OFDM水声通信中最大多普勒频偏估计算法研究

在水声通信中,发射与接收端相对运动引起的多普勒频移会严重恶化OFDM系统传输效果.因此多普勒频移估计是水声通信的关键技术之一.本文采用的最大Doppler频移估计算法,是利用信道功率谱的分布对接收的导频信号进行频域差分检测来进行估计.这种方法复杂度低,精度较高,无需获得正确的信道时域冲击响应信息,也无需再对信号进行自相关运算,而且特别适用于低信噪比的水声通信系统.     

多普勒频移补偿在水声自适应跳频通信系统中的应用

传统的自适应跳频通信可以克服水声多径信道时变、频变、空变给通信质量带来的影响,在频率选择性衰落的信道上保证高质量的数据传输。基于概率的自适应跳频通信在传统的自适应跳频通信的基础上,根据信道通信质量的好坏分配不同的使用概率,能够进一步提高系统的可靠性和安全性。在水声通信中,多普勒现象非常明显。在水声基于概率的自适应跳频通信系统中,引入多普勒频移补偿能够进一步提高通信系统的可靠性。     

OFDM水声通信信道估计技术研究

水声通信系统的目的在于高速、可靠地将信源信息经过海洋声信道传输到信宿。正交频分复用(OFDM)技术具有抗多径、频带利用充分、传输数据率高的优点,在水声通信中具有重要的发展前景。但随着传输距离的增加,信道条件恶化,多径、多普勒频移影响加剧,需要设计相适应的OFDM系统并采取有效手段改善系统性能。介绍了基于导频的OFDM信道估计的基本算法,主要研究了LS(最小平方误差)、LMMSE(线性最小均方误差)、LRLMMSE(低秩线性最小均方误差)算法,对各种算法的性能和复杂度进行了分析。     

基于数据样本方差的正交频分复用水声通信多普勒频移估计方法

针对正交频分复用(OFDM)水声移动通信易受时变多普勒频移影响的缺点,该文提出一种基于数据样本方差的多普勒频移估计方法。利用前序符号的信道估计值恢复当前符号的有效数据序列及其频域分集副本,计算分集副本与数据序列的比值并搜索该比值序列在不同多普勒补偿因子下的方差,选取方差最小时对应的补偿因子作为多普勒频移估计值,利用稀疏贝叶斯学习和判决反馈信道估计算法获得修正后的信道频域响应并传递给后序符号,实现对多普勒频移的实时跟踪。数值仿真验证了该方法的可行性和优越性,海上试验证明,该方法实现了基于无人水下航行器的OFDM水声移动通信,能够对时变多普勒频移进行有效估计。     

移动通信中一种精确的多普勒频移估计方法

该文利用有效到达径上信道参数的估计值,统计其包络的电平通过率(Level Crossing Rate,LCR),据此估计出时变多径信道的多普勒频移。同时还在理论上分析了噪声环境下LCR方法的估计偏差,进而提出了精确的改进方法,使得通信系统在各种移动速度和信噪比下都具有较高的估计精度。     

移动水声通信多径传输非一致多普勒估计方法研究

现有水声通信中的多普勒估计方法一般都假定多径信道每条路径的多普勒因子相等,当各路径多普勒因子存在差异时,该类方法往往无法正确估计出每径的多普勒因子,导致补偿后的信号存在较大误差。为此该文在分析水声信道稀疏特性的基础上,提出一种基于稀疏表示理论的非一致多普勒估计方法。该方法结合水声多径信道与非一致多普勒稀疏特性生成包含信道信息的超完备字典,将每径的非一致多普勒因子估计转化为基于超完备字典的稀疏重构求解,实现非一致多普勒因子的准确估计。仿真结果表明,所提方法不仅能估计出差异较大的非一致多普勒因子,而且对于大多普勒频偏的估计也相当有效,特别适合于高速移动水声通信中的多普勒估计。     

MFSK水声远程通信技术及试验研究

海洋水声信道是多途、时变和频散的信道。论文针对浅海远程80～100km,设计出基于MFSK高数据率水声远程信息传输最佳系统。本系统经湖试验证表明:该技术在远程通信恶劣条件下具有较强抗多径干扰和信道衰落能力,对同步精度要求低,可实现80～100km,40bit/s,10-4量级通信精度的信息可靠传输,性能稳健,易于工程实现。     

采用时域联合稀疏恢复的多输入多输出水声信道压缩感知估计

多输入多输出(MIMO)水声通信技术可以在极其有限的水声信道频带资源内提高信道容量,但多径和同道干扰的同时存在,使传统信道估计算法如最小二乘算法、压缩感知估计算法的性能急剧下降。考虑到通信数据块间水声信道多径结构存在一定的相关性,该文利用这种数据块间多径结构的时间域相关性建立水声MIMO信道的时域联合稀疏模型,并利用同步正交匹配追踪算法进行多个数据块联合稀疏恢复信道估计,提高MIMO信道多径稀疏位置的检测增益并抑制同道干扰,提高水声MIMO信道的估计性能。仿真和MIMO水声通信海试实验表明了所提方法的有效性。     

基于DET的OFDM信道估计算法及分析

在高速无线通信中,由于多径衰落和多普勒频移引起的子载波间串扰,严重影响了OFDM系统的性能。因此,接收端的信道估计显得至关重要。基于离散进化变换进行时频分析的方法,可以较完全获得一个受多径衰落、频率选择性衰落干扰的信道的特性。仿真表明,在大多普勒频移影响的时变信道中,采用时频分析的方法比常规信道估计方法的性能有明显提高。     

水声信道中的迭代分组判决反馈均衡器

 与传统的时域均衡相比,频域均衡为单载波传输系统提供了一种计算更为有效的方法。该文对有大多径时延和多普勒频移的水声信道中的频域分组迭代判决反馈均衡器(IB-DFE)进行了研究,提出了一种联合迭代均衡和频域信道估计(JECE)算法,分析了JECE算法的性能以及不同数据参数对IB-DFE性能的影响,并计算了IB-DFE的计算复杂度。仿真结果表明,所提出的JECE算法对时变水声信道有更好的适应性。与TD-DFE相比,IB-DFE有3.6 dB以上的信噪比增益和约19%的复杂度增益,而且随着多径时延和多普勒频移的增加,这种性能增益会进一步增加。     

Synchronization techniques for underwater acoustic communications

Underwater acoustic channel is a challenging medium for communication due to the presence of significant multipath, high noise, frequency-dependent propagation loss, and high and non-uniform Doppler spread. Doppler shift is non-negligible in underwater communication due to the low velocity of underwater signals. Synchronization and Doppler estimation are important requirements for achieving good performance in this channel. Synchronization algorithms that give good performance in radio communication do not work well in underwater communication. Hence, this area has received a lot of attention from researchers. This paper surveys important works in the area. The techniques proposed in the literature for frame synchronization, frequency and phase synchronization, and timing synchronization in single carrier communications are reviewed here. The synchronization techniques proposed for OFDM, MIMO OFDM, and spread spectrum communication are also surveyed. Doppler estimation methods proposed in the literature are also reviewed. It is found that most of the recent works in underwater acoustic communication focus on OFDM synchronization. Deep learning-based methods proposed in the literature are also reviewed. Key open problems and areas that require future research attention in the field of synchronization and Doppler estimation in underwater communications are highlighted in this paper. The area needing most attention of underwater communication researchers was found to be MIMO OFDM due to the difficulty in synchronization in such systems while used in underwater communication. Reducing the computational complexity of the algorithms used is also important for future work. Schemes that work with Doppler due to relative velocity over 10 m/s also need to be developed.     

A prefix-based approach for joint Doppler and channel estimation in underwater communication

Developing a reliable and robust underwater acoustic communication system is a difficult task due to the complicated nature of the underwater channel, non-stationary noise, and several other factors. Indeed, channel estimation or equalization presents numerous challenges in this non-stationary, highly Doppler, multipath environment; as a result, traditional equalizers and PLL-based methods have limited performance. Generally, communication over such time-varying channels is accomplished via packets that contain a prefix/preamble signal for training, a payload containing the actual data, and a silent period for proper alignment. The prefix signal must be designed properly because it is used to estimate the channel and determine the start of the packet. In this paper, we propose a new prefix signal based on the hyperbolic chirp signal, which has Doppler invariance properties. These properties enable the extraction of the entire packet even under severe multipath and Doppler. Our new proposed prefix signal can accurately and efficiently characterize an underwater channel by estimating the multipath delay, amplitude, and Doppler scales. Our new method has been validated through extensive simulations using various channel models for its robustness and effectiveness under various conditions. It has also been tested on a real-world channel.     

长时延环境下多普勒频偏估计与补偿技术

长时延信道模型中相对多普勒频偏比一般的无线信道要大若干个数量级.典型的长时延信道有:水声信道,卫星通信信道等.水声信道中多普勒频偏不仅表现在频域上,还表现在时域上的压缩和扩展.文中对水声OFDM系统中宽带多普勒频偏提出了估计与补偿算法,可以有效消除宽带多普勒频偏的影响.     

Detection, Synchronization, and Doppler Scale Estimation with Multicarrier Waveforms in Underwater Acoustic Communication

In this paper, we propose a novel method for detection, synchronization and Doppler scale estimation for underwater acoustic communication using orthogonal frequency division multiplex (OFDM) waveforms. This new method involves transmitting two identical OFDM symbols together with a cyclic prefix, while the receiver uses a bank of parallel self-correlators. Each correlator is matched to a different Doppler scaling factor with respect to the waveform dilation or compression. We characterize the receiver operating characteristic in terms of probability of false alarm and probability of detection. We also analyze the impact of Doppler scale estimation accuracy on the data transmission performance. These analytical results provide guidelines for the selection of the detection threshold and Doppler scale resolution. In addition to computer-based simulations, we have tested the proposed method with real data from an experiment at Buzzards Bay, MA, Dec. 15, 2006. Using only one preamble, the proposed method achieves similar performance on the Doppler scale estimation and the bit error rate as an existing method that uses two linearly-frequencymodulated (LFM) waveforms, one as a preamble and the other as a postamble, around each data burst transmission. Compared with the LFM based method, the proposed method works with a constant detection threshold independent of the noise level and is suited to handle the presence of dense multipath channels. More importantly, the proposed approach does not need to buffer the whole data packet before data demodulation, which facilitates future development of online realtime receivers for multicarrier underwater acoustic communications.     

分数阶Fourier变换在深海远程水声通信中的应用

提出深海远程水声通信方案,利用深海声道特性,将通信节点置于声道轴以减小传播损失,提高通信距离.将分数阶Fourier变换(FRFT)应用于水声通信同步检测及Pattern时延差编码(PDS)水声通信体制,可减小多普勒频偏的影响.通过计算机仿真研究表明,分数阶Fourier变换相对于拷贝相关将更适用于存在多普勒频偏的相干多途水声信道条件,有助于实现低误码率通信.     

Performance evaluation and implementation of convolution coded OFDM modem in wireless underwater acoustic communication

High‐data transmission rate and reliable communication in underwater acoustic channel is challenging because of limited bandwidth, multipath propagation, and Doppler shift, which results in poor bit error performance. Under this constraint, this paper explains the simulation results of underwater wireless acoustic data transmission system by using quadrature phase shift keying modulation with convolution encoder at the transmitter and proposed Viterbi decoder at the receiver. The decoder algorithm in comparison with MATLAB inbuilt function shows asserting improved results. This paper evaluates the performance of convolution coded orthogonal frequency division multiplexing modem, which is studied over typical underwater channel through an extensive computer simulation and a semirealistic experimentation. The performance of convolution coded orthogonal frequency division multiplexing system is measured in time domain plots, bit error rate curves, and number of bit errors per frame over additive white Gaussian noise and Rayleigh channel conditions.     

差分混沌移位键控在水声通信中的应用

水声信道作为目前已知最严酷、最复杂的无线通信信道之一，具有强多途干扰、时—频率双扩展、高噪、带宽窄等特征。水声信道的时变特性，使得估计与跟踪信道很困难，因此，研究无需信道估计与均衡的非相干稳健水声通信调制方法具有重要的意义。首先介绍了基于正交频分复用（OFDM）和扩频调制的水声调制技术的研究进展，然后，分析了差分混沌移位键控在水声信道下的可行性，提出了两种基于OFDM的多载波差分混沌移位键控方案，给出两个方案的调制解调器原理。在时—频双扩展信道和水声信道下，对两个方案进行了性能仿真和分析。性能结果表明所提出的方案在水声信道下具有良好的稳健性。     

Mobile direct-sequence spread-spectrum underwater acoustic communication based on improved differential energy detector

A Doppler estimation method based on the structural features of spread spectrum signal was proposed,which could achieve real-time Doppler estimation for received spread spectrum signals by energy detecting the result of matching correlation calculation of adjacent received spreading symbols.In addition,this method made full use of the spread spectrum processing gain and could do Doppler estimation under low SNR condition.Improved differential energy detector was further proposed,which can effectively deal with the rapid carrier phase fluctuation interference caused by Doppler effect and time-varying multi-path interference by detecting the output energy of two correlators.Simulation verified the robustness of improved differential energy detector algorithm and the Doppler estimation method based on spread spectrum signal.At-sea data shows that improved differential energy detector combined with the proposed Doppler estimation method can achieve low bit error rate communication for direct-sequence spread spectrum underwater acoustic communication with large time-varying Doppler interference when SNR is ?10 dB.     

MIMO-HFM: A MIMO System with Hyperbolic Frequency Modulation for Underwater Acoustic Communication

Reliable transmission and high data rate over underwater acoustic channels are considerably challenging. In this paper, we propose Multiple-Input and Multiple-Output (MIMO) scheme using a Hyperbolic Frequency Modulation (HFM) waveform. Our proposed system combines the advantages of both systems-special multiplexing of MIMO and immunity against Doppler shift of HFM. To increase the spectral efficiency, we employ M-ray HFM and overlapped sub-channels by leveraging the high temporal resolution characteristic. To verify effectiveness of our system, we have designed a theoretically enhanced acoustic simulator, which especially focuses on the reflection phenomenon by utilizing approved reflection loss models. Based on our acoustic simulator, we could verify that our system is robust against for multipath fading and Doppler shifting while keeping the multiplexing benefit of MIMO, while maintaining a very low complexity and system overhead. In addition, the results provide a useful insight for physical layer design in acoustic communication systems.