单载波频域均衡中的水声信道频域响应与噪声估计

本文对单载波频域均衡(SC-FDE)中的信道和噪声估计进行研究,理论分析了形成误码率平台的可能原因,提出了基于Chu序列的联合信道时频域响应和噪声功率估计算法.利用水声信道响应的稀疏特性和门限确定信道能量集中区域,进行信道估计的去噪处理和噪声功率计算.借助于水声射线模型对所提出的算法进行了仿真,分析了噪声估计对SC-FDE性能的重要影响.仿真结果表明,所提出的信道和噪声估计算法可以有效减缓或消除SC-FDE中的误码率平台.     

水声通信低复杂度最大似然联合均衡译码方法

现有Turbo均衡采用迭代联合均衡译码结构,这一迭代过程在信噪比高于某一门限后才能产生正向的迭代增益,当信道码间干扰较为严重或者采用高阶调制方式时,该信噪比门限较高;同时,现有Turbo均衡的性能还受限于其交织器长度,故当码块长度较短时,其性能并不理想。针对以上问题,提出了一种基于Spinal码的联合均衡译码方法。首先,该方法采用非迭代的最大似然联合均衡译码结构,有效避免了Turbo均衡迭代结构所带来的信噪比门限高问题;其次,该方法采用信息混合能力更强的Spinal码,有助于提高码块长度受限时的误码率性能;最后,该方法利用Spinal码可以有效增加相似信息序列间汉明距离的特点,将联合均衡译码时间复杂度降低为仅随信息序列长度与信道响应长度呈线性增长,解决了本方法难以实际应用于实时水声通信的难题。在抚仙湖湖试中,同样实现速率为3.34kbps数据通信,该方法的误码率优于Turbo均衡方法两个数量级以上。时变水声信道下的仿真以及湖试结果表明,该方法在码长受限的情况下仍在复杂水声信道中表现出更好性能,其所需的最低输入信噪比显著低于现有Turbo均衡方法。      

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。      

Frequency-domain channel estimation and equalization for shallow-water acoustic communications

A new frequency-domain channel estimation and equalization (FDE) scheme, combined with a new group-wise phase correction scheme, is proposed for single-carrier (SC) underwater acoustic communications systems employing single transducer and multiple hydrophones. The proposed SC-FDE scheme employs a 2N-point Fast Fourier Transform (FFT) to estimate and equalize the channel in frequency domain, where N is the number of symbols in a data block. Both the frequency-domain channel estimation and equalization are designed by the linear minimum mean square error criterion. Initial channel estimation is performed by a pilot signal block and later updates are achieved using the detected data blocks. The proposed phase correction scheme utilizes a few pilot symbols in each data block to estimate the initial phase shift and then correct it for the block to combat the large phase rotation due to the instantaneous Doppler drifts in the acoustic channels. Time-varying instantaneous phase drifts are re-estimated and compensated adaptively by averaging the phase variation across a group of symbols. The proposed SC-FDE and phase correction method is applied to the AUVFest’07 experimental data measured off the coast of Panama City, Florida, USA, June 2007. With the Quadrature Phase Shift Keying (QPSK) modulation and a symbol rate of 4 ksps, the proposed scheme achieves an average uncoded bit error rate on the order of 1×10^?4 for fixed-to-fixed channels with the source–receiver range of 5.06 km. For the moving-to-fixed source–receiver channels where the source–receiver range is 1–3 km, the multipath delay spread is 5 ms, the average Doppler shifts are ±20 Hz, and the maximum instantaneous Doppler drifts from the mean is ±4 Hz, the proposed scheme achieves an average uncoded bit error rate on the order of 1×10^?3.     

Orthogonal M-ary code shift keying spread spectrum underwater acoustic communication

M-ary and code shift keying（CSK）spread spectrum is generally applied to overcoming the spreading gain versus data rate limitation in underwater acoustic（UWA）communications.In addition,the concept of orthogonal M-ary CSK is introduced to UWA communication to gain higher rates.We propose a scheme employing dual-orthogonal-channel M-ary and CSK spread spectrum UWA communication based on Gold sequences owing large code set and good self and cross correlation.The modulation and demodulation scheme of the orthogonal M-ary CSK is derived algebraically and the factors critical to its performance are discussed.Furthermore,its bit error rate（BER）curve is evaluated via simulations and compared with the dual-channel M-ary CSK.Finally,the validity of the simulations is verified in tank experiments.1096.8 bps data rate of the proposed communication scheme is realized for 10 kbits data volume with no error.Overall through deduction,numerical simulation,and experiment,it is shown that the proposed method offers significantly improved performance.     

稀疏贝叶斯学习水声信道估计与脉冲噪声抑制方法

水声信道具有显著的稀疏特性,利用稀疏贝叶斯学习(SBL)算法能够实现稀疏水声信道的有效估计。针对SBL计算复杂度较高的问题,将广义近似消息传递-稀疏贝叶斯学习(GAMP-SBL)引入水声信道估计。该方法在SBL的框架下结合GAMP以消息传递的方式计算信道冲激响应,能够有效降低SBL的计算复杂度。针对假设背景噪声服从高斯分布的信道估计方法在脉冲噪声环境下性能下降问题,提出了基于GAMP-SBL的脉冲噪声抑制水声信道估计方法:首先利用脉冲噪声时域稀疏特性,采用GAMP-SBL估计脉冲噪声并进行抑制,然后再次利用GAMP-SBL实现水声信道估计.基于第九次北极科考冰下脉冲噪声的两次仿真结果表明,所提出的方法在归一化均方误差上相对于未进行脉冲噪声抑制的GAMP-SBL最大分别降低了18.71%,6.61%,在信道解码前误码率上最大分别降低了1.66%,4.05%,并且相对于Clipping方法更加稳健。在信噪比为20 dB时,误码率可低于10-2。      

Channel temporal correlation-based optimization method for imperfect underwater acoustic channel state information

The rapid time variations and large channel estimation errors in underwater acoustic (UWA) channels mean that transmitters for adaptive resource allocation quickly become outdated and provide inaccurate channel state information (CSI). This results in poor resource allocation efficiency. To address this issue, this paper proposes an optimization approach for imperfect CSI based on a Gauss–Markov model and the per-subcarrier channel temporal correlation (PSCTC) factor. The proposed scheme is applicable to downlink UWA orthogonal frequency division multiple access systems. The proposed PSCTC factors are measured, and their long-term stability is verified using data recorded in real-world sea tests. Simulation and experimental results show that the optimized CSI effectively mitigates the effects of the temporal variability of UWA channels. It demonstrates that the resource allocation scheme using optimized CSI achieves a higher effective throughput and a lower bit error rate than both imperfect CSI and the CSI predicted by the recursive least-squares (RLS) algorithm.     

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.     

水声正交频分多址上行通信稀疏信道估计与导频优化

针对水声正交频分多址(OFDMA)上行通信中用户导频数量少、分布不均匀, 导致传统内插信道估计方法产生误码平层的问题, 提出一种稀疏信道估计与导频优化方法. 基于压缩感知(CS)理论估计稀疏信道冲激响应, 并依据CS理论中测量矩阵互相关最小化原理, 提出基于随机搜索的导频图案和导频功率联合优化算法. 仿真结果表明, 所提方法在不同多径扩展信道下的性能均优于基于线性内插的最小二乘估计、未经导频优化的CS信道估计以及单纯基于导频图案优化的CS信道估计. 水池实验分别验证了交织式和广义式子载波分配的水声OFDMA上行通信性能, 在接收信噪比高于10 dB时利用所提方法实现了两用户接入的可靠通信.     

Joint iterative compressed sensing algorithm eliminating clipping noise in underwater acoustic communication system

Due to the intricacy characteristics of the Underwater Acoustic(UWA) channel especially the limited bandwidth, Orthogonal Frequency Division Multiplexing(OFDM) is used because of its high spectrum efficiency. However, relatively high Peak-to-Average-Power Ratio(PAPR) limits the efficiency of OFDM in UWA communication, leading to saturation in the power amplifier and consequent distortion of the signal. Clipping and C companding as the most classic and convenient algorithms, are widely applied to address the high PAPR issue. However clipping introduces additional noise which degrades the system's performance and traditional C companding is also not suitable for underwater acoustic field. Thus, an improved C companding combined with clipping is proposed here. Due to the sparseness of clipping noise, Compressed Sensing(CS) can be utilized to estimate it. The scheme exploits pilot tones and data tones as observations instead of reserve tones, which is different from the previous works and improves data rate. Furthermore, to minimize the effect of the underwater acoustic channel, the channel is also estimated using the CS technique, which provides more accurate channel characteristics than Least Square(LS) or Minimum Mean Square Error(MMSE) estimation algorithms. The effectiveness of the proposed algorithm is proved in computer simulations as well as in a pool experiment.     

Spatial diversity and combination technology using amplitude and phase weighting method for phase-coherent underwater acoustic communications

The UWA channel is characterized as a time-dispersive rapidly fading channel,which in addition exhibits Doppler instabilities and limited bandwidth. To eliminate intersymbol interference caused by multipath propagation, spatial diversity equalization is the main technical means. The paper combines the passive phase conjugation and spatial processing to maximize the output array gain. It uses signal-to-noise-plus-interference to evaluate the quality of signals received at different channels. The amplitude of signal is weighted using Sigmoid function. Second order PLL can trace the phase variation caused by channel, so the signal can be accumulated in the same phase. The signals received at different channels need to be normalized. It adopts fractional-decision feedback diversity equalizer(FDFDE) and achieves diversity equalization by using different channel weighted coefficients. The simulation and lake trial data processing results show that, the optimized diversity receiving equalization algorithm can improve communication system's ability in tracking the change of underwater acoustic channel,offset the impact of multipath and noise and improve the performance of communication system.The performance of the communication receiving system is better than that of the equal gain combination. At the same time, the bit error rate(BER) reduces 1.8%.     

一种基于Optisystem的静态与动态色散补偿相结合方案研究

介绍色散补偿光纤DCF及动态啁啾光纤光栅FBG的补偿技术并比较其优缺点,提出在传统光传输系统末端加入动态FBG的方案,通过Optisystem仿真软件搭建40 Gbit/s的光传输系统,用FBG仿真光传输400 km的Q值为3.745,误码率为7.419 42e-5,用DCF静态混合补偿和静态与动态相结合的补偿方案分别仿真并比较两种方案传输相同距离的Q值和误码率的大小,证明提出的静态加动态的方案提高了光传输性能。     

变步长似p范数约束稀疏水声信道估计方法

针对范数约束类归一化最小均方(NLMS)算法在正交频分复用(OFDM)稀疏水声信道估计中误码率较高的问题,提出一种改进的变步长似p范数约束信道估计方法。采用改进双Logistic函数构造步长,并将误差信号自相关函数引入其中,实时调整步长和零吸引项,使得收敛速度和估计精度能够很好地折中。算法仿真结果表明,在浅海多径稀疏水声信道下,相比于传统方法,所提出的信道估计获得的最高性能提升为收敛速度提高72.3%,稳态误差降低95.9%。湖上试验数据处理结果显示,相比于传统信道估计方法,所提出的方法能使通信误码率降低2~3个数量级,实现零误码水声通信。      

Adaptive and efficient nonlinear channel equalization for underwater acoustic communication

We investigate underwater acoustic (UWA) channel equalization and introduce hierarchical and adaptive nonlinear (piecewise linear) channel equalization algorithms that are highly efficient and provide significantly improved bit error rate (BER) performance. Due to the high complexity of conventional nonlinear equalizers and poor performance of linear ones, to equalize highly difficult underwater acoustic channels, we employ piecewise linear equalizers. However, in order to achieve the performance of the best piecewise linear model, we use a tree structure to hierarchically partition the space of the received signal. Furthermore, the equalization algorithm should be completely adaptive, since due to the highly non-stationary nature of the underwater medium, the optimal mean squared error (MSE) equalizer as well as the best piecewise linear equalizer changes in time. To this end, we introduce an adaptive piecewise linear equalization algorithm that not only adapts the linear equalizer at each region but also learns the complete hierarchical structure with a computational complexity only polynomial in the number of nodes of the tree. Furthermore, our algorithm is constructed to directly minimize the final squared error without introducing any ad-hoc parameters. We demonstrate the performance of our algorithms through highly realistic experiments performed on practical field data as well as accurately simulated underwater acoustic channels.     

Delay-Doppler domain decision feedback turbo equalization for OTFS modulation

Emerging evidence indicates that orthogonal time–frequency space (OTFS) modulation is a potential candidate modulation scheme for high mobility wireless communications. However, OTFS may experience significant inter-symbol interference (ISI) and inter-Doppler interference (IDI) in the receiver. In this paper, we propose a soft decision feedback turbo equalization for OTFS transmission over delay-Doppler channels to jointly combat both interferences. A novel block decision feedback equalization (BDFE) algorithm is constructed using the band feature of the channels in the delay-Doppler domain. The feedforward and feedback filters are designed by the delay-time channels coefficients. According to the designed filter, an equivalent system model is employed to allow turbo equalization. The posterior probability is established using the soft prior information and feedback filter, and then fed back to the channel decoder as external information. Both theoretical analysis and simulation results demonstrate the effectiveness of the proposed scheme in improving the bit error rate (BER) performance and combat various interference. Numerical simulations are finally provided to justify the validity of the proposed scheme in improving the bit error rate (BER) performance and combating various interference.     

An embedded pilot power based channel estimation and low-complexity feedback equalization scheme for OTFS system

Orthogonal Time Frequency Space modulation (OTFS) has evolved as an astounding modulation technique for high-speed communication in a doubly dispersive channel. In any wireless communication system, channel estimation and equalization are essential at the receiver to recover the transmitted data. To accomplish this for the emerging OTFS based systems, a modified embedded pilot-based channel estimation technique and low complexity feedback equalization algorithm for integer Doppler shifts in the delay-Doppler domain are proposed in this paper. Our channel estimation scheme exploits embedded-pilot arrangement, and the symbol equalization relies on the Interference calculation and its mitigation iteratively. To achieve this we contemplate a prudent arrangement of symbols in the OTFS frame in such a way that the Guard symbols prevent the interference between data symbols and the pilot symbol at the receiver. Two distinct lumps of received data of the same OTFS frame will be engaged in channel estimation and data detection. An analytical expression of the theoretical Cramer Rao Lower Bound (CRLB) is derived and plotted for the proposed channel estimation scheme. The attained simulation results for Bit-Error-Rate (BER) under the proposed scheme show a significant error rate improvement over the Minimum Mean Squared Error (MMSE) equalization algorithm. Further, a lower computational complexity is also achieved in comparison with modified MMSE detection and MP detection algorithms.     

Experimental demonstration of underwater acoustic communication using bionic signals

This paper applies dolphin whistles to covert underwater acoustic (UWA) communication and proposes a UWA communication scheme based on M-ary bionic signal coding. At the transmitter end, the scheme maps multiple information bits into a dolphin whistle through a signal selector. At the receiver end, passive time reversal mirror (PTRM) is used for channel equalization and source information is restored according to the decision of which whistle is transmitted. The scheme has high spread spectrum gain. The anti multi-path performance is greatly improved when using PTRM. Different from traditional covert UWA communication methods, this mimicked signal is unlikely to alert an adversary even in high SNRs because of its real existence in marine environment. A tank experiment is conducted for the scheme, at communication rate of 50 bit/s with SNR −5 dB user information is recovered at a very low bit error rate. The results of tank experiment demonstrate the feasibility of this covert UWA communication scheme.     

Pilot design and doubly-selective channel estimation for faster-than-Nyquist signaling

Being capable of enhancing the spectral efficiency (SE), faster-than-Nyquist (FTN) signaling is a promising approach for wireless communication systems. This paper investigates the doubly-selective (i.e., time- and frequency-selective) channel estimation and data detection of FTN signaling. We consider the intersymbol interference (ISI) resulting from both the FTN signaling and the frequency-selective channel and adopt an efficient frame structure with reduced overhead. We propose a novel channel estimation technique of FTN signaling based on the least sum of squared errors (LSSE) approach to estimate the complex channel coefficients at the pilot locations within the frame. In particular, we find the optimal pilot sequence that minimizes the mean square error (MSE) of the channel estimation. To address the time-selective nature of the channel, we use a low-complexity linear interpolation to track the complex channel coefficients at the data symbols locations within the frame. To detect the data symbols of FTN signaling, we adopt a turbo equalization technique based on a linear soft-input soft-output (SISO) minimum mean square error (MMSE) equalizer. Simulation results show that the MSE of the proposed FTN signaling channel estimation employing the designed optimal pilot sequence is lower than its counterpart designed for conventional Nyquist transmission. The bit error rate (BER) of the FTN signaling employing the proposed optimal pilot sequence shows improvement compared to the FTN signaling employing the conventional Nyquist pilot sequence. Additionally, for the same SE, the proposed FTN signaling channel estimation employing the designed optimal pilot sequence shows better performance when compared to competing techniques from the literature.     

Chaotic modulations and performance analysis for digital underwater acoustic communications

Two modulation schemes, M-ary phase shift keying (MPSK) and M-ary frequency shift keying (MFSK), are commonly used for coherent and incoherent digital communications, respectively. Despite wide applications in underwater acoustic (UWA) communications, they are not suitable for confidential applications for their well-known generating processes and signal features. In this paper, two corresponding chaotic modulation methods are proposed to improve their security, namely chaotic MPSK (CMPSK) and chaotic MFSK (CMFSK). By application of chaotic sequences into the modulation procedures, they can prevent the unauthorized receivers from extracting information from the intercepted signals even with high SNR. The confidential performance of chaotic modulations is evaluated by a designed automatic modulation classification (AMC) system. Simulation results indicate a success identification rate of more than 90% for the MPSK and MFSK signal at the SNR from −10 dB to 40 dB, but only an identification rate of nearly zero for the CMPSK and CMFSK signal. Therefore, chaotic modulations have lower available probability and can achieve higher confidential performance. Also, an experiment was conducted to verify the performance of chaotic modulations in actual UWA communications. The experimental results show that chaotic modulations can achieve similar bit error ratio (BER) compared with conventional digital modulations, which verifies potential applications of chaotic modulations in confidential UWA communications.     

一种DCSK通信系统的延迟合并接收方案

提出一种多径衰落信道下DCSK通信系统的延迟合并接收方案. 理论分析和仿真结果证实了该方案的误码性能. 结果表明，和原DCSK通信系统相比，使用了延迟合并接收方案的DCSK通信系统误码性能有所提高；不同接收路径延迟对应的误码性能基本相同；各接收路径采用等增益合并方式时误码性能最好；其理论上限随着接收路径数增加而逐渐降低.