Underwater Multipath Channel Research Articles

An Iterative Orthogonal Frequency Division Multiplexing Receiver with Sequential Inter-Carrier Interference Canceling Modified Delay and Doppler Profiler for an Underwater Multipath Channel

In 2023, we proposed the modified delay and Doppler profiler (mDDP) as an inter-carrier interference (ICI) countermeasure for underwater acoustic orthogonal frequency division multiplexing (OFDM) mobile communications in a multipath environment. However, the performance improvement in the computer simulation and pool experiments was not significant. In a subsequent study, the accuracy of the channel transfer function (CTF), which is the input for the mDDP channel parameter estimation, was considered insufficient. Then a sequential ICI canceling mDDP was devised. This paper presents simulations of underwater OFDM communications using an iterative one- to three-step mDDP. The non-reflective pool experiment conditions are a two-wave multipath environment where the receiving transducer moves at a speed of 0.25 m/s and is subjected to a Doppler shift in the opposite direction. As NumCOL, the number of taps in the multitap equalizer which removes ICI, was increased, the bit error rate (BER) of 0.0526661 at NumCOL = 1 was significantly reduced by a factor of approximately 45 to a BER of 0.0011655 at NumCOL = 51 for the sequential ICI canceling mDDP.

Recognition method for biomimetic camouflage communication signal imitating cetacean click in underwater multipath channels

Focusing on the need of biomimetic camouflage communication signal recognition in underwater multipath channels, we propose a recognition method for biomimetic camouflage click communication train (BCCCT) based on time delay difference (TDD) coding in multipath channels. For the problem of multipath interference, we perform multipath determination on the received signal by autocorrelation, filter relevant peak points, determine the time delays between multipaths, and remove the multipath interference signals according to the determined time delays. A click extraction method based on the double sliding window is proposed, which locates the boundary of click by signal envelope. By analyzing the distribution characteristics of the time interval of adjacent clicks (TIAC) of BCCCT and real click train (RCT), we model the TIACs based on Gaussian kernel probability density estimation (PDE), and recognize the BCCCTs by the Convolutional Neural Networks (CNN), which takes the PDE curve as the system input. The numerical simulations and a lake experiment are carried out to measure the effectiveness of the recognition method proposed under different signal-to-noise ratios (SNR), coding parameters, and channels. The recognition accuracy can achieve 80 % at 0 dB in the constructed channels. Moreover, in the real underwater communication environment, the recognition accuracy can achieve 95 %.

Accurate Identification for CW Direct Signal in Underwater Acoustic Ranging

The underwater channel is bilateral, heterogeneous, uncertain, and exhibits multipath transmission, sound line curvature, etc. These properties complicate the structure of the received pulse, causing great challenges in direct signal identification for ranging purposes and impacts on back-end data processing, even accurate acoustic positioning. Machine learning (ML) combined with underwater acoustics has emerged as a prominent area of research in recent years. From a statistical perspective, ML can be viewed as an optimization strategy. Nevertheless, the existing ML-based direct-signal discrimination approaches rely on independent assessment, utilizing a single sensor (beacon or buoy), which is still insufficient for adapting to the complex underwater environment. Thus, discrimination accuracy decreases. To address the above issues, an accurate CW direct signal detection approach is performed using the decision tree algorithm, which belongs to ML. Initially, the pulse parameter characteristics in the underwater multipath channel are investigated and the parameter models are built. Then, based on multi-sensor localization performance feedback, fusion characteristics for diverse pulse are created. Next, the pulse parameter characteristics are preprocessed to mitigate the impact of varying magnitudes and units of magnitude on data processing. Then, the decision tree is built to obtain the desired output results and realize accurate recognition of the ranging direct signals. Finally, the feasibility and reliability of this paper’s method are verified by computer simulation and field testing.

Multipath time delay estimation based on the improved Wigner-ville distribution

The accuracy of underwater multipath channel time delay is crucial in underwater acoustic signal processing. The temporal resolution of the traditional matched filter is restricted by the transmitted signal bandwidth. Wigner-Ville distribution (WVD) which has excellent energy concentration characteristics and effectively avoids the interaction between temporal and frequency resolutions, and is widely used in non-stationary signal processing. However, considering the multipath signal processing, the cross-term in echo WVD would cause inaccurate time delay estimation. Therefore, in this paper, a novel multipath signal time delay estimation method based on the improved WVD is proposed. By utilizing the difference in the continuity of energy changes between auto-term and cross-term in the time-frequency domain, the Seeded Region Growing algorithm is applied and improved to extract auto-term in the WVD. The simulation results show that the method removes cross-term and maintains high concentration characteristics of auto-term, compared with pseudo-WVD and smooth pseudo-WVD, etc. In addition, it improves the accuracy of multipath signal delay estimation and has some anti-noise ability.

An underwater target velocity estimation based on cepstrum and autocorrelation for acoustic echo

Due to the target highlights scattering in underwater multipath channels, the underwater target echoes are generally considered as a superposition of sub-echoes, which are represented by different amplitude attenuation factors, time delays and Doppler factors, and the superposition usually degrades the target velocity estimation performance greatly. With a developed multi-highlight target echo model in underwater multipath channels, and based on the great performance of cepstrum and autocorrelation for time delay estimation, a target velocity estimation method is proposed in this paper, where the transmitted signal of the active sonar is designed to be a combined signal. In this method, the received echo is subjected to matched filter first, then the autocorrelation and cepstrum are applied successively to the echo. Since the cepstrum and autocorrelation exist peaks at the same location, they are found to be useful for improving the accuracy of the target velocity estimation. The proposed method is compared with the conventional matched filter method in both simulations and experiments. The results demonstrate that using a lower computational effort, the proposed method can estimate the target velocity more precisely at a lower signal-to-noise ratio.

Time of arrival estimation for underwater acoustic signal using multi-feature fusion

Underwater target detection (UTD) is widely applied in ocean exploration. A key issue for the time-of-arrival (ToA)-based UTD algorithm is to localize the first direct path in an underwater multipath channel. However, the conventional Matched Filter (MF) suffers from a deterioration of the estimation accuracy when the direct arrival of the received signal is non-strongest. Therefore, this paper proposes a multi-feature fusion ToA estimation algorithm. First, a multi-decision strategy including the coarse ToA range estimation and the fine ToA estimation is proposed to realize a balance between computational complexity and estimation accuracy. Second, the coarse ToA range estimation is realized by multi-level spectrum analysis. Then, the coarse ToA range is used to localize the fine ToA utilizing the multi-feature fusion ToA estimation. Finally, the simulation and experimental results are provided to validate the proposed ToA estimation algorithm. It clearly shows that without training data, the proposed algorithm can not only deal with the multipath acoustic channel problem but also improve the accuracy of the ToA estimation compared to other algorithms.

UFMC-Based Underwater Voice Transmission Scheme with LDPC Codes

An underwater universal filtered multicarrier (UFMC)-based voice transmission scheme is proposed using a 512-point inverse discrete Fourier transform, utilizing 10 sub-bands, and that each had 20 subcarriers. In this proposed UFMC method, the adaptive modulation technologies with 4 quadrature amplitude modulation (QAM), 16-QAM, and low-density parity-check (LDPC) channel coding were integrated. Additionally, the bit error rate (BER), transmission power weighting, the ratios of power-saving, and underwater voice transmission performance with perfect channel estimation (PCE), and 5% and 10% channel estimation errors (CEEs) were investigated. The underwater voice transmission had a BER quality of service 10−3. Simulation results showed that the PCE outperformed 5% and 10% CEEs, under 4-QAM, with gains of 0.5 and 0.9 dB, respectively, and a BER of 4×10−4. The PCE outperformed 5% and 10% CEEs, under 16-QAM, with gains of 0.5 and 2.4 dB, respectively, and a BER of 8.5×10−4. The proposed UFMC scheme can be applied to underwater voice transmission with a BER below 10−3 The proposed system showed a superior capability to contend with additive white Gaussian noise, underwater multipath channel fading, and CEEs.

Time Reversal and Fractional Fourier Transform-Based Method for LFM Signal Detection in Underwater Multi-Path Channel

Fractional Fourier transform (FrFT) is a useful tool to detect linear frequency modulated (LFM) signal. However, the detection performance of the FrFT-based method will deteriorate drastically in underwater multi-path environment. This paper proposes a novel method based on time-reversal and fractional Fourier transform (TR-FrFT) to solve this problem. We make use of the focusing ability of time-reversal to mitigate the influence of multi-path, and then improve the detection performance of FrFT. Simulated results show that, compared to FrFT, the difference between peak value and maximum pseudo-peak value of the signal processed by TR-FrFT is improved by 8.75 dB. Lake experiments results indicate that, the difference between peak value and maximum pseudo-peak value of the signal processed by TR-FrFT is improved by 7.6 dB. The detection performance curves of FrFT and TR-FrFT detectors with simulated data and lake experiments data verify the effectiveness of proposed method.

Pulse Ranging Method Based on Active Virtual Time Reversal in Underwater Multi-Path Channel

Aiming at improving the accuracy of pulse ranging measurement in underwater multi-path environment, this paper proposes a novel pulse ranging algorithm based on active virtual time reversal (AVTR). By using the focusing characteristics of AVTR, the received signal can be focused at the receiving end, which eliminates the negative influence of multiple pseudo-peaks. In order to extract the received signal, we propose an energy-based adaptive windowed method which preserves the signal focus peak while truncates the side peak component. Numerical simulations are provided and outfield experiments are conducted. The results demonstrate the effectiveness of the proposed method comparing with correlation-based method.

Underwater Acoustic Time Delay Estimation Based on Envelope Differences of Correlation Functions.

This paper proposes underwater acoustic time delay estimation based on the envelope differences of correlation functions (EDCF), which mitigates the delay estimation errors introduced by the amplitude fluctuations of the correlation function envelopes in the traditional correlation methods (CM). The performance of the proposed delay estimation method under different time values was analyzed, and the optimal difference time values are given. To overcome the influences of digital signal sampling intervals on time delay estimation, a digital time delay estimation approach with low complexity and high accuracy is proposed. The performance of the proposed time delay estimation was analyzed in underwater multipath channels. Finally, the accuracy of the delay estimation using this proposed method was demonstrated by experiments.

Fully generalised spatial modulation technique for underwater communication

A novel transmission scheme called fully generalised spatial modulation (FGSM) is proposed for underwater communication, where any subset of available transmitting antennas ( N t ) is activated at a time instant to transmit the data constellation symbol and the index of the active antenna is also harnessed to carry information. The FGSM offers better energy efficiency (EE) than previous spatial modulation (SM) and generalised SM systems. The proposed FGSM system is tested in an acoustic underwater multipath channel. Simulation results show that it can significantly improve the average bit error rate (ABER) as well as EE.

Experimental study on characteristics of echoes reflected by a cylindrical object in underwater multi-path channel

Research work on characteristics of echoes scattered by a cylindrical object located in underwater multi-path is introduced in the paper. A sound scattering experiment of a cylindrical object in water-filled tank is carried out, the purpose of which is to give answers to questions of how the multi-path channel corrupt the characteristics of echoes from an elastic body in underwater multi-path channel and how many target-related features can still be extracted in both time and frequency domain which can be used for classification or feature-based detection. Detailed data processing results are shown and analyzed, which may be useful to research on signal processing method of sonar target detection and classification.

Passive time reversal communication with cyclic shift keying over underwater acoustic channels

Abstract Many advanced channel estimation and equalization algorithms have been proposed for high-rate coherent underwater acoustic communications. However, these algorithms are usually computationally demanding or ineffective under low signal-to-noise ratio (SNR). Energy-efficient or reliable communications over complex underwater multipath channels are required in many applications. In this paper, passive time reversal with cyclic shift keying (CSK) spread spectrum using hyperbolic frequency modulated (HFM) waveform is proposed for reliable point-to-point underwater acoustic communication. At the transmitter, multiple bits information is represented by the cyclically shifted version of the basic HFM waveform. Therefore, the CSK scheme provides a higher data rate than conventional direct sequence spread spectrum. At the receiver, the peak position of the cyclic correlator output is estimated to recovery information. A modified passive time reversal receiver for the proposed HFM based CSK is designed to: (1) focus the multipath signal without estimating the channel impulse response to increase the received SNR; (2) and alleviate the requirement for accurate symbol synchronization. Simulation and lake experimental results verify the proposed method and show its potential for low cost and reliable underwater acoustic communication systems.

Experimental Evaluation of Zp-Ofdm Scheme in Time Varying Underwater Multipath Channel

An experimental analysis to evaluate the practical suitability of the Underwater Acoustic (UWA) communication scheme is the major aim of this study. Oceanic channel imposes two major challenges (i.e., multipath propagation and Doppler effects) for underwater acoustic communication. Being fastest possible underwater information exchanger, OFDM is considered as the most viable option for mitigation of multipath. However, wideband nature of oceanic channel introduces severe Doppler shifts when dealing with OFDM subcarriers. Slight dynamics in a channel may cause overlapping of subcarriers such that entire signal can get completely distorted and sued as ICI (inter-carrier interference) induced transmission. In this study, we are presenting the algorithm to rectify the effects of Doppler shifts and to make OFDM technique more robust and more feasible for underwater environments. This study assures the performance of Zero-Padded Orthogonal Frequency Division Multiplexing (ZP-OFDM) based communication system in dynamic channel conditions. Data transmission using various packets structures are used to demonstrate the effectiveness of ZP-OFDM scheme. The narrow rectangular water tank of acoustic lab, Harbin Engineering University is utilized as a dynamic aquatic channel. The experimental results endorse the successful performance and suggest ZP-OFDM as a viable choice for high-rate communications over dynamic and high multipath oceanic channels.

Performance Evaluation of ZP-OFDM Scheme in Underwater Multipath Channel

One of the major problems while dealing with underwater acoustic communication (UWAC) is the occurrence of unwanted multipath having long delay spreads. Higher probability of reflections (from wavy sea surface, uneven sea bottom, other obstacles, etc.) and refraction of sound are the main causes of these grievous multipath signals. Multipath effects further offer severe degradation in the packet (s) and mutilation of entire signal in terms of inter-symbolic interference (ISI) is occurred. During last decade, multicarrier modulation in terms of OFDM was considered as one of the viable solutions to mitigate the multipath effect and provide high-rate communication. In this article, we are performing the practical evaluation of Zero-Padded Orthogonal Frequency Division Multiplexing (ZP-OFDM) based communication system. The data using various packet structures are used to demonstrate the effectiveness of ZP-OFDM scheme in the flat multipath channel of the water pool of acoustic lab, Harbin Engineering University. Satisfactory performance results are obtained even within high noisy environment. These results suggest that ZP-OFDM is a viable choice for high-rate communications over multipath induced underwater channels.

Time reversal mirror localization technology based on a high-resolution MVDR algorithm

Minimum variance distortionless response (MVDR) algorithm has broken through the Rayleigh limit restrictions and makes higher resolutions possible. However, they are easily affected by complex underwater acoustic environments and are not sufficiently robust, so it is difficult to use them in real underwater systems. Time reversal mirror technology is a new method of localization, which may be effectively applied to target detection and localization. To improve its robustness, the MVDR algorithm is combined with time reversal mirroring for passive detection and location. This method makes full use of underwater multipath channel information and reduces the effects of complex underwater environments. The performance of time-reversal MVDR that is based on fixed diagonal loading and robust capon beamforming (RCB) was also studied. Simulations, water tank experiment and sea trial proved it is feasible and practical Keywords: MVDR; time reversal mirror; robust capon beamforming; passive location