulse position modulation scheme with real piling sounds. At the receiver, basis pursuit algorithm under the theory of compressive sensing is used for channel estimation, and passive phase conjugation technique is used for channel equalization to eliminate the impacts of multipath channel. The feasibility and efficiency of the proposed method are verified by simulation and pool experiment.

Study on the Structure of an Efficient Receiver for Covert Underwater Communication Using Direct Sequence Spread Spectrum

This paper proposes an underwater communication receive model based on the spread spectrum technique in order to provide the characteristic of a low probability of interception. To do this, turbo equalization techniques employing Bahl-Cocke-Jelinek-Raviv (BCJR) decoding to improve performance through repetition are applied to offer excellent performance even at a low signal to noise ratio (SNR) of transmitted signals due to the spread spectrum technique. A turbo equalization model based on RAKE which increase signal power by summing the received signal through the multipath is proposed to compensate distorted data due to multipath channel and the performance improvements were proven by applying the threshold and weighted coefficient in the RAKE receiver model. The model was applied to covert underwater communication in a multi-sensor environment, and the efficiency of the proposed method was proven through underwater experiments.