基于扩频码的单载波迭代频域均衡水声通信

单载波时域均衡在长时延扩展水声信道中计算量大,并对接收机参数的选择较为敏感,可靠性低,而正交频分复用信号峰均功率比高、对频率偏移敏感. 针对这些问题,提出基于扩频码的单载波块传输高速率水声通信方法和基于T/4分数间隔迭代频域均衡的接收机算法. 该接收机利用已知扩频码进行信道估计以及对由多普勒偏移引起的旋转相位进行估计,并通过一种低复杂度迭代频域均衡算法改善系统性能. 开展了湖上实验研究,结果表明在浅水1.8 km距离且复杂多径干扰条件下,利用BPSK/QPSK调制可实现10^-2–10^-4的误码率并达到1500–3000 bit/s的有效数据率. 关键词： 水声通信 单载波 频域均衡 迭代处理     

一种鲁棒性强的OFDM 水声通信系统*

为了在不同衰落水声信道下实现正交频分复用水声通信,该文提出了一种鲁棒性强的正交频分复用水声通信方案,方案包括编码调制、信道估计和多普勒估计等内容。为了使该系统更稳健,整个信道编码分为两个步骤。首先,循环冗余校验编码器和里德-所罗门编码器用于编码整个数据包,然后循环冗余校验编码器和Turbo 编码器用于编码每个数据帧,其中比特交织编码调制技术用来对抗信道的时变特性。为了得到水声信道估计,使用线性最小均方误差估计器来处理导频数据得到信道估计值。多普勒估计包括帧的多普勒估计和符号的多普勒估计。实验结果表明该系统在不同的衰落信道下都能实现正确的跟踪和译码,系统的鲁棒性能优越。此外,该系统算法计算简单,易于实现,具有良好的工程应用价值。     

水声通信实验信道仿真研究

水声信道的时变多途特性对其中通信信号传输构成严重影响，为了分析评估现实实验信道条件下水声通信系统的性能，给出了一种基于实验测量的水声信道仿真方法。此方法通过发射线性调频脉冲以检测信道多径结构，同时分别采用自回归模型与多普勒时间压扩来仿真信道中各路径的时变幅度与时变延迟。通过一个具体的直接序列扩频水声通信系统，表明在仿真与实验信道条件下系统性能具有较好的一致性。     

一种利用海豚叫声的仿生水声通信方法

针对水下通信隐蔽性的需求, 克服传统固定载波调制方式带来的声源暴露问题, 提出一种基于海豚叫声的仿生伪装水声通信方法, 使通信信号被当作海洋生物噪声排除, 达到隐蔽通信的效果. 研究了海豚叫声信号特点, 利用海豚哨声信号实现同步与识别, 采用差分脉冲位置调制方法, 信息调制在相邻海豚嘀嗒声信号的时间间隔, 采用压缩传感体制下的匹配追踪技术估计信道, 虚拟时反技术实现信道均衡. 湖试结果验证了该方法的有效性和可行性, 接收声信号与发射信号声音上具有很高的相似度, 可以达到伪装隐蔽的效果. 实验中水平距离2 km, 通信速率不小于29 bps时,误码率可以达到10^-4以下. 关键词： 水声通信 仿生 海豚 隐蔽     

时变信道下的被动时间反转扩频水声通信

本文研究了时变信道条件下采用被动时间反转的直接序列扩频水声通信方案。多通道被动时间反转可通过对信道多径进行时间、空间聚焦实现信道匹配,但低信噪比、时变特性造成的信道特性失配对被动时间反转处理的性能造成严重影响。在垂直阵接收的基础上,本文采用码片级信道估计获取水声信道特性并进行周期性更新,并采用已判决码元产生的扩频码片作为信道估计训练序列,结合应用稀疏信道估计算法抑制零值抽头上的估计噪声,从而可有效改善时变、低信噪比条件下的被动时间反转处理的时、空多径聚焦效果,提高扩频通信性能。通过湖试实验比较了采用稀疏信道估计、传统信道估计算法的时反扩频接收机,以及经典直扩接收机的通信性能,实验结果表明：本文方案可在低信噪比获得较好的性能,并有效抑制时变信道对时反扩频通信性能的影响。     

块稀疏水声信道的改进压缩感知估计

压缩感知信道估计可利用信道稀疏特性提高估计性能,但对于具有典型块稀疏分布的水声信道,经典的l₀或l₁范数无法很好地描述块稀疏特性。利用水声信道块稀疏分布规律特性提出一种能够识别块稀疏结构的块稀疏似零范数,并在稀疏恢复信道估计算法中引入块稀疏似零范数约束项,进一步推导了复数域块稀疏似零范数恢复迭代算法,该算法通过对块稀疏似零范数进行梯度下降迭代并将梯度解投影至解空间来获得水声信道的块稀疏似零范数估计。数值仿真和海上水声通信实验结果表明该算法相对经典的稀疏信道估计算法有较明显的性能改善。通过算法推导、仿真和实验可获取结论:利用水声信道的块稀疏特性进行压缩感知重构可有效提高信道估计性能。      

MFSK浅海远程通信多径抑制的频率分组编码方法

多频移键控调制技术（MFSK）对信道衰落有良好的适应性，是远程水声通信的重要技术。但是，用于浅海远程水声通信时，由于多径传播的影响，导致严重的码间干扰（ISI），使得远程微弱信号的频率估计产生较大偏差，误码率显著增加。本文针对远程水下信道和MFSK调制的特点，提出了一种新的频率分组编码（FGC）方法。该方法改变了传统MFSK的数据帧格式，相邻符号采用不同组的频率进行编码映射，有效抑制了多径干扰。在深入研究新方法统计特性的基础上进行了湖试验证，结果表明，采用FGC新方法的通信系统，多径抑制有效，性能稳健，更适合于80-100km水声远程通信。     

CO-OFDM系统基于时频域卡尔曼滤波的相位噪声补偿算法

本文基于高阶正交幅度调制(QAM)和大线宽的相干光正交频分复用(CO-OFDM)系统,提出了一种基于时域和频域卡尔曼滤波联合估计的相位噪声补偿算法。该算法在发射端时域插入训练符号和导频序列,首先在接收端基于训练符号进行频域卡尔曼滤波得到信道估计。其次进行时域扩展卡尔曼滤波得到相位噪声粗略估计值。因此时相位估计值为复数值,为获得较好的补偿效果,根据导频序列数目,将每个OFDM符号分割为若干个亚符号。同时在导频序列处进行时域扩展卡尔曼滤波,通过在每个亚符号最后一个导频处进行线性插值得到每个采样点的相位噪声粗略估值。将粗略相位噪声补偿后的频域数据进行预判决后,最后进行时域相位噪声精细补偿。基于50Gb/sCO-OFDM系统传输100km进行了仿真验证,该算法较其他方法相比取得了较好的补偿效果。在激光器线宽为1MHz且16QAM以及线宽为800KHz且32QAM两种情况下,误码率性能可达前向纠错(FEC)上限。该算法能极大促进CO-OFDM系统在长距离接入网和城域网中的应用。     

水声通信中后验符号概率密度分布及概率成形容量分析

考虑到实际水声信道复杂特性对水声通信的影响,对系统的通信容量进行计算分析,并研究了一种适用于水声信道的容量评估算法。文中通过对不同条件下的均衡器输出的后验符号特性进行统计分析,提出了一种基于后验高斯分布的符号方差分段转移函数模型。在此基础上,采用Maxwell-Boltzmann分布来改变均衡器输出符号的成形分布,即引入概率成形技术对实际的水声通信系统容量进行评估分析。最后通过对水池和湖试多径信道数据的处理,获得了不同调制方式下的通信容量。结果显示,在特定的信噪比区间内,概率成形技术比传统的等概率映射的容量提升最大可达0.5 bits/symbol。      

垂直水声通信中的伪随机序列精确定时算法*

针对垂直水声通信中时变多普勒引起的定时偏差问题,研究一种基于伪随机序列的迭代定时估计及补偿算法。该方法利用伪随机序列作为同步信号,分三步估计时变多普勒：采用低复杂度的模糊函数法粗补偿接收信号内的平均多普勒;通过迭代插值法实现残留时变多普勒的精确估计及补偿;利用基于信道相关函数的相位信息,纠正均衡后信号的偏转相位。为实现高阶海试数据的有效解调,采用基于伪随机序列均方误差的多通道加权合并方式,获得空间分集增益。仿真及海试数据处理结果证明所提方法具有良好的时变多普勒估计及补偿性能,同时对30个通道内的1024QAM数据进行合并处理,在500 m的通信距离下,误码率为0.04,信道容量达到7.6 bits/symbol。与传统数据帧结构相比,无需使用线性调频信号,可以提高有效数据传输率。     

OFDM水声通信CS限幅失真补偿与LS信道估计优化算法*

正交频分复用技术应用于水声通信系统时,会引起较高的峰均比,当采用限幅法对峰均比进行抑制时,会产生非线性失真。另外,系统采用最小二乘法进行信道估计受噪声的影响较大。针对以上问题,提出了一种基于压缩感知技术的补偿限幅非线性失真与最小二乘信道估计相组合的新算法,在接收端利用导频数据采用压缩感知算法对限幅失真进行补偿,同时对最小二乘信道估计进行优化运算,以减小噪声对其影响。理论分析和仿真结果均表明,新算法不但能够有效抑制限幅法造成的系统非线性失真问题,而且能够降低高斯白噪声对于信道估计的影响。     

Deep learning and expert knowledge based underwater acoustic OFDM receiver

In this paper, a deep learning and expert knowledge based receiver is proposed for underwater acoustic (UWA) orthogonal frequency division multiplexing (OFDM). Different from the existing deep learning based UWA OFDM receivers, the proposed receiver combines deep learning with the classical expert knowledge of block-based signal processing in UWA OFDM to improve system performance and interpretability. It performs joint channel estimation and signal detection by designing skip connection (SC) convolutional neural network (CNN) cascaded attention mechanism (AM) enhanced bi-directional long short-term memory (BiLSTM) network, abbreviated as SC-CNN-AM-BiLSTM network (SCABNet). Specifically, the channel estimation subnet is designed with SC-CNN to utilize the thought of image super-resolution to reconstruct the entire channel frequency response of all subcarriers. The signal detection subnet is designed with AM-BiLSTM to extract the correlations of received sequential data for signal detection. Especially with the AM, the signal detection subnet can focus more on effective information of the received distorted signal to train the optimal network weights to improve the accuracy of data recovery. The proposed SCABNet is evaluated by experimental data, and the results have demonstrated that the SCABNet has the lowest BER and robust performance compared to the traditional linear algorithm, deep learning based black-box receiver, and ComNet receiver. And the proposed SCABNet is effective and robust when multiple nonideal factors co-exist.     

基于矢量传感器的频率估计算法在水声通信中的应用

本文给出了基于矢量传感器的ESPRIT频率估计算法，并将其应用于频率调制水声通信系统中。与声强频率估计算法相比，ESPRIT频率估计可在小样本的情况下，获得高精度的频率估计。仿真和湖试结果表明，基于矢量传感器的ESPRIT频率估计算法可以提高通信速率并降低对通信系统的带宽要求。本算法对信噪比的要求较高，目前看，较适用于近程高速水声通信。     

Bandwidth-efficient frequency-domain equalization for single carrier multiple-input multiple-output underwater acoustic communications

This paper proposes a single carrier (SC) receiver scheme with bandwidth-efficient frequency-domain equalization (FDE) for underwater acoustic (UWA) communications employing multiple transducers and multiple hydrophones. Different from the FDE methods that perform FDE on a whole data block, the proposed algorithm implements an overlapped-window FDE by partitioning a large block into small subblocks. A decision-directed channel estimation scheme is incorporated with the overlapped-window FDE to track channel variations and improve the error performance. The proposed algorithm significantly increases the length of each block and keeps the same number of training symbols per block, hence achieving better data efficiency without performance degradation. The proposed scheme is tested by the undersea data collected in the Rescheduled Acoustic Communications Experiment (RACE) in March 2008. Without coding, the 2-by-12 MIMO overlapped-window FDE reduces the average bit error rate (BER) over traditional SC-FDE schemes by 74.4% and 84.6% for the 400 m and 1000 m range systems, respectively, at the same data efficiency. If the same BER performance is required, the proposed algorithm has only 8.4% transmission overhead, comparing to over 20% overhead in other existing UWA OFDM and SC-FDE systems. The improved data efficiency and/or error performance of the proposed FDE scheme is achieved by slightly increased computational complexity over traditional SC-FDE schemes.     

正交M元码元移位键控扩频水声通信

M元和码元移位键控(CSK)扩频在水声通信中被广泛采用来克服扩频增益对通信速率的限制。为了获得更高的通信速率,文章基于Gold序列较大的码本数量、良好的自相关和互相关特性,提出正交双通道的M元和CSK相结合的水声通信方法。通过公式推导描述了正交M元CSK的调制和解调流程及影响其性能的因素。并且,通过与双通道M元CSK的仿真比较,得到正交M元CSK的误码率曲线。最后,通过实验验证了仿真的有效性。并实现了10 kbits数据量下1096.8 bps通信速率的无误码传输。通过推导、仿真和实验,可以得到这样的结论:正交M元CSK提供了优良的通信性能。      

Multi-array iterative receiver for underwater acoustic OFDM communications with EXIT chart evaluation

In this paper, a multi-array iterative receiver based on log-likelihood ratio (LLR)-combining detection involving joint sparse channel estimation and decoding is proposed for underwater acoustic OFDM communication. First, Extrinsic information transfer (EXIT) chart analysis is applied to evaluate the convergence behavior of the iterative receiver using the real data collected from the Kauai Acomms MURI 2011 (KAM11) experiment. This experiment was conducted in about 106 m-depth shallow water west of Kauai, HI, in June 2011, with a 20 kHz bandwidth (12–32 kHz) at range up to 3 km. It helps to explain the impact of different data configurations, detectors, and the diversity combinations in a highly inhomogeneous underwater environment and to predict the bit-error rate (BER) performance of the proposed receiver. Then the BERs as a function of the number of combined elements are illustrated to verify the prediction and analysis via the EXIT chart. Data transmission using 16QAM modulation achieves a BER of 10⁻⁴ at a data rate of 21 kb/s. The results provide guidance for the design of system parameters including the data configurations, the number of iterations for both iterative processing and low density parity check (LDPC) decoding, which are beneficial to achieve a good efficiency-performance tradeoff.     

Time reversal acoustic communication receivers: DSP implementation and fast channel estimation

A communication system is implemented on digital signal processors (DSPs) for the underwater acoustic environment. The implemented receiver uses time reversal multi-channel combining followed by a single-channel decision feedback equalizer. Periodic channel estimation is employed to track the channel fluctuations. These techniques are used to mitigate time-varying inter-symbol interference, which is the main challenge in the underwater acoustic channel at operating frequencies greater than 10 kHz. Various optimization tasks are performed to reduce the receiver computational complexity. A fast implementation of the matching pursuit algorithm is tested on the DSP platform. Its performance, in terms of accuracy and run-time, is compared with that of the basic matching pursuit algorithm. Experimental results of the transmission and demodulation of binary phase-shift keying signals at three different symbol rates were obtained in the local Delaware Bay. The low bit error rates demonstrate the effectiveness of our implementation.     

The study of underwater acoustic communication technology based-on the acoustic vector sensor

Underwater acoustic (UWA) communication based on an acoustic vector sensor is studied. The method of joint weighted sound pressure and velocity processing is used in phase modulation high-speed UWA communication system combined with coherent demodulation and adaptive equalization algorithm to demodulate and decode. Whereas the sound intensity could be used instead of pressure for frequency decoding in frequency modulation UWA communication system. The results of theory analysis, simulation calculations and lake trials have shown that either in phase modulation or in frequency modulation UWA communication system, the processing gain can be evidently increased, so that the BER (bit error rate) can be effectively reduced and the telemetry distance can be enlarged by using the acoustic vector sensor.     

隐蔽通信中基于水声信道的密钥生成技术*

水声传感器网络实现了高度智能化、自主性强、分布式、全天候的水下信息采集、传输、处理及融合,是水下目标的监测、定位、跟踪与分类等应用的最佳选择之一。针对隐蔽传输中的加密,提出了基于水声信道响应特征产生密钥的方法,通过利用水声信道的短时相关性,通信双方实时的产生加密密钥,以保证信息的保密性能。通过将信息隐藏技术和密钥生成技术相结合,确保水声信息的隐蔽传输。仿真与试验结果表明,基于提出的密钥生成方法能够生成匹配密钥,为水声隐蔽通信提供加密支持。