This letter proposes an adaptive modulation and coding (AMC) scheme based on deep learning for underwater acoustic (UWA) communications. To achieve good communication performance in fast time-varying UWA channels, the proposed AMC scheme is implemented on the orthogonal time-frequency space (OTFS) modulation system. We design an end-to-end deep convolutional neural network (CNN) to capture the channel features and determine the optimal modulation and coding scheme. Additionally, we utilize a meta-learning algorithm to address environment mismatch in real-world UWA applications. This algorithm effectively adapts the CNN model from a given UWA environment to a new UWA environment with only a small amount of data. The performance of the proposed scheme is verified through real-world measured channels. Simulation results demonstrate that the proposed method outperforms existing machine learning-based AMC and fixed modulation and coding schemes in various UWA scenarios, offering better communication throughput and stronger learning capabilities.

Optical intelligent reflecting surface (IRS) is regarded as a forward-looking technology to deal with line-of-sight limitations in optical wireless communications. This paper proposes an iterative photon counting nonorthogonal multiple access-multiple-input-multiple-output (NOMA-MIMO) system based on optical IRS. By analyzing the geometric and misalignment losses under turbulence effect in the optical IRS-based atmospheric channel, we construct the channel fading coefficient matrix of the NOMA-MIMO system. At the receiver, a novel iterative parallel interference cancellation algorithm based on logarithmic likelihood ratio is developed for signal detection. Through the simulation of bit error rate (BER), the reliability of the proposed system is proved. Quantitatively, the BER performance can achieve 2 x 10- 5 $2\times {10}<^>{-5}$ at the - 165 $-165$ dBJ per bit for the scenario of 3 x 5 $3\times 5$ MIMO system. Furthermore, the numerical results of the external information transfer process indicated that the proposed system has a rapid convergence performance.

Orthogonal time-frequency-space (OTFS) modulation has shown significant error performance advantages over orthogonal frequency division multiplexing (OFDM) in time-varying channels. However, the symbols in OTFS frames still suffer from inter-Doppler interference (IDI) and inter-symbol interference (ISI). In this letter, we propose a two dimensional (2D) adaptive multichannel decision feedback equalizer (DFE) to combat the interference. The proposed receiver constructs the input signal and feedback signal of the DFE based on the characteristic of OTFS modulation. Meanwhile, by exploiting the sparsity feature of the channels in delay-Doppler domain, the iterative improved proportionate normalized least mean squares (IPNLMS) algorithm is employed to achieve fast convergence and fast tracking. The simulation results show that the proposed equalizer could achieve satisfied BER performance with lower complexity.

This paper presents an optimal design of QAM-CCK modulation based on maximizing mapping diversity for ARQ transmission scheme, aiming at improving the unsatisfying performance of QAM-CCK in underwater channels with single transmission. In order to achieve the optimal BER performance, this paper first derives the pair-wise error probability after re-transmission and then obtains the optimal construction of QAM-CCK by minimizing the target function. Simulation results demonstrate that optimum desigend QAM-CCK achieves 2 dB SNR gain compared to randomly selected QAM-CCK when BER equals 10 -4 . Furthermore, optimum desigend QAM-CCK achieves 5 dB SNR gain compared to randomly selected QAMCCK when BER equals 10 -5 under tested acoustic channels from UNet06 sea experiments.

This work proposes a novel interleave-division multiple access (IDMA) system based on cyclic shift keying (CSK) modulation, named CSK-IDMA, for multiuser underwater acoustic (UWA) communications. The CSK modulation uses a circular cyclic shift of the spreading sequence to represent the information, which can be viewed as a special $M$-ary spread spectrum modulation. Compared with conventional direct-sequence spread spectrum (DSSS) technique, CSK overcomes the spreading gain versus data rate limitations. The proposed scheme adopts the IDMA technique to reduce the cochannel interference. At the receiver, we employ passive time reversal (PTR) technique to compress the channels and improve the signal-to-interference-noise ratio (SINR) at first. In order to perfectly incorporate with the IDMA receive structure, we propose a soft CSK demodulation which can iteratively exchange extrinsic information with a soft channel decoder to form a turbo equalization. Simulation results show the proposed CSK-IDMA scheme can offer significantly improved performance. The data processing results from a lake experiment show that the proposed scheme can support 8 users and 10 users with zero error bits for 2.3 km transmission and 2.7 km transmission, respectively. The corresponding bit error rate (BER) for 16 users are $\text{8.4} \times \text{10}<^>{-4}$ and $\text{9.7} \times \text{10}<^>{-4}$, respectively. The data rate is 62.5 bit/s per user within 4 kHz bandwidth and spreading sequences of 256 chips. To the best of our knowledge, this is the first work which can support more than 8 users for multiuser UWA communications with a high bandwidth efficiency.

Underwater acoustic (UWA) channels exhibit double selectivity in both time and frequency domains, posing significant challenges to reliable UWA communications. This correspondence paper presents a novel spatial block coding scheme integrated with complementary code keying (CCK) modulation to exploit both spatial diversity and code diversity against doubly selective fading channels. The proposed spatial-CCK modulation utilizes multiple transmit transducers and extends the traditional CCK code set to improve receiver performance. We introduce novel code design criteria by considering maximum likelihood detection receivers under fast Rayleigh fading channel condition. Simulation and lake experiment results demonstrate substantial performance gain by the proposed spatial-CCK scheme over the conventional space-time codes without incurring additional high complexity cost for UWA channels.

This letter presents a novel design of high rate complementary code keying (CCK) modulation scheme (named QAM-CCK) that is suitable for bandwidth efficient wireless link adaptation. The proposed QAM-CCK can increase the bandwidth efficiency of CCK signaling by simultaneously implementing quadrant and phase modulations. We introduce an optimal design criterion based on weighted minimum Euclidean distance to optimize the QAM-CCK mapping with minimum average bit error probability. In order to utilize the inherent coding gain of QAM-CCK, we propose an iterative soft maximum likelihood receiver to enhance the decoding performance. Compared with traditional QPSK of the same bandwidth efficiency, the proposed QAM-CCK achieves a 4-dB SNR gain under additive white Gaussian noise channels when used in conjunction with proposed iterative receiver.

Shallow seawater acoustic channel has serious multipath effect. We proposed hybrid time-frequency domain Turbo equalization (HTFDTE) and bidirectional time-frequency domain Turbo equalization (Bi-HTFDTE) for multiple-input multiple-output (MIMO) single carrier (SC) underwater acoustic system with PN code to improve the quality of underwater acoustic communication. At the receiving end, the channel is estimated using the known PN code. The frequency domain equalization with minimum mean square error (MMSE) criterion and the time domain Turbo iterative processing are combined together to further improve performance. To eliminate error propagation effect, the Bi-HTFDTE includes two parallel HTFDTE structures, which requires roughly twice as many computational operations per iteration as the HTFDTE but achieving a lower bit error rate (BER). From the simulation results, we can find that the BER performance of HTFDTE is better than frequency domain Turbo equalization (FDTE). At the order of 10(-4), the HTFDTE with three iterations gains about 0.5dB and 1dB over the HTFDTE with first iteration and the FDTE with first iteration for QPSK modulation, respectively. The Bi-HTFDTE with three iterations gains about 0.15dB over the HTFDTE with three iterations.

Driven by the huge demand to explore oceans, underwater wireless communications have been rapidly developed in the past few decades. Due to the complex physical characteristics of water, acoustic wave is the only media available for underwater wireless communication at any distance. As a result, underwater acoustic communication (UAC) is the major research field in underwater wireless communication. In this paper, characteristics of underwater acoustic channels are first introduced and compared with terrestrial communication to demonstrate the difficulties in UAC research. To give a general impression of the UAC, current important research areas are mentioned. Furthermore, different principal modulation-based schemes for short- and medium-range communications with high data rates are investigated and summarized. To evaluate the performance of UAC systems in general, three criteria are presented based on the research publications and our years of experience in high-rate short- to medium-range communications. These three criteria provide useful tools to generally guide the design and evaluate the performance of underwater acoustic communication systems.

针对水下声传感器网络参考节点布设成本高、难度大所带来的问题,研究了一种基于迭代的大规模水下传感器网络定位方法,通过少量初始锚节点定位普通节点,评估定位成功节点的精度保持情况,升级定位精度高的普通节点为锚节点参与定位余下的普通节点.研究结果表明:以一定的精度损失为代价,迭代定位方法显著提高了定位覆盖率,而联合三维欧几里得距离估计的迭代方法可进一步提升定位覆盖率.

In this letter, we present a novel spatial modulation (SM) transmission with multiple antennas by applying non-orthogonal complementary code keying (CCK). We propose a high-performance turbo receiver for this CCK-SM signaling over frequency-selective fading channels. Compared with conventional SM, our proposed CCK-SM transceiver utilizes the advantages of CCK to achieve substantial coding gain without the cost of high complexity. Our novel receiver incorporates iterative block decision feedback equalization with a CCK soft decoder to achieve clear performance gain. Simulation test shows substantial performance gain by the proposed CCK-SM transceiver over the conventional SM.

Phase-coherent underwater acoustic (UWA) communication systems typically employ multiple hydrophones in the receiver to achieve spatial diversity gain. However, small underwater platforms can only carry a single transducer which can not provide spatial diversity gain. In this paper, we propose single-carrier with frequency domain equalization (SC-FDE) for phase-coherent synthetic aperture acoustic communications in which a virtual array is generated by the relative motion between the transmitter and the receiver. This paper presents synthetic aperture acoustic communication results using SC-FDE through data collected during a lake experiment in January 2016. The performance of two receiver algorithms is analyzed and compared, including the frequency domain equalizer (FDE) and the hybrid time frequency domain equalizer (HTFDE). The distances between the transmitter and the receiver in the experiment were about 5 km. The bit error rate (BER) and output signal-to-noise ratio (SNR) performances with different receiver elements and transmission numbers were presented. After combining multiple transmissions, error-free reception using a convolution code with a data rate of 8 kbps was demonstrated.

Complementary code keying (CCK) is a high rate spread spectrum coded modulation designed for frequency selective channels. CCK has been shown as an effective signaling technology in underwater communications. To improve the performance of CCK receivers, this work presents a joint channel estimation and detection receiver based on a Markov Chain Monte Carlo (MCMC) approach. We simplify the receiver complexity by introducing a reduced state detector based on the concept of set partitioning for the CCK modulation and incorporated with the MCMC channel estimation mechanism. The proposed method demonstrates significant performance gain at modest computational complexity over several existing receiver algorithms.

Single-carrier with frequency domain equalization (SC-FDE) has been considered for bandwidth efficiency underwater acoustic (UWA) communication recently due to its reduced computational complexity and low peak-to-average power ratio. A multi-channel time-frequency domain equalization method for pseudurandom noise (PN) based SC-FDE is proposed in this paper. The proposed equalizer includes a multi-channel frequency domain equalizer followed by a low order multi-channel adaptive time domain decision feedback equalizer (DFE). The proposed algorithm is applied to the real data receiving from a lake test conducted in November 2011. It is demonstrated that the uncoded error-free data rates of around 1500 and 3000 bps are achieved using one transmitter and six-channel receiving hydrophone array at a distance of 1.8 km. Experiment results shows that the performance can be enhanced by 4.5-5.5 dB in terms of output signal-to-noise ratio (SNR).

Recently, single carrier block transmission (SCBT) has received much attention in high-rate phase-coherent underwater acoustic communication. However, minimum-mean-square-error (MMSE) linear FDE may suffer performance loss in the severely time dispersive underwater acoustic channel. To combat the channel distortion, a novel multi-channel receiver with maximum ratio combining and a low complex T/4 fractional iterative frequency domain equalization (FDE) is investigated to improve diversity gain and the bit error rate (BER) performance. The proposed method has been verified by the real data from a lake underwater acoustic communication test in November 2011. At 1.8 km, the useful data rates are around 1500 and 3000 bits/ s for BPSK and QPSK respectively. The results show the improvements of system performance. Compared with MMSE FDE system, the output SNR improvement is 6.9 dB, and the BER is from 10-3 to no error bits for BPSK. The output SNR improvement is 5.3 dB, and the BER is from 1.91×10-2 to 2.2×10-4 for QPSK.

In this paper, a novel method based on ensemble empirical mode decomposition (EEMD) and autoregressive (AR) spectrum is presented to fault diagnosis of rolling bearing. This method can carry out ensemble empirical mode decomposition and extract feature information of different machine parts in condition monitoring and fault diagnosis of machinery. The criterion of adding white noise in EEMD method is established. EEMD is used for avoiding mode mixing in signal decomposition, and it is combined with the AR spectrum in this paper. Then the AR model estimation is applied to each intrinsic mode function and the AR spectrum is obtained. Finally, the proposed method is applied to analyze the rolling bearing vibration signal and the result confirms the advantage of the proposed method.

The vibration signals acquired from rotating machinery are often complex, and fault features are masked by background noise. Feature extraction and denoising are the key for rotating machinery fault detection, and advanced signal processing method is needed to analyze such vibration signals. In this paper, an optimal lifting multiwavelet denoising method is developed for rotating machinery fault detection. Minimum energy entropy is used as the metric optimize the lifting multiwavelet coefficients, and the optimal lifting multiwavelet is constructed to capture the vibration signal characteristics. The improved denoising threshod method is used to remove the background noise. The proposed method is applied to turbine generator and rolling bearing fault detection to verify the effectiveness. The results show that the method is a robust approach to reveal the impulses from background noise, and it performs well for rotating machinery fault detection.

The performance of frequency-domain equalization in a SC-FDE system is affected by the precision of channel estimation results and estimation methods based on compressive sensing have better performance in sparse channel condition such as underwater acoustic communication channels. However, the commonly used greedy algorithm in sparse channel estimation requires the sparsity to terminate the recursive process. Unlike many existing methods in which the sparsity is treated as a known factor, we propose a sparse channel estimation method with sparsity predetermined by wavelet decomposition. Typical LS estimation method is applied first and wavelet decomposition results of the estimated channel impulse response are used to set the threshold for determining the channel sparsity. With the predetermined sparsity, sparse channel estimation technique based on compressive sensing can achieve a better performance.