Quadratic chirp modulation for underwater acoustic digital communications

Signal integration, as an effective method of detecting weak target, is widely used in areas of radar, sonar, etc. In previous studies of long-time coherent integration, researchers usually established a multi-pulse echo model with linear frequency modulation (LFM) signal due to its good compression performance and large Doppler tolerance. Then, perfect analytical formula can be deduced to compensate for range migration and Doppler spread, which is helpful in analyzing the mechanism of long-time coherent integration in depth. However, besides LFM, a wide variety of signal waveforms are also used in modern sonar and underwater guidance system to meet the requirements for diverse applications. For instance, continuous wave (CW) pulse is often used in signal detection, high resolution direction of arrival (DOA) estimation, and velocity estimation, while large time-bandwidth product waveforms such as modulated signal, coded signal, and pseudo-random signal are utilized for special tasks like anti-interference detection, channel matching, and active concealed detection. Therefore, the formulas and corresponding instructive conclusions deduced by LFM have no generality when other sonar waveforms are used in pulse integration. In this paper, we focus on long-time coherent integration for arbitrary signal reflected by underwater target moving with a uniform velocity and propose a motion-compensated coherent integration method for arbitrary complex envelop signal. A kind of general ambiguity function (GAF) for transmitted signal is defined to present a unified expression based on GAF for the output of the matched filter. The operation not only helps us to describe and calculate the pulse compression form of the arbitrary complex envelop by using a general mathematical model, but also provides information about the range migration and Doppler frequency shift of the multi-pulse echo, which is needed in pulse range alignment and FFT integration. For the matched filter output expressed by the GAF, Keystone transform is utilized to correct the complex envelop of the multi-pulse echo and eliminate the range walk. Then, Doppler frequency shift is compensated for by performing FFT transform, and the long-time coherent integration for arbitrary complex envelop is realized. To verify the correctness of the proposed method, we carry out the computer simulation on both signal integration and detection performance by using four sonar waveforms, i.e., CW signal, LFM signal, m-sequence phase-coded signal, and Costas frequency hop coded signal. The simulation results show that the proposed motion-compensated coherent integration method is applicable to arbitrary complex envelop signal. We also design an anechoic water tank experiment scheme which can successfully obtain the multi-pulse echoes of constant moving target. The motion-compensated coherent integration of the experimental data of the above-mentioned four waveforms further validates the effectiveness of the proposed method.

Aiming to address the performance degradation of OFDM systems caused by Doppler shift in underwater acoustic (UWA) communications, this paper proposes a two-step algorithm that combines the traditional block estimation method with an Attention-based Long Short-Term Memory Network (ALSTM) to mitigate the Doppler effect. In the first step, the Doppler factor is coarsely estimated using a conventional block estimation technique based on linear frequency modulation (LFM) signals. The estimated value is then used to resample the received data, which is subsequently fed into a neural network model to predict the carrier frequency offset (CFO) of the resampled signal. Simulation results demonstrate that the proposed algorithm outperforms both traditional methods and basic neural network approaches in terms of mean square error (MSE) and system bit error rate (BER), and achieves superior accuracy and robustness in Doppler shift estimation, offering a reliable solution for communication in complex UWA environments.

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This paper focuses on the wake-up signal detection design for underwater acoustic communication (UAC) terminals. A wake-up signal detection unit can considerably reduce the power consumption of the terminals. Compared with terrestrial wireless counterparts, the wake-up signal detection design for UAC terminals is challenged by the severe underwater acoustic channels, which is characterized as doubly selective fading and low signal-to-noise ratio (SNR). This paper proposes a wake-up signal detection approach called channel-adaptive detection and location-assisted joint decision (ChAD-LaJD), for UAC terminals. ChAD-LaJD applies a group of linear frequency modulation (LFM) signals as a wake-up signal. In order to increase the detection probability while keeping a low false alarm rate, ChAD-LaJD consists of two procedures: channel-adaptive detection (ChAD) and location-assisted joint decision (LaJD). Besides a pre-determined threshold, ChAD procedure defines two special parameters which reflect instantaneous channel states to detect wake-up signals adaptively. LaJD procedure further exploits the location relationships of LFM signals detected by ChAD to achieve a joint decision. The simulations and field experiments are conducted to evaluate the performance of ChAD-LaJD. The results show that ChAD-LaJD outperforms the conventional methods that consider a fixed threshold (FixTh) and/or constant false alarm rate (CFAR).

Background: For the traditional Fourier Transform (FT), it cannot effectively detect the frequency of non-stationary signals with time. Analyzing the local characteristics of time-varying signal by using FT is hard to achieve. Therefore, many time-frequency analysis methods which can meet different needs have been proposed on the basis of the traditional Fourier transform, like the Short Time Fourier Transform (STFT), the widely used Continuous Wavelet Transform (CWT), Wigner-Ville Distribution (WVD) and so on. However, the best time and frequency resolution cannot be achieved at the same time due to the uncertainty criterion. Methods: From the point of view of optimizing time-frequency performance, a new Generalized S Transform (GST) window function optimization method is proposed by combining time frequency aggregation with an improved genetic algorithm in this paper. Results: In the noiseless condition, the Linear Frequency Modulation (LFM), Nonlinear Frequency Modulation (NLFM) and binary Frequency Shift Keying (2FSK) signals are simulated. The simulation results prove that the method can improve the time-frequency concentration and decreasing Rényi entropy effectively. Compared with other four traditional time-frequency analysis methods, the improved GST has more advantages. Conclusion: In this paper, a new method of optimizing the window function in GST based on improved GA is proposed in this paper. Experiments on LFM, NLFM and 2FSK signals show that the proposed method not only has the advantages of high resolution, but also determines the optimal parameters via the time frequency focusing criterion and the Rényi entropy. Compared with the other four kinds of time-frequency analysis methods, the optimized GST based on improved GA in this paper has the best time-frequency focusing.

Aiming at the countering doppler shift, a mobile underwater acoustic communication scheme of M-ary via FDM-PLFM is proposed. The cross-correlation between piecewise linear frequency modulation(PLFM) and traditional LFM signal is analyzed based on the cross-correlation function. Combining with frequency division multiplexing(FDM), the spectrum utilization of traditional underwater acoustic LFM communication scheme is improved. With the underwater acoustic LFM communication scheme, the new scheme does not require the doppler frequency shift compensation at the low doppler shift. The simulation results show that the present scheme has lower bit error rate than that for the traditional underwater acoustic M-ary LFM communication method under higher data rate conditions.

In recent years, underwater acoustic (UWA) communications have received much attention as their applications have begun to shift from military toward commercial. Digital communications through UWA channels differ substantially from those in other media, such as radio channels, due to severe signal degradations caused by multipath propagation and high temporal and spatial variability of the channel conditions. The design of underwater acoustic communication systems has until recently relied on the use of noncoherent modulation techniques. However, to achieve high data rates on the severely band-limited UWA channels, bandwidth-efficient modulation techniques must be considered, together with array processing for exploitation of spatial multipath diversity. The new generation of underwater communication systems, employing phase-coherent modulation techniques, has a potential of achieving at least an order of magnitude increase in data throughput. The emerging communication scenario in which the modern underwater acoustic systems mill operate is that of an underwater network consisting of stationary and mobile nodes. Current research focuses on the development of efficient signal processing algorithms, multiuser communications in the presence of interference, and design of efficient modulation and coding schemes. This paper presents a review of recent results and research problems in high-speed underwater acoustic communications, focusing on the bandwidth-efficient phase-coherent methods. Experimental results are included to illustrate the state-of-the-art coherent detection of digital signals transmitted at 30 and 40 kb/s through a rapidly varying one-mile shallow water channel.

Immune particle swarm optimization of linear frequency modulation in acoustic communication

The complexity of the underwater environment including long time-delay, large background noise, and excessive transmission loss, leads to two main challenges for the current mid-distance and long-distance underwater acoustic (UWA) communications: limited transmission rate and system reliability. To improve the performance of mid-range and long-range underwater acoustic communication, this paper proposes an M-ary multi-sequence superimposed spread spectrum(M-MSSS) UWA communication scheme based on a virtual time-reversal mirror (VTRM). Multiple sequences with periodic correlation characteristics of the original spread spectrum waveform are superimposed, and M-ary multi-sequence modulation is performed in each data block at the transmitter. A VTRM matched filter is designed for the receiver. The VTRM technology uses the physical characteristics of the typical coherent multipath of the underwater acoustic wave channel to reduce the signal amplitude through its strong spatial focusing ability. The inter-symbol interference is suppressed by reconstructing multipath signals and improved receiver. Simulations show that the scheme not only significantly improves the communication rate, but also ensures the system's better robustness in terms of bit error rate.

As the dominant information carrier in water, the acoustic wave is widely used for underwater detection, communication and imaging. Even though underwater acoustic communication has been greatly improved in the past decades, it still suffers from slow transmission speeds and low information capacity. The recently developed acoustic orbital angular momentum (OAM) multiplexing communication promises a high efficiency, large capacity and fast transmission speed for acoustic communication. However, the current works on OAM multiplexing communication mainly appear in airborne acoustics. The application of acoustic OAM for underwater communication remains to be further explored and studied. In this paper, an impedance matching pentamode demultiplexing metasurface is designed to realize multiplexing and demultiplexing in underwater acoustic communication. The impedance matching of the metasurface ensures high transmission of the transmitted information. The information encoded into two different OAM beams as two independent channels is numerically demonstrated by realizing real-time picture transfer. The simulation shows the effectiveness of the system for underwater acoustic multiplexing communication. This work paves the way for experimental demonstration and practical application of OAM multiplexing for underwater acoustic communication.

For pulse compression characteristics and not easily affected by noise, linear frequency modulation signal are widely used in underwater acoustic communication. This paper analyzes the characteristics of hyperbolic frequency modulation signal over underwater acoustic channels. Compared with linear frequency modulation signal, hyperbolic frequency modulation has the same performance of strong anti-noise and anti-multipath, what’s more, hyperbolic frequency modulation signal is better resist the influence of doppler. And discussed the influence of doppler on signal, simulation results show that the hyperbolic frequency modulation signal detection rate is better than linear frequency modulation signal in the doppler environment. DOI: http://dx.doi.org/10.11591/telkomnika.v11i3.2183 Full Text: PDF

Underwater acoustic communication is paid more and more attention with the development of marine researches in recent years. Underwater acoustic communication channel is considered to be one of the most complicated channels, it suffers from the less available bandwidth, the more serious multipath effect and the even complex noise. Due to the severe distortion of the received signals, the underwater acoustic communication always experiences a high Bit Error Rate (BER). To solve this problem, Carrier Waveform Inter-Displacement (CWID) modulation was proved to be an effective method to decrease BER in our previous work. The Linear Frequency Modulation (LFM) carrier-waves are used in CWID modulation. The frequency band of LFM carrier-waves has an important effect on the communication performance. The Particle Swarm Optimization (PSO) is introduced to search the optimal frequency band of the LFM carrier-waves in this paper, due to its excellent performance in solving complicated optimization problems. The multiple objective optimization nature of the PSO gives a clear hint to deduce the relationship of upper band and lower band of the LFM carrier-waves. Simulations verify the improved performance and effectiveness of the optimization method.

We focus on the problem of compressive sampling of Linear Frequency Modulation(LFM) signal via Matching Pursuit. We designed an adaptive matching pursuit algorithm through dynamic dictionary optimization. Results from the sparse property of LFM under Chirplet Basis, we use Chirplet atom to generate dictionary. Then by feeding the information of signal back into dictionary generating process, the grid error can be reduced significantly. It also allows us to get better parameter estimation results and lower signal sampling rate. We demonstrate computing performance on a LFM signal sampling example.

Recent advancements in underwater technology have made cooperative exploration, an efficient underwater exploration technique, possible. This technique networks mobile and fixed platforms, sensors and systems, and makes it possible to accomplish underwater tasks efficiently by integrated operations over a wide area and balancing exploration and exploitation. As a networking technique, underwater acoustic (UWA) communication is a technology of choice today, since it can enables mobility and flexibility in cooperative exploration systems. However, UWA communication still has several design challenges which need to be carefully addressed. To provide a reliable UWA modem suitable for cooperative exploration, in this study we designed a Doppler-resilient orthogonal signal division multiplexing with time diversity (D-OSDM-TD) and evaluated its performance in simulations and experiments. We evaluated the D-OSDM-TD in simulations and experiments and found that the D-OSDM-TD achieved a far better bit-error-rate performance in UWA channels with delay and Doppler spreads using low signal power and without using a receiver array. We expect that D-OSDM-TD can become a viable alternative offering highly reliable communication for underwater operations, and we believe it is suitable for underwater cooperative operations.

A linear frequency modulation (LFM) signal and its corresponding de-chirp operation are one of the basic methods for wideband radar signal processing, which can reduce the burden of the radar system sampling rate and is more suitable for large-bandwidth signal processing. More importantly, most existing methods against interrupted sampling repeater jamming (ISRJ) are based on time–frequency (TF) or frequency domain analysis of the de-chirped signal. However, the above anti-ISRJ methods cannot be directly applied to multiple-input multiple-output (MIMO) radar with multiple beams, because the angular waveform (AW) in mainlobe directions does not possess the TF properties of the LFM signal. Consequently, this work focuses on the co-optimization of transmit beampattern and AW similarity in wideband MIMO radar systems. Different from the existing works, which only concern the space–frequency pattern of the transmit waveform, we recast the transmit beampattern and AW expressions for wideband MIMO radar in a more compact form. Based on the compact expressions, a co-optimization model of the transmit beampattern and AWs is formulated where the similarity constraint is added to force the AW to share the TF properties of the LFM signal. An algorithm based on the alternating direction method of multipliers (ADMM) framework is proposed to address the aforementioned problem. Numerical simulations show that the optimized waveform can form the desired transmit beampattern and its AWs have similar TF properties and de-chirp results to the LFM signal.