Joint block estimation and attention-based long short-term memory network for doppler shift mitigation in UWA communication systems

The basic idea of this letter is concerned with the large Peak-to-Average Power Ratio (PAPR) problem in multi-carrier Under-Water Acoustic (UWA) communication systems that implement Orthogonal Frequency Division Multiplexing (OFDM). A Tangent Rooting (TanhR) companding technique is presented in this paper to reduce the PAPR in UWA communication systems. This technique achieves a noticeable reduction of the PAPR in UWA systems, while maintaining a good Bit Error Rate (BER) performance . Channel equalisation with Linear Minimum Mean Square Error (LMMSE) and Zero-Forcing (ZF) equalisers is considered in this paper. Simulation results ensure superiority of the proposed technique compared to the Selected Mapping (SLM) and the Partial Transmit Sequence (PTS) techniques from both the PAPR and BER perspectives.

Underwater acoustic communications are required for controlling and networking a variety of underwater or sea surface objects. Due to the unique characteristics of the underwater acoustic channel, e.g., complicated multipath propagation, selective frequency fading, extreme Doppler effect and heavy ambient noise, FH-MFSK techniques play a vital role in robust underwater acoustic communication, especially in shallow water. However, based on detecting the frequency of the receiving signals, the demodulation scheme for FH-MFSK may be sensitive to the frequency shifting caused by underwater Doppler shifts. This paper focuses on how to design the frequency demodulation scheme and to improve the performance of underwater FH-MFSK acoustic communication system. By using an original Double-Line Interpolation algorithm with Hanning window, we propose a demodulation scheme to enhance the tolerance of the Doppler shift and improve the detection rate. Simulations and demodulation performance were tested. Experimental results show a promising future for FH-MFSK demodulation in underwater acoustic communication system.

Underwater acoustic (UWA) communication in shallow water has been challenging owing to multipath-induced signal distortion and motion-induced Doppler distortion. In this study, we investigate the performance of a UWA communication system using orthogonal signal division multiplexing (OSDM) on intersymbol interference (ISI) and Doppler shift in a simulation, by comparing it with an existing communication system that uses coherent modulation with an adaptive filter, to clarify whether OSDM is suitable for UWA communication in shallow water. The obtained results clarify that OSDM is robust to heavy ISI, and it is expected to be suitable for a rapidly time-varying channel. For example, OSDM can improve the output signal-to-noise ratio (SNR) of 9.4 dB in a shallow water channel with a multipath delay time dispersion of 20 symbols, compared with the UWA communication system with decision feedback equalizer (DFE). However, it is also clarified that OSDM is more sensitive to the Doppler effect compared with the existing communication system, and there is the necessity of applying frequency-offset compensation, because the permissible Doppler shift, when the output SNR falls to -3 dB, for OSDM is less than half that for the UWA communication system with DFE.

A signal transmitted in an Underwater Acoustic Communication (UAC) system operating in a shallow-water channel suffers from strong time dispersion due to multipath propagation. This causes the Inter-Symbol Interference (ISI) observed in the received signal, which significantly limits the communication system’s reliability and transmission rate. In such propagation conditions, the Direct-Sequence Spread Spectrum (DSSS) method is one of the solutions that make reliable data transmission possible. In systems with one-to-one communication, it ensures communication with a satisfactory Bit Error Rate (BER). Additionally, it makes it possible to implement the Code-Division Multiple Access (CDMA) protocol in underwater acoustic networks. This paper presents the results of simulation and experimental communication tests on a DSSS-based UAC system using three types of spreading sequence, namely m-sequences, Kasami codes and Gold codes, and occupying different bandwidths from 1 kHz to 8 kHz around a carrier frequency equal to 30 kHz. The UAC channel was simulated by impulse responses calculated by the virtual sources method and the UAC chanel models available in the Watermark simulator. The experimental tests were conducted in a model pool. Based on the obtained results, a transmission rate was estimated, which is possible to achieve in strong multipath propagation conditions, assuming reliability expressed as BER less than 0.001.

This paper presents the application of the hyperbolic frequency modulated (HFM) signal to acoustic propagation in order to improve the performance of underwater acoustic (UWA) communications. Due to the large delay spread caused by multipath propagation and the severe Doppler Effect of the channel, we propose the using of double HFM signals as preambles for Doppler estimation, frame synchronization in UWA communications. A theoretical analysis about Doppler-invariability of HFM signals was provided firstly, then some numerical simulations about Doppler estimation were implemented, and experiments on testing performance of double-HFM preambles for frame synchronization in the pool were carried out as well. The simulation and experimental results show that using double-HFM signals as preambles has the capability to take an explicit Doppler estimation and retiming for frame synchronization, demonstrating that it has a good prospect in underwater acoustic communication system, especially for moving platforms.

TCN-based M-ary mobile spread spectrum underwater acoustic communication

The channel phase estimates comprise the information of both the carrier frequency offset (CFO) and the Doppler shift (DS), thus this paper presents a joint estimator of CFO and DS by exploiting the phase difference of fading channel. In order to enhance the estimation stability, we propose an iterative strategy. First, the CFO is estimated and then compensated in the channel estimate stage. Second, both the CFO and DS are re-estimated by exploiting the compensated channel estimates. Simulations show a good performance in a wide range of signal-to-noise ratios (SNR) and velocities.

An underwater acoustic multimedia communication (UWAMC) system is proposed with 2400 transmission modes according to time-varying multipath underwater acoustic (UWA) channel conditions. The orthogonal variable spreading factor (OVSF) scheme and Gold sequence (GS) scramble code are integrated into multi-input multi-output UWAMC system based on orthogonal frequency-division multiplexing to achieve the quality of service of multimedia transmission in the UWA channel. Binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK) adaptive modulation, direct mapping (DM) or space–time block code (STBC) transmission strategies, convolution channel code with rate 1/2 and 1/3, and a power assignment mechanism were adopted in the proposed system. Simulation results show that the bit error rate (BER) and power saving ratio (PSR) performance of the STBC strategy with transmission diversity is superior to that of the DM strategy without transmission diversity, and the performance of the BERs and PSRs of the transmission scheme with the GS scramble code surpasses that of the scheme without the code. The performance of the BERs and PSRs of BPSK modulation with a channel code rate of 1/3 is better than that of BPSK modulation with a channel code rate of 1/2, and the performances of BERs and PSRs of BPSK modulation with a channel code rate of 1/3 are better than that of QPSK modulation with a channel code rate of 1/3. As the length of the OVSF codes increases, the UWAMC system’s BERs decrease, and its PSRs increase. The UWAMC system can achieve either maximum transmission speed or maximum transmission power efficiency.

In uplink orthogonal frequency division multiple access (OFDMA), carrier frequency offsets (CFO) and/or timing offsets (TO) of other users with respect to a desired user can cause significant multiuser interference (MUI). In this paper, we derive analytical bit error rate (BER) expressions that quantify the degradation of BER due to the combined effect of both CFOs as well as TOs; such an analytical BER derivation has not been reported so far for uplink OFDMA in the presence of CFOs and TOs. For the case when the desired user is perfectly frequency/time aligned (and other users have non-zero CFOs/TOs), we obtain an exact closed-form expression for the BER. For the case when the desired user also has non-zero CFO/TO, we obtain an approximate BER expression involving a single integral. Analytical and simulation results are shown to match quite well.

A real demand for underwater acoustic (UWA) communications exists in oceanography, ocean exploration and undersea navigation. A new Doppler resilient digital communication, based on quadratic frequency modulations (QFM) is presented. The binary information is transmitted using two orthogonal QFM chirps. This signal modulation is suitable for low-data-rate communication such as telemetry. The first motivation of this paper resides in the performance of the noncoherent detector for binary QFM signal detection. It is shown that for some spreading factors, the detection of QFM signal waveform gives better performance than a linear frequency modulation (LFM) signal waveform in terms of bit error rate. The second motivation is that the non-coherent receiver is more Doppler resilient for QFM waveforms than LFM. An analytical demonstration is given, which predicts the simulated results. Real underwater commutations were accomplished on the Atlantic Ocean on February 2015.

Performance of both orthogonal frequency division multiplexing (OFDM) and single carrier (SC) systems suffers from a carrier frequency offset (CFO) due to Doppler effect and the carrier instability between the transmitter and the receiver. We investigate the bit error rate (BER) performance degradation of OFDM and SC systems due to the frequency offset in an additive white Gaussian noise (AWGN) channel as well as multipath Rayleigh fading channels. We consider three effects to the BER degradation, i.e., phase shift, useful power decrease and intercarrier interference (ICI). We also derive the approximate expressions of BER under binary phase shift keying (BPSK) and quaternary phase shift keying (QPSK) for both OFDM and SC systems in the presence of CFO. In general, SC is more robust to CFO in the AVVGN channel than OFDM in terms of BER, while both of them suffer similarly from CFO in the multipath Rayleigh fading channels assuming the the same CFO.

To address the inability to quantitatively analyze the performance of the direct sequence spread spectrum (DSSS) communication system in the presence of short-period pseudo-noise (PN) sequences, this study derives the interference components resulting from correlation processing of single-frequency interference. Given the short period of the spread spectrum sequence used in underwater acoustic systems, a theoretical expression is proposed for analyzing the anti-single-frequency interference performance of DSSS communication systems in underwater acoustics. This expression combines the interference power output by the system to enable quantitative analysis. Simulations are conducted to validate the accuracy of the theoretical expression. The results reveal that the interference sequence generated by correlation processing assumes either a direct current (DC) or sinusoidal distribution. The bit error rate (BER) of the system is influenced by signal-to-interference ratio (SIR), interference frequency offset, phase deviation, and the structure of the spreading code itself. The BER exhibits regular variation with changes in the interference phase. In all working conditions where the interference frequency offset is an integer multiple of the bit bandwidth, single-frequency interference with a frequency offset equivalent to the bit bandwidth has the greatest impact on the system. The concise theoretical expression presented in this study has significant practical value as it allows for quick prediction of variation in the anti-single-frequency interference performance of underwater acoustic DSSS communication systems using short-period PN sequences.

Due to the simultaneous existence of severe difficulties caused by multi-path, Doppler, and environmental noise caused by underwater acoustic channels, designing a stable and reliable underwater acoustic communication system (UWACS) is a challenging task. In addition to channel estimation and equalization, the technique of channel codes, which is capable of approaching Shannon’s capacity at a low encoding and decoding complexity, offers an efficient and reliable solution to improve the performance of the UWACS and thus draws extensive attention from the research community. By seeking a balance between communication efficiency and channel tolerance via multiple coding rate tuning, it has been recognized that the rate-compatible codes can achieve smooth rate-adaptive transmission under highly diverse underwater acoustic channel states. While there has been a substantial number of investigations on rate-compatible channel codes for the UWACS, in this paper, the research of rate-compatible channel codes has been summarized, reviewed, and compared, mainly focusing on the low-density parity-check (LDPC) and polar codes, which render flexibility in terms of code length and rate. Furthermore, typical research was evaluated and compared from the practical viewpoint of performance under real noise in a shallow water environment. Finally, problems encountered by the LDPC and polar codes for the UWACS were highlighted, and several future research issues were discussed.

The single-carrier frequency-division multiple access (SC-FDMA) or the discrete Fourier transform (DFT) pre-coded orthogonal frequency-division multiple access (OFDMA) technique has been adopted in the uplink transmission of the long term evolution (LTE) terrestrial communication standard, for its lower peak-to-average power ratio (PAPR) and similar receiver complexity, compared with OFDM. It is, however, rarely investigated in underwater acoustic (UWA) communications. This paper presents an SC-FDMA transceiver scheme for multiple-input, multiple-output (MIMO) UWA communications, and studies the performance of turbo equalization on the receiver side. The signalling of the pilot symbols bypasses the DFT precoding module, so the channel estimation and the carrier frequency offset (CFO) estimation are performed in a similar way as in a conventional OFDM system. A recently proposed high-performance frequency-domain turbo equalization technique is adopted for detecting the data symbols, which suffer inter-symbol interference (ISI) due to the DFT spreading operation. Experimental results clearly show the performance gain attributed to the turbo equalization, especially for high-order modulations.

Linear frequency modulation (LFM) signal is widely used in pulse compression radars, whereas faced with the time delay bias caused by Doppler shift of radar echoes, which must be accounted for in some applicative occasions where the linear frequency modulation ratio is very low or Doppler shift is very large. The method of time delay estimation based on cross-ambiguity function (CAF) could solve the problem but need two-dimensional search around the entire time delay and Doppler shift plane, which increases computational complexity heavily and brings great challenge to real-time processing. An effective time delay estimation method is proposed in this paper based on the relationship of time delay and Doppler shift, which can reduce two-dimensional search to one-dimensional search along LFM signal energy line, and diminish the computation complexity greatly with the same performance of parameters estimation. Simulations proved the validity of the method proposed.