Virtual Time Reversal Mirror Research Articles

Design of Deep Learning Acoustic Sonar Receiver with Temporal/ Spatial Underwater Channel Feature Extraction Capability

In this study, deep learning network technology is employed to solve the problem of rapid changes in underwater channels. The modulation techniques employed are frequency-shift keying (FSK) and the BELLHOP module of MATLAB; they are used to create water with multipath, Doppler shifts, and additive Gaussian white noise such that underwater acoustic receiving signals simulating the actual ocean environment can be obtained. The southwest coastal area of Taiwan is simulated in the manuscript. The results reveal that optimizing the environment by using the virtual time reversal mirror (VTRM) technique can generally mitigate the bit error rates (BERs) of the deep learning network’s model receiver and traditional demodulation receiver. Lastly, seven deep learning networks are deployed to demodulate the FSK signals, and these approaches are compared with traditional demodulation techniques to determine the deep learning network techniques that are most suitable for marine environments.

Virtual Space-Time DiversityTurbo Equalization Using Cluster Sparse Proportional Recursive Least Squares Algorithm for Underwater Acoustic Communications

The oceanic positioning, navigation and timing (PNT) network requires high-quality underwater acoustic message transmission. Turbo equalization technology has exhibited superior performance for underwater acoustic (UWA) communications compared with conventional channel equalizers. To overcome the performance reduction caused by severe doubly selective UWA channels, the virtual space-time diversity soft direct-adaptation turbo equalization is proposed for UWA communications. The proposed scheme improves the ability of the typical turbo equalizer to deal with both Doppler and multipath effects for time varying channels. We utilize a fractionally spaced soft interference cancellation equalizer (FS-SE) instead of a hard decision to constitute the soft-input soft-output (SISO) equalizer. Combined with another virtual time-reversal mirror equalizer component, we can obtain virtual space and time diversity with only a single receiving transducer and mitigate the error propagation phenomenon of the feedback filter. To satisfy the sparse UWA channel, the ℓp,q-PRLS algorithm is applied to adaptive updates for FS-SE. In the proposed scheme, an adjustable interpolator and digital phase-locked loop are embedded into the equalizer to overcome the residual Doppler frequency shift and recover the timing distortion. Results of simulations and field lake trial show that the proposed scheme achieves better performance than existing ones under the same equalizer order.

Vertical array signal recovery method based on normalized virtual time reversal mirror

Focusing on the vertical array source signal recovery method, aiming at the estimation deviation caused by environment mismatch in the engineering application of virtual time reversal mirror technology, this paper proposes the normalized virtual time reversal technology through theoretical derivation and analyzes the performance of the virtual time reversal technology under the change of environment and signal-to-noise ratio through simulation. Finally, the practicability of the normalized virtual time reversal method and the effectiveness of model correction are verified through experimental data. The data processing results show that the source level estimation error of vertical array based on normalized virtual time reversal algorithm is less than 5.2dB. In the experimental environment of this paper, the correlation of signal spectral structure recovered by this method is about 5% higher than that by the traditional method.

Recursive Decoding Based Multi-channel M-ary Spread Spectrum Underwater Acoustic Communication

The spread spectrum communication method has strong anti-noise and anti-fading capability, but it also brings the problem of low information transmission efficiency. The paper proposes a multi-channel M-ary spread spectrum communication method, using a recursive decoding algorithm to reduce the communication bit error rate. In combination with the channel equalization technique based on the virtual time reversal mirror technique, the scheme improves the communication rate of spread spectrum communication. It is verified through computer simulation and sea trial data that the communication method can effectively reduce the mutual interference between different channel signals, which provides a new idea for the design of hydroacoustic spread spectrum communication scheme.

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.

M-ary nonlinear sine chirp spread spectrum for underwater acoustic communication based on virtual time-reversal mirror method

Linear chirp spread spectrum technique is widely used in underwater acoustic communication because of their resilience to high multipath and Doppler shift. Linear frequency modulated signal requires a high spreading factor to nearly reach orthogonality between two pairs of signals. On the other hand, nonlinear chirp spread spectrum signals can provide orthogonality at a low spreading factor. As a result, it improves spectral efficiency and is more insensitive to Doppler spread than the linear counterpart. To achieve a higher data rate, we propose two variants (half cycle sine and full cycle sine) of the M-ary nonlinear sine chirp spread spectrum technique based on virtual time-reversal mirror (VTRM). The proposed scheme uses different frequency bands to transmit chirp, and VTRM is used to improve the bit error rate due to high multipath. Its superior Doppler sensitivity makes it suitable for underwater acoustic communication. Furthermore, the proposed method uses a simple, low-power bank of matched filters; thus, it reduces the overall system complexity. Simulations are performed in different underwater acoustic channels to verify the robustness of the proposed scheme.

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.

M-ary Cyclic Shift Keying Spread Spectrum Underwater Acoustic Communications Based on Virtual Time-Reversal Mirror.

Underwater acoustic communications are challenging because channels are complex, and acoustic waves when propagating in the ocean are subjected to a variety of interferences, such as noise, reflections, scattering and so on. Spread spectrum technique thus has been widely used in underwater acoustic communications for its strong anti-interference ability and good confidentiality. Underwater acoustic channels are typical coherent multipath channels, in which the inter-symbol interference seriously affects the performance of underwater acoustic communications. Time-reversal mirror technique utilizes this physical characteristic of underwater acoustic channels to restrain the inter-symbol interference by reconstructing multipath signals and reduce the influence of channel fading by spatial focusing. This paper presents an M-ary cyclic shift keying spread spectrum underwater acoustic communication scheme based on the virtual time-reversal mirror. Compared to the traditional spread spectrum techniques, this method is more robust, for it uses the M-ary cyclic shift keying spread spectrum to improve the communication rate and uses the virtual time-reversal mirror to ensure a low bit error rate. The performance of this method is verified by simulations and pool experiments.

Experimental Demonstration of Long-Range Underwater Acoustic Communication Using a Vertical Sensor Array.

This paper proposes a composite channel virtual time reversal mirror (CCVTRM) for vertical sensor array (VSA) processing and applies it to long-range underwater acoustic (UWA) communication in shallow water. Because of weak signal-to-noise ratio (SNR), it is unable to accurately estimate the channel impulse response of each sensor of the VSA, thus the traditional passive time reversal mirror (PTRM) cannot perform well in long-range UWA communication in shallow water. However, CCVTRM only needs to estimate the composite channel of the VSA to accomplish time reversal mirror (TRM), which can effectively mitigate the inter-symbol interference (ISI) and reduce the bit error rate (BER). In addition, the calculation of CCVTRM is simpler than traditional PTRM. An UWA communication experiment using a VSA of 12 sensors was conducted in the South China Sea. The experiment achieves a very low BER communication at communication rate of 66.7 bit/s over an 80 km range. The results of the sea trial demonstrate that CCVTRM is feasible and can be applied to long-range UWA communication in shallow water.

Biologically inspired covert underwater acoustic communication by mimicking dolphin whistles

Focusing on the need of covert underwater acoustic communication, a camouflage underwater acoustic communication approach is proposed in this paper. The idea is to deceive the interceptor to actively exclude the communication signal as it resembles the original whistle. Based on the time and frequency modulation feature of dolphin whistles, a series of segmented chirp rate modulated signals are designed to mimic the dolphin whistles by sound characteristics. A specialized communication frame structure is designed to facilitate the information demodulation, where the original whistle is used as the synchronization signal and the mimicry signal is followed to convey information. Besides, the original synchronization whistle is also used as a transmit reference to extract the whistle contour and furthermore to get the center frequency of each chirp. Virtual time reversal mirror technique is employed to mitigate channel effect. Fractional Fourier Transform and correlation approaches are adopted to realize demodulation respectively. The feasibility and efficiency of the proposed method are verified by both simulation and sea experiment. Bit error rate less than 10-3 with chirp duration of 20ms at a distance of 5.5km is achieved in the sea trial.

Joint virtual time reversal communications with an orthogonal chirp spread spectrum over underwater acoustic channel

In this paper, a mathematical derivation was performed to prove that there are three characteristics of chirp signals, self-compression, cyclic shift and chirp rate orthogonality. Based on these three features, a new method is proposed to improve the spectrum efficiency by joining virtual time reversal together with an orthogonal carrier. The advantages of the method can be summarized as follows. (1) By using the self-compression feature of a chirp signal, it is easy to obtain the spread gain by increasing the production of the time and bandwidth, instead of designing a complex pseudo random code. (2) The bit rate can be improved by taking the cyclic-shift feature into account and designing the M-ary modulation for the chirp carrier. (3) Chirp carriers have a certain orthogonality if they are under the considered design, that is, it is possible to use multi-carriers to improve the bit rate and spectrum efficiency. (4) Furthermore, by using the VTRM (Virtual Time Reversal Mirror) technique, the ISI (Inter Symbol Interference) of the Chirp carrier can be decreased and the robustness can be increased. Simulations, pool and sea experiments verified its feasibility and performance. Outfield experiments demonstrated that the BER (Bit Error Rate) of the method can be stabilized at a magnitude of 10−3 without the channel code, while the bit rate can reach 1kbps by using a 2kHz bandwidth. Thus, the spectrum efficiency can be over 0.5b/s/Hz and can be useful in reality.

Design of multistatic sonar space-time code detection signal and time reversal copy-correlation detection technology

A space-time code (STC) detection signal is designed for multistatic sonar to distinguish the multiple access echoes in the shallow water, and a time reversal (TR) copy-correlation detection method is presented for solving the problem of multi-path sub-channel differences when the vertical array is oblique. STC detection signal has a good orthogonality through modulating with pseudo-random signals, can distinguish multiple access target echoes while combating with sub-channel attenuation. The vertical array affected by current is oblique, its multi-path sub-channel differences causing signals from different sub-channels can not focus on the receiving end, so the array gain decreases. Meanwhile, it causes resolution to decline and the error rate of signal judgment to rise. Channel training signal is designed to estimate the sub-channel multipath environment, and the best matched detection signal for sub-channel differences is got through virtual time reversal mirror (VTRM) with the estimated channels and the received signal being detected by TR copy-correlation. Simulation results of tristatic sonar (two transmitters and one receiver) show that the SPC detection signal and TR copy-correlation can meet the requirements of multistatic sonar detection and distinguish the multiple access echoes in low SIR and complex channel environment. The best matched detection signal is got through VTRM (virtual time reversal mirror) with the estimated channels, and the received signal is detected by TR copy-correlation. Simulation results of tristatic sonar (two transmitters and one receiver) show that the STC detection signal and TR copy-correlation can meet the requirements of multistatic sonar detection and distinguish the multiple access echoes in low SIR environment.

Chirp spread spectrum of orthogonal frequency division multiplexing underwater acoustic communication system based on multi-path diversity receive

Aiming at the problem of traditional orthogonal frequency division multiplexing (OFDM) communication system performance degradation when the channel is subjected to complex multi-path and frequency-selective deeply fading, a chirp spread spectrum (CSS) of OFDM underwater acoustic communication system called CSS-OFDM is proposed in this paper. The CSS-OFDM system spreads the spectrum of traditional OFDM signals, whose sub-carriers are modulated into the same chirp rate, different center frequency orthogonal chirp signals with overlapped bandwidth. Dispreading after underwater acoustic coherent multi-path channel, at the receiving end, the multi-path signals will be expanded from each other in the frequency domain. Applying virtual time reversal mirror technology, the energies of the multi-path signals are focused to complete the multi-path channel diversity receiving. The system performance is improved not only by suppressing the frequency selective fading, but also taking full advantage of multi-path energy of the channel. The effectiveness and reliability of this system are verified through a number of simulations and lake trials.

Study on time reverse mirror in underwater acoustic communication

Time reversal mirror (TRM) can adaptively match the sound channel without any prior knowledge. In this paper, active TRM, passive TRM and virtual TRM which are all based on a single array element and the application of TRM in the underwater acoustic communication including single-user communication and multi-user communication are studied. Single sensor TRM which has time compression performance lacks array processing space gain, however, it can meet the requirements for underwater acoustic communication nodes being in the pursuit of simple structure and low power consumption. It is verified that TRM could focus multipath signal and achieve real-time adaptive channel equalization through computer simulation and test results, which could suppress the inter-symbol interference(ISI) and improve the signal-to-noise ratio(SNR).

Time-reversal passive localization for underwater radiated noise source

Time reversal (TR) is a technique of passive acoustical sources localization using a time reversal mirror (TRM) and is especially useful in multi-path environments. TR is commonly used to localize noise sources in aero acoustics and in many cases pulsed waveforms are used. A TR passive localization method for long duration waveforms was presented in this paper. The TR is applied to actual radiated noise recorded by only one hydrophone to estimate the source distance and depth. Compared to a standard time reversal approach, the proposed technique is based on the virtual time reversal mirror which can match the acoustic channel automatically and leads to an adaptive spatial focusing and temporal compressing as the conventional TRM. The processing result of simulated data and experiment data show this method could mitigate the multi-path interference on passive localization.

Application of time reverse mirror in underwater acoustic networks communication

Time reversal mirror (TRM) could match the acoustic channel impulse responses (CIR) automatically to achieve channel equalization without any transcendental knowledge. Active time reversal mirror (ATRM) needs monostatic sensor, and the signal propagates through the acoustic channel twice leading to inefficient. Further, the array processing enhances the complexity. In order to overcome disadvantages, the single sensor passive time reversal mirror (PTRM) and virtual time reversal mirror (VTRM) are presented. Based on the properties of underwater acoustic channel, using the uncorrelated character among different users' CIR, the single-element time reversal mirror is proposed to apply to underwater acoustic networks. The scheme could focus the desired user's information and suppress the undesired users' information. Furthermore, we have performed results of numerical simulations. Key words: time reversal mirror (TRM); channel equalization; underwater acoustic communication; networks

Photoacoustic tomography in scattering biological tissue by using virtual time reversal mirror

On the basis of the time reversal invariance of photoacoustic waves, a photoacoustic tomography method using a virtual time reversal mirror is presented to tackle acoustical scattering in biomedical imaging. Simulations show its advantages over the traditional photoacoustic tomography method in imaging scattering tissues. The consideration of system errors guarantees its robustness in practical applications. The proposed method could serve as an alternative for imaging complex biological tissues, where strong scatterers such as bones and cavities exist.

Non-invasive transcranial ultrasound therapy based on a 3D CT scan: protocol validation and in vitro results

A non-invasive protocol for transcranial brain tissue ablation with ultrasound is studied and validated in vitro. The skull induces strong aberrations both in phase and in amplitude, resulting in a severe degradation of the beam shape. Adaptive corrections of the distortions induced by the skull bone are performed using a previous 3D computational tomography scan acquisition (CT) of the skull bone structure. These CT scan data are used as entry parameters in a FDTD (finite differences time domain) simulation of the full wave propagation equation. A numerical computation is used to deduce the impulse response relating the targeted location and the ultrasound therapeutic array, thus providing a virtual time-reversal mirror. This impulse response is then time-reversed and transmitted experimentally by a therapeutic array positioned exactly in the same referential frame as the one used during CT scan acquisitions. In vitro experiments are conducted on monkey and human skull specimens using an array of 300 transmit elements working at a central frequency of 1 MHz. These experiments show a precise refocusing of the ultrasonic beam at the targeted location with a positioning error lower than 0.7 mm. The complete validation of this transcranial adaptive focusing procedure paves the way to in vivo animal and human transcranial HIFU investigations.

Multiuser underwater acoustic communication using single-element virtual time reversal mirror

Pattern time delay shift coding (PDS) scheme is introduced and combined with spread spectrum technique called SS-PDS for short which is power-saving and competent for long-range underwater acoustic networks. Single-element virtual time reversal mirror (VTRM) is presented in this paper and validated by the lake trial results. Employing single-element VTRM in multiuser communication system based on SS-PDS can separate different users’ information simultaneously at master node as indicated in the simulation results.

Concurrent detection, classification and localization of seabed targets using virtual time reversal

During the last decade, the use of unmanned vehicles to detect seabed objects has been revolutionized by the rapid development of AUV technology. The reduced power budget imposed by autonomous operations limits the amount of processing allowable onboard and calls for simple and efficient detection algorithms. Time reversal techniques have proved to be a robust way of focusing sound on reflective objects in complex environments and only require minimal computation. The method proposed here uses a virtual time reversal mirror in monostatic configuration to localize the origin of the field scattered by objects located on the seafloor. The waveguide is insonified with a low frequency source (∼kHz) mounted on the AUV and the resulting scattered field is sampled by a receiving array towed behind the vehicle. The recorded signals are then used to simulate the time reversed transmissions onboard, generating a map of reflectors present on the seabed. The rejection of clutter using singular value decomposition and the sensitivity of this method to environmental misknowledge are addressed along with the presentation of experimental results. [Work supported by ONR.]